ATLAS OF FUNCTIONAL NEUROANATOMY

Transcription

ATLAS OF FUNCTIONAL
NEUROANATOMY
By
WALTER J. HENDELMAN, M.D., C.M.
Professor
Department of Cellular and Molecular Medicine
Faculty of Medicine
University of Ottawa
Ottawa, Canada
©2000 CRC Press LLC
Library of Congress Cataloging-in-Publication Data
Hendelman, Walter.
Atlas of functional neuroanatomy/by Walter J. Hendelman.
p. cm.
Includes bibliographical references and index.
ISBN 0-8493-1177-2 (alk. paper)
1. Neuroanatomy--Atlases. I. Title.
[DNLM: 1. Central Nervous System--anatomy & histology-Atlases. WL17 H495a 2000]
QM451 .H347 2000
611.8’022’2--dc21
99-087837
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DEDICATION
To my wife, Teena, and our children, Lisanne and Devra
To my many teachers and mentors, particularly in remembrance of
Dr. Donald Hebb
Dr. Richard Bunge
Dr. Malcolm Carpenter
each an inspiring tutor at a particular point in my growth and development
as a neuroscientist
And finally, to all students of the Brain …
PREFACE
The instructional goal of this Atlas is to assist the student of the brain to achieve an understanding and a three-dimensional visualization of the human central nervous system (CNS).
The Atlas of Functional Neuroanatomy is written for medical students who are studying the CNS for the first time, students in allied health fields, and professionals-in-training (physicians, nurses, physical and occupational therapists) who
require a visual reference to the structures of the CNS, as well as students in certain undergraduate courses, particularly
neuroscience and psychology. Regardless of the student, the challenge to the teaching faculty is the same — how could
we improve, enhance, and facilitate the learning process? We, as teachers, must see the learning task from the perspective
of the student — how can he/she learn, understand, and assimilate this very complex subject matter, particularly with
the volume of material to be learned and the short period of time typical of new curricula?
Clearly the challenge to any author is to try to organize and reduce the information load and present core material with
adequate explanation. The Atlas of Functional Neuroanatomy contains both diagrams and text, an optimum way of
guiding the student through the complexity of the structure and function of the CNS, with some clinical references to
make the information relevant to the real world of people with diseases. The illustrations include diagrams and photographs, each labeled to the degree necessary for a student learning the material for the first time, or for a professional
requiring a resource review of the CNS. The focus is on the illustrations, each of which is accompanied by explanatory
text on the facing page, supplemented by a brief introduction to various sections (e.g., brainstem, motor systems,
limbic).
This Atlas is built upon three previous editions [titled Student’s Atlas of Neuroanatomy] which began with some illustrations, then photographic material, and subsequently added air brush diagrams on the basal ganglia, thalamus, and
limbic system. After use in our course here and much feedback from students, the material of the Atlas has been significantly reorganized and rethought, and much of the text rewritten. The approach of this revised Atlas is to integrate the
structure and function so that a student can understand the neurological approach to disease of the nervous system,
with a focus on where the information has been interrupted.
The Atlas starts with an Orientation to the various parts of the nervous system, presented from the spinal cord upward
to the brain. Radiographic material has been added, since this is the way the CNS will be viewed and investigated by all
our students in clinics. The second section, Functional Systems, presents the sensory and motor pathways as they traverse the nervous system. The addition of color to these diagrams contributes substantially to their visual impact.
The third section, Neurological Neuroanatomy, has both an anatomical and neurological orientation. Sufficient information is given to allow the student to work through the neurological question — where is the disease process occurring (i.e., neurological localization)? The emphasis in this section is on the brainstem. To assist in this goal, a select
series of cross sections of the human brainstem is included. In addition, new illustrations have been added on the blood
supply to the brain, using color and graphic overlays, since vascular lesions are still most common and relate closely to
the functional neuroanatomy. This section is supplemented with cross sections of the human brainstem.
The section on the Limbic System has been completely revised and much reduced in content from the previous edition.
It is placed as the last section of the Atlas because it can be taught at various points in the medical curriculum, e.g., as
part of “mind” or psychiatry. Other courses might not include this specialized topic.
Consistent with the computer/digital revolution, many of the illustrations were converted into computer graphics, with
tones of shading. Color has been added to facilitate the visualization of the CNS pathways, in both system-based
(Section B) and cross-sectional diagrams (Section C). Some students might still want to add color to the illustrations;
coloring the illustrations has assisted many students by adding an active component to the learning process. (A guide to
color coding is included after the list of illustrations.)
Much of the subject matter’s difficulty is terminology — complex, difficult to spell, sometimes inconsistent, with a Latin
base, and sometimes with names of individuals (used often by neurologists, neurosurgeons, and neuroradiologists). A
glossary of terms is appended to help the student through this task.
Students might wish to consult more complete texts on the anatomy and physiology of the nervous system and certainly
some neurology books. A guide to this reference material is included in the annotated bibliography. Added to this are
suggestions for material available on CD-ROMs, as well as the Internet. Students are encouraged to seek out additional
resources of this nature.
The digital revolution has led to the expectation of a visual presentation with clear graphics on the screen! Therefore,
this edition of the Atlas is being published with an accompanying CD-ROM, allowing the use of full-color illustrations,
where relevant. As in the book, each graphic has a brief explanatory text. We hope that students will have easy access to
view this CD and that this additional resource will enhance learning! The author is grateful to CRC Press for agreeing to
publish the Atlas with the CD.
Many individuals have contributed to the Atlas. Their efforts are deeply appreciated. We have worked collaboratively to
try to present a clear understandable view of the structure and function of the CNS. All have worked under my direction
and therefore the ultimate value of the Atlas, whether favorable or unfavorable, rests on the shoulders of the author. You,
the learner, will be our best judge.
Comments are welcome and can be e-mailed to the author ([email protected]).
Special thanks are extended to the members of CRC Press LLC without whose help this project would not have been
completed.
Walter J. Hendelman, M.D., C.M.
AUTHOR BIOGRAPHY
Dr. Walter Hendelman is a Canadian, born and raised in Montreal. He completed his undergraduate studies at McGill
University with an honors program in psychology. His first experimental work was with rats that had lesions of the hippocampus, which was then a little-known area of the brain. At that time, Professor Donald Hebb was the chair of the
Psychology Department and was gaining prominence for his theory known as “cell assembly” (how the brain functions).
Dr. Hendelman then proceeded to do his medical studies at McGill, in the shadow of the world-famous Montreal
Neurological Institute (MNI) where Dr. Wilder Penfield and colleagues were forging a new frontier in the understanding
of the brain. Dr. Hendelman then completed an internship and a year of pediatric medicine, again in Montreal.
Dr. Hendelman’s next decision was between clinical (pediatiric) neurology or brain research — he chose the latter and
completed four years of postgraduate studies in the U.S., in the emerging field of developmental neuroscience, using
what were then the new techniques of nerve tissue culture and electron microscopy. His research mentor at Columbia
Medical Center in New York was Dr. Richard Bunge, and his neuroanatomy mentor was Dr. Malcolm Carpenter, the
author of a neuroanatomy textbook.
Ottawa, Canada has been the site for Dr. Hendelman’s entire academic career, at the Faculty of Medicine of the
University of Ottawa. The Department of Anatomy became the Department of Anatomy and Neurobiology, which then
merged with physiology and pharmacology, forming what is now the Department of Cellular and Molecular Medicine.
Dr. Hendelman continued his research, using nerve tissue culture and studying the development of the cerebellum;
more recently, he has been involved in studies on the development of the cerebral cortex. He is a member of various
neuroscience and anatomy professional organizations, has attended and presented at their meetings, and has numerous
research publications, often in collaboration with other scientists.
In addition to research, teaching, and the usual academic “duties,” Dr. Hendelman was involved with a University committee on research ethics. He has also been very active in curriculum planning and teaching matters in the Faculty. In
addition, after some further training, Dr. Hendelman participated in a clinic for the assessment of children with learning problems at the Children’s Hospital of Eastern Ontario.
Dr. Hendelman is a student of the brain and has been deeply engaged as a teacher of the subject throughout his career.
He is dedicated to assisting those who wish to learn functional neuroanatomy – he has produced teaching videotapes
and three previous editions of this Atlas. More recently, he has authored (with collaborators) two computer learning
modules, one on the spinal cord based upon the disease syringomyelia, and the other on voluntary motor pathways.
Both modules have original graphics to assist the learning of this challenging and fascinating subject matter, the human
brain.
ACKNOWLEDGMENTS
The illustrations in the Atlas have evolved and could not have been created without the efforts of the following individuals:
Illustrations
Mr. Jean-Pierre Morrissey, a medical student at the time of the first edition of Student’s Atlas of Neuroanatomy (line
drawings of pathways and cerebellum)
Dr. Andrei Rosen (the airbrush diagrams of the basal ganglia, thalamus, limbic system)
Photography
Mr. Stanley Klosovych, formerly director of the Health Sciences Communication Services, University of Ottawa (all
photographs of the brain)
Medical Artist
Mr. Emil Purgina, of the same service, now retired (Figure 21 and improvements to the third edition of Student’s Atlas
of Neuroanatomy)
Medical Illustrator
Mr. Gordon Wright (all digital enhancements)
Radiographs
Colleagues and staff at the Ottawa Hospital (General Campus) and the Children’s Hospital of Eastern Ontario
Brainstem Sections
Colleagues and staff of the Department of Pathology, Children’s Hospital of Eastern Ontario
Parts of the Student’s Atlas of Neuroanatomy , First, Second, and Third Editions which also appear in this book, were
supported by grants from Teaching Resources Services of the University of Ottawa. Also very much appreciated and acknowledged are the contributions of the University of Ottawa Press and W.B. Saunders to the previous editions.
The support of my home department and its members at the Faculty of Medicine of the University of Ottawa, including
secretaries and other support staff, is gratefully acknowledged; originally called Anatomy, then Anatomy and
Neurobiology, the department is now the Department of Cellular and Molecular Medicine.
Finally, I gratefully acknowledge the many classes of students who have provided their comments, suggestions, and feedback over the years.
With thanks,
Walter J. Hendelman, M.D., C.M.
TABLE OF CONTENTS
Preface
Acknowledgements
List of Illustrations
Suggestions for Color Coding
Section A: Orientation
Spinal Cord
Brainstem and Cranial Nerves
Cerebellum
Diencephalon (Thalamus)
Cerebral Hemispheres
Cortex
Corpus Callosum and White Matter
Ventricles and CSF
Basal Ganglia
Internal Capsule
Section B: Functional Systems
Part I: Sensory Systems
Somatosensory and Trigeminal Systems
Special Senses (Audition and Vision)
Part II: Reticular Formation and Pain
Part III: Motor Systems
Cortical
Brainstem
Basal Ganglia
Cerebellum
Section C: Neurological Neuroanatomy
Blood Supply
Thalamus
Brainstem Histology
Midbrain
Pons
Medulla
Spinal Cord
Brainstem (Human) Cross Sections
Section D: The Limbic System
Limbic Lobe
Limbic Structures
Hippocampus
Amygdala
Limbic Diencephalon
Hypothalamus and Septal Region
Olfactory System
Basal Forebrain
Annotated Bibliography
Glossary of Terms
LIST OF ILLUSTRATIONS
Section A: Orientation
FIGURE 1A: Spinal Cord I — Spinal Cord: Longitudinal View
FIGURE 1B: Spinal Cord — Spinal Cord MRI: Longitudinal View
FIGURE 2A: Spinal Cord II — Spinal Cord Cross Section: C8 Level
FIGURE 2B: Spinal Cord — Spinal Cord MRI: Axial View
FIGURE 3: Brainstem I — Brainstem and Diencephalon: Ventral View
FIGURE 4: Brainstem II — Brainstem and Diencephalon:
Ventral (Photographic)view
FIGURE 5: Brainstem III — Cranial Nerve Nuclei: Motor
FIGURE 6: Brainstem IV — Cranial Nerve Nuclei: Sensory
FIGURE 7: Brainstem V — Brainstem: Dorsal View (Cerebellum Removed)
FIGURE 8: The Cerebellum — Cerebellum and Brainstem (Photographic View)
FIGURE 9: The Diencephalon — Thalamus: Orientation
FIGURE 10: Thalamus — Thalamus: Nuclei
FIGURE 11: Cerebral Hemispheres I — Cerebral Cortex: Dorsal View
FIGURE 12: Cerebral Hemispheres II — Cerebral Cortex: Dorsolateral View
FIGURE 13: Cerebral Hemispheres III —Cerebral Cortex: Inferior View
FIGURE 14: Cerebral Hemispheres IV — Inferior Surface: Brainstem Removed
FIGURE 15: Cerebral Hemispheres V — Corpus Callosum: Dorsal View
FIGURE 16: Cerebral Hemispheres VI — Cerebral Cortex: Medial View
FIGURE 17: Radiologic View of Hemispheres — MRI: Sagittal View
FIGURE 18: Corpus Callosum — Cerebral Hemispheres: Dissected View
FIGURE 19A: White Matter — Cerebral Hemispheres: Association Fibers
FIGURE 19B: White Matter — Cerebral Hemispheres: Association Fibers
FIGURE 20A: Ventricles — Ventricles: Lateral View
FIGURE 20B: Ventricles — Ventricles: Anterior View
FIGURE 21: Cerebrospinal Fluid — Schematic of CSF Circulation
FIGURE 22: Basal Ganglia I — Basal Ganglia: Orientation
FIGURE 23: Basal Ganglia II — Basal Ganglia: Nuclei A
FIGURE 24: Basal Ganglia III — Basal Ganglia: Nuclei B
FIGURE 25: Basal Ganglia IV — Basal Ganglia and Ventricles
FIGURE 26: Internal Capsule I — Internal Capsule: White Matter
FIGURE 27: Internal Capsule II — Horizontal Section of Hemispheres (Photographic View)
FIGURE 28A: Horizontal View — Horizontal View: CT
FIGURE 28B: Horizontal View — Horizontal View: MRI
FIGURE 29: Coronal View — Coronal Section of Hemispheres (Photographic View)
FIGURE 30: Coronal View — Coronal View: MRI
Section B: Functional Systems
Part I: Sensory Systems
FIGURE 31: Anterolateral System — Pain, Temperature, Crude Touch
FIGURE 32: Dorsal Column — Medial Lemniscus Pathway — Discriminative Touch, Joint Position,
Vibration
FIGURE 33: Trigeminal System — Discriminative Touch, Pain, Temperature
FIGURE 34: Sensory Systems— Somatosensory and Trigeminal Pathways
FIGURE 35: Audition: Hearing — Auditory Pathway I
FIGURE 36: Auditory System — Auditory Pathway II
FIGURE 37: Auditory System — Auditory Gyri (Photographic View)
FIGURE 38: Sensory Systems — Ascending Tracts and Sensory Nuclei
FIGURE 39A:Visual System: A — Visual Pathway
FIGURE 39B: Visual System: B — Visual Reflexes
Part II: Reticular Formation
FIGURE 40A: Reticular Formation I — Organization
FIGURE 40B: Reticular Formation II — Nuclei
FIGURE 41: Pain — Descending Control System
Part III : Motor Systems
FIGURE 42: Cortico-spinal Tract: The Pyramidal System — Direct Voluntary Pathway
FIGURE 43: Cortico-bulbar (and Cortico-pontine) Fibers — Brainstem Motor System
FIGURE 44: Rubro-spinal Tract — Non-Pyramidal Motor System
FIGURE 45: Descending Tracts and Motor Nuclei — Motor Pathways and Nuclei
FIGURE 46: Pontine (Medial) Reticulo-spinal Tract — Indirect Voluntary Pathway
FIGURE 47: Medullary (Lateral) Reticulo-spinal Tract — Indirect Voluntary Pathway
FIGURE 48: Lateral Vestibulo-spinal Tract — Non-pyramidal Motor System
FIGURE 49A: Vestibular System — Vestibular Nuclei
FIGURE 49B: Medial Longitudinal Fasciculus — MLF and Associated Tracts
FIGURE 50: Basal Ganglia: Circuitry — Motor Regulatory Systems
FIGURE 51: Thalamus: Motor Circuits — Motor Regulatory Systems
FIGURE 52: Cerebellum I — Functional Lobes
FIGURE 53: Cerebellum II — Afferents
FIGURE 54A: Cerebellum III — Circuitry
FIGURE 54B: Cerebellum IV — Efferents
FIGURE 55: Cerebellum V — Superior Cerebellar Peduncle
Section C: Neurological Neuroanatomy
FIGURE 56: Blood Supply I — The Arterial Circle of Willis (Overlay)
FIGURE 57: Blood Supply II — MR Angiogram (MRA)
FIGURE 58: Blood Supply III — Cortical: Dorsolateral (Overlay)
FIGURE 59: Blood Supply IV — Cortical: Medial (Overlay)
FIGURE 60: Blood Supply V — Internal Capsule and Basal Ganglia
FIGURE 61: Thalamus — Nuclei: Histological
FIGURE 62: Brainstem: Histology — Ventral View: Schematic
FIGURE 63: Brainstem: Histology — Sagittal View: Schematic
FIGURE 64: Upper Midbrain — Cross Section (B1)
FIGURE 65: Lower Midbrain — Cross Section (B2)
FIGURE 66: Upper Pons — Cross Section (B3)
FIGURE 67: Mid Pons — Cross Section (B4)
FIGURE 68: Lower Pons — Cross Section (B5)
FIGURE 69: Upper Medulla — Cross Section (B6)
FIGURE 70: Mid Medulla — Cross Section (B7)
FIGURE 71: Lower Medulla — Cross Section (B8)
FIGURE 72: Spinal Cord Tracts — C8 level
FIGURE 73: Spinal Cord — Cross-sectional Views
Brainstem Cross-Sections (Human)
Section D : The Limbic System
FIGURE 74: Limbic Lobe
FIGURE 75: Limbic Structures
FIGURE 76: Hippocampus (Photographic View)
FIGURE 77A: Hippocampal Formation
FIGURE 77B: Hippocampal Formation: 3 Parts
FIGURE 78: Coronal Brain Section (Photographic View)
FIGURE 79A: Amygdala
FIGURE 79B: Amygdala — Connections
FIGURE 80: Limbic Structures and the Lateral Ventricle
FIGURE 81: Limbic Diencephalon — Anterior Nucleus
FIGURE 82: Limbic Diencephalon — Dorsomedial Nucleus
FIGURE 83A: Hypothalamus and Limbic Midbrain
FIGURE 83B: Septal Region and Medial Forebrain Bundle
FIGURE 84: Olfactory System
FIGURE 85A: Basal Forebrain — Basal Nucleus.
FIGURE 85B: Basal Forebrain — Basal Ganglia
SUGGESTIONS FOR COLOR CODING
Learning styles vary from person to person, but for many people color seems to add a significant beneficial dimension
to the learning of neuroanatomy. Although color has been added to several diagrams in the current edition, particularly
the pathways (in Sections B and C), many students have been helped in their learning by adding color to the illustrations. The process adds information and depth to the diagrams and adds an active component to the learning process.
We have consistently used various gray tones in the cross sections of the brainstem and spinal cord (Section C) to unify
— visually — the functional aspects of the nuclei and tracts. While this presentation scheme might suffice for some
students, others might want to use color. For convenience, numbers have been inserted beside certain structures to indicate which color to use (e.g., Figure 64). The same color coding has been used in the accompanying CD-ROM.
Note to student: It is recommended that you view the colors used on the CD-ROM and select your coloring choices to
match.
The following color scheme is suggested:
SENSORY
Dorsal column – medial lemniscus
#7
royal blue
Anterolateral
#7B
blueberry
Trigeminal
#4
jade green
Special senses
#11
dark brown
#10
orange
MOTOR
Voluntary (cortico-spinal)
Reticular formation
#1
yellow
Vestibular (nuclei and tracts)
#5
salmon
Cerebellum (nuclei and tracts)
#9
light blue
Other motor pathways
#8
green
Parasympathetic
#13
violet
Substantia Nigra
#12
black
Red Nucleus (and tract)
#3
red
Locus Ceruleus
#6
dark blue
Other
#2
peach
SPECIAL NUCLEI
Background
pencil shading
Some students may even wish to add color to some of the airbrush diagrams, including the basal ganglia, thalamus, and
limbic system.
It’s your choice!
Section A
ORIENTATION
An understanding of the central nervous system — the
CNS — requires a knowledge of its component parts
and their specialized function. This section introduces
the student to the CNS from an anatomical and functional viewpoint; subsequent sections use these components to build the functional units that are the sensory
and motor systems.
FUNCTIONAL NEUROHISTOLOGY
The CNS is composed of neurons and supporting cells,
the glia. The neuron has a cell body (also called soma,
or perikaryon), dendrites which extend a short distance
from the soma, and an axon which connects one neuron
with others, either close by (for interneurons) or at a
distance. Neuronal membranes are specialized for electrochemical events, which allow these cells to receive and
transmit messages to other neurons. The dendrites and
cell bodies of the neurons receive information, and the
axons transmit the firing pattern of the cell to the next
neuron. Communication between neurons occurs
almost exclusively at specialized junctions called synapses,
using biological molecules called neurotransmitters.
These modify ion movements across the neuronal membranes of the synapse and alter neurotransmission —
they may be excitatory, inhibitory, or modulatory in
their action. The post-synaptic neuron will modify its
firing pattern depending upon the summative effect of
all the synapses acting upon it at any moment in time.
Within the CNS, neurons that share a common function
are often grouped together; such collections are called
nuclei. In other parts of the brain, the neurons are
positioned at the surface, forming a cortex. In the
cortical organization, cells are arranged in layers, and
the neurons in each layer are functionally distinct. This
arrangement enhances, so it seems, the complexity of
information processing. Older cortical areas have three
layers (e.g., the cerebellum); more recently evolved
cortices have six layers (the cerebral cortex) and
sometimes sublayers.
Much of the remainder of the brain consists of axons
which connect one part of the brain with other areas.
These fibers serve to link the various parts of the brain
with each other. Many of the axons are myelinated,
which serves to increase the speed of axonal conduction;
the thicker the myelin sheath the faster the conduction.
Axons originating from one area (cortex or nucleus) and
destined for another are usually grouped together and
form a tract, also called a pathway (or fasciculus).
There are two types of glial cells. Astrocytes are involved
in supportive structural and metabolic events.
Oligodendrocytes are responsible for the formation and
maintenance of the myelin which ensheaths the axons.
Some of the early maturation that we see in infants and
children can be accounted for by the progressive myelination of the various pathways within the CNS.
ORGANIZATION OF THE CNS
One approach to an understanding of the nervous
system is to conceptualize a number of functional
modules, starting with simpler ones and moving to the
higher primates and humans with a more complex
organizational network of cells and connections.
The basic unit of the CNS is the spinal cord (see Figures
1A and 1B), which connects the central nervous system
with the periphery. It receives sensory information from
the skin and body wall and sends motor commands to
the muscles. Reflex circuits and other motor patterns
are organized in the spinal cord. These are under the
influence of motor areas in other parts of the brain.
Afferent and efferent information concerning the autonomic nervous system is also part of the functioning of
the spinal cord. The incoming sensory nerves and the
outgoing motor nerves organize the spinal cord into
segments (e.g., cervical, lumbar) and levels (see Figure 73).
As the systems of the brain become more complex, new
control “centers” have evolved. These are often spoken of
as higher centers. The first set of these is located in the
brainstem, which is situated above the spinal cord and
within the skull (in humans). The brainstem includes
three distinct areas — the medulla, pons, and midbrain
(see Figures 3 and 4). Some nuclei within the brainstem
are concerned with essential functions, such as the regulation of blood pressure, pulse, and respiration. Other
nuclei within the brainstem are involved in setting our
level of arousal and play an important role in maintaining our state of consciousness. Special nuclei in the
brainstem are responsible for some basic types of movements in response to gravity or sound. Many nuclei in
the brainstem are related to the cerebellum. In addition,
most of the cranial nerves and their nuclei (which
supply the structures of the head and neck) are anchored in the brainstem.
The cerebellum is situated behind the brainstem (see
Figure 8). This “little brain” is involved in motor coordination and also in the planning of movements. How this
is accomplished will be understood once the
input/output connections of the various parts of the
cerebellum are studied. Parts of the cerebellum are quite
old in the evolutionary sense, and other parts are relatively new. The cerebellum has a simpler form of cortex
which consists of only three layers.
Next in the hierarchy of the development of the CNS is
the area of the brain called the diencephalon (see
Figures 3 and 9). Its largest part, the thalamus, develops
in conjunction with the cerebral hemispheres and acts as
the gateway to the cerebral cortex. The thalamus consists
of a group of nuclei which project to the cortex and
receive reciprocal connections from the cortex. The
hypothalamus, a much smaller part, serves mostly to
control the neuroendocrine system and also organizes
the activity of the autonomic nervous system. Parts of
the hypothalamus are intimately connected with the expression of basic drives (e.g., hunger and thirst), the regulation of water in our bodies, and emotional behavior
as part of the limbic system (see below).
With the continued evolution of the brain there is encephalization, which has culminated in the development
of the cerebral hemispheres. Buried within the cerebral
hemispheres are the basal ganglia, large collections of
neurons (see Figure 22) which are involved mainly in
the initiation and organization of motor movements.
These neurons affect motor activity through their influence on the cerebral cortex.
The surface of the cerebral hemispheres is occupied by
cortex, the cerebral cortex (see Figures 11 and 12), most
of which is six-layered (also called the neocortex). We
need our cerebral cortex for thinking, consciousness,
and language, and many other functions related to the
sensory and motor systems. In the human, the cerebral
cortex is thrown into ridges (gyri; singular, gyrus) and
valleys (sulci; singular, sulcus). The expansion of the
cerebral cortex in the human, both in terms of size and
complexity, has resulted in this part of the brain becoming the dominant controller of the CNS, being capable,
so it seems, of overriding most of the other regulatory
systems.
A number of areas of the brain are involved in behavior
which is characterized by the reaction of the animal or
person to situations. This reaction is often termed “emotional,” and in humans it consists of both psychological
and physiological changes. Various parts of the brain are
involved with these activities, and collectively they have
been named the limbic system. This network of neurons
includes those found in the cortex, various subcortical
areas, parts of the basal ganglia, the hypothalamus and
parts of the brainstem. (The limbic system is described
in the final section of the Atlas.)
In summary, the nervous system has evolved so that its
various parts have assigned tasks. In order for the
nervous system to function properly, there must be
communication between the various parts. Some of
these links are the major sensory and motor pathways,
called tracts (or fascicles). Much of the mass of tissue in
our brain is made up of these pathways (e.g., see Figures
32 and 42). One of the major puzzles in the growth and
development of the nervous system involves understanding the mechanisms whereby the various parts of
the nervous system link with each other in a seemingly
precise manner.
STUDY OF THE CNS
Early studies of the normal brain were generally descriptive. Brain tissue does not have a firm consistency and
the brain needs to be fixed for gross and microscopic examination. One of the most common fixatives used to
preserve the brain for study is formalin; after being preserved in formalin, the brain can be handled and sectioned. Areas containing predominantly neuronal cell
bodies (and their dendrites and synapses) become
grayish in appearance after formalin fixation and are
traditionally called gray matter. Tracts containing myelinated axons become white in color with formalin fixation and such areas are likewise simply called the white
matter (see Figures 27 and 29).
We have learned much about the normal function of the
human CNS through study of diseases and injuries to
the nervous system. Diseases of the nervous system can
involve the neurons, either directly (e.g., metabolic
disease) or by reducing the blood supply which is critical for the viability of nerve cells. Neurons are highly
specialized cells and do not have the ability to regenerate; once lost, they cannot be replaced. Some degenerative diseases affect a particular group of neurons. Other
diseases can affect the cells supporting the myelin
sheath, thereby disrupting neurotransmission.
Biochemical disturbances may disrupt the balance of
neurotransmitters and cause functional disease states.
The recent introduction of functional imaging of the
nervous system is revealing fascinating information
about the functional organization of the CNS. We are
slowly beginning to piece together an understanding of
what is considered by many as the last and most important frontier of human knowledge, an understanding of
the brain.
FIGURE 1A
SPINAL CORD I
SPINAL CORD: LONGITUDINAL
VIEW
The spinal cord is an elongated part of the CNS that is
located in the vertebral canal. It consists of gray matter
(neurons), organized as nuclei (see Figure 2A), and
white matter, the various pathways (see Section B; also
Figures 72 and 73). Nerve roots enter and exit from the
spinal cord, giving the appearance of a segmented structure. The spinal cord is covered with meninges — dura,
arachnoid, and pia.
The four vertebral levels — cervical, thoracic, lumbar,
and sacral — are indicated (on the right side). The
spinal cord ends at the level of L2 (second lumbar vertebra) in the adult. Hence, the levels of the spinal cord do
not match the vertebral levels. One must be very aware
of which reference point is being used when discussing
spinal cord injuries, the vertebral or spinal.
Macroscopically, there are two enlargements of the cord:
at the cervical level is the brachial plexus (for the upper
limb), and at the lumbosacral level are the lumbar and
sacral plexuses (for the lower limb). The cord’s lowermost portion tapers and is called the conus medullaris.
Below that level of L2/vertebral in the adult, inside the
vertebral canal, are numerous nerve roots collectively
called the cauda equina; these are found within the
lumbar cistern (see MRI in next illustration; also discussed with Figure 21), an enlargement of the subarachnoid space. This is where a lumbar puncture (LP) is
performed to obtain a cerebrospinal fluid (CSF) sample.
The spinal cord, notwithstanding its relatively small size
(compared with the rest of the brain), is absolutely essential for our normal function. It is the connector
between the CNS and the body (other than the head and
neck). On the sensory side, the information arriving
from the skin, muscles, and viscera informs the CNS
about what is occurring in the periphery; this information then ascends to higher centers in the brain. On the
motor side, the nerves leaving the spinal cord control
our muscles (acting through the anterior horn cells and
their axons). Although the spinal cord has a functional
organization within itself, the neurons of the spinal cord
receive their instructions from higher centers, including
the cerebral cortex, via several descending tracts. This
enables us to carry out normal movements, including
walking and voluntary activities. The spinal cord also
has a motor output to the viscera and glands, part of the
autonomic nervous system. The area of the conus is responsible for the autonomic control of bowel and
bladder, subject to commands from higher centers (including the cortex).
Developmental Aspects
Embryologically, the spinal cord commences as a tube of
uniform size. In those segments that innervate the limbs
(muscles and skin), all the neurons reach maturity.
However, in the intervening portions, there is massive
programmed cell death during development because
there is less peripheral tissue to be supplied. In the adult,
therefore, the spinal cord has two enlargements: the cervical for the upper limb, and the lumbo-sacral for the
lower limb, each giving rise to the nerve plexuses for the
upper and lower limbs, respectively.
In the fetus, the spinal cord and vertebral column are the
same length. After birth, the vertebral column continues
to grow while the spinal cord does not. In the adult, the
spinal cord ends at the level of the second lumbar vertebra. Therefore, the exiting roots for the lower extremity
travel within the subarachnoid space (the lumbar
cistern) for a fair distance before exiting, forming the
cauda equina.
The incoming sensory and outgoing motor roots which
innervate the periphery allow us to discuss the spinal cord
in terms of segments. This “segmentation” has an embryological explanation: areas of skin are supplied by certain
nerve segments, called dermatomes (e.g., umbilical region
= T10), and various muscles are supplied by certain segments called myotomes (e.g., biceps of the upper limb =
C5 & 6; quadriceps of the lower limb = L3 & 4).
Clinical Aspects
The segmental organization of the spinal cord and the
known pattern of innervation allow a knowledgeable
practitioner, after performing a detailed neurological examination, to develop an accurate idea of the location
— the spinal cord segmental level — of a lesion of the
spinal cord.
Dura mater
Cervical
Arachnoid
mater
Subarachnoid
space
Thoracic
Pia mater
Conus
medullaris
Lumbar
Cauda
equina
Lumbar
cistern
Sacral
FIGURE 1A: Spinal Cord I — Spinal Cord: Longitudinal View
FIGURE 1B
SPINAL CORD
SPINAL CORD MRI: LONGITUDINAL
VIEW
This is an MRI — magnetic resonance image — of the
vertebral column and spinal cord, viewed in a mid-sagittal plane. It is a TI-weighted image, in which the cerebrospinal fluid (CSF) is dark. (The various radiological
techniques used to image the nervous system are discussed with Figure 17.) This image is from an older
child, in which no pathology was found in the spinal
cord radiological examination.
Because of the length of the spinal cord, it is shown in
two parts, upper and lower. The upper portion (image
on the left) shows the spinal cord to be a continuation of
the medulla of the brainstem, at the lowermost border
of the skull. A white asterisk has been placed in the enlargement of the subarachnoid space — the cisterna
magna (see Figure 20A) — below the cerebellum. The
spinal cord tissue is located in the middle of the vertebral column, surrounded by the meninges (which
cannot be visualized), with the dura separating the subarachnoid space containing CSF from the space outside
the meninges, the epidural space (between the meninges
and vertebra themselves). The epidural space in the
lower thoracic region and in the lumbar and sacral
regions often contains fat, which appears bright in
the image.
The lower portion of the spinal cord (image on the
right) shows the spinal cord itself tapering and terminating as the conus medullaris, around the level of vertebra
L1-L2. Below that level is the enlarged subarachnoid
space, called the lumbar cistern within which are the
nerve roots (dorsal and ventral) — the cauda equina —
for the lower extremity.
The nerve roots exit the spinal cord at the appropriate
intervertebral level. The roots to the lower extremity,
particularly those exiting between L4–L5 and L5–S1 are
the ones most commonly involved in everyday back injuries that affect many adults. The student should be familiar with the signs and symptoms that accompany
degenerative disc disease in the lumbar region.
The spinal cord can be affected by tumors, either within
the cord (intramedullary) or outside the cord (extramedullary). There is a large plexus of veins on the
outside of the dura of the spinal cord and this is a site
for metastases from pelvic (including prostate) tumors.
These tumors press upon the spinal cord as they grow
and cause symptoms as they interfere with the various
pathways.
Clinical Aspects
Sampling of CSF is done by placing a needle between
the vertebra, below the level of the L2, into the lumbar
cistern. This procedure is called a lumbar puncture
(LP)(further discussed with Figure 1A).
FIGURE 1B: Spinal Cord — Spinal Cord MRI: Longitudinal View
FIGURE 2A
SPINAL CORD II
SPINAL CORD CROSS SECTION:
C8 LEVEL
This diagram is a cross section of the spinal cord at the
C8 level, the eighth cervical segmental level of the spinal
cord, not the vertebral level. The gray matter is said to
be arranged in the shape of a butterfly or somewhat like
the letter “H.” The gray matter of the spinal cord contains a variety of cell groups, i.e., nuclei, which subserve
different functions. Although hard to visualize, these
groups are continuous longitudinally throughout the
length of the spinal cord.
The dorsal region of the gray matter, called the dorsal or
posterior horn, is associated with the incoming dorsal
root and is thus related to sensory functions. The ventral
gray matter, called the ventral or anterior horn, is the
motor portion of the gray matter and has the large
motor neurons, the anterior horn cells. An anterior horn
cell and its axon, along with all the muscle fibers innervated, forms the functional motor unit. The area
between is usually called the intermediate gray and has
a variety of cell groups with some association-type functions. The neurons controlling the autonomic nervous
system (sympathetic and parasympathetic) are found in
this region.
The nuclei of the spinal cord gray matter have both
names and numbers. The names are older and descriptive. A newer classification of these nuclei is based upon
the functional aspect of these nuclei and uses a numbering system: these are the so-called Rexed lamina. This
cross section of the spinal cord shows the descriptive
names on the right side and the Rexed lamina on the
left; these are listed below with their associated function:
Name
Lamina
Function
Posteromarginal nucleus
Substantia gelatinosa
Proper sensory nucleus
Intermediate gray
Dorsal nucleus
(of Clarke)
Ventral horn
Anterior horn cells
I
II
III, IV
V, VI, VII
Part of VII
Sensory
Sensory
Sensory
Association
Relay to cerebellum
VIII
IX
medial = proximal
lateral = distal
X
Motor (axial muscles)
Limb musculature
Commissural neurons
Unknown
The central canal of the spinal cord (see Figures 20A and
20B) is located in the center of the commissural gray
matter. It represents the remnant of the neural tube and
is filled with CSF. In the adult, the central canal of the
spinal cord is probably not patent throughout the whole
spinal cord.
Physiologically, the spinal cord is the basis of a number
of simple and complex reflexes. The basic motor reflex is
the stretch reflex, also called the myotatic reflex —
tapping on a tendon stretches the tendon, activates the
muscle spindle, and causes a reflex contraction of the
muscle. This is a monosynaptic reflex and perhaps one
of the most important for a neurological examination
(also called a deep tendon reflex, DTR). Other reflexes,
such as the withdrawal from a painful stimulus, are multisynaptic. All these reflexes involve hard-wired circuits
of the spinal cord but are influenced by information descending from higher levels of the nervous system.
Recent studies indicate that complex motor patterns are
present in the spinal cord, such as stepping movements
with alternating movements of the limbs, and that influences from higher centers provide the organization for
these built-in patterns of activity. The integrity of the
spinal cord is needed for normal bowel and bladder
function.
Clinical Aspects
Lesions of the spinal cord in humans are usually devastating in their effects. Often these occur as a result of
diving accidents (into shallow water) or following car
accidents.
The immediate effect of a spinal cord transection in the
human is a complete shut-down of all spinal cord activity. This is referred to as spinal shock. After a period of a
few weeks, intrinsic spinal reflexes appear, now no
longer modified from higher control centers. (The
details of the pathways involved are discussed in Section
B.) The result is a dramatic increase in muscle tone
(spasticity) and hyperactive deep tendon reflexes (discussed with Figure 47). Thereafter, a number of abnormal or excessive reflex responses occur. Such patients
require exceptional care by the nursing staff.
FIGURE 2A: Spinal Cord II — Spinal Cord Cross Section: C8 Level
FIGURE 2B
SPINAL CORD
SPINAL CORD MRI: AXIAL VIEW
There are two axial MRI views of the spinal cord — the
upper image at the C7 cervical cord level and the lower
image at the T1 thoracic cord level. The CSF is again
dark.
The spinal cord can be easily visualized within the vertebral canal. The size difference between the C7 level and
the T1 level should be noted. The C7 spinal cord is at
the level of the brachial plexus enlargement (discussed
with Figure 1A). The T1 level is the high thoracic level
of the spinal cord where the cord is smaller (see also
Figure 73).
Not much detail can be seen in this normal radiograph.
The dorsal (indicated by a white arrow) and ventral
roots can be seen in the upper image (but not in the
lower one), with the emerging spinal nerve.
Clinical Aspects
Any abnormal protrusion of a vertebra or disc could be
visualized, as well as tumors within the vertebral canal
or of the cord itself. A small arterio-venous (A-V) malformation can also be visualized with MRI.
Syringomyelia, an uncommon disease, involves a pathological cystic enlargement of the central canal. The enlargement often interrupts the pain and temperature
fibers in their crossing anterior to the central canal (see
Figures 31 and 72). Usually this occurs in the cervical
region and the patients complain of the loss of these
modalities in both upper limbs.
If the spinal cord is completely transected (i.e., cut
through completely), all the tracts are interrupted. For
the ascending pathways, this means that sensory information from the periphery is no longer available to the
brain. On the motor side, all the motor commands
cannot be transmitted to the anterior horn cells, the
final common pathway for the motor system. The
person therefore is completely cut off on the sensory
side and loses all voluntary control below the level of the
lesion. Bowel and bladder control are also lost.
C7 level
T1 level
FIGURE 2B: Spinal Cord — Spinal Cord MRI: Axial View
FIGURE 3
BRAINSTEM I
BRAINSTEM AND DIENCEPHALON:
VENTRAL VIEW
The brainstem is the lowermost part of the brain,
situated inside the skull and above the spinal cord. This
region of the brain is responsible for key vital functions
including blood pressure, pulse, and respiration (lower
portions), as well as consciousness (upper portions).
Many motor activities are organized in the brainstem.
The brainstem and cerebellum occupy the posterior
cranial fossa of the skull.
The brainstem is a relatively small mass of tissue that is
packed with various nuclei and tracts. Firstly, it is the site
of origin or termination of ten of the cranial nerves
(CN III to CN XII). Of great clinical significance is the
fact that all the ascending (sensory) pathways pass
through the brainstem, and all the motor (descending)
pathways either originate in or go through the brainstem
(see Section B). In addition, many of the connections to
the cerebellum, including pathways and nuclei, are
found in the brainstem.
The brainstem is divided anatomically into three parts
— the narrow midbrain, the pons with its ventral bulge,
and the medulla lowest. Each of these parts is recognizably distinct when one sees a gross brain specimen or a
microscopic cross section. The specimen in Figure 3 was
obtained by isolating the brainstem from the remainder
of the brain. The diencephalon (to be discussed with
Figure 9) is also included. A photographic view of such a
specimen is presented in the next illustration. The cerebellum is located behind the brainstem (to be described
with Figure 8).
pontine nuclei).
• The medulla has two distinct elevations known as the
pyramids on each side of the midline. The direct voluntary motor pathway from the cortex to the spinal
cord — the cortico-spinal tract — is located within
the pyramid. Behind each is a prominent bulge, the
olive, which in fact represents a major nucleus of the
medulla, the inferior olivary nucleus.
One of the keys to understanding the brainstem is identifying the cranial nerves. In almost all cases the attachment site of each cranial nerve (CN) to the brainstem is
a marker to the location of the cranial nerve nucleus
within the brainstem (see Figures 4, 5, 6 and 7; discussed
in Section C). Knowledge of this is critical in determining the clinical location of a lesion of the brainstem
region. The cranial nerves are presented in numerical
order, starting at the midbrain level. (Details concerning
their function will be discussed with Figures 4, 5 and 6.)
Midbrain Level
• CN III, the oculomotor nerve, emerges ventrally
between the cerebral peduncles.
• CN IV, the trochlear nerve, which exits posteriorly, is a
thin nerve that wraps around the lowermost border of
the cerebral peduncle.
Pontine Level
• CN V, the trigeminal nerve, is a massive nerve attached along the middle cerebellar peduncle.
• CN VI, the abducens nerve, is seen exiting at the junction between the pons and medulla.
• CN VII, the facial nerve, and CN VIII, the vestibulocochlear nerve, are both attached to the brainstem at
the ponto-cerebellar angle.
Medullary Level
The distinguishing features of the three parts of the
brainstem visualized on this ventral view include the following (from above downwards):
• CN IX, the glossopharyngeal, and CN X, the vagus,
are attached to the lateral margin of the medulla,
behind the inferior olive.
• The midbrain region (mesencephalon). It has two
large “pillars” anteriorly, called the cerebral peduncles,
which consist of millions of axons descending from
the cerebral cortex to various levels of the brainstem
and spinal cord.
• CN XI, the spinal accessory nerve, is the part that
exits from the uppermost region of the spinal cord,
enters the skull, and then exits from the skull as if it
were a cranial nerve.
• The pons portion is distinguished by its anterior
bulge, an area which is composed of nuclei (the
• CN XII, the hypoglossal nerve, emerges by a series of
rootlets between the inferior olive and the pyramid.
Diencephalon
Midbrain
Middle cerebellar
peduncle
Cerebellum
Pons
Flocculus
Medulla
Tonsil
CP = Cerebral peduncle
P = Pons
Py = Pyramid
O = Inferior olive
FIGURE 3: Brainstem I — Brainstem and Diencephalon: Ventral View
FIGURE 4
BRAINSTEM II
BRAINSTEM AND DIENCEPHALON:
VENTRAL (PHOTOGRAPHIC) VIEW
The specimen in Figure 4 was obtained by isolating the
brainstem and diencephalon from the remainder of the
brain. It is the same specimen as in Figure 3. The three
parts of the brainstem can be differentiated on this
ventral view (from above downwards):
• The midbrain region has the two large “pillars” anteriorly called the cerebral peduncles. The fossa between
the cerebral peduncles is the interpeduncular fossa,
with the emerging CN III.
• The pontine portion is distinguished by its bulge anteriorly, the pons proper, an area composed of nuclei
(the pontine nuclei);
• The medulla is distinguished by the pyramids, two
distinct elevations on each side of the midline, within
which is located the direct voluntary motor pathway
from the cortex to the spinal cord — the corticospinal tract (see Figure 42). Behind each pyramid is
the olive (inferior olivary nucleus).
As mentioned previously, one of the keys to understanding the brainstem is locating the cranial nerves. In
almost all cases, knowledge of the attachment site of
each cranial nerve (CN) to the brainstem is a marker to
the location of the cranial nerve nucleus within the
brainstem (see Figures 5 and 6). Unfortunately, not all of
these nerves have been preserved on this specimen
because stripping of the meninges often also removes
these nerves.
Midbrain Level
• CN III, the oculomotor nerve, emerges ventrally from
the interpeduncular fossa. It supplies several extraocular muscles, and includes parasympathetic innervation
to the pupil.
• CN IV, the trochlear nerve, exits posteriorly (see
Figure 7). It is a thin nerve that wraps around the lowermost border of the cerebral peduncle (on the right
side in the photograph). It supplies one extraocular
muscle.
Pontine Level
• CN V, the trigeminal nerve (not labeled), is a massive
nerve attached along the middle cerebellar peduncle
(see Figures 5 and 7). It is the major sensory nerve of
the head and also has a motor component.
• CN VI, the abducens nerve, exits at the junction
between the pons and medulla (it is preserved only on
the left side on the photograph). It supplies one extraocular muscle.
• CN VII, the facial nerve, has been torn; it is attached
to the brainstem just above CN VIII (the acoustic
nerve), both at the ponto-cerebellar angle. The facial
nerve supplies the facial muscles and has other components. CN VIII is a special sense nerve for balance
and hearing.
Medullary Level
• CN IX, the glossopharyngeal, and CN X, the vagus,
cannot be seen on this view of the brainstem. (They
are attached to the lateral margin of the medulla,
behind the inferior olive. See Figure 7).They innervate
the muscles of the pharynx and larynx; the vagus is
the major parasympathetic nerve to the viscera.
• CN XI, the spinal accessory nerve, is not clearly visible
on this specimen. It innervates the large muscles of
the neck and is not a true cranial nerve.
• CN XII, the hypoglossal nerve, emerges by a series of
rootlets between the inferior olive and the pyramid
(preserved only on the left side in the photograph). It
supplies the muscles of the tongue.
The fibers projecting from the cerebral cortex to the
spinal cord (the cortico-spinal tract) traverse the whole
brainstem and are located in the cerebral peduncles, the
pons proper, and the pyramids (see Figure 42). The
medulla ends where these cortico-spinal fibers cross the
midline as the pyramidal decussation. Below this is the
cervical spinal cord (not labeled).
Knowledge of the brainstem is necessary during the
study of almost all parts of the CNS. As would be expected, this information is essential for the diagnosis of
clinical syndromes that involve this part of the brain.
A lesion might interrupt either one or more sensory
and/or motor pathways. Because of the close relationship with the cerebellum, there may be cerebellar signs
as well. The accompanying cranial nerve deficits
would assist a neurologist to pinpoint the brainstem
level involved.
Optic chiasm
Fibers of internal capsule
Mammillary body
CN III
Interpeduncular fossa
Cerebral peduncle
CN IV
CN VI
CN VIII
Middle cerebellar
peduncle
Flocculus
CN XII
Pyramidal decussation
Inferior cerebellar peduncle
D = Diencephalon
Md = Midbrain
P = Pons
Py = Pyramid
O = Inferior olivary nucleus
M = Medulla
T = Tonsil
FIGURE 4: Brainstem II — Brainstem and Diencephalon: Ventral (Photographic) View
FIGURE 5
BRAINSTEM III
CRANIAL NERVE NUCLEI: MOTOR
The cranial nerves are the peripheral nerves that supply
the head and neck region (except CN I and CN II). Each
cranial nerve is unique and does not follow a general
pattern as with the spinal nerves; each of the cranial
nerves may have one or more functional components,
either sensory or motor, or both, and some also have an
autonomic (parasympathetic) component. The motor
aspects are reviewed in this diagram; the sensory ones
are considered in the next diagram.
On the motor side, there are three kinds of motor
functions:
(i) The motor supply to the muscles derived from
somites, including CN III, IV, VI, and XII;
(ii) The motor supply to the muscles derived from
the branchial arches, branchiomotor, including
CN V, VII, IX, and X (and the cranial part of XI);
(iii) The parasympathetic supply to smooth muscles
and glands of the head and viscera, including CN
III, VII, IX, and X.
Figure 5 shows the location of the motor nuclei of the
cranial nerves, superimposed upon the ventral view of
the brainstem. These nuclei are also shown in Figure 45,
in which the brainstem is presented from a dorsal perspective. Detailed information regarding the cranial
nerve nuclei within the brainstem is presented in Section
C (Figures 64–71).
Midbrain Level
• CN III, the oculomotor nerve, has both motor and autonomic fibers. The somatic motor nucleus of the
oculomotor nerve, which supplies most of the muscles
of the eye, is found at the upper midbrain level. This is
the level of the superior colliculus. The parasympathetic nucleus known as the Edinger-Westphal
nucleus, is associated with the nucleus of III. These
fibers supply the pupillary constrictor muscle and the
muscle that controls the curvature of the lens.
• CN IV, the trochlear nerve, is a motor nerve to one
eye muscle, the superior oblique muscle. The trochlear
nucleus is found at the lower midbrain level,
the level of the inferior colliculus.
Pontine level:
• CN V, the trigeminal nerve, is the major sensory nerve
of the head region and also has a motor component;
this branchiomotor nucleus supplies the muscles of
mastication. The nucleus is located at the mid-pontine
level; the small motor nerve is attached to the brainstem at the mid-pontine level, along the middle cerebellar peduncle, with the sensory root.
• CN VI, the abducens nerve, is a motor nerve which
supplies one extraocular muscle, the lateral rectus
muscle. The somatic motor nucleus is located in the
lower pontine region.
• CN VII, the facial nerve, is a mixed cranial nerve. The
branchiomotor nucleus, which supplies the muscles of
facial expression, is found at the lower pontine level.
The parasympathetic fibers (to salivary and lacrimal
glands) come from the superior salivatory nucleus.
Medullary Level
• CN IX, the glossopharyngeal nerve, and CN X, the
vagus nerve, are mixed cranial nerves attached to the
medulla along its lateral margin, behind the inferior
olive. Both have a branchiomotor component, which
supplies the muscles of the pharynx (IX) and larynx
(X), originating from the nucleus ambiguus. In addition, each has a parasympathetic component: the
fibers of IX come from the inferior salivatory nucleus
(to the parotid gland); those of X from the dorsal
motor nucleus of the vagus supply the organs of the
thorax and abdomen. Both nuclei are found throughout the mid and lower portions of the medulla.
• CN XI, the spinal accessory nerve, has two portions.
The cervical or spinal component, which originates
from a cell group in the upper 4–5 segments of the
cervical spinal cord, supplies the large muscles of the
neck (the sternomastoid and trapezius).
• CN XII, the hypoglossal nerve, innervates all the
muscles of the tongue. It has an extended nucleus in
the medulla situated alongside the midline. Its fibers
exit between the pyramid and the olive. (Note: In the
diagram it appears that the nucleus ambiguus is
supplying the fibers to CN XII. This is merely a visualization problem.)
Edinger-Westphal n.
Oculomotor n.
Trochlear n.
Motor trigeminal n.
Abducens n.
CN II
CN III
Facial n.
CN IV
Superior salivatory n.
CN V
Inferior salivatory n.
CN VI
CN VII
CN VIII
CN IX
CN X
CN XII
CN XI
Dorsal motor n.
Hypoglossal n.
Ambiguus n.
Spinal accessory n.
FIGURE 5: Brainstem III — Cranial Nerve Nuclei: Motor
FIGURE 6
BRAINSTEM IV
CRANIAL NERVE NUCLEI: SENSORY
The cranial nerve nuclei with sensory functions are presented in this diagram. Sensory information from the
region of the head and neck includes the following:
1.
2.
3.
General sensations, consisting of touch (both discriminative and crude touch), pain, and temperature. These come from the skin, the surface of the
eye (cornea and conjunctiva), the lips, and the
mucous membranes of the mouth (including
the tongue) and the nose, via branches of the
trigeminal nerve. These are classified as somatic
afferents.
Sensory input from the pharynx and other homeostatic receptors of the neck (e.g., for blood pressure) and from the organs of the thorax and
abdomen. This afferent input is carried mainly by
the vagus but also by the glossophayngeal nerve.
These are also called visceral afferents.
Special senses, consisting of auditory (hearing)
and vestibular (balance) afferents, as well as the
special sense of taste.
The sensory nuclei are also shown in Figure 38, which
presents the brainstem from a dorsal (posterior) perspective. It should be noted that the olfactory nerve (CN
I) and the optic nerve (CN II) are not attached to the
brainstem and not considered at this stage.
CN V Trigeminal Nerve
The major sensory nerve of the head region is the
trigeminal nerve through its three divisions. The sensory
components of the trigeminal nerve are found at several
levels of the brainstem. The principal nucleus, which is
responsible for the discriminative aspects of touch, is
located at the mid-pontine level, adjacent to the motor
nucleus of CN V. From this region extending caudally is
a long column of cells that relay pain and temperature
information from the teeth, oral mucosa, and skin of the
face. This cell group, known as the spinal nucleus of V,
or the descending trigeminal nucleus, reaches the upper
cervical levels of the spinal cord.
Another group of cells extends into the midbrain region,
the mesencephalic nucleus of V. It is an unusual cell type
for the CNS in that its cells appear to be morphologically similar to neurons of the dorsal root ganglia. These
neurons are thought to be the sensory proprioceptive
neurons for the muscles of mastication.
CN VIII, the vestibulocochlear nerve, consists of:
Cochlear nuclei: The auditory fibers from the spiral
ganglion (in the cochlea) are carried to the CNS, in the
VIIIth nerve, and form their first synapses in the
cochlear nuclei. These nuclei are situated along the
course of the nerve, as it enters the brainstem at the
uppermost level of the medulla (see Figure 7). Tonotopic
localization is maintained in these nuclei.
Vestibular nuclei: Vestibular afferents enter the CNS as
part of CN VIII. The vestibular nuclei are located in the
upper medullary level as well as the lower pontine level.
There are four nuclei: the medial and inferior located in
the medulla, the lateral located at the ponto-medullary
junction, and the small superior nucleus located in the
lower pontine region. The vestibular afferents terminate
in these nuclei. (The vestibular nuclei are further discussed with Figure 49A.)
Visceral Afferents and Taste: The nucleus that receives
the visceral afferents from CN IX and X is the solitary
nucleus, found in the medulla. The special sense of taste,
mainly carried in CN VII, and also in nerves IX and X,
also terminates in the solitary nucleus.
Note: Additional sensory nuclei of the brainstem are discussed with Figure 7, in which the brainstem is viewed
from the posterior (dorsal) perspective.
Diencephalon
Optic chiasm
Pituitary stalk
Mesencephalic n. of V
Mammillary n.
Medial vestibular n.
Superior vestibular n.
CN V
Principal n. of V
Cochlear n.
Solitary n.
Lateral vestibular n.
Spinal (descending) n. of V
FIGURE 6: Brainstem IV — Cranial Nerve Nuclei: Sensory
Inferior vestibular n.
FIGURE 7
BRAINSTEM V
BRAINSTEM: DORSAL VIEW
(CEREBELLUM REMOVED)
The view of the brainstem that is used for some of the
diagrams is an oblique perspective of the brainstem
from behind — a dorsal view — with the cerebellum
removed. This dorsal perspective is useful for presenting
the combined visualization of many of the cranial nerve
nuclei and the various pathways of the brainstem (e.g.,
Figures 38 and 45).
Additional structures of the brainstem are seen from this
perspective:
• The dorsal part of the midbrain has four elevations,
the superior and inferior colliculi (see also Figure 8).
These colliculi form the quadrigeminal plate, also
called the tectal plate or tectum. The upper ones are
the superior colliculi, and they are functionally part of
the visual system, a center for visual reflexes. The
lower ones are the inferior colliculi, and these are
relay nuclei in the auditory pathway. (The brachium
of the inferior colliculus and the medial geniculate
body will be discussed with the auditory pathway; see
Figure 36.) This view also shows the back edge of the
cerebral peduncle, the most anterior structure of the
midbrain (see Figures 3 and 4). The trochlear nerves
(CN IV) emerge at the lower level of the midbrain,
below the inferior colliculi (as in Figure 8).
• The dorsal aspect of the pons is represented, with
most of the fourth ventricle “unroofed.” (The ventricles of the brain are discussed with Figures 20A and
20B.) The roof of the upper portion of the fourth ventricle is shown and bears the name superior
medullary velum; more relevant, it contains an important connection of the cerebellum, the superior
cerebellar peduncles (which are discussed with Figure
55). As seen from this perspective, the fourth (IVth)
ventricle has a “floor”; noteworthy are two large
bumps, called the facial colliculus (discussed with the
pons in Section C of the Atlas). Because the cerebellum has been removed, the cut edges of the middle
and inferior cerebellar peduncles, the other cerebellar
connections, are seen. CN V emerges through the
middle cerebellar peduncle.
• The posterior aspect of the medulla (including the
floor of the fourth ventricle) has some special structures, including the entering CN VIII and the inferior
cerebellar peduncle. Below the fourth ventricle, two
large protuberances are seen on either side of the
midline — the gracilis and cuneatus nuclei, which
belong to the ascending somatosensory pathway (discussed with Figure 32). More anteriorly, from this
oblique view, are the fibers of the glossopharyngeal
(CN IX) and vagus (CN X) nerves, as these emerge
from the lateral aspect of the medulla, behind the inferior olive.
A representative cross section of the spinal cord is also
shown from this posterior perspective.
Brachium of inferior
colliculus
Red n.
Medial geniculate n.
Superior colliculus
Inferior colliculus
Cerebral peduncle
CN IV
Superior cerebellar
peduncle
Superior medullary
velum
CN V
Fourth ventricle
Middle cerebellar
peduncle
Inferior cerebellar
peduncle
Facial colliculus
CN VIII
Acoustic stria
CN IX
CN X
Inferior olive
Cuneatus n.
Gracilis n.
Cervical spinal cord
FIGURE 7: BRAINSTEM V — Brainstem: Dorsal View (Cerebellum Removed)
FIGURE 8
THE CEREBELLUM
CEREBELLUM AND BRAINSTEM
(PHOTOGRAPHIC VIEW)
This specimen of the brainstem, with the cerebellum attached, is shown from the dorsal or posterior perspective, as in the previous figure. The paired diencephalon
are again seen (discussed with Figure 9), separated from
each other by the third ventricle. The colliculi of the
midbrain are in view, with CN IV exiting posteriorly.
The cerebellum, sometimes called the “little brain,” is
easily recognizable by its surface which is composed of
narrow ridges of cortex, called folia (singular is folium).
The cerebellum is located beneath a thick sheath of the
meninges, the tentorium cerebelli, inferior to the occipital lobe of the hemispheres (see Figure 16), in the posterior cranial fossa of the skull.
The cerebellum is involved with motor control and is
part of the motor system, influencing posture, gait, and
voluntary movements (discussed in more detail with the
motor system). Its function is to facilitate the performance of movements by coordinating the action of the
various participating muscle groups. This is often
spoken of simply as “smoothing out” motor acts.
Although it is rather difficult to explain in words what
the cerebellum does in motor control, damage to the
cerebellum leads to quite dramatic alterations in ordinary movements (discussed with Figure 55). Lesions of
the cerebellum result in the decomposition of the activity, or fractionation of movement, so that the action is no
longer smooth and coordinated. Certain cerebellar
lesions also produce a tremor which is seen when performing voluntary acts, better known as an intention
tremor.
There are two distinct ways of dividing the cerebellum:
1.
an anatomical approach, which is outlined below,
and
2.
a functional approach, which is explained in the
discussion of the cerebellum as part of the motor
system (Section B).
Anatomically, the cerebellum can be described by
looking at its appearance in a number of ways. The
human cerebellum in situ has an upper or superior
surface, as seen in this photograph, and a lower or inferior surface (see Figure 4). The central portion is known
as the vermis. The lateral portions are called the cerebellar hemispheres.
Sulci separate the folia, and some of the deeper sulci are
termed fissures. The horizontal fissure is located at the
margin between the superior and inferior surfaces.
Using these sulci and fissures, the cerebellar cortex has
traditionally been divided into a number of different
lobes, but most of these do not have a distinctive functional or clinical importance, so only a few are mentioned. The primary fissure, located on the superior
surface of the cerebellum, separates the anterior lobe of
the cerebellum — part of the functional spinocerebellum (discussed with Figure 52). The other functional
area that can be visualized — on the ventral view of the
cerebellum (see Figure 4) — is the flocculus (also discussed with Figure 52).
The cerebellar peduncles are the connections between
the brainstem and the cerebellum, and there are three
pairs of them. In the inferior view of the brainstem and
cerebellum (see Figure 4), two of them can be seen: the
inferior cerebellar peduncle attaching the medulla and
the cerebellum, and the prominent middle cerebellar peduncle from the pons to the cerebellum. (These are also
shown from the dorsal perspective in Figure 7.) Details
of the information carried in these pathways is outlined
in the discussion of the functional aspects of the cerebellum with the motor system. The superior cerebellum peduncle is located in Figure 7 on the dorsal aspect of the
brainstem, in the roof of the fourth ventricle.
Clinical Aspect
The cerebellar lobule adjacent to the medulla is known
as the cerebellar tonsil (see ventral view of the cerebellum, Figure 4). The tonsils are found just inside the
foramen magnum of the skull. Should there be an increase in the mass of tissue occupying the posterior
cranial fossa (e.g., a tumor or hemorrhage), the cerebellum would be pushed downward. This pressure may
force the cerebellar tonsils into the foramen magnum,
thereby compressing the medulla. The compression, if
severe, may lead to a compromising of function of the
vital centers located in the medulla (discussed with
Figure 40A). The complete syndrome is known as
tonsillar herniation, or coning, and is a life-threatening
situation which can cause cardiac and/or respiratory
arrest.
The pineal is discussed with Figure 9.
Third ventricle
Fibers of internal
capsule
Superior
colliculus
Pineal
Inferior
colliculus
CN IV
Primary
fissure
Horizontal
fissure
Vermis of cerebellum
D = Diencephalon
FIGURE 8: The Cerebellum — Cerebellum and Brainstem (Photographic View)
FIGURE 9
THE DIENCEPHALON
THALAMUS: ORIENTATION
The diencephalon translates as “between brain.” The diencephalon is composed of both thalamus and hypothalamus as well as some other subparts. It is situated
between the brainstem and the cerebral hemispheres,
deep within the brain. As shown photographically (see
Figure 4) and also diagrammatically (see Figure 3), the
diencephalon sits “atop” the brainstem. During development of the human brain the enormous growth of the
cerebral hemispheres has virtually hidden, or “buried,”
the diencephalon (somewhat like a weeping willow tree)
so that it can no longer be visualized from the outside,
except from the inferior view (see hypothalamus in
Figure 13). It is important to note that there are two
thalami; these are often connected across the midline by
the massa intermedia (as seen in Figure 3).
The thalamus makes up the bulk of the diencephalon. It
has many nuclei which are strongly linked with the cerebral cortex, even during development. This feature
becomes clearer in one of the principles of thalamic
function, namely that most thalamic nuclei that project
to the cerebral cortex also receive input from that area
— these are called reciprocal connections. This principle
does not apply, however, to all of the nuclei (see below).
The major function of the thalamic nuclei is to process
information before forwarding it to the select area of the
cerebral cortex. This is especially true for all the sensory
systems, except the olfactory. Crude forms of sensation,
including pain, may possibly be “appreciated” in the thalamus, but localization of the sensation requires the involvement of the cortex. Likewise, two subsystems of the
motor system, the basal ganglia and the cerebellum,
relay in the thalamus before sending their information
to the motor areas of the cortex. In addition, the limbic
system also has circuits that involve the thalamus. Other
thalamic nuclei are related to association areas of the
cerebral cortex (explained below — these are areas not
specifically related either to sensory or motor functions
of the cortex). Parts of the thalamus play an important
role in the maintenance and regulation of the state of
consciousness, alertness, and also possibly attention.
The hypothalamus and other nuclear areas of the diencephalon are discussed with the limbic system (Section
D). The pineal (visible in Figure 8) is sometimes considered a part of the diencephalon. This gland is thought to
be involved with the regulation of circadian rhythm.
Many people now take melatonin, which is produced by
the pineal, to regulate their sleep cycle and to overcome
jetlag. The subthalamus is an area between the thalamus
and midbrain; the subthalamic nucleus, located in this
small zone, is an important nucleus involved with the
circuitry of the basal ganglia and substantia nigra and is
discussed with those structures (see Figure 24).
Additional Detail
The following topographic information will be understood only after studying the hemispheres (see Figures
11–16). It is recommended that students review this material at that time. As shown in the diagram, the diencephalon is situated within the brain below the level of
the body of the lateral ventricles (see also Figures 29 and
30). In fact, the thalamus forms the “floor” of this part
of the ventricle (see Figure 16). In a horizontal section of
the hemispheres, the two thalami are located at the same
level as the lentiform nucleus (see Figures 27, 28A, and
28B); on each side, the thalamus forms one of the
boundaries of the posterior limb of the internal capsule.
Between the two thalami is the third ventricle (see
Figures 8 and 20B).
Lateral ventricle (body)
Corpus callosun
Caudate nucleus (body)
Thalmus (paired)
Brainstem
FIGURE 9: The Diencephalon — Thalamus: Orientation
FIGURE 10
THALAMUS
THALAMUS: NUCLEI
There are two ways of dividing up the nuclei of the
thalamus: topographically and functionally.
1.
2.
Topographically, the thalamus is subdivided by
bands of white matter into a number of component parts. The main white matter band that runs
within the thalamus is called the internal
medullary lamina and it is shaped like the letter
“Y” (see also the previous illustration). It divides
the thalamus into a lateral mass, a medial mass,
and an anterior group of nuclei.
Functionally, the thalamus has three different
types of nuclei:
• The specific relay nuclei relay incoming
sensory information to specific sensory areas of
the cerebral cortex. Included with these are the
medial and lateral geniculate bodies, relay
nuclei for the auditory and visual systems. In
addition, motor regulatory information from
the basal ganglia and cerebellum is also relayed
in the thalamus as part of this set of nuclei.
These nuclei are located in the lateral nuclear
mass.
• The association nuclei are connected to broad
areas of the cerebral cortex known as the association areas. One of the most important nuclei
of this group is the dorsomedial nucleus,
located in the medial mass of the thalamus.
• The non-specific nuclei are scattered nuclei
that have other and/or multiple connections.
Some of these nuclei are located within the internal (medullary) lamina and are often referred to as the intralaminar nuclei. This
functional group of nuclei does not have the
strong reciprocal connections with the cortex
like the other nuclei. Some of these nuclei form
part of the ascending reticular activating system
which is involved in the regulation of our state
of consciousness and arousal (discussed with
Figure 40A). The reticular nucleus which lies
on the outside of the thalamus is also part of
this functional system.
The nuclei are organized as follows.
Specific relay nuclei (and function)
These nuclei are reciprocally connected to specific
primary sensory or motor areas of the cerebral cortex.
VA
ventral anterior (motor)
VL
ventral lateral (motor)
VPL
ventral posterolateral (somatosensory)
VPM
ventral posteromedial (trigeminal)
MG
medial geniculate (body) nucleus (auditory)
LG
lateral geniculate (body) nucleus (vision)
Association nuclei (and association cortex)
These nuclei are reciprocally connected to association
areas of the cerebral cortex.
DM
dorsomedial nucleus (prefrontal cortex)
AN
anterior nucleus (limbic lobe)
P
pulvinar (visual cortex)
LP
lateral posterior (parietal lobe)
Nonspecific nuclei
These nuclei relay to widespread areas of the cerebral
cortex.
IL
intralaminar
CM
centromedian (not illustrated — see Figures
51 and 61)
R
reticular (not illustrated — see Figure 61)
For schematic purposes, this presentation of the thalamic nuclei, which is also shown in a number of textbooks,
is quite usable. A more histological view of the thalamus
is shown in Section C (see Figure 61).
Note to student: The thalamus is introduced at this
point because it is involved in all the functional systems.
The student should learn the names and understand the
general organization of the various nuclei at this point.
The student should also consult this diagram as the cerebral cortex is described (see the following figures). Each
of the specific relay nuclei involved in one of the pathways is discussed again with the functional system
(Section B), and at that point the student should return
to this and the previous illustration. Various nuclei are
involved with the limbic system (Section D). After
studying all of these systems, it is worthwhile returning
again to Figures 9 and 10 for better understanding and
integration of the thalamus.
AN
DM
IL
VA
VL
LP
P
VP(L)(M)
LG
FIGURE 10: Thalamus — Thalamus: Nuclei
MG
FIGURE 11
CEREBRAL HEMISPHERES I
CEREBRAL CORTEX: DORSAL VIEW
When people talk about the brain, they are generally referring to the cerebral hemispheres, also called the cerebrum. The brains of the higher apes and humans are
dominated by the cerebral hemispheres. The nervous
tissue of the hemispheres, particularly the outer layer of
neurons — the cerebral cortex — is responsible for consciousness, language, thinking, memory, movements,
sensory perceptions, and certain aspects of emotion. In
short, neuronal activities in the cerebral cortex determine to a large extent our capabilities. It is not that the
other parts of the CNS are not important, but working
in and adapting to our complex modern world depends
upon proper functioning of the cerebral hemispheres.
The hemispheres are organized in the following way: billions of neurons and their dendrites (and synapses) are
located at the surface, forming a cortex, the cerebral
cortex. Most of the cerebral cortex is organized in six
layers, the neocortex, with the neurons of each layer
having a different function. In formalin-fixed material,
the neuronal cortex takes on a grayish appearance and is
often referred to as the gray matter.
The surface of the hemispheres in humans and some
other species is thrown into irregular folds. These ridges
are called gyri (singular, gyrus) and the intervening
crevices are called sulci (singular, sulcus). A very deep
sulcus is called a fissure. This arrangement allows for a
greater surface area to be accommodated within a confined space, the skull. The cerebral hemispheres occupy
the interior of the skull, the cranial cavity, which is
divided into the three cranial fossa.
The surface of the cerebral hemispheres can be visualized from a number of perspectives: from above (dorsal
view, as seen in Figure 11), from the side (the dorsolateral view, as in Figure 12), and from below (inferior view,
as in Figures 13 and 14 ). In addition, dividing the two
hemispheres along the interhemispheric fissure (in the
midline) shows the hemispheres have a medial surface as
well (see Figure 16). The photograph in Figure 11 shows
the cerebral hemispheres from above, a dorsal view.
Different parts of the cortex have different functions.
Some parts have a predominantly motor function,
whereas other parts are receiving areas for one of the
major sensory systems. Most of the cerebral cortex in
humans has an association function, a term that can
perhaps be explained functionally as interrelating the
various activities in the different parts of the brain.
Each of the hemispheres is divided into four lobes:
frontal, parietal, temporal, and occipital. Two prominent
fissures allow this subdivision to be made — the central
fissure and the lateral fissure. The central fissure, which
is easier to identify on the right side of the specimen in
Figure 11, divides the area anteriorly, the frontal lobe,
from the area posteriorly, the parietal lobe. The parietal
lobe extends posteriorly to the parieto-occipital fissure,
which is more easily seen on other views (see Figure 16).
The brain area behind the parieto-occipital fissure is the
occipital lobe. The temporal lobe and the lateral fissure
cannot be seen on this view of the brain (see Figure 12).
Large areas of the frontal lobes have a predominantly
motor function. The most anterior parts of the frontal
lobe are the newest in evolution and are known as the
prefrontal cortex. This broad cortical area seems to be
the chief “executive” part of the brain. The parietal
areas are connected to sensory inputs and have a major
role in integrating sensory information from the various
modalities. The occipital lobe is concerned with the processing of visual information (see Figure 16).
The meningeal layers (arachnoid and pia) have not been
removed from this specimen, which means that the
blood vessels are also present. Under the arachnoid
membrane is the subarachnoid space, which is collapsed
in this fixed specimen; it is normally filled with cerebrospinal fluid (CSF) (which is discussed with Figure
20A). This photographic view shows some coral-like
whitish material lying adjacent to the interhemispheric
fissure; this material is collectively called the arachnoid
granulations and is part of the CSF circulation, returning the CSF to the venous circulation (discussed with
Figure 21).
Anterior
Arachnoid
granulations
Central fissure
Interhemispheric
fissure
Parieto-occipital
fissure
F = Frontal lobe
P = Parietal lobe
O = Occipital lobe
FIGURE 11: Cerebral Hemispheres I — Cerebral Cortex: Dorsal View
FIGURE 12
CEREBRAL HEMISPHERES II
CEREBRAL CORTEX:
DORSOLATERAL VIEW
With the meninges removed, it is possible to identify the
sulci and fissures with more certainty. The central fissure
(often called the fissure of Rolando) is now seen more
completely, dividing the frontal lobe anteriorly from the
parietal lobe posteriorly.
Some cortical areas are directly connected functionally
with either a sensory or motor system; these are known
as the primary areas. The gyrus in front of the central
fissure is called the precentral gyrus (see also Figure 51)
and is the primary motor area, specialized for the
control of voluntary movements. The frontal eye field,
an area in the frontal lobe (outlined), has a motor function related to eye movements. The gyrus behind the
central fissure, the postcentral gyrus (see also Figure
34), has a somatosensory function for information from
the skin (and joints). (Other sensory primary areas are
identified where appropriate.)
Those cortical areas that are not directly linked to either
a sensory or motor function are called association
cortex. The area in front of the frontal eye fields previously mentioned, the prefrontal cortex, is a typical
example of an association area. Large parts of the parietal and temporal lobes are association cortex.
The cortex has been studied by many people using different techniques. It is possible to recognize distinct histological (microscopic) features between different
cortical areas, and these might reflect the differing functions of each particular area. One of the most commonly
used sub-parcelations of the cerebral cortex is that of
Brodmann who divided brain areas numerically. Some
of these numbers are sometimes used interchangeably
with the names, such as area 4 for the precentral gyrus
(the motor strip), area 8 for the frontal eye field, area 6
for the lateral premotor cortex (in-between) (see also
Figure 51), and areas 3, 1, and 2 which are synonymous
for the postcentral somatosensory gyrus.
Some cortical functions are not equally divided between
the two hemispheres. One hemisphere is therefore said
to be dominant for that function. This is the case for
language ability, which, in most right-handed people, is
located in the left hemisphere. The photograph in Figure
12 shows the left hemisphere, and the two language
areas are indicated: Broca’s area for the motor aspects of
speech, and Wernicke’s area for the comprehension of
written and verbal language.
The lateral fissure (fissure of Sylvius) divides the temporal lobe below from the frontal and parietal lobes
above. Extending the line of the lateral fissure
posteriorly continues the demarcation between the
temporal and parietal lobes. In the parietal lobe there are
two gyri whose association type of function is known;
they have been labeled the supramarginal and angular
gyri. These areas, particularly on the nondominant side,
seem to be involved in visuo-spatial activities.
The temporal lobe is a large area of association cortex
whose function is still being defined. The areas exposed
on this dorsolateral view — other than the portions involved with the auditory system and language (on the
dominant side) — are, in fact, still to be assigned a functional role. Other portions of the temporal lobe include
the inferior parts (to be discussed with the subsequent
figures) and the medial portion which is part of the
limbic system (see Section D).
The specialized cortical areas for audition are located
within the lateral fissure (as shown in Figure 37). It
should be noted that the lateral fissure has a large
number of blood vessels within it, branches of the
middle cerebral artery (discussed with Figure 58). They
have been removed from the specimen in the figure.
The location of the parieto-occipital fissure is indicated
on this photograph. This fissure separates the parietal
lobe from the occipital lobe, which can be seen in Figure
16. The cerebellum lies below the occipital lobe, with the
large dural sheath — the tentorium cerebelli — separating these parts of the brain (see Figure 16).
It is most important to delineate anatomically the
various functional areas of the cortex. This forms the
basis for understanding the possible clinical implications
of lesions in the various parts of the brain, a task becoming more sophisticated with the help of modern
imaging techniques, such as computed tomography
(CT) and magnetic resonance imaging (MRI — see, for
example, Figure 17).
Central
fissure
Supramarginal
gyrus
Postcentral
Angular
gyrus
gyrus
(areas 3, 1, 2)
Parieto-occipital
fissure
Anterior
Frontal eye field
(area 8)
Precentral
gyrus
(area 4)
Broca’s
area
Auditory
gyri
Lateral fissure
Wernicke’s
area
Cerebellum
P = Parietal lobe
F = Frontal lobe
T = Temporal lobe
O = Occipital lobe
(areas 18, 19)
FIGURE 12: Cerebral Hemispheres II — Cerebral Cortex: Dorsolateral View
FIGURE 13
CEREBRAL HEMISPHERES III
CEREBRAL CORTEX: INFERIOR VIEW
Figure 13 is a photographic view of the intact brain, including the brainstem and the cerebellum, seen from
below. The medulla and pons, parts of the brainstem,
can be identified, but the midbrain is mostly hidden
from view. The cranial nerves are still attached to the
brainstem, and the arteries to the brainstem and cerebellum are also present.
The inferior surface of the frontal lobe extends from the
frontal pole to the anterior tip of the temporal lobe (and
the beginning of the lateral fissure). These gyri rest on
the roof of the orbit and are sometimes referred to as
the orbital gyri. This cortex is an association type and
has strong connections with the limbic system (discussed in Section D).
The next area is the inferior surface of the temporal
lobe. The temporal lobe extends medially towards the
midbrain and ends in a blunt knob of tissue which is
known as the uncus. Moving laterally from the uncus,
the first sulcus visible is the collateral sulcus/fissure
(seen clearly on the right side of the photograph in
Figure 13). It demarcates the parahippocampal gyrus,
part of the limbic system (discussed with Figure 74),
which is the most medial gyrus of the temporal lobe. It
should be noted that the uncus is the most medial protrusion of this gyrus. (The clinical significance of the
uncus and the discussion of uncal herniation are discussed with Figure 14.)
Parts of the olfactory and visual sensory afferent systems
are seen on this view; both, in fact, are CNS tracts and
are not peripheral cranial nerves although they are generally called CN I and CN II. The true olfactory nerves
penetrate the roof of the nose (the cribriform plate) as a
number of filaments and then synapse in the olfactory
bulb (discussed with Figure 84). Olfactory information
is then carried in the olfactory tract to the uncal region
of the temporal lobe. The optic nerves exit from the
orbit and continue to the optic chiasm where there is a
partial crossing of visual fibers (see Figures 39A and
39B). Posterior to the chiasm is the area of the hypothalamus, part of the diencephalon, which can be seen
more clearly in Figure 14.
The brainstem and cerebellum occupy the posterior part
(the posterior cranial fossa) and obscure the visualization of the occipital lobe (shown in Figure 14). Various
cranial nerves can be identified (as in Figure 4). The
oculomotor nerve, CN III, exits from the midbrain. The
trigeminal nerve, CN V, exits along the middle cerebellar
peduncle. Cranial nerves VII and VIII are seen attached
to the brainstem at the ponto-cerebellar angle.
The arterial system is also seen in this illustration. The
two vertebral arteries unite to form the basilar artery
(which is displaced from the midline of the pons in this
specimen). The arterial supply to the brainstem and
cerebellum comes from these arteries. There are three
pairs of cerebellar arteries — posterior inferior, anterior
inferior, and superior. The basilar artery gives off the
two superior cerebellar arteries at the upper level of the
pons, and ends by dividing into the posterior cerebral
arteries to supply the occipital regions of the brain. Parts
of the arterial Circle of Willis are also seen on this specimen. The arterial supply to the hemispheres is fully
described in Section C (see Figure 56).
Anterior
Olfactory bulb
Olfactory tract
Lateral fissure
Optic chiasm
Hypothalamus
Uncus
CN III
Parahippocampal
gyrus
Posterior
cerebral artery
Basilar artery
Collateral sulcus
Superior cerebellar
CN V
CN VII and CN VIII
Middle cerebellar peduncle
Flocculus
Anterior inferior
cerebellar artery
Posterior inferior
cerebellar artery
Vertebral
artery
Tonsil
F = Frontal lobe
T = Temporal lobe
C = Cerebellum
P = Pons
M = Medulla
FIGURE 13: Cerebral Hemispheres III —Cerebral Cortex: Inferior View
FIGURE 14
CEREBRAL HEMISPHERES IV
INFERIOR SURFACE: BRAINSTEM
REMOVED
In Figure 14, the brainstem has been sectioned at the
level of the midbrain, and most of the brainstem itself
and the attached cerebellum have been removed. The cut
surface of the midbrain exposes for view the pigmented
cells of the substantia nigra; since this specimen has not
been processed for microscopy, the pigment is retained
(discussed with the basal ganglia and see Figure 24; also
shown with the cross sections of the midbrain in Figure
64).
This dissection reveals the inferior surface of both the
temporal and the occipital lobes. It is not possible to
define the boundary between these two lobes on this
view. Some of these gyri are involved with the processing
of visual information, including color, as well as facial
recognition. The parahippocampal gyri should be noted
on both sides, with the collateral sulcus demarcating the
lateral border of this gyrus (discussed with Figure 13).
The optic nerves lead to the optic chiasm. Behind the
optic chiasm is the median eminence and the mammillary bodies, both of which belong to the hypothalamus.
The median eminence is an elevation of tissue that contains some hypothalamic nuclei. The pituitary stalk is attached to the median eminence and connects the
hypothalamus to the pituitary gland. (The pituitary stalk
is not present in this photograph.) Behind are the paired
mammillary bodies, two nuclei of the hypothalamus
(discussed with the limbic system, see Figure 83A).
A thick sheath of dura separates the occipital lobe from
the cerebellum below — the tentorium cerebelli (as it
covers the cerebellum, it is not present on this specimen). The tentorium divides the cranial cavity into an
area above it, the supratentorial space, and an area
below, the infratentorial space, which is the posterior
cranial fossa. The sheath of dura splits to allow the
brainstem to pass through; this split in the tentorium is
called the tentorial notch (hiatus).
The uncus is clearly seen, with its blunted tip pointed
medially. Note that it lies just above the free edge of the
tentorium cerebelli. The occurrence of a tumor or a
large cerebral hemorrhage in the cerebral hemispheres,
or swelling of the brain for any reason, will lead to increased tissue mass in the cranial cavity above the tentorium, accompanied by increased intracranial pressure
(ICP). As the volume of brain tissue increases, the hemispheres are forced out of their supratentorial space and
the only avenue is in a downward direction, through the
tentorial notch. The uncus becomes the leading edge of
this pathological event. The whole process is clinically
referred to as uncal herniation. Since the edges of the
tentorium cerebelli are very rigid, the extra tissue in the
area causes a compression of the brain matter, leading to
brainstem compression and a progressive loss of consciousness. CN III is usually compressed as well, damaging it and causing a fixed and dilated pupil on that side,
an ominous sign in any lesion of the brain. (The pupillary light reflex is discussed with the introduction to the
midbrain in Section C.)
Anterior
Mammillary
bodies
Optic nerve
Median eminence
Substantia nigra
Midbrain
Parahippocampal gyrus
T = Temporal lobe
O = Occipital lobe
FIGURE 14: Cerebral Hemispheres IV — Inferior Surface: Brainstem Removed
FIGURE 15
CEREBRAL HEMISPHERES V
CORPUS CALLOSUM: DORSAL VIEW
In the photograph in Figure 15, the brain is again
viewed from above, as in Figure 11. The interhemispheric fissure is opened. The dura between the hemispheres,
the falx cerebri, has been removed from the interhemispheric fissure. This thick sheath of dura holds the two
halves of the hemispheres in place within the cranial
cavity. The superior sagittal venous sinus has also been
removed. A whitish structure is seen in the depths of the
fissure — the corpus callosum.
One of the other major features of the cerebral cortex is
the number of neurons devoted to communicating with
other neurons of the cortex. These interneurons are essential for the processing and elaboration of information, whether generated in the external world or
internally by our thoughts. This intercommunicating
network is reflected in the vast interconnections between
cortical areas. Therefore, one would expect to find
various bundles of axons that course within the hemispheres (further discussed with Figures 19A and 19B).
These interconnecting axons are located within the
depths of the hemispheres. They have a white coloration
after fixed in formalin, and these regions are usually
called the white matter (see Figures 27 and 29).
There are three kinds of white matter bundles within the
hemispheres: those connecting cortical areas across the
midline (commissural bundles), those interconnecting
the various cortical areas on the same side (association
bundles), and those connecting the cerebral cortex with
various subcortical structures (called projection fibers).
The corpus callosum is the largest of the commissural
bundles, as well as the latest in evolution. This is the
anatomic structure required for each hemisphere to be
kept informed of the activity of the other hemisphere.
Functionally, the axons connect to and from the lower
layers of the cerebral cortex, and in most cases the connections are between homologous areas and are reciprocal.
If the brain is sectioned in the sagittal plane along this
interhemispheric fissure, the corpus callosum will be
divided (see Figure 16). This sectioning will reveal the
medial aspect of the brain. It is difficult from the view in
Figure 15 to appreciate the depth of the corpus callosum. In fact, there is a considerable amount of cortical
tissue on the medial surface of the hemispheres, as represented by the frontal, parietal, and occipital lobes. It
should be noted that the cerebral ventricles are located
below (i.e., inferior to) the corpus callosum (see Figures
9 and 16).
Note on the safe handling of brain tissue:
Figure 15 shows a rather old photograph of a brain
specimen. Current guidelines recommend the use of disposable gloves when handling any brain tissue (as is seen
in Figures 37 and 76).
Anterior
Corpus
callosum
FIGURE 15: Cerebral Hemispheres V — Corpus Callosum: Dorsal View
FIGURE 16
CEREBRAL HEMISPHERES VI
CEREBRAL CORTEX: MEDIAL VIEW
The view in Figure 16 is one of the most important for
understanding the gross anatomy of the brain, brainstem, and ventricles. In this figure, the brain has been
sectioned in the midline, mid-sagittally, through the
corpus callosum and brainstem.
The medial aspects of the lobes of the brain are in view.
The central fissure does extend onto this part of the
brain (although not as deep, usually, as on the dorsolateral surface). The medial surface of the frontal lobe is
situated anteriorly, with the parietal lobe behind.
Moving posteriorly, the parieto-occipital fissure has
been opened. The occipital lobe is visible, divided by a
deep fissure, the calcarine fissure, into upper and lower
portions. The primary visual area, the cortical area
where the visual fibers first arrive in the cerebral cortex,
is located along the banks of the calcarine fissure. This
area is commonly called area 17 (described with Figures
39A and 39B). The adjacent areas of the occipital lobe
are visual association areas, also known as areas 18 and 19.
The corpus callosum in this particular specimen does
not have the usual white matter appearance that would
be expected. The septum pellucidum, a membrane
which divides the lateral ventricle of one hemisphere
from that of the other side (see Figure 28A), has been
removed, thereby exposing one of the lateral ventricles
which is seen to be situated inferior to the corpus callosum. Above the corpus callosum is an important gyrus
which is part of the limbic system, the cingulate gyrus
(see Figures 74 and 75).
This medial view of the brain exposes one-half of the
paired diencephalon (see Figures 3, 4, and 9). The thalamic portion is separated from the hypothalamic part by
a groove, the hypothalamic sulcus. This sulcus starts at
the foramen of Monro (the interventricular foramen,
discussed with the ventricles; see Figure 20B) and ends
at the aqueduct of the midbrain. The optic chiasm is
found at the anterior aspect of the hypothalamus (see
also Figures 13 and 14). The pineal body (sectioned) is
located off the posterior aspect of the diencephalon (see
also Figure 8).
The three parts of the brainstem can be distinguished
from this view — the midbrain, the pons (with its bulge
anteriorly), and the medulla (refer to the ventral view
shown in Figure 4). Through the midbrain is a narrow
channel for CSF, the cerebral aqueduct, also known as
the aqueduct of the midbrain or the aqueduct of Sylvius
(see Figure 20B). The posterior aspect of the midbrain
(behind the aqueduct) consists of the superior and inferior colliculi (see Figure 8).This aqueduct opens into the
fourth ventricle, which separates the pons and medulla
from the cerebellum. The fourth ventricle is said to have
a floor, which is the brainstem, and a roof (see Figure
20A). The roof is divided into an upper and lower
portion. The upper part consists of a band of white
matter known as the superior medullary velum (see also
Figure 7). The lower part of the roof of this ventricle is
occupied by choroid plexus, which has not been preserved on this specimen.
The cerebellum lies behind (or above) the fourth
ventricle. It has been sectioned through its midline
portion, the vermis. Although it is not necessary to
name all of the various parts of the vermis, it is useful to
know two of them: the lingula and the nodulus. (The
reason for knowing these is evident when describing
the cerebellum; see Figure 52). The lingula is that part
of the vermis lying immediately above the superior
medullary velum. The nodulus is that part of the vermis
lying adjacent to the lower portion of the roof of the
fourth ventricle.
Corpus callosum
Cingulate gyrus
Central fissure
Pineal
Parieto-occipital fissure
Lateral ventricle
Fornix
Foramen of
Monro
Area 17
Calcarine
fissure
Hypothalamic
sulcus
Optic chiasm
Superior medullary
Superior and inferior colliculi
Cerebral aqueduct
T = Thalamus
Hyp = Hypothalamus
Md = Midbrain
P = Pons
M = Medulla
Fourth ventricle
F = Frontal lobe
P = Parietal lobe
O = Occipital lobe
FIGURE 16: Cerebral Hemispheres VI — Cerebral Cortex: Medial View
L = Lingula
N = Nodulus
FIGURE 17
RADIOLOGIC VIEW OF
HEMISPHERES
MRI: SAGITTAL VIEW
The radiological image in Figure 17 was obtained by
magnetic resonance imaging (MRI), which shows the
brain as clearly as the actual brain itself. MRI imaging is
the way the brain is seen in clinical settings. The view
presented in this figure is a T1-weighted image (see
below). Note that the cerebrospinal fluid is dark in this
image, including the subarachnoid space and cisterns
(see Figure 21). The bones (tables) of the skull are
visible as dark lines; bone marrow, including its replacement by fatty tissue, and layers of soft tissue and fatty
tissue of the scalp are well demarcated.
By comparing this view with the photographic view of
the brain shown in Figure 18, the various structures of
the brain can be easily identified: various fissures (e.g.,
parieto-occipital, calcarine), cortical gyri (e.g., area 17,
cingulate), the corpus callosum, lateral ventricle, and
thalamus. The three parts of the brainstem — midbrain,
pons and medulla — can be identified, with the tectum
(colliculi) seen behind the aqueduct of the midbrain.
The fourth ventricle separates the cerebellum from the
brainstem. The location of the cerebellar tonsil(s) should
be noted, adjacent to the medulla and immediately
above the foramen magnum, the “opening” at the base
of the skull (see discussion on tonsillar herniation with
Figure 8). The location of the cerebello-medullary
cistern (the cisterna magna) behind the medulla and just
above the foramen magnum is easily seen (see Figure
20A; also Figure 1B).
The remaining structures are those of the nose and
mouth, which are not within the subject matter in
this Atlas.
Note on radiological imaging
Ordinary x-rays show the skull and its bony structures
but not the brain. A remarkable revolution occurred in
clinical neurology and our understanding of the brain
when imaging techniques were developed that allowed
for visualization of the brain. These techniques now
include computed tomography (CT) and magnetic
resonance imaging (MRI):
• CT (often pronounced as a “cat” scan — see Figure
28A) uses x-rays and is a computer reconstruction of
the brain after a series of views are taken from many
perspectives. In this view the bones of the skull are
bright, the CSF is dark, and the brain tissue gray, not
clear. This image can be obtained in several seconds,
even with a very sick patient.
• MRI does not use x-rays; the image is created by capturing the energy of the hydrogen ions of water on
their return to a steady state. It uses an extremely
strong magnet, and requires more time. Again there is
a computer reconstruction of the images. The brain
itself looks “anatomic.” The view can be weighted
during acquisition of the image to produce a TI
image, in which the CSF is dark (as in Figure 17), or a
T2 image in which the CSF is white (see Figure 30).
An intermediate setting allows the structures of the
interior of the brain to be seen; this method produces
a proton density image. With MRI, the bones of the
skull are dark, while fatty tissue (including bone
marrow) is bright.
As imaging and technology improve, we are able to visualize the brain during functional activity. Functional
MRIs allow us to see which areas of the brain are particularly active during a certain task, based upon the
metabolic rate (oxygen saturation). They are becoming
more widely available.
Other techniques also visualize the living brain and its
activity, such as the positron emission tomography
(PET) scan. PET uses a very short-acting radioactive
compound which is injected into the arterial system. Its
use is usually restricted to specialized neurologic centers
involved in research on the human brain.
1. Corpus callosum
2. Thalamus
3. Tectum of midbrain
4. Optic chiasm
5. Pons
6. Tonsil of cerebellum
7. Cisterna magna
8. Spinal cord
FIGURE 17: Radiologic View Of Hemispheres — MRI: Sagittal View
FIGURE 18
CORPUS CALLOSUM
CEREBRAL HEMISPHERES:
DISSECTED VIEW
Structures within the depths of the cerebral hemispheres
include the white matter, cerebral ventricles, and basal
ganglia, all of which are described in the following figures.
The white matter consists of the myelinated axonal
fibers connecting brain regions. In the spinal cord these
are called tracts; in the hemispheres these bundles are
classified in the following way (also discussed with
Figure 15):
• association bundles which connect cortical areas on
the same side;
• projections fibers, connecting the cortex with subcortical structures in the diencephalon, brainstem, and
spinal cord; and
• commissural connections, across the midline — the
largest of these is the corpus callosum.
In the dissection of this specimen, the brain is again
seen from the medial view. Its anterior aspect is on the
right side of the photograph. Cortical tissue has been
removed (as shown in Figure 16), using blunt dissection
techniques. If done successfully, the fibers of the corpus
callosum can be followed to the cerebral cortex. These
fibers intermingle with the other fiber bundles which
make up the mass of white matter in the depth of the
hemisphere.
The dissection in Figure 18 shows the white matter of
the corpus callosum, followed to the cortex. The corpus
callosum is the massive commissure of the forebrain,
connecting homologous regions of the two hemispheres
of the cortex across the midline (see also Figure 15). In a
sagittal section, the thickened anterior aspect of the
corpus callosum is called the genu, and the thickened
posterior portion the splenium (neither has been labeled
in this figure).
Clinical Aspects
In a clinical setting, the corpus callosum is sectioned
surgically in individuals with intractable epilepsy
(epilepsy that could not be controlled with anti-convulsant medication). The idea behind the surgery is to stop
the spread of the abnormal discharges from one hemisphere to the other. Studies of these individuals have
helped to clarify the role of the corpus callosum in
normal brain function. Generally, the surgery has been
helpful in well selected cases and there is apparently no
noticeable change in the person, or his or her level of
brain function. Under laboratory conditions, it has been
possible to demonstrate in these individuals how the two
hemispheres of the brain function independently, after
the sectioning of the corpus callosum. These studies
show how each hemisphere responds differently to
various stimuli, and the consequences of information
not being transferred from one hemisphere to the other.
Corpus callosum
FIGURE 18: Corpus Callosum — Cerebral Hemispheres: Dissected View
FIGURE 19A
WHITE MATTER
ASSOCIATION FIBERS
The dorsolateral aspect of the brain is shown in Figure
19A. Under the cerebral cortex is the white matter of the
brain. Various fiber bundles can be dissected (not easily)
using a blunt instrument (e.g., a wooden tongue depressor). Some of these, functionally, are the association
bundles, fibers that interconnect different parts of the
cerebral cortex on the same side.
The specimen in Figure 19A has been dissected to show
one of the association bundles within the hemispheres.
There are also shorter association fibers between
adjacent gyri.
These association bundles are extremely important in
informing different brain regions of ongoing neuronal
processing, allowing for integration of activities (for
example, sensory with motor and limbic). The various
names of these association bundles are usually not of
much importance in a general introduction to the CNS
(except as in Figure 19B).
White matter
association bundle
FIGURE 19A: White Matter — Cerebral Hemispheres: Association Fibers
FIGURE 19B
WHITE MATTER
ASSOCIATION FIBERS
The arcuate bundle is a specific group of association
fibers of some importance, particularly on the side
dominant for language (the left hemisphere in most
people). This fiber bundle connects the two cortical
areas for language (see Figure 12), Broca’s anteriorly
with Wernicke’s posteriorly.
Damage to these fibers in humans causes a disruption of
language, called conduction aphasia. Aphasia is a
general term for a disruption or disorder of language. In
conduction aphasia, the person has normal comprehension (intact Wernicke’s area) and fluent speech (intact
Broca’s area). The only deficit is an inability to repeat
what has been heard. This is usually tested by asking the
patient to repeat single words or phrases whose meaning
cannot be readily understood (e.g., the phrase “no ifs,
ands, or buts”).
Arcuate bundle
FIGURE 19B: White Matter — Cerebral Hemispheres: Association Fibers
FIGURE 20A
VENTRICLES
that the “hole” in the middle of the third ventricle represents the massa intermedia, linking the two thalami
across the midline (see Figure 3; described with Figure 9).
VENTRICLES: LATERAL VIEW
The ventricular system then narrows considerably as it
goes through the midbrain and is now called the aqueduct of the midbrain, the cerebral aqueduct, or the
aqueduct of Sylvius (see Figures 16 and 17). In the hindbrain region, the area consisting of pons, medulla, and
cerebellum, the ventricle widens again to form the
fourth ventricle (see Figures 16 and 17). The channel
continues within the CNS and becomes the very narrow
central canal of the spinal cord (Figure 2A).
The ventricles are cavities within the brain filled with
CSF, cerebrospinal fluid. The formation, circulation, and
locations of the CSF are explained with Figure 21.
The ventricles of the brain are what remain of the original neural tube, the tube that was present during development. The cells of the nervous system, both neurons
and glia, originated from a germinal matrix adjacent to
the lining of this tube. The cells multiply and migrate
away from the walls of the neural tube, forming the
nuclei and cortex. As the nervous system develops, the
mass of tissue grows and the size of the tube diminishes,
leaving various spaces in different parts of the nervous
system.
The parts of the tube that remain in the hemispheres
are called the cerebral ventricles, or the lateral
ventricles. In Figure 20A, the lateral ventricle of one
hemisphere is shown from the lateral perspective. It is
shaped like a reversed letter “C”; it curves posteriorly to
enter into the temporal lobe. Its various parts are
• the anterior horn, which lies deep to the frontal lobes;
• the central portion or body, which lies deep to the
parietal lobes;
• the atrium or trigone where the ventricle widens and
curves into the inferior horn, which goes into the
temporal lobes.
In addition, there may be an extension into the occipital
lobes, called the occipital or posterior horn. These
lateral ventricles are also called ventricles I and II
(assigned arbitrarily).
Each lateral ventricle is connected to the midline third
ventricle by an opening, the foramen of Monro (interventricular foramen — seen in the medial view of the
brain, Figure 16). The third ventricle is a narrow, slitlike ventricle between the thalamus on either side and is
also called the ventricle of the diencephalon (see also
Figure 8). Sectioning through the brain in the midline
(as in Figure 16) passes through the third ventricle. Note
Within the ventricles is specialized tissue, the choroid
plexus. It is made up of the lining cells of the ventricles,
called the ependyma, and pia with blood vessels (discussed with Figure 21). The choroid plexus is the tissue
responsible for the formation of most of the CSF. It is
found in the body and inferior horn of the lateral ventricle, in the roof of the third ventricle, and in the lower
half of the roof of the fourth ventricle. The tissue forms
large invaginations into the ventricles in each of these
locations (unfortunately, none of these are visible in the
photographic views).
CSF flows through the ventricular system, from the
lateral ventricles, through the interventricular foramina
into the third ventricle, then through the narrow aqueduct and into the fourth ventricle. At the bottom of the
fourth ventricle, CSF exits from the ventricular system
and enters the subarachnoid space. The exit points are
foramina in the fourth ventricle. The major exit is the
foramen of Magendie in the midline. There are two additional exits of the CSF laterally from the fourth ventricle, the foramina of Luschka (seen in Figure 20B).
From the foramen of Magendie, the CSF then enters one
of the enlargements of the subarachnoid space, called a
cistern, in this case the cisterna magna, the cerebellomedullary cistern. It lies below the cerebellum and is
found inside the skull, just above the foramen magnum
(see Figure 1B). The CSF then flows in the subarachnoid
space downwards around the spinal cord and upwards
around the brain (discussed with Figure 21).
LATERAL VENTRICLE
Anterior horn
Body
Inferior horn
Posterior horn
Cisterna magna
Foramen of Monro
Third ventricle
Aqueduct of Sylvius
Fourth ventricle
Foramen of Magendie
Central canal
FIGURE 20A: Ventricles — Ventricles: Lateral View
FIGURE 20B
VENTRICLES
VENTRICLES: ANTERIOR VIEW
The ventricular system is viewed from the anterior perspective in Figure 20B. Both lateral ventricles and the
short interventricular foramen (of Monro) are visible on
both sides, connecting each lateral ventricle with the
midline third ventricle. It is important to note that the
thalamus (diencephalon) is found on each side of the
third ventricle (see also Figure 8).
Sectioning of the brain in the coronal (frontal) plane, if
done at the appropriate plane, reveals the spaces of the
lateral ventricles within the hemispheres (see Figure 29).
Likewise, sectioning of the brain in the horizontal axis, if
done at the appropriate level, shows the ventricular
spaces of the lateral and third ventricles (see Figure 27).
These can also be visualized with radiographic imaging
(CT and MRI; see Figures 28A , 28B, and 30).
The ventricular channel continues through the aqueduct
of the midbrain. CSF then enters the fourth ventricle,
which also straddles the midline. The fourth ventricle is
diamond-shaped, and the lateral recesses carry CSF into
the cisterna magna through the foramina of Luschka,
the lateral apertures, one on each side.
Clinical Aspects
The flow of CSF can be interrupted or blocked at
various key points within the ventricular system. One of
the most common is the cerebral aqueduct, the aqueduct
of the midbrain (Sylvius). Since most of the CSF is
formed upstream, in the lateral (and third) ventricles, a
blockage at the narrowest point at the level of the aqueduct of the midbrain will create a damming effect. In
essence, this causes a marked enlargement of the ventricles. The CSF flow can be blocked in a variety of ways,
such as developmentally, following a meningitis, or by a
tumor in the region. The result is an enlargement of the
ventricles, called hydrocephalus, which can be seen with
imaging techniques (e.g., CT scan).
Not uncommonly, hydrocephalus in infancy occurs
spontaneously, for unknown reasons. Since the sutures
of the infant’s skull are not yet fused, hydrocephalus
leads to an enlargement of the head. Clinical assessment
of normal infants should include measuring the size of
the head and charting this in the same way as one charts
height and weight. Untreated hydrocephalus eventually
leads to a compression of the nervous tissue of the
hemispheres and damage to the developing brain.
Clinical treatment of this condition, after evaluation of
the causative factor, includes shunting the CSF out of
the ventricles into one of the body cavities.
In adults, hydrocephalus caused by a blockage of the
CSF flow leads to an increase in intracranial pressure,
since the sutures are fused and skull expansion is not
possible. The cause in adults is usually a tumor, and, in
addition to experiencing the specific symptoms, the
patient will usually complain of headache, often in the
early morning.
Lateral ventricle
Foramina of Luschka
Foramen of Monro
Third ventricle
Central canal
Aqueduct of Sylvius
Fourth ventricle
T = Thalamus
FIGURE 20B: Ventricles — Ventricles: Anterior View
FIGURE 21
CEREBROSPINAL FLUID
SCHEMATIC OF CSF CIRCULATION
Figure 21 presents a schematic representation of the relationship between the brain tissue, CSF, and cerebral
blood vessels. The CSF is formed within the ventricles,
flows through the various channels, exits from within
the brain, circulates in the subarachnoid space and cisterns around the brain and spinal cord, and is finally reabsorbed into the venous system (the venous sinuses).
The physiological barriers between the various compartments is also indicated in the illustration.
The ventricles of the brain are lined with a layer of cells
known as the ependyma. In certain loci within each of
the ventricles the ependymal cells and the pia meet, thus
forming the choroid plexus, which invaginates into the
ventricle. Functionally, the choroid plexus has a vascular
layer, i.e., the pia, on the inside, and the ependymal layer
on the ventricular side. The blood vessels of the choroid
plexus are freely permeable, but there is a cellular barrier
between the interior of the choroid plexus and the ventricular space — the blood-CSF barrier (labeled B-CSFB in the diagram). The barrier consists of tight junctions
between the ependymal cells that line the choroid
plexus. CSF is actively secreted and an enzyme is involved. The ionic and protein composition of CSF is
different from that of serum.
CSF leaves the ventricular system from the fourth ventricle, as indicated schematically in the diagrams. In the
intact brain, this occurs via the medially placed foramen
of Magendie and the two laterally placed foramina of
Luschka (described in the previous illustrations). CSF
flows through the subarachnoid space (SAS), between
the pia and arachnoid. The CSF fills the enlargements of
the subarachnoid spaces around the brainstem — the
various cisterns — and also flows downwards around
the spinal cord to fill the lumbar cistern (see Figures 1A
and 1B).
This slow circulation is completed by the return of
CSF to the venous system. The return is through the
arachnoid villi which protrude into the venous sinuses
of the brain, particularly the superior sagittal sinus
(located in the interhemispheric fissure). These can be
seen as collections of villi, called arachnoid granulations, on the surface of the brain (shown diagrammatically; see also Figure 11).
There is no real barrier between the intercellular tissue
of the brain and the CSF, either through the ependyma
(at all sites other than the choroid plexus) or the pia.
This lack of barrier is depicted by the arrows, which indicate a free communication between these compartments. Therefore, substances found in detectable
amounts in the intercellular spaces of the brain may be
found in the CSF.
On the other hand, there is a real barrier, both structural
and functional, between the blood vessels and the brain
tissue. This is called the blood-brain barrier (BBB) and
is situated at the level of the brain capillaries. Only
oxygen, carbon dioxide, glucose, and other select, small
molecules are normally able to cross the BBB.
Sampling of CSF for clinical evaluation, including inflammation of the meninges (meningitis), is performed
almost always in the lumbar cistern (discussed with
Figures 1A and 1B).
A
SAS
Pia
Brain
Ependyma
B
Ventricle
B-CSF-B
B
B
Choroid plexus
Vein
BBB = Blood-brain barrier
A = Artery
B-CSF-B = Blood-CSF barrier
SAS = Subarachnoid space
FIGURE 21: Cerebrospinal Fluid — Schematic of CSF Circulation
FIGURE 22
BASAL GANGLIA I
BASAL GANGLIA: ORIENTATION
The basal ganglia are large collections of neurons belonging to the forebrain. This neuronal mass is found
deep within the cerebral hemispheres. Our understanding of the functional role of the basal ganglia is derived
largely from disease states affecting these neurons. In
general, humans with lesions in the basal ganglia have
some form of motor dysfunction (movement disorder)
— dyskinesia.
This large group of neurons is now thought to be involved in the control of complex patterns of motor activity, such as skilled movements (e.g., writing). There
are two aspects to this involvement. The first concerns
the initiation of the movement, and the second concerns
the quality of the performance of the motor task. It
seems that different parts of the basal ganglia are concerned with the speed and magnitude of a movement. In
addition, some of the structures that make up the basal
ganglia are thought to influence cognitive aspects of
motor control, helping to plan the sequence of tasks
needed for purposeful activity. This process is sometimes referred to as the selection of motor strategies.
From the strictly anatomical point of view, the basal
ganglia are collections of neurons located within the
hemispheres. Traditionally, this would include the
caudate nucleus, the putamen, the globus pallidus,
and the amygdala. Although the caudate and putamen
are separated from each other anatomically, they are
histologically the same neurons, and are known as the
striatum. The putamen and globus pallidus are anatomically found together and are called the lentiform or
lenticular nucleus. From the functional point of view
and based upon the complex pattern of interconnections, two other nuclei should be included with the description of the basal ganglia: the subthalamic nucleus
(part of the diencephalon) and the substantia nigra
(located in the midbrain). The amygdala is now
included with the limbic system (see Section D).
Clinical Aspects
Overall, the basal ganglia receive much of their input
from the cortex, the motor areas of the cortex, as well as
wide areas of association cortex. There are intricate connections between the various parts of the system (involving different neurotransmitters), and the output is
directed via the thalamus mainly to premotor, supplementary motor, and frontal cortical areas. Therefore, the
basal ganglia act as a subloop of the motor system by altering cortical activity (to be fully discussed with Figures
50 and 51).
Note on terminology: The term ganglia refers to a collection of cells in the peripheral nervous system.
Therefore, the anatomically correct name for this group
of neurons should be the basal nuclei. Few texts use this
term. Most clinicians would be hard-pressed to change
to the use of this term, so the traditional name remains
in use.
The functional role of this large collection of neurons is
best illustrated by clinical conditions affecting this neuronal system — abnormal movements, such as chorea
(jerky movements), athetosis (writhing movements),
and tremors (rhythmic movements). The most common
condition that affects this group of neurons is
Parkinson’s disease. A person with this disease has difficulty initiating movements, loses facial expressiveness
taking on a mask-like appearance, and has muscular
rigidity, slowing of movements (bradykinesia), and a
tremor of the hands when at rest that goes away with
purposeful movements (and in sleep).
The other major disease that affects the basal ganglia is
Huntington’s Chorea, an inherited degenerative condition.
Corpus callosum
Caudate nucleus
Cerebellum
Lenticular nucleus
Brainstem
FIGURE 22: Basal Ganglia I — Basal Ganglia: Orientation
FIGURE 23
BASAL GANGLIA II
BASAL GANGLIA: NUCLEI A
The basal ganglia, from the point of view of strict neuroanatomy, are located within the cerebral hemispheres.
There are three major nuclei in the hemispheres:
• The caudate nucleus has three portions:
the head, located deep within the frontal lobe;
the body, located deep in the parietal lobe; and
the tail, which goes into the temporal lobe.
• The lentiform or lenticular nucleus, so-named
because it is lens-shaped. In fact, it is composed of
two nuclei (see Figure 24): putamen and globus pallidus. It is situated laterally and is deep in the hemispheres within the central white matter. Sectioning of
the brain in the horizontal plane (see Figure 27) and
in the coronal (frontal) plane (see Figure 29) shows
the location of the lentiform nucleus in the depths of
the hemispheres. Both the caudate and the lentiform
nuclei are found below the level of the corpus
callosum.
In Figure 23 the basal ganglia are shown in isolation
from the lateral perspective, as well as from above, allowing a view of the caudate nucleus of both sides. The
various parts of the caudate nucleus are easily recognized — head, body and tail. The head of the caudate
nucleus is large and actually intrudes into the space of
the anterior horn of the lateral ventricle (see Figures 27
and 28A). The body of the caudate nucleus is considerably smaller and lies above the thalamus (see Figure 29).
The tail follows the inferior horn of the lateral ventricle
into the temporal lobe (see Figure 25). In a coronal
section (Figure 80), the tail of the caudate is found
above the inferior horn of the lateral ventricle.
The lentiform (lenticular) nucleus is only a descriptive
name, meaning lens-shaped. The nucleus is in fact composed of two distinct parts — the putamen, laterally,
and the globus pallidus, medially (see Figures 24 and
27). When viewing the basal ganglia from the lateral perspective, one sees only the putamen part.
Strands of tissue are seen connecting the caudate
nucleus with the lentiform nucleus; in fact, they connect
the caudate with the putamen (as discussed with Figure
26). The caudate and the putamen are histologically
alike and are collectively called the neostriatum, or
simply the striatum.
The amygdala, though part of the basal ganglia by definition, has its functional connections with the limbic
system and is discussed with it (see Figures 79A and
79B). The inferior portions of the putamen and globus
pallidus are found at the level of the anterior commissure. These ventral parts of the lentiform nucleus may
have a limbic connection (discussed with Figure 85B).
The anterior commissure connects the amygdala and
other temporal lobe structures of the two sides (discussed with Figure 74).
The other two nuclei of the functional basal ganglia —
the subthalamus and substantia nigra — are not shown
in this illustration.
Caudate nucleus
(body)
Lenticular nucleus
(putamen)
Caudate nucleus
(tail)
Caudate nucleus
(head)
Amygdala
Anterior commissure
FIGURE 23: Basal Ganglia II — Basal Ganglia: Nuclei A
FIGURE 24
BASAL GANGLIA III
BASAL GANGLIA: NUCLEI B
The view in Figure 24 has been obtained by removing all
parts of the basal ganglia of one hemisphere, except the
tail of the caudate and the amygdala. This view exposes
the lentiform nucleus of the “distal” side; the lentiform
nucleus is thus visualized from a medial perspective. The
two portions of this nucleus are seen: the putamen laterally and the globus pallidus, which is medially placed.
The caudate nucleus and the putamen receive the inputs
into the basal nuclei. As has been explained previously,
the caudate and putamen are, in fact, the same neurons
embryologically. Together they are known as the
neostriatum. (In some texts they are simply called the
striatum.) Strands of cells may be seen connecting the
various portions of the caudate with the putamen. This
nuclear structure has been separated into two distinct
components by groups of axons, which collectively are
called the internal capsule (see Figure 26). These fiber
bundles “fill the spaces” between the cellular strands.
The globus pallidus is one of the efferent nuclei of the
basal ganglia. It is composed of two segments — the
medial and lateral segments, also known as internal and
external segments, respectively. (These segments can also
be seen in the horizontal section of the brain, Figure 27).
This subdivision of the globus pallidus is quite important functionally.
This view exposes the two additional nuclei of the basal
ganglia — the subthalamic nucleus (part of the diencephalon) and the substantia nigra (of the midbrain).
The functional connections of these nuclei are discussed
as part of the motor system (see Figures 50 and 51).
A distinct collection of neurons is also found in the
ventral region, composed in part of neurons belonging
to the basal nuclei — the nucleus accumbens. The
nucleus accumbens is unique in that it seems to consist
of a mix of neurons from the basal ganglia and from the
limbic structures in the region (discussed with the
limbic system, Figure 85B).
Putamen
Caudate nucleus
Globus pallidus
(external segment)
Subthalamic nucleus
Globus pallidus
(internal segment)
Substantia nigra
Red nucleus
Caudate nucleus
(tail)
N. accumbens
Midbrain
Amygdala
Anterior commissure
FIGURE 24: Basal Ganglia III — Basal Ganglia: Nuclei B
FIGURE 25
BASAL GANGLIA IV
BASAL GANGLIA AND VENTRICLES
In humans, the three forebrain nuclei of the basal
ganglia have a complex arrangement in space, and visualization of their location is made easier by understanding their relationship with the cerebral ventricles (also
illustrated in Figure 80).
In Figure 25 the basal ganglia are visualized deep in the
hemisphere, from the lateral perspective. The various
parts include the caudate nucleus, the lentiform nucleus,
and also the amygdala. Included in this view is the ventricular system (as in Figure 20A) and the thalamus,
which lies adjacent to the third ventricle (see Figures 16
and 20B).
All three parts of the caudate nucleus — the head, body,
and tail — are situated adjacent to the lateral ventricle.
In fact, the head protrudes into the anterior horn of the
lateral ventricle (see Figure 27). The body of the caudate
lies adjacent to the body of the ventricle (see Figure 9),
with the tail following the ventricle into the temporal
lobe (see Figure 80).
The lentiform nucleus (which includes the putamen and
globus pallidus), is located deep within the hemispheres,
not adjacent to the ventricle. This “nucleus” is found
lateral to the thalamus, which locates the lentiform
nucleus in a horizontal section as lateral to the third
ventricle.
In the view provided by Figure 25, one can see that the
caudate and the lentiform nuclei are connected by
strands of tissue between them; in fact, these neurons
are the same embryologically. (These connecting strands
are shown in the previous diagrams.) As fiber systems
develop, namely the internal capsule (see Figure 26),
these nuclei become separated from each other (by the
anterior limb of the internal capsule). Some connecting
strands of tissue can still be found in the adult brain on
a few horizontal sections through the lowermost parts of
these nuclei.
The internal capsule, which consists of a collection of
fibers (described with Figure 26) is situated between the
lentiform nucleus and the head of the caudate, and
between the lentiform nucleus and the thalamus. A horizontal section through the brain at the level of the lateral
fissure would reveal all these structures (see Figure 27).
The amygdala is located in front of the tip of the inferior
horn of the lateral ventricle, under the uncus (see
Figures 13 and 14).
Connection between lentiform and caudate nn.
CAUDATE NUCLEUS
Head
Body
Tail
Amygdala
Lentiform nucleus
Third ventricle
Lateral ventricle
Thalamus
FIGURE 25: Basal Ganglia IV — Basal Ganglia and Ventricles
FIGURE 26
INTERNAL CAPSULE I
INTERNAL CAPSULE: WHITE
MATTER
The white matter bundles that course between parts of
the basal ganglia and the diencephalon are collectively
grouped together and called the internal capsule. The
internal capsule is a group of fibers located at a specific
location within the cerebral hemispheres — between the
diencephalon (the thalamus) and the various parts of
the basal ganglia. These so-called projection fibers are
axons going to and coming from the cerebral cortex,
linking the cortex with the diencephalon (thalamus),
brainstem, and spinal cord.
The internal capsule has 3 parts:
• As was noted earlier, a group of fibers has separated
off the two parts of the neostriatum from each other,
the caudate from the putamen. This fiber system
forms its anterior limb;
• At the level of the lentiform nucleus, situated medially,
is the thalamus, part of the diencephalon (see Figure
27). The fiber system that runs between the thalamus
(medially) and the lentiform nucleus (laterally) is the
posterior limb of the internal capsule. The posterior
limb carries sensory information from thalamus to
cortex, the reciprocal connections from cortex to thalamus, and most of the descending fibers to the brainstem (cortico-bulbar, Figure 43) and spinal cord
(cortico-spinal, Figure 42).
• The internal capsule can be seen in a horizontal
section of the brain (see Figure 27), and with neuroradiological imaging (CT, see Figure 28A, and MRI, see
Figure 28B). In this view, the internal capsule (of each
side) is seen to be V-shaped. The anterior portion
between the caudate and lentiform is the anterior
limb. The portion between the lentiform nucleus and
the thalamus is the posterior limb. The bend of the
“V” is called the genu and it is situated medially.
In addition, there are numerous axons descending from
the cortex destined for the cerebellum (and that will
synapse first in the pontine nuclei; discussed with Figure
53). These cortico-pontine fibers descend in both the
anterior and posterior limbs of the internal capsule.
Other fiber systems emanating from the thalamus are
often described in relation to the internal capsule. For
example, the visual radiation is situated posterior to the
internal capsule, yet functionally is part of it (see Figures
39A and 39B). The auditory radiation, which runs inferiorly (see Figure 36), is similar.
Below the level of the internal capsule is the midbrain.
The descending fibers of the internal capsule are continuous with those found in the cerebral peduncle of the
midbrain. A parcellation of the descending fibers occurs
in the cerebral peduncle, where the cortico-pontine
fibers are found in the outer and inner thirds of the peduncle, and the cortico-bulbar and cortico-spinal fibers
are located in the middle third (see also Figure 43).
In summary, at the level of the internal capsule, there are
both the ascending fibers from thalamus to cortex, as
well as the descending fibers from widespread areas of
the cerebral cortex to the thalamus and the rest of the
neuraxis. These ascending and descending fibers are all
called projection fibers (discussed with Figure 18). This
fiber system is sometimes likened to a funnel, in which
the top of the funnel is the cerebral cortex and the stem
is the cerebral peduncle. The base of the funnel would
be the internal capsule. The main point is that the
various fiber systems, both ascending and descending,
are condensed together in the region of the internal
capsule.
Clinical Aspects
The area of the internal capsule is clinically important
because of the frequency of lesions here. The blood
vessels that supply the internal capsule come from the
middle cerebral artery, as it courses in the lateral fissure;
they are known as the striate arteries (see Figure 60). For
reasons that are not clearly understood, these blood
vessels are prone to occlusion which destroys the surrounding axons. Because of the high packing density of
the axons in this region, a small lesion can create extensive disruption of descending and/or ascending pathways; this is a common form of “stroke.” These small
lesions are seen as small holes with neuroimaging and
called “lacunes” (discussed with Figure 60).
Posterior limb
Genu
Visual
radiation
Anterior
limb
Descending
fibers
Midbrain
Thalamus
Caudate nucleus
(head)
Lenticular
nucleus
Cortico-pontine fibers
Anterior commissure
FIGURE 26: Internal Capsule I — Internal Capsule: White Matter
Cortico-pontine
fibers
Cortico-spinal fibers
Cortico-bulbar fibers
FIGURE 27
INTERNAL CAPSULE II
HORIZONTAL SECTION OF
HEMISPHERES (PHOTOGRAPHIC
VIEW)
The brain specimen in Figure 27 has been sectioned in
the horizontal plane, at the level of the lateral fissure.
This view exposes the white matter of the hemispheres
and the basal ganglia, as well as parts of the ventricular
system. Understanding the topography of the structures
seen in this view is immeasurably important when the
student enters the clinical setting.
The basal ganglia are observable when the brain is
sectioned at this level. The head of the caudate nucleus
is seen, protruding into the lateral ventricle. The
lentiform nucleus is shaped somewhat like a lens and
is demarcated by white matter. The outer part, the
putamen, has neurons that are identical to the caudate
nucleus, and, therefore, the two nuclei look the same.
The inner portion, the globus pallidus, is functionally
different and contains many more fibers and therefore is
lighter in color. Depending upon the level of the section,
it is sometimes possible (as in the right side of the photograph in Figure 27) to see the two subdivisions of the
globus pallidus, the internal and external segments (see
Figure 24).
The white matter medial to the lentiform nucleus is the
internal capsule. It is divisible into an anterior portion,
the anterior limb, and a posterior limb; the base of the
“V” is known as the genu. The anterior limb separates
the lentiform nucleus from the head of the caudate
nucleus. This portion of the caudate nucleus is related to
the anterior horn of the lateral ventricle, which is cut
through its lowermost part and is represented in this
photograph by a very small cavity. Some strands of gray
matter located within the anterior limb represent the
tissue that unites the caudate nucleus with the putamen
(as shown in Figure 25). The posterior limb of the internal capsule separates the lentiform nucleus from the thalamus. The major ascending sensory tracts and the
descending motor tracts from the cerebral cortex are
found in the posterior limb.
Lateral to the lentiform nucleus is another thin strip of
tissue, the claustrum. The functional contribution of this
small strip of tissue is not really known. Lateral to this is
the cortex of the insula (see Figure 37). Posteriorly,
behind the thalamus, the cerebellum is visible.
It is also possible to see the atrium portion of the lateral
ventricle, deep within the parietal lobe. The ventricle is
sectioned at this level as it enters into the temporal lobe
and is becoming the inferior horn of the lateral ventricle
(see Figure 20A). The third ventricle can be seen situated
between the thalamus of both sides.
A view similar to this is commonly presented in brain
scans of patients — CTs and MRIs — as shown in the
following figures.
Anterior
Lateral ventricle
(anterior horn)
Anterior limb of
internal capsule
Head of caudate
nucleus
Putamen
Lentiform nucleus
Globus pallidus
Claustrum
Posterior limb of
internal capsule
Thalamus
Third ventricle
Lateral
ventricle
C = Cerebellum
FIGURE 27: Internal Capsule II — Horizontal Section of Hemispheres (Photographic View)
FIGURE 28A
HORIZONTAL VIEW
HORIZONTAL VIEW: CT
The radiological view of the brain is not in the exact
horizontal plane as the anatomical specimen shown in
the previous illustration. The radiological images of the
brain are done at an angle in order to minimize the
dense bones of the posterior cranial fossa, which impair
the viewing of the structures (brainstem and cerebellum) of the posterior cranial fossa.
A CT image (refer to Figure 17) shows the skull bones
and the relationship of the brain to the skull. The outer
cortical tissue can be seen, with gyri and sulci. The
structures seen in the interior of the brain include the
white matter and the ventricular spaces. Note that the
CSF is dark (black) in the ventricles (#2 and #3 in the illustration) and in the cisterns (#7 in the illustration).
The ventricular space in the frontal lobes is the anterior
horn of the lateral ventricle on each side (see Figure 27,
also Figures 20A and 20B), separated in the midline by
the septum pellucidum. (The septum has been removed
from the specimen shown in Figure 16, a mid-sagittal
view of the brain and brainstem.)
Although the basal ganglia and thalamus can be identified (see #1, #4, and #6 in the illustration), there is little
tissue definition. Note that the head of the caudate
nucleus protrudes, or bulges, into the anterior horn of
the lateral ventricle (see also Figure 27). The area of the
internal capsule can be seen as well (#5 in the illustration; compare with the MRI in the next illustration,
Figure 28B). The cerebellum can be recognized with its
characteristic folia (#8 in the illustration). The CSF
cistern, called the quadrigeminal plate cistern, behind
the tectal plate (the colliculi; also known as the
quadrigeminal plate; discussed with Figure 7) is a very
important landmark for the neuroradiologist (seen also
in mid-sagittal views, but not labeled, in Figures 16 and
17).
Clinical Aspects
A regular CT can show areas of increased density (e.g.,
fresh hemorrhage) or of decreased density (e.g., an
infarct), as well as changes in the size and shifting of the
ventricles. Tumors may be seen as an abnormal area of
either increased or decreased density. A CT can also be
enhanced by injecting an iodinated compound into the
blood circulation and noting whether it escapes into the
brain tissue because of leakage in the blood-brainbarrier (BBB; see Figure 21). This examination is invaluable in the assessment of a neurological patient in the
acute stage of illness (e.g., tumor, head injury), and it is
most frequently used because the image can be captured
in seconds.
1. Head of caudate nucleus
2. Lateral ventricle (anterior horn)
3. Third ventricle
4. Lentiform nucleus
5. Internal capsule (posterior limb)
6. Thalamus
7. Quadrigeminal plate cistern
8. Cerebellum
FIGURE 28A: Horizontal View — Horizontal View: CT
FIGURE 28B
HORIZONTAL VIEW
HORIZONTAL VIEW: MRI
Figure 28B is a view of the brain taken in the same plane
as used for Figure 28A, but with MRI adjusted for a T2weighted image (see explanation with Figure 17). In this
view, the CSF of the ventricles is white. The lateral ventricle posteriorly (see number 5) is cut at the level of its
widening — the atrium — as it curves into the temporal
lobe (see Figure 20A).
This MRI shows the brain as if it were an anatomical
specimen (compare with Figure 28A), showing the
cortex and white matter. There is a clear visualization of
the basal ganglia and its subdivisions (head of the
caudate, putamen), as well as of the thalamus. In addition, the area of the internal capsule is also seen.
Clinical Aspects
The MRI has proved to be invaluable in assessing lesions
of the CNS — infarcts, tumors, plaques of multiple sclerosis, and numerous other lesions.
1. Head of caudate nucleus
2. Lentiform nucleus
3. Thalamus
4. Internal capsule (posterior limb)
5. Lateral ventricle (atrium)
FIGURE 28B: Horizontal View — Horizontal View: MRI
FIGURE 29
CORONAL VIEW
CORONAL SECTION OF
HEMISPHERES (PHOTOGRAPHIC
VIEW)
The photographic view of the brain in Figure 29 is sectioned in the coronal plane and shows the internal
aspect of the hemispheres. The plane of this section is
somewhat asymmetric, and includes the parietal and
temporal lobes.
The external aspect of the hemispheres is composed of
the cerebral cortex, seen as the gray matter. It is thrown
into various gyri with sulci between. The deep interhemispheric fissure is seen (not labeled; see Figure 15),
along with the lateral fissure (also not labeled), with the
insula within the depths of this fissure (see Figure 37).
The white matter is seen internally; it is not possible to
separate the various fiber systems of the white matter.
The fibers of the corpus callosum are seen crossing the
midline at the bottom of the interhemispheric fissure
(see also Figure 18). Below the corpus callosum are the
two spaces, the cavities of the lateral ventricle, represented at this plane by the body of the ventricles. Because
the section was not cut symmetrically, the inferior horn
of the lateral ventricle is found only on the right side of
the photograph, in the temporal lobe.
The brain is sectioned in the coronal plane through the
diencephalic region. At this plane of section, the third
(midline) ventricle is not present between the thalami,
except for a rather small space, likely because the section
passes through the connecting link between the two
thalami — the massa intermedia (discussed with Figures
9 and 20A).
Various parts of the basal ganglia are seen in this view, as
has been described with the illustration of the ventricles
(see Figure 25). At the outer margins of the ventricle
(body) is a dark nuclear structure, the body of the
caudate nucleus. More laterally is the lentiform nucleus;
because the brain has not been sectioned symmetrically,
more of this nucleus is found on the left side of the
photograph.
Lateral to the thalamus and medial to the lentiform
nucleus is the internal capsule, its posterior limb. (The
student should refer to the horizontal section in Figure
27 and note why this is the posterior limb of the internal
capsule.)
The structures noted in this section should be compared
with a similar (coronal) view of the brain taken more
posteriorly (see Figure 78).
The fornix is explained with the limbic system (Section D).
Corpus callosum
Thalamus
Fornix
Insula
Internal capsule
(posterior limb)
Lentiform nucleus
Lateral ventricle
(inferior horn)
P = Parietal lobe
T = Temporal lobe
FIGURE 29: Coronal View — Coronal Section of Hemispheres (Photographic View)
FIGURE 30
CORONAL VIEW
CORONAL VIEW: MRI
Figure 30 presents the same coronal view of the brain as
shown in Figure 29, but with MRI. It shows the cortex,
white matter, a little of the basal ganglia, the thalamus,
and the ventricular spaces. This is a T2-weighted view,
with the CSF white.
The cortex and white matter can be easily differentiated
in the figure. The plane of this section includes the body
of the caudate nucleus (compare with the coronal
section of the brain shown in Figure 78).
The region of the thalamus is seen adjacent to the
midline third ventricle, interrupted by the massa intermedia. This view of the ventricles is similar to that presented in Figure 20B. The area of the internal capsule
(posterior limb) is lateral to the thalamus, with the
lentiform nucleus lateral to it. This view also includes
the brainstem (the pontine region).
1. Caudate nucleus (body)
2. Thalamus
3. Pontine region
FIGURE 30: Coronal View — Coronal View: MRI
Section B
FUNCTIONAL SYSTEMS
INTRODUCTION
The functioning nervous system requires a hierarchical
organization to carry out its activities. A simple reflex
(e.g., touching a hot surface) involves incoming information from the periphery, some central processing, and
a response. The incoming fibers are the afferent
(sensory) fibers, the outgoing are the efferent (motor)
fibers, and the central processing portions are the interneurons of the spinal cord (see Figure 2A).
In order to go beyond the simple reflex state, more elaborate central processing stations have evolved, creating
functional systems — both sensory and motor. These
involve nuclei of the CNS at the level of the brainstem
and forebrain, and the connections between these
nuclei. The axons connecting various nuclei usually run
together, forming a distinct bundle of fibers, called a
tract or pathway. Along their way, these axons may distribute information to several other parts of the CNS by
means of axon collaterals. In almost all functional
systems in humans, the cerebral cortex is involved. Part I
of this section discusses the sensory tracts or pathways
and their connections in the CNS. Part II introduces the
reticular formation which has sensory, motor, and other
functions. Part III discusses the pathways and brain
regions concerned with motor control.
PART I: SENSORY SYSTEMS
Sensory systems, also called modalities (singular,
modality), share many features. All sensory systems
begin with receptors, some of which are highly specialized, such as those in the skin for touch and vibration
sense, and the hair cells in the cochlea for hearing, as
well as the rods and cones in the retina. These receptors
are hard-wired to activate the peripheral sensory fibers
appropriate for that sensory system. The peripheral
nerves have their cell bodies in sensory ganglia that
belong to the peripheral nervous system (PNS). For the
body (neck down), these are the dorsal root ganglia
located in the intervertebral spaces. The trigeminal
ganglion serves the sensory fibers of the head. The
central process of these peripheral neurons enters the
CNS and synapses in the nucleus appropriate for that
sensory system (again hard-wired).
Generally speaking, the older systems both peripherally
and centrally involve axons that are thinly myelinated or
unmyelinated, with a slow rate of conduction. In
general, these pathways consist of fibers-synapses-fibers,
creating a multisynaptic chain with many opportunities
for spreading the information, but thereby making
transmission slow and quite insecure. The newer pathways that have evolved have axons that are more thickly
myelinated and that conduct at a more rapid speed.
These form rather direct connections with few, if any,
collaterals. The latter type of pathway transfers information more securely and is more specialized functionally.
Because of the upright posture of humans, the sensory
systems go upwards, or ascend, to the cortex — the
ascending systems. The sensory information is processed by various nuclei along the pathway. Three
systems are concerned with sensory information from
the skin, two from the body region, and one (with subparts) from the head:
1.
The anterolateral system, an older system which
carries pain and temperature and some less discriminative forms of touch sensations (formerly
called the lateral spino-thalamic and anterior
(ventral) spino-thalamic tracts, respectively).
2.
The dorsal column-medial lemniscus pathway, a
newer system for the somatosensory modalities
of discriminative touch, joint position, and
“vibration.”
3.
The trigeminal pathways, carrying sensations
from the face area (including discriminative
touch, pain, and temperature), involve both older
and newer components.
The central nuclei for the afferents from the body region
are located in the spinal cord. The trigeminal nuclei are
found in the brainstem.
The auditory and visual systems are two special senses
that will be studied in some detail. Each has some
unique features which are described. Other sensory
pathways, such as vestibular and taste, are reviewed
below. All these pathways relay in the thalamus before
going to the cerebral cortex (refer to Figure 10). The olfactory system (smell) is considered with the limbic
system (see Figure 84).
Reticular Formation
Interspersed with the consideration of the pathways is
the reticular formation, located in the core of the brainstem. This group of nuclei comprises a rather old system
with multiple functions — some generalized, and some
involving both sensory and motor systems. Some
sensory pathways have collaterals to the reticular formation, some do not. The explanation of the reticular formation is presented after the sensory pathways; the
motor aspects are discussed with the motor systems.
Clinical Aspects
Destruction of the nuclei and pathways due to disease or
injury leads to a neurological loss of function. How does
the physician or neurologist diagnose what is wrong? He
or she does so on the basis of a detailed knowledge of
the pathways and their position within the central
nervous system; this knowledge is a prerequisite for the
part of the diagnosis that locates where the disease is occurring in the nervous system localization. The disease
can sometimes be recognized by experienced physicians
because of the pattern of the disease process; at other
times, specialized investigations are needed to make the
disease-specific diagnosis.
There is an additional caveat — almost all of the pathways cross the midline, each at a unique and different
location; this is called a decussation. The important
clinical correlate is that destruction of a pathway may
affect the opposite side of the body, depending upon the
location of the lesion.
Learning Plan
Studying pathways in the CNS necessitates visualizing
them, a challenging task for many. The pathways studied
here extend longitudinally through the CNS, going from
spinal cord to thalamus and cortex. Various texts and
atlases attempt to facilitate this visualization exercise for
the student by presenting diagrams; color adds to the
ability to visualize these pathways.
The tracts are presented in two ways. On the left side of
the page is a schematic of the CNS, with the spinal cord,
brainstem, and forebrain including the internal capsule.
(Note: T = thalamus, C = head of caudate nucleus, L =
lentiform nucleus.) This diagram is used to convey the
overall course of the tract, and particularly at what level
the fibers cross in the CNS. This information will assist
the student in correlating the anatomy of the pathway
with the clinical symptomatology.
On the right side, for each pathway, are cross sections
through the brainstem and spinal cord. The course of
the pathway is followed through the various levels on
one side; the exact position of the tract within the brainstem or spinal cord is indicated (in black) in the cross
sections on the other side. These brainstem sections are
similar to those shown in Section C, and the level of
each is identified. That section is titled “Neurological
Neuroanatomy” because it allows precise localization of
an injury or disease. At that stage, the student is presented with details of the histological anatomy of the spinal
cord and brainstem.
In this overview of the pathways, the student is advised
to return to the description of the thalamus and the
various specific relay nuclei (see Figures 9 and 10).
Likewise, reference to the cortical illustrations (see
Figures 11–16) will inform the student which areas of
the cerebral cortex are involved in the various sensory
modalities and will assist in integrating the anatomical
information presented in the previous section.
FIGURE 31
ANTEROLATERAL SYSTEM
PAIN, TEMPERATURE, CRUDE
TOUCH
Figure 31 shows the pathway that carries the modalities
of pain and temperature and a form of touch sensation
called crude or light touch. Also conveyed in this fiber
system are the sensations of itch and tickle, and other
forms of sensation (e.g., sensations of a sexual nature).
In the periphery the receptors are usually simply free
nerve endings, without any specialization. The nerves
are unmyelinated or thinly myelinated and conduct
slowly. The nerve cell bodies for these peripheral fibers
are located in the dorsal root ganglia (sometimes referred to as the first order neuron).
These fibers enter the spinal cord and synapse in the
dorsal horn (see Figure 2A). There are many collaterals
within the spinal cord that are the basis of several protective reflexes. The number of synapses formed is variable and somewhat uncertain, but eventually a neuron is
reached that will project its axon up the spinal cord
(sometimes referred to as the second order neuron).
This axon will cross — decussate — in the spinal cord,
usually within two to three segments above the level of
entry of the peripheral fibers. This crossing occurs in the
white matter in front of the central canal and commissural neurons (see Figures 2A and 72) and is called the
anterior (ventral) white commissure of the spinal cord.
These axons then form the anterolateral tract, located in
that portion of the white matter of the spinal cord. In
fact, it was traditional to speak of two pathways, that for
pain and temperature, the lateral spino-thalamic tract,
and that for light (crude) touch, the anterior spinothalamic tract. These are now considered together
under one name.
The tract ascends in the same position through the
spinal cord (see also Figure 72). As fibers are added from
the upper regions of the body, they are positioned medially, pushing the fibers from the lower body more laterally. Thus, there is a topographic organization to this
pathway in the spinal cord. In the brainstem collaterals
are given off to the reticular formation which are
thought to be quite significant for the function of the
nervous system.
Further consideration of the connections and pathways
for pain sensation have led to the notion of an older and
a newer pain system. The older pathway (also called the
paleospinothalamic) involves the reported sensation of
an ache, or diffuse pain that is poorly localized. The
fibers underlying this pain system are likely unmyelinated both peripherally and centrally, and the central connections are probably very diffuse; most likely these
fibers terminate in the nonspecific thalamic nuclei (see
Figure 10) and influence the cortex widely. The newer
pathway, sometimes called the neospinothalamic system,
involves thinly myelinated fibers in the PNS and CNS,
likely ascends to the VPL nucleus of the thalamus, and
from there is relayed to the postcentral (sensory) gyrus.
Therefore, the sensory information in this pathway can
be well localized. The common example for these different pathways is a paper cut: immediately one knows
exactly where the cut has occurred; this is followed many
seconds later by a diffuse poorly localized aching sensation.
Clinical Aspects
Lesions of the anterolateral pathway from the point of
crossing in the spinal cord upward result in a loss of the
modalities of pain, temperature, and crude touch on the
opposite side of the body. The exact level of the lesion
can be quite accurately ascertained because the sensation
of pain can be quite simply tested at the bedside by
using the end of a pin. (The tester should be aware that
this is a very uncomfortable or unpleasant sensation for
the patient being tested.)
Neurological Neuroanatomy
The cross-sectional levels for this pathway include spinal
cord levels L3 (lumbar) and C8 (cervical), and brainstem
levels B7 (mid-medulla), B4 (mid-pons), and B1 (upper
midbrain).
In the spinal cord, this pathway is found amongst the
various pathways in the anterolateral region of the white
matter (see Figure 72), hence its name.
In the brainstem, the tract is small and cannot usually be
seen as a distinct bundle of fibers. In the medulla, this
pathway is situated dorsal to the inferior olivary nucleus.
In the pons, it is again located laterally. In the uppermost pons and certainly in the midbrain, the fibers join
the medial lemniscus (to be described with the next
pathway in Figure 32).
FIGURE 31: Anterolateral System — Pain, Temperature, Crude Touch
FIGURE 32
DORSAL COLUMN: MEDIAL
LEMNISCUS PATHWAY
DISCRIMINATIVE TOUCH, JOINT
POSITION, VIBRATION
The dorsal column–medial lemniscus pathway, shown in
Figure 32, carries the following modalities from the
body: discriminative touch sensation, joint position,
and the somewhat artificial “sense” of vibration.
Discriminative touch is the ability to discriminate the
distance between two points touched simultaneously; it
is usually tested by asking the patient to identify objects
placed in the hand (with the eyes closed). Joint position
is tested by moving a joint and asking the patient to
report the direction of the movement (again with the
eyes closed). Vibration is tested by placing a tuning fork
which has been set into motion onto a bony prominence
(e.g., the wrist, the ankle). In the periphery, the sensory
receptors are quite specialized, and the fibers are thickly
myelinated. The neurons in the dorsal root ganglia are
also large in size.
The axons enter the spinal cord but do not synapse
immediately; instead they ascend, after giving off local
collaterals which form the basis of various reflexes.
Those fibers entering below the level of about T6 form
the fasciculus (another word for tract) gracilis, the
gracile tract; those entering above T6, particularly those
from the upper limb, form the fasciculus cuneatus, the
cuneate tract, which is situated more laterally. These
tracts ascend the spinal cord in the dorsal area, between
the two dorsal horns, forming the dorsal column (see
Figures 2A and 72). Their first synapse occurs in the
lowermost brainstem, in the nuclei that have the same
names — the nuclei gracilis and cuneatus (see Figures 7
and 38). Topographical organization, called somatotopic,
is maintained in this well-myelinated pathway.
After neurophysiological processing in these nuclei,
which includes sharpening the focus of the sensations,
axons emanate and cross the midline. This stream of
fibers can be recognized in suitably stained sections of
the lower medulla; they are called the internal arcuate
fibers (see Figures 38 and 71). After crossing in the lower
medulla, the fibers then group to form the medial
lemniscus, which ascends through the brainstem.
It should be noted that this pathway does not give off
collaterals to the reticular formation in the brainstem.
The medial lemniscus terminates in the ventral posterolateral nucleus of the thalamus, usually called the VPL
(see Figures 10 and 34). After synapsing there, fibers
enter the internal capsule, its posterior limb, and travel
to the somatosensory cortex, terminating along the postcentral gyrus (see Figures 12, 16, and 34). The postcentral gyrus has a representation of the body called the
sensory homunculus. The face and hand are represented
on the dorsolateral surface (see Figure 12), with the
lower limb on the medial aspect of the hemisphere (see
Figure 16). The hand, particularly the thumb, has an extensive area of representation on the post-central gyrus.
Clinical Aspects
Lesions involving this tract will result in the loss of the
sensory modalities carried in this pathway. A lesion of
the dorsal column in the spinal cord will cause a loss on
the same side; after the synapse and the crossing in the
lower brainstem, any lesion of the medial lemniscus (or
internal capsule) will result in the loss occurring on the
opposite side of the body. With cortical lesions, the area
of the body affected by a lesion will be determined by
the area of the post-central gyrus involved.
The cross-sectional levels for this pathway include spinal
cord levels L3 and C8, and brainstem levels B8 (lower
medulla), B4 (mid-pons), and B1 (upper midbrain).
In the spinal cord, this dorsal column pathway is found
in the dorsal region, between the two dorsal horns, as a
well-myelinated bundle of fibers.
The medial lemniscus tract is a heavily myelinated tract
that is easily seen in myelin-stained sections of the
brainstem (see sections of the brainstem, Figures 64–71).
It is located initially between the inferior olivary nuclei
and is oriented in the dorsal-ventral position (see
Figures 70 and 71). In the pons, it changes to a mediallateral orientation (see Figure 68); in the upper part of
the pons, the tract moves more laterally (see Figure 66).
This shift of position continues in the midbrain.The
fibers in the medial lemniscus are topographically organized, with the leg represented laterally and the upper
limb medially.
FIGURE 32: Dorsal Column: Medial Lemniscus Pathway — Discriminative Touch, Joint Position, Vibration
FIGURE 33
TRIGEMINAL SYSTEM
terminate in the VPM nucleus of the thalamus and other
thalamic nuclei, and follow the same course as those of
the anterolateral system (discussed with Figure 34).
DISCRIMINATIVE TOUCH, PAIN,
TEMPERATURE
Clinical Aspects
The sensory fibers from the face include all the modalities. The fibers enter the brainstem at the level of the
middle cerebellar peduncle (see Figures 3 and 4). Within
the CNS there is a differential handling of the modalities, which allows us to describe the system using the
analogy of the dorsal column-medial lemniscus and the
anterolateral pathways.
Those fibers carrying the sensations of discriminative
touch synapse in the principal (main) nucleus of CN V,
in the mid-pons, at the level of entry of the nerve (see
Figure 6). The axons from this nucleus cross and join the
medial lemniscus. These fibers terminate in the ventral
posteromedial (VPM) nucleus of the thalamus (see
Figures 10 and 34), and are relayed to the post-central
gyrus via the posterior limb of the internal capsule. The
face area is located on the dorsolateral surface and is
very well represented on the post-central gyrus (see
Figure 12). Some sensory input from the midline area of
the face ascends on the same side (ipsilaterally), without
crossing (this is not shown; see Figure 34).
The fibers carrying the modalities of pain and temperature, including those from the teeth and gums, and the
mucous membranes of the eyes, nose, and mouth,
descend within the brainstem. They form a tract that
starts at the mid-pontine level, descends through the
medulla, and reaches the upper level of the spinal cord
(see Figure 6). The tract is called the descending or
spinal tract of V (also known as the spinal trigeminal
tract). Immediately medial to this tract is a nucleus,
with the same name. The fibers terminate in this nucleus
and, after synapsing, cross to the other side and ascend
(see Figure 38). Therefore, these fibers decussate over a
wide region, from the lower pons to the spinal cord, and
do not form a compact bundle of crossing fibers. This
system will give off collaterals to the reticular formation
(not shown), just like those of the anterolateral system.
Just as the anterolateral pathway joins the medial
lemniscus in the pons, so do the trigeminal fibers join
the medial lemniscus (see Figures 34 and 38). The fibers
Lesions of the lateral medulla, such as an infarct or
tumor, will disrupt the descending pain and temperature
fibers and result in a loss of these sensations on the same
side of the face. Such a lesion will leave the fibers for discriminative touch sensation (the medial lemniscus)
intact. This vascular lesion can and does occur in the
lateral medullary (Wallenberg) syndrome (discussed
with the introduction to the medulla in Section C). A
lesion of the medial lemniscus above the mid-pontine
level will involve all trigeminal sensations from the opposite side. Internal capsule and cortical lesions will also
involve trigeminal sensations of the opposite side (with
some midline representation maintained), as well as
other pathways.
There is a particular affliction of the trigeminal nerve
called trigeminal neuralgia, also known as tic
douloureux. The patients report that they have “electriclike” sensations of intense pain in the distribution of one
of the branches of the trigeminal nerve. The cause of
this disorder is sometimes viral (like shingles, which is
caused by the virus of chicken pox) but often unknown.
The painful sensations can be brought on by any object
touching the skin or even by air currents. As one can
imagine, this is an extremely unpleasant and disabling
condition. Treatment, involving drugs or surgery, is
fraught with difficulties.
The cross-sectional levels for this pathway include brainstem levels B8, B7, and B6 (medulla), B4 (mid-pons),
and B2 (lower midbrain).
The principal nucleus of CN V is seen at the midpontine level (see also Figure 6). The descending trigeminal tract is found in the lateral aspect of the medulla,
with the nucleus situated immediately medially (see
Figures 69–71). The crossing pain and temperature
fibers join the medial lemniscus over a wide area and are
thought to have completely crossed by the lower pontine
region (see Figure 38). The collaterals of these fibers to
the reticular formation are not shown.
FIGURE 33: Trigeminal System — Discriminative Touch, Pain, Temperature
FIGURE 34
SENSORY SYSTEMS
SOMATOSENSORY AND
TRIGEMINAL PATHWAYS
The diagram in Figure 34 presents all the somatosensory
and trigeminal pathways through the midbrain, into the
thalamus and to the cortex. The view is a dorsal perspective (as in Figure 7); the cross-sectional representations
through the midbrain are also shown.
The pathway that carries discriminative touch sensation
and information about joint position (as well as vibration) from the body is the medial lemniscus (Figure 32).
The equivalent pathway for the face comes from the
principal nucleus of the trigeminal, which is located at
the pontine level (see Figures 7 and 33). Most of the
trigeminal system is crossed (TTc); there are also ascending trigeminal fibers that come from midline portions of the face that remain ipsilateral (TTi).
By the level of the midbrain, all of these sensory pathways merge together, including also the anterolateral
system (see Figure 31). This merging is shown in a series
of cuts (in the lower portion of the diagram) through
the midbrain region. At the level of the lower midbrain,
these pathways are located near to the surface, dorsal to
the substantia nigra; as they ascend they are found
deeper within the midbrain, dorsolateral to the red
nucleus (shown in cross section in Figures 64 and 65).
The two pathways carrying the modalities of fine touch
and position sense (and vibration) terminate in different
specific relay nuclei of the thalamus (see Figure 10):
• the medial lemniscus from the body, in the VPL,
ventral posterolateral nucleus;
• the trigeminal pathways from the face, in the VPM,
ventral posteromedial nucleus.
Sensory modality and topographic information is retained in these nuclei. There is physiologic processing of
the sensory information and some type of sensory perception occurs at the thalamic level. Precise localization
and two-point discrimination are cortical functions.
After the synaptic relay, the pathways continue as the superior thalamo-cortical radiation through the posterior
limb of the internal capsule, between the thalamus and
lenticular nucleus (see Figures 26 and 27). The fibers are
then found within the white matter of the hemispheres.
The somatosensory information is distributed to the
cortex along the postcentral gyrus (see the small diagrams of the brain) areas 3, 1, and 2, also called SI. The
information from the face and hand is topographically
located on the dorsolateral aspect of the hemispheres
(see Figure 12). The information from the lower limb is
localized along the continuation of this gyrus on the
medial aspect of the hemispheres (see Figure 16). This
cortical representation is called the sensory homunculus,
a distorted representation of the body and face with the
trunk and lower limbs having very little area, whereas
the face and fingers receive considerable representation.
Further elaboration of the sensory information occurs in
the parietal association areas adjacent to the postcentral
gyrus (known as areas 5 and 7). This allows us to learn
to recognize objects by tactile sensations (e.g., food in
the mouth, coins in the hand).
The pathways carrying pain and temperature from the
body (the anterolateral system) and the face (descending
trigeminal) terminate in part in the specific relay nuclei,
ventral posterolateral and posteromedial (VPL and
VPM), respectively, but mainly in the intralaminar
nuclei. These latter terminations may well be involved
with the emotional correlates that accompany many
sensory experiences (e.g., pleasant or unpleasant).
Lesions of the thalamus may sometimes give rise to pain
syndromes.
Axons projecting from the specific relay nucleus of the
thalamus enter the posterior limb of the internal capsule
and are conveyed to several cortical areas, including the
post-central gyrus and area SII (a secondary sensory
area) which is located in the lower portion of the parietal lobe, as well as other cortical regions. The output
from the intralaminar nuclei of the thalamus goes to
widespread cortical areas.
Thalamo-cortical radiation
Caudate nucleus (head)
Ventral posterolateral n.
Ventral posteromedial n.
Putamen
Trigeminal lemniscus (TL)
Red nucleus (RN)
Globus pallidus
RN
SN
Trigeminal-thalamic
tract — ipsilateral (TTi)
Trigeminal-thalamic
tract — crossed (TTc)
Substantia nigra
(SN)
ML
TL
Medial lemniscus (ML)
ML
Anterolateral
system
TTi
TTc
FIGURE 34: Sensory Systems— Somatosensory and Trigeminal Pathways
FIGURE 35
AUDITION: HEARING
AUDITORY PATHWAY I
The auditory pathway is more complex than the other
sensory pathways. Firstly, the pathway is bilateral; secondly, there are more synaptic stations (nuclei) along
the way, with numerous connections across the midline.
It also has a unique feature — a feedback pathway from
the CNS to cells in the cochlea.
The specialized hair cells in the cochlea respond maximally to certain frequencies (pitch) in a tonotopic
manner. The peripheral ganglion for these sensory fibers
is the spiral ganglion. The afferent fibers from the ganglion project to the first brainstem nuclei at the level of
entry of the eighth nerve at the ponto-medullary junction — the dorsal and ventral cochlear nuclei. After this,
the pathway can follow a number of different routes. In
an attempt to make some semblance of order, these
routes are discussed in sequence, even though an axon
may or may not synapse in each of these nuclei.
At the level of the lower pons is the superior olivary
complex, which consists of a number of nuclei (see
Figure 68). Most of the fibers leaving the cochlear nuclei
will synapse in the superior olivary complex, either on
the same side or on the opposite side. Crossing fibers are
found in a structure known as the trapezoid body (see
Figures 38 and 68). The main function of the superior
olive is sound localization; sound coming from one side
does not reach each ear at the same moment. This differential is processed by the dendrites of the neurons in
the superior olive.
Fibers from the superior olivary complex either ascend
on the same side or cross (in the trapezoid body) and
ascend on the other side. They form a tract, the lateral
lemniscus, which begins just above the level of these
nuclei (see Figure 38). The lateral lemniscus carries the
auditory information upwards to the inferior colliculus
of the midbrain. There are nuclei scattered along the
way, interspersed with the lateral lemniscus, and some
fibers terminate or relay in these nuclei. The lateral lemnisci are interconnected across the midline throughout
the brainstem.
Almost all the axons of the lateral lemniscus terminate
in the inferior colliculus. Next, the fibers ascend to the
medial geniculate nucleus of the thalamus (a specific
relay nucleus; see Figure 10) and project to the auditory
cortex (shown in the following illustrations).
In summary, audition involves a complex pathway, has
bilateral representation, with numerous opportunities
for synapses along its course. The name “lemniscus” is
rather unfortunate because it does not transmit information in the efficient manner of the medial lemniscus.
It is important to note that although the pathway is predominantly a crossed system, there is also a significant
ipsilateral component. There are also numerous interconnections between the two sides. Therefore, a lesion of
the auditory pathway on one side will not lead to a total
loss of hearing on the opposite side.
The auditory pathway has a feedback system, from the
higher levels to lower levels (e.g., from the inferior colliculus to the superior olivary complex). The final link in
this feedback is unique in the mammalian CNS, for it
influences the cells in the receptor organ itself. This
pathway, known as the olivo-cochlear bundle, has its
cells of origin in the vicinity of the superior olivary
complex. It has both a crossed and an uncrossed component. Its axons reach the hair cells of the cochlea by
traveling in the eighth nerve. This system changes the
responsiveness of the peripheral hair cells.
The auditory system is shown at various levels of the
brainstem, including B6 (upper medulla), B5, B4, and
B3 (all pontine), and B2 (lower midbrain, inferior
collicular level).
The cochlear nuclei are the first CNS synaptic relay for
the auditory fibers from the peripheral spiral ganglion of
the internal ear; these nuclei are found along the incoming eighth nerve at the level of the upper medulla (B6
level; see Figure 69). The superior olivary complex, consisting of several nuclei, is located in the lower pontine
level (B5; see Figure 68). The trapezoid body, containing
the crossing auditory fibers, is also found at this level. By
mid-pons (level B4; see Figure 67), the lateral lemniscus
can be recognized. These fibers move towards the outer
margin of the upper pons (level B3) and terminate in
the inferior colliculus (level B2; see Figure 65).
FIGURE 35: Audition: Hearing — Auditory Pathway I
FIGURE 36
AUDITORY SYSTEM
AUDITORY PATHWAY II
The illustration in Figure 36 shows the projection of the
auditory system fibers from the level of the inferior colliculus (lower midbrain) to the thalamus and then to the
cortex.
Auditory information is carried via the lateral lemniscus
to the inferior collicular level (see Figures 35 and 38)
after several synaptic relays. There is another synapse in
this nucleus, making the auditory pathway overall different and more complex than the medial lemniscal and
visual pathways. The inferior colliculi are connected to
each other by a small commissure (not labeled in the
illustration).
The auditory information is next projected to a specific
relay nucleus of the thalamus, the medial geniculate
(MG) body (nucleus; see Figure 10). The tract that connects the two, the brachium of the inferior colliculus,
can be seen on the dorsal aspect of the midbrain (see
Figure 7; Figure 8 not labeled) and this is shown diagrammatically in Figure 36.
From the medial geniculate nucleus the auditory
pathway continues to the cortex. This projection, which
courses beneath the lenticular (lentiform) nucleus of the
basal ganglia (see Figure 22), is called the sublenticular
pathway, the inferior limb of the internal capsule, or
simply the auditory radiation. The cortical areas involved with receiving this information are the transverse
gyri of Heschl, situated on the superior temporal gyrus,
within the lateral fissure. The location of these gyri is
shown in the inset as the primary auditory areas (a photographic view is shown in Figure 37).
The medial geniculate nucleus is likely involved with
some analysis and integration of the auditory information. More exact analysis occurs in the cortex. Further
elaboration of auditory information is carried out in the
adjacent cortical areas. On the dominant side for language, these cortical areas overlap Wernicke’s area (see
Figure 12).
Sound frequency, known as tonotopic organization, is
maintained all along the auditory pathway, starting in
the cochlea. This can be depicted as a musical scale with
high and low notes. The auditory system localizes the direction of a sound in the superior olivary complex (discussed with Figure 35) by analyzing the difference in the
timing that information reaches each ear and by the difference in sound intensity reaching each ear. The loudness of a sound would be represented physiologically by
the number of receptors stimulated and by the frequency of impulses, as in other sensory modalities.
This view of the brain includes the body of the lateral
ventricle (cut) and adjoining structures. The thalamus is
seen to form the floor of the ventricle; the body of the
caudate nucleus lies above the thalamus and on the
lateral aspect of the ventricle.
This diagram also includes the lateral geniculate body
(nucleus) which subserves the visual system and its
projection, the optic radiation (to be discussed
subsequently). The temporal lobe structures are also
shown, including the inferior horn of the lateral ventricle, the hippocampus proper, and adjoining structures
(relevant to the limbic system, which is discussed in
Section D).
Putamen (lenticular n.)
Superior
temporal gyrus
Lateral fissure
Thalamus
Primary auditory
areas
Association auditory
areas
Lateral geniculate n.
Brachium of inferior colliculus
Stria terminalis
Inferior colliculus
Lateral lemniscus
Midbrain
Caudate
nucleus
(tail)
Optic radiation
Lateral ventricle
(inferior horn)
Auditory radiation
Hippocampus proper
FIGURE 36: Auditory System — Auditory Pathway II
FIGURE 37
AUDITORY SYSTEM
AUDITORY GYRI
(PHOTOGRAPHIC VIEW)
In Figure 37 the photographic view of the right hemisphere is shown from the lateral perspective. The lateral
fissure has been opened slightly, exposing two gyri oriented transversely. These gyri are the areas of the cortex
that receive the incoming auditory sensory information
first. They are named the transverse gyri of Heschl (see
also Figure 36).
The lateral fissure (see Figure 12) completely separates
this part of the temporal lobe and the frontal and parietal lobes above. The auditory gyri occupy the superior
aspect of the temporal lobe, namely, the superior temporal gyrus.
Further opening of the lateral fissure reveals some cortical tissue which is normally completely hidden from
view. This area is the insula, and a small part of it is seen
in this photograph in the depth of the lateral fissure. It is
important not to confuse the auditory gyri and insula.
The position of the insula in the depth of the lateral
fissure is also shown in the coronal slice of the brain
(see Figure 29). The function of this area of cortex is still
not clear.
Cortical representation of sensory systems reflects the
particular sensation (modality). The auditory gyri are
organized according to pitch, giving rise to the term
tonotopic localization. This representation is similar to
that of the somatosensory system on the postcentral
gyrus (somatotopic localization; the sensory homunculus — see Figure 34). The precentral strip has the equivalent, a motor homunculus, again with large areas
devoted to the hand and face, reflecting the fine control
possible with these muscles.
It should be noted that the lateral fissure usually has
within it a large number of blood vessels, branches of
the middle cerebral artery. These branches emerge and
then distribute to the cortical tissue seen on the dorsolateral surface, including the frontal, temporal, parietal,
and occipital cortex (discussed with Figure 58). Small
branches are distributed to the internal capsule and
basal ganglia within the lateral fissure (discussed with
Figure 60).
Insula
Auditory gyri (transverse gyri of Heschl)
FIGURE 37: Auditory System — Auditory Gyri (Photographic View)
FIGURE 38
SENSORY SYSTEMS
olivary complex. From this point, the tract known as the
lateral lemniscus is formed. It terminates in the inferior
colliculus.
ASCENDING TRACTS AND
SENSORY NUCLEI
The other sensory cranial nerves in this illustration are
CN VII, the facial nerve (sensory afferents and taste) and
CN IX and X, the glossopharyngeal and vagus nerves
(sensory and visceral afferents, and taste).
Figure 38 is a diagrammatic presentation of the internal
structures of the brainstem shown from the dorsal perspective (as in Figures 7 and 8). The information concerning the major sensory systems is presented in an
abbreviated manner, as most of this has been reviewed
with previous figures. The orientation of the cervical
spinal cord representation should be noted.
Dorsal column-medial lemniscus — The dorsal
columns (cuneate and gracile tracts) of the spinal cord
terminate (synapse) in the nuclei gracilis and cuneatus
in the lowermost medulla. Axons from these nuclei then
cross the midline (decussate) as the internal arcuate
fibers (not labeled; see Figure 71), forming a new bundle
called the medial lemniscus. These fibers ascend
through the medulla, change orientation in the pons,
and move laterally, occupying a lateral position in the
midbrain.
Anterolateral system — Having already crossed, this
tract ascends from the spinal cord through the brainstem. In the medulla it is posterior to the inferior olive.
At the upper pontine level, it becomes associated with
the medial lemniscus, and the two lie adjacent to each
other in the midbrain region. Some of its fibers enter the
superior colliculus (not labeled).
Trigeminal system — The sensory afferents for discriminative touch synapse in the principal nucleus of V; the
fibers then cross at the level of the mid-pons and form
the trigemino-thalamic tract, which joins the medial
lemniscus. The pain and temperature fibers descend and
form the descending trigeminal tract through the
medulla with the nucleus medial to it. These fibers
synapse and cross, over a wide area, eventually joining
the trigemino-thalamic tract and medial lemniscus in
the uppermost pons.
Lateral lemniscus — The auditory fibers (CN VIII)
enter the brainstem at the uppermost portion of the
medulla. After the initial synapse in the cochlear nuclei,
many of the fibers cross the midline, forming the trapezoid body. Some of the fibers synapse in the superior
Facial nerve — Sensory fibers entering in the facial
nerve (CN VII) include some afferents from the ear lobe
and taste fibers from the anterior two thirds of the
tongue. The sensory afferents join those of the descending tract and nucleus of V. The taste fibers synapse in the
solitary nucleus (not shown) which lies adjacent to the
descending tract and nucleus of V (see Figure 6; also
seen in cross section in Figure 70) in the medulla.
Glossopharyngeal and vagus nerves — These nerves
convey mainly visceral afferents, as well as taste fibers
from the posterior one third of the tongue (IX) and also
the epiglottis region (X). These fibers synapse in the
solitary nucleus. The few sensory fibers from the ear and
meninges join with the descending tract and nucleus of V.
Clinical Aspects
The diagram in Figure 38 allows the visualization of all
the pathways together which assists in understanding
lesions of the brainstem. The cranial nerve nuclei affected help locate the level of the lesion. One of the classic
lesions of the brainstem is an infarct of the lateral
medulla (see Figure 33), known as the Wallenberg syndrome. (The blood supply of the brainstem is reviewed
with Figure 56.) This lesion affects the lateral pathways
including the anterolateral tract and the lateral lemniscus, but not the medial lemniscus. The descending
trigeminal system is also involved, as are the nuclei of
CN IX and X.
The red nucleus is one of the prominent structures of
the midbrain.
The superior cerebellar peduncles are shown, located
within the superior medullary velum, the roof of the
fourth ventricle (see Figure 7). This cerebellar efferent
pathway decussates in the lower midbrain (shown in
cross section, Figure 65).
Red nucleus
Decussation of superior
cerebellar peduncles
Inferior colliculus
Superior cerebellar
peduncle
Anterolateral system
Trigemino-thalamic pathway
Lateral lemniscus
CN V
Principal nucleus of V
Medial lemniscus
Trapezoid body
CN VII
Superior olivary
complex
CN VIII
Cochlear nuclei
Cuneatus and
gracilis nuclei
CN IX
CN X
Descending (spinal) nucleus of V
Dorsal column tracts
FIGURE 38: Sensory Systems — Ascending Tracts and Sensory Nuclei
FIGURE 39A
VISUAL SYSTEM: A
VISUAL PATHWAY
The visual image exists in the visual fields; because of
the lens of the eye, the visual information from the
upper visual field is seen in the lower retina (likewise the
lower visual field is seen in the upper retina). The visual
fields are also divided into temporal (lateral) and nasal
(medial) portions. Again, because of the lens of the eye,
the temporal visual field is projected onto the nasal part
of the retina of the ipsilateral eye, and onto the temporal
part of the retina of the contralateral eye. The primary
purpose of the visual apparatus — muscles, cornea, lens
— is to have the visual image fall on corresponding
points on the retina of both eyes.
The central portion of the visual field is seen by the
macular area of the retina, composed of cones; it is the
area for discriminative (e.g., reading) and color vision.
Visual processing begins in the retina with the photoreceptors (rods and cones). The bipolar neurons, the next
neurons in the chain, are functionally equivalent to cells
in the dorsal root ganglion in the somatosensory system.
The axons of the next neuron in any sensory system (the
second order neuron) cross the midline and project to
the thalamus (compare to the nuclei gracilis and cuneatus, Figure 32). In the visual system, these neurons are
the ganglion cells of the retina whose axons are carried
in the optic nerve (CN II). After exiting from the orbit,
these nerves undergo only a partial crossing (decussation) in the optic chiasm, and the axons continue as the
optic tract. Some fibers go to the superior colliculus and
other nuclei of the midbrain (discussed with Figure
39B), but most terminate in the lateral geniculate
nucleus (LG). In fact, this entire pathway, from ganglion
cells to geniculate, is a pathway of the CNS, with the
oligodendrocyte being the myelin-forming cell (as for
the CNS).
As indicated, there is a partial crossing of fibers in the
optic chiasm. The fibers from the nasal retina, representing the temporal visual fields, cross in the optic chiasm.
Therefore, the information from the visual field of one
side — the temporal visual field of one eye and the nasal
visual field of the other — is brought together in the
optic tract on the opposite side. (The best way of learning this is to sketch the visual pathway.)
The fibers that terminate in the lateral geniculate
nucleus, a specific relay nucleus of the thalamus (see
Figures 9 and 10) synapse in specified layers, and after
processing they project to the primary visual cortex,
area 17. The projection is unusual (shown also in the
next illustration), with some of the fibers sweeping
forward alongside the inferior horn of the lateral ventricle in the temporal lobe, called Meyer’s loop, while
others project directly posteriorly (see also Figure 36).
The geniculo-calcarine (optic) radiation is arranged in
the following manner:
• The fibers representing the lower retinal field sweep
forward into the temporal lobe, as Meyer’s loop.
Destruction of only these fibers results in a loss of
vision in the upper visual field of both eyes on the
side opposite the lesion, specifically the upper quadrant of both eyes.
• Those fibers representing the upper retinal field
project posteriorly without this unusual looping,
passing deep within the parietal lobe. Destruction of
only these fibers results in the loss of the lower visual
field of both eyes on the side opposite the lesion,
specifically the lower quadrant of both eyes.
The visual information goes to area 17 (shown in black
in the insets), the primary visual area, also called the
calcarine cortex.
The visual area located at the occipital pole (looking at
the external aspect of the brain; see Figure 12) is where
macular vision is represented; the visual cortex on the
medial surface along the banks of the calcarine fissure
(see Figure 16) represents the peripheral areas of the
retina. The adjacent cortical areas of the occipital lobe
are association areas for vision, usually called areas 18
and 19.
Clinical Aspects
The visual pathway is easily testable, even at the bedside.
Students should be able to draw the visual field defect in
both eyes that would follow a lesion (e.g., homonymous
hemianopsia). Visual loss can occur for many reasons,
including a lesion in the retina, the temporal lobe, or the
loss of blood supply to the cortical areas.
Primary visual area (area 17)
Lateral ventricle (cut)
Stria terminalis
Caudate nucleus (tail)
Association visual areas (18, 19)
Optic radiation
Optic
tract
Optic radiation (temporal loop)
Optic chiasm
Lateral ventricle (inferior horn)
FIGURE 39A:Visual System: A — Visual Pathway
Primary visual area (area 17)
FIGURE 39B
VISUAL SYSTEM: B
VISUAL REFLEXES
The vast majority of fibers of the optic tract project to
the lateral geniculate nucleus, the LG. This nucleus has a
six-layered structure, and each of the eyes projects to
three of the layers. After synapsing, these fibers relay to
the primary visual area (area 17)
The illustration in Figure 39B shows some fibers from
the optic tract which project to the superior colliculus
(bypassing the lateral geniculate) via the brachium of
the superior colliculus. This nucleus serves as an important center for visual reflex behavior, particularly that involving eye movements. Fibers leave this nucleus and
connect with the nuclei of the extra-ocular and neck
muscles via the medial longitudinal fasciculus (the MLF,
discussed with Figures 49A and 49B). There is also a
projection to the spinal cord via a small pathway (the
tecto-spinal tract), which is found incorporated with
the MLF throughout the brainstem and the upper
spinal cord.
Other fibers are illustrated emerging from the pulvinar,
the visually related association nucleus of the thalamus
(see Figure 10). These are carried in the optic radiations
and go to areas 18 and 19, the visual association areas of
the cortex (shown in Figure 39A, alongside area 17).
Some other fibers terminate in the suprachiasmatic
nucleus of the hypothalamus (located above the optic
chiasm), which is involved in the control of diurnal
(day-night) rhythms.
A small but extremely important group of fibers from
the optic tract (not shown) project to the pretectal area
situated in the upper portion of the midbrain. The reflex
adjustment of the diameter of the pupil — the pupillary
light reflex — is coordinated in the pretectal nucleus.
Reflex adjustments of the visual system are also required
for seeing objects nearby (such as for reading), known as
the accommodation reflex.
Pupillary light reflex — Some of the visual information
(from certain ganglion cells in the retina) is carried in
the optic nerve and tract to the midbrain. A nucleus
located in the area in front of the colliculi, called the
pretectal area (the other name for the colliculi is the
tectal area; see Figures 7 and 8) is the site of synapse for
the pupillary light reflex. Shining light on the retina
causes a constriction of the pupil on the same side; this
is the direct pupillary light reflex. Fibers also cross to the
nucleus on the other side and the pupil there reacts as
well; this is the consensual light reflex. The efferent part
of the reflex involves the parasympathetic nucleus
(Edinger-Westphal) of the oculomotor nucleus (discussed with the midbrain and also Figure 64). The efferent fibers course in CN III, synapsing in the ciliary
ganglion in the orbit before innervating the smooth
muscle of the iris.
Clinical Aspects
The student is encouraged to draw this pathway and to
work out the clinical picture of a lesion involving the afferent fibers and the efferent fibers. In addition, diseases
such as multiple sclerosis can diminish the number of
fibers in the optic nerve and can lead to a condition
called a relative afferent pupillary defect; a specific test
for this condition is the swinging light test. In this condition, because of the diminished afferent input to the
pretectal nucleus, the pupil on the affected (lesion) side
reacts less to the light stimulus than does the normal
eye, and it therefore dilates paradoxically.
CN III is oftentimes involved in brain herniation syndromes, particularly uncal herniation (discussed with
Figure 14). This results in a fixed dilated pupil on one
side, a critical sign when one is concerned about increased intracranial pressure from any cause. The significance and urgency of this situation must be understood
by everyone involved in critical care.
Accommodation reflex — The second reflex associated
with incoming visual fibers is the accommodation reflex,
involving looking at a nearby object, such as in reading.
Three events occur simultaneously: convergence of both
eyes (involving both medial recti muscles), a change
(rounding) of the curvature of the lens, and pupillary
constriction. This reflex requires the visual information
to be processed at the cortical level. The descending
(cortico-bulbar) fibers go to the oculomotor nucleus
and influence both the motor portion (the medial recti
muscles), and the parasympathetic portion (via the
ciliary ganglion) to the smooth muscle of the lens and
the pupil.
Optic radiation
Calcarine fissure
Lateral ventricle (cut)
Lateral ventricle
(occipital horn)
Pulvinar
Superior colliculus
Brachium of
superior colliculus
Lateral geniculate
nucleus
Medial geniculate
nucleus
Primary visual area
(area 17)
Lateral geniculate nucleus
Optic tract
Red nucleus
Substantia nigra
FIGURE 39B: Visual System: B — Visual Reflexes
PART II: RETICULAR
FORMATION
FIGURE 40A
RETICULAR FORMATION I
ORGANIZATION
The reticular formation is the name for a group of
neurons found throughout the brainstem. Using the
ventral view of the brainstem, the reticular formation
occupies the central portion or core area of the brainstem from midbrain to medulla (shown on the right
portion of this illustration; see also cross sections in
Figures 64–71).
This collection of neurons is a phylogenetically old set of
neurons that functions like a network or reticulum, from
which it derives its name These nuclei receive afferents
from most of the sensory systems and project to
virtually all parts of the nervous system. Functionally,
it is possible to localize different subgroups within the
reticular formation:
• Cardiac and respiratory “centers”: Subsets of neurons
within the medullary reticular formation are responsible for the control of the vital functions of heart rate
and respiration.
• Motor areas: Both the pontine and medullary nuclei
of the reticular formation contribute to motor control
via the cortico-reticulo-spinal system (discussed with
Figures 46 and 47). In addition, these nuclei exert a
very significant influence on muscle tone.
• Ascending projection system: Fibers from the reticular
formation ascend to the thalamus and project to
various nonspecific thalamic nuclei. From these
nuclei, fibers are distributed diffusely to the cerebral
cortex. This whole system is concerned with consciousness and has been called the ascending reticular
activating system (ARAS).
• Pre-cerebellar nuclei: Numerous nuclei in the brainstem located within the boundaries of the reticular
formation project to the cerebellum. These nuclei
are not always included in discussions of the reticular
formation.
It is also possible to describe the reticular formation
topographically. The neurons appear to be arranged in
three longitudinal sets (these are shown on the left hand
side of this illustration):
• The lateral group consists of neurons that are small in
size. These are the neurons that receive the various
inputs to the reticular formation, including those
from the anterolateral (pain and temperature) and
trigeminal systems, as well as auditory and visual
input.
• The next group of neurons (medially) is called the
central group. These cells are larger in size and project
their axons upwards and downwards. The ascending
projection from the midbrain area is particularly involved with the consciousness system. Within this
group are the well-known nucleus gigantocellularis of
the medulla and the pontine reticular nuclei, caudal
(lower) and oral (upper) portions, which give origin
to the two reticulo-spinal tracts (discussed with Figure
40B, see also Figures 46 and 47).
• A set of neurons, called the raphe nuclei, occupies the
midline region of the brainstem. The best-known
nucleus of this group is the nucleus raphe magnus,
which plays an important role in the descending pain
system (discussed with Figure 41).
In addition, both the locus ceruleus and the periaqueductal gray are considered part of the reticular formation (discussed with Figure 40B).
In summary, the reticular formation is connected with
almost all parts of the CNS. Although it has a generalized influence within the CNS, it also contains subsystems that are directly involved in specific CNS functions.
Ascending
projection fibers
Locus ceruleus
Lateral group
Central group
Raphe nuclei
Reticulo-spinal tracts
FIGURE 40A: Reticular Formation I — Organization
FIGURE 40B
RETICULAR FORMATION II
NUCLEI
In this diagram, the reticular formation is viewed from
the dorsal (posterior) perspective. Various nuclei of the
reticular formation — RF — which have a known significant functional role are depicted, as well as the descending tracts emanating from some of these nuclei.
There are afferent and efferent nuclei in the reticular
formation and groups of neurons that are distinct
because of the catecholamine neurotransmitter used,
either noradrenaline or serotonin. This understanding of
the reticular formation overlaps with the topographical
description, as being arranged in three longitudinal sets
of neurons, as discussed with Figure 40A.
• The neurons that receive the various inputs to the RF
are found in the lateral group. In Figure 40B, these
neurons are shown receiving collaterals (or terminal
branches) from the ascending anterolateral system,
carrying pain and temperature (see Figure 31). Similar
information is received from the descending fibers of
the trigeminal nerve (see Figure 33). The RF also receives visual and auditory information. The medial
lemniscus, though, does not give off collaterals to
the RF.
• The central group of neurons are larger and are the
output neurons of the reticular formation, at various
levels. These cells project their axons upwards and/or
downwards. The nucleus gigantocellularis of the
medulla, and the pontine reticular nuclei, caudal and
oral portions, give rise to the descending tracts that
emanate from these nuclei — the medial and lateral
reticulo-spinal pathways, part of the indirect voluntary motor system (discussed in the introduction to
the motor system and with Figures 46 and 47).
• A set of neurons, called the raphe nuclei, occupies the
midline region of the brainstem. All the neurons of
this group use the neurotransmitter serotonin. The
serotonergic raphe nuclei project to all parts of the
CNS. Recent studies seem to indicate that serotonin
plays a significant role in emotional equilibrium, as
well as in the regulation of sleep. One nucleus of this
group, the nucleus raphe magnus, located in the
upper part of the medulla, plays a special role in the
descending pain pathway (described with Figure 41).
Other nuclei in the brainstem seem to functionally
belong to the reticular formation yet are not located
within the core region. These include the periaqueductal
gray and the locus ceruleus.
The periaqueductal gray of the midbrain (for its location see Figures 64 and 65) includes neurons that are
found around the aqueduct of the midbrain (see also
Figure 20B). This area also receives input (illustrated but
not labeled) from the ascending sensory systems conveying pain and temperature, the anterolateral and trigeminal. As shown in Figure 41, some of the neurons of the
periaqueductal gray as well as the nucleus raphe magnus
of the medulla are part of a descending pathway to the
spinal cord that is concerned with pain control.
The locus ceruleus is a small nucleus in the upper pons
region (see Figure 66). In some species (including
humans), the neurons of this nucleus accumulate a
pigment that can be seen when the brain is sectioned
(prior to histological processing). Output from this
small nucleus is distributed widely throughout the brain
to virtually every part of the CNS, including all cortical
areas, subcortical structures, the brainstem and cerebellum, and the spinal cord. The neurotransmitter involved
is noradrenaline. Although the functional and electrophysiogical role of this nucleus is still not clear, the locus
ceruleus has been thought to act like an “alarm system”
in the brain; it has also been implicated in a wide variety
of CNS activities, such as mood, reaction to stress, and
various autonomic activities.
The cerebral cortex sends fibers to the RF nuclei,
forming part of the so-called cortico-bulbar fibers (see
Figure 43). Those nuclei that give off the pathways to the
spinal cord form part of an indirect motor system — the
cortico-reticulo-spinal pathways. These pathways are
known to have an important role in the voluntary
control of the muscles of the spine (axial musculature)
and those of the large joints (proximal joints of the
shoulder and hip). In addition, this system is known to
play an extremely important role in the control of
muscle tone. Lesions of the cortical input to the reticular
formation have a very significant impact on muscle tone
and reflexes (discussed with Figure 47).
Aqueduct of midbrain
RF - central group
Periaqueductal gray
RF - lateral group
Fourth ventricle
Pontine reticular nn.
(oral and caudal)
Locus ceruleus
N. gigantocellularis
N. raphe magnus
RF - raphe nn.
RF - lateral group
Reticulo-spinal tracts
FIGURE 40B: Reticular Formation II — Nuclei
FIGURE 41
PAIN
DESCENDING CONTROL SYSTEM
Pain is a unique sensory modality — it both warns of
tissue injury and can distract the organism from performing essential functions. Pain may be perceived at
several points, and there is good evidence that some of
this occurs at the thalamic level. Localization of pain requires the cortex of the post-central gyrus (SI); SII is
also likely involved in the perception of pain (discussed
with Figure 31). Widespread areas of the limbic system
and association cortex of the frontal lobe are involved
with the human reactions to pain, particularly to
chronic pain (e.g., such as from cancer or arthritis).
Humans have a built-in system for dampening the influences of pain — the descending pain modulation
pathway(s). Some of this modulation occurs at the
brainstem level and some in the spinal cord. Substantial
evidence suggests that the transmission of pain from the
periphery can be influenced by nuclei located in the
brainstem. Those areas that have been found to exert the
major influence include the periaqueductal gray of the
midbrain and one of the midline serotonergic nuclei of
the medulla, the nucleus raphe magnus.
The system apparently functions as follows. The neurons
of the periaqueductal gray can be activated in a number
of ways. In terms of physiology, many ascending fibers
from the anterolateral system (and trigeminal system)
activate neurons in this area (see Figure 38). These are
either collaterals of ascending pain fibers or direct
endings of these fibers in the midbrain. This area is also
rich in opiate receptors, and it seems that neurons of
this region can be activated by circulating endorphins.
In experiments, one can activate these neurons by direct
stimulation or by a local injection of morphine. In addition, descending cortical fibers (cortico-bulbar) may also
be available to activate these neurons.
The axons of some of the neurons of the periaqueductal
gray descend and terminate in one of the raphe nuclei in
the medulla — the nucleus raphe magnus. This nucleus
is one of the serotonin-containing raphe system of
nuclei. From here, there is a descending, crossed,
pathway which is located in the dorsolateral funiculus of
the spinal cord. The serotonergic fibers terminate in the
substantia gelatinosa of the spinal cord (see Figure 2A),
a nuclear area of the dorsal horn of the spinal cord. The
axons are thought to terminate on small interneurons
that contain enkephalin. It is postulated that these
enkephalin-containing spinal neurons inhibit the transmission of the pain afferents in the spinal cord. Thus,
descending influences are thought to activate a local
circuit.
This idea is based upon the proposed mechanism that
these same interneurons can be activated by stimulation
of other sensory afferents, particularly those conveying
information from the mechanoreceptors; these are
anatomically large, well-myelinated peripheral nerve
fibers. This is the physiological basis for the gate control
theory of pain.
It should be pointed out that the same circuitry is thought
to be operative in the descending nucleus of the trigeminal
nerve, the lower portion of which is responsible for pain
(and also temperature) transmission. This nucleus is in
fact continuous with the substantia gelatinosa.
Clinical Aspects
Some of the current treatments for pain are based upon
the structures and neurotransmitters being discussed
here. The gate control theory of pain underlies the use
of transcutaneous stimulation which is one of the
current therapies offered for the relief of pain. More
controversial and certainly less certain is the postulated
mechanism(s) for the use of acupuncture in the treatment of pain. The role of the limbic (emotional) system
is discussed in Section D.
Most discussions concerning pain refer to acute pain.
Chronic pain, a particularly tragic state of being for
many people, should be thought of very differently. The
pain of chronic arthritis or cancer is extremely difficult
to treat. Many of these people are now referred to pain
clinics, where a team of physicians (such as anesthetists
and neurologists) and others (such as social workers and
psychologists) try to assist people, using a variety of
therapies, to alleviate their disabling condition.
Periaqueductal gray
N. raphe magnus
FIGURE 41: Pain — Descending Control System
PART III: MOTOR SYSTEMS
INTRODUCTION
The motor system is a complex subject to understand
because of the multiple centers involved in motor
control. All of our voluntary motor output is controlled
by the motor regions of the cortex. Other motor activity
may be controlled from several brainstem nuclei, some
of which are under the control of the motor cortices. In
addition, there are various spinal motor activities and
reflexes. Finally, there is the regulation of muscle tone
and reflexes, some of which depends upon the afferents
from the muscle spindle and some of which is regulated
by descending influences from higher centers. In addition, there are two important large areas of the brain,
the cerebellum and basal ganglia, devoted to motor regulation, “working behind the scene.”
Voluntary motor control involves both direct and indirect pathways:
• the direct voluntary pathway includes the corticospinal fibers which are found within the pyramids and
the lateral cortico-spinal tract of the spinal cord,
mainly for the control of fine motor movements; and
• the indirect voluntary pathway, an older system for
control of proximal and axial musculature, involving
the reticular formation of the brainstem. The pathway
goes from the motor areas of the cerebral cortex to the
reticular formation (via cortico-bulbar fibers), and
then from the reticular formation to the spinal cord
(via the reticulo-spinal pathways).
Other pathways from the brainstem (from the reticular
formation, vestibular nuclei, possibly the red nucleus)
also control proximal joint movements and axial musculature, as well as muscle tone and reflex responsiveness.
Each of these gives rise to a descending pathway. Some
but not all of these nuclei are under the influence of the
cortex. A typical human lesion of the brain usually
affects several of the descending systems in addition to
the cortico-spinal tract, and results in a more profound
weakness or paralysis of movement. Most important,
because of the involvement of other parts of the motor
control system, there is in most cases an eventual
increase in muscle tone (spasticity) and reflexes
(discussed with Figures 46 and 47). The control of the
muscles of the head and neck (eye, tongue, facial, mastication, swallowing, phonation) is discussed
separately.
The basal ganglia and cerebellum, which might be called
motor regulatory systems, calibrate the output from the
motor cortex. They are discussed after the pathways are
described.
The motor pathways (tracts) are called descending
because they commence in the cortex or brainstem and
influence motor cells lower in the neuraxis, either in the
brainstem or spinal cord. Those neurons in the cortex or
brainstem (including the reticular formation) giving rise
to these pathways are collectively called the upper motor
neurons. The motor neurons in the spinal cord or brainstem that give rise to the peripheral efferent fibers
(spinal and cranial nerves) are often collectively called
the lower motor neuron. In the spinal cord, these
neurons are located in the ventral or anterior horn and
are (histologically) the anterior horn cells. Physiologists
call these neurons the alpha motor neurons. In the
brainstem, these neurons include the motor neurons of
the cranial nerves (see Figure 5). Since all of the descending influences converge upon the lower motor
neurons, these neurons have also been called, in a functional sense, the final common pathway.
There are a number of descending tracts or pathways:
• Cortico-spinal tract originates in motor areas of the
cerebral cortex and travels from cortex to spinal cord.
It is a relatively new tract and one of the most important for voluntary motor control in humans, particularly for fine movements of the hand and digits — the
direct voluntary motor pathway.
• Cortico-bulbar fibers is a descriptive term that is
poorly defined and includes all fibers going to the
brainstem, both cranial nerve nuclei and other brainstem nuclei. The fibers going to the reticular formation include those that form part of the indirect
voluntary motor pathway.
• Rubro-spinal tract originates from the red nucleus of
the midbrain. Its connections are such that it may play
a role in voluntary motor activity; this may be the case
in higher primates, but its precise role in humans is
not clear.
• Reticulo-spinal tracts are involved in the voluntary
(indirect) pathways, as well as in the underlying
control of muscle tone and reflex responsiveness. Two
tracts descend (on each side) from the reticular formation, one from the pontine region (the medial
reticulo-spinal tract) and one from the medulla (the
lateral reticulo-spinal tract).
• Lateral vestibulo-spinal tract comes from the lateral
vestibular nucleus in the pons. This nucleus plays an
important role in the regulation of our responses to
gravity (vestibular afferents). It is under control of the
cerebellum, not the cerebral cortex.
• Medial longitudinal fasciculus (usually called MLF)
is a complex pathway of the brainstem and upper
spinal cord which serves to coordinate various eye and
neck reflexes. Descending vestibular influences from
other vestibular nuclei join this pathway. There are
also ascending fibers within the MLF. Descending
fibers from the superior colliculus (the tecto-spinal
pathway) form part of the MLF.
In humans, there is one abnormal reflex that indicates
there has been a lesion interrupting the cortico-spinal
pathway, at any level (cortex, white matter, internal
capsule, brainstem, spinal cord). Normally stroking the
bottom of the foot (a most uncomfortable sensation for
most people) results in flexion of the toes, called a
plantar response, and often an attempt to withdraw the
limb. After a lesion interrupting the cortico-spinal
pathway, stroking of the bottom of the foot results in an
upward movement of the big toe (extension), and a
fanning apart of the other toes. The whole response is
called a Babinski reflex and it is found almost immediately after any lesion that interrupts any part of the
cortico-spinal pathway, from cortex through to spinal
cord. Most interestingly, the Babinski reflex is normally
present in infants and disappears somewhere in the
second year of life, concurrent with the myelination that
occurs in this pathway.
FIGURE 42
CORTICO-SPINAL TRACT —
THE PYRAMIDAL SYSTEM
DIRECT VOLUNTARY PATHWAY
The cortico-spinal tract is a direct pathway linking the
cortex with the spinal cord. From the evolutionary point
of view, it is one of the newer pathways. The corticospinal tract is the most important one for voluntary
motor movements in humans. It is also known as the
pyramidal system, particularly in the clinical setting.
The neurons giving rise to this pathway are mostly
located in the motor areas of the cerebral cortex (see
Figures 12 and 16; discussed below and with Figure 51).
The axons are well myelinated and descend through the
white matter of the hemispheres, through the posterior
limb of the internal capsule (see Figures 26 and 27).
After descending through the midbrain and pons, the
fibers are found within the medullary pyramids (see
Figures 3 and 4). Hence, the cortico-spinal pathway is
often called the pyramidal tract. At the lowermost part
of the medulla, most (90%) of the cortico-spinal fibers
decussate (cross) in the pyramidal decussation, and
form the lateral cortico-spinal tract in the spinal cord
(see Figure 72). Many end directly on the anterior horn
cells. The entire pathway is involved with controlling the
individualized fine movements, particularly of our
fingers and hands — the distal limb musculature.
Those fibers that do not cross in the pyramidal decussation form the anterior (or ventral) cortico-spinal tract.
The ventral pathway is found in the ventral portion of
the white matter of the spinal cord (see Figure 72).
Many of these axons will cross before terminating, while
others supply motor neurons on both sides. The ventral
pathway is concerned with movements of the proximal
limb joints and axial movements. Other pathways are
also involved in the control of this musculature.
Other areas of the cortex, including the sensory cortical
areas (e.g., the postcentral gyrus), contribute to the
cortico-spinal pathway. This part of the pathway presumably carries “instructions” from the cortex to the
dorsal horn of the spinal cord (its sensory portion; see
Figure 2A) that may modify the transmission of sensory
information at the spinal cord level (also discussed with
Figure 43).
Clinical Aspects
Lesions involving the cortico-spinal tract in humans are
quite devastating because they rob the individual of voluntary motor control, particularly fine (skilled) motor
movements. This pathway is quite commonly involved
in strokes, as a result of vascular lesions of the cerebral
arteries or of the deep arteries to the internal capsule
(reviewed with Figures 58 and 60). These lesions result
in a weakness or paralysis of the muscles on the opposite
side. A Babinski reflex would be found almost immediately after interruption of this pathway. Damage to the
tract in the spinal cord is seen after traumatic injuries
(e.g., automobile and diving accidents). Amyotrophic
lateral sclerosis (ALS) (also known as Lou Gehrig’s
disease) involves a degeneration of both upper (cortical)
and lower (spinal and brainstem) motor neurons.
The cross-sectional levels for following this pathway
through the brainstem include B1 (upper midbrain), B4
(mid-pons), B7 (mid-medulla), and spinal cord levels
C8 and L3.
After emerging from the internal capsule, the corticospinal tract is found in the midportion of the cerebral
peduncles (see Figures 26, 43, and 64) in the midbrain.
The cortico-spinal fibers are then dispersed in the
pontine region and are seen as bundles of axons among
the pontine nuclei (see Figure 67). The fibers collect
again in the medulla as a single tract, one on each side of
the midline. These fibers are located within the elevations known as the pyramids (see Figures 3 and 4). At
the lowermost level of the medulla, 90% of the fibers decussate and form the lateral cortico-spinal tract, situated
in the lateral aspect of the entire spinal cord (see Figure
72). The ventral cortico-spinal tract is found in the
anterior portion of the white matter of the spinal cord
(see Figure 72).
FIGURE 42: Cortico-spinal Tract — The Pyramidal System-Direct Voluntary Pathway
FIGURE 43
CORTICO-BULBAR (AND
CORTICO-PONTINE) FIBERS
weakness, not paralysis, of the muscles supplied. For
example, a lesion on one side might result in difficulty
in swallowing or phonation, and often these problems
dissipate in time. There are two exceptions to this rule,
which are very important in the clinical setting.
BRAINSTEM SYSTEM
1.
The major exception is the cortical input to the
facial nucleus. In humans, there is a most important difference between the innervation to the
upper and lower face. The part of the facial
nucleus controlling the lower facial muscles receives only a crossed input from the cortex. The
cortical input to the part of the facial nucleus
controlling the upper facial muscles is supplied
from both hemispheres. Therefore, a patient with
a lesion of the appropriate area of the motor
cortex or of the cortico-bulbar fibers on one side
will be able to wrinkle his/her forehead normally
on both sides, but will not be able to show the
teeth or smile symmetrically on the side opposite
the lesion; often there will be a drooping of the
lower face. This will also affect the muscle of the
cheek (the buccinator muscle) and cause some
difficulties with drinking, eating, and chewing
(the food gets stuck in the cheek and often has to
be manually removed); sometimes there is also
drooling. This functional loss must be distinguished from a lesion of the facial nerve itself
(most often seen with Bell’s palsy, a lesion of the
facial nerve as it emerges from the skull) which
affects the muscles of both the upper and lower
face, on one side.
2.
The cortical innervation to the hypoglossal
nucleus is not always bilateral. In some individuals, there is a predominantly crossed innervation.
The term cortico-bulbar is a descriptive one and does not
indicate a single pathway. The word bulb (i.e., bulbar)
refers to the brainstem and is borrowed from an older
descriptive literature.
Many of the fibers descending from the cerebral cortex
to all parts of the neuraxis are involved in motor
control. These axons, part of the so-called projection
fibers of the hemispheres, course via the internal capsule
(see Figure 26) and continue into the cerebral peduncles
of the midbrain. Some of these are part of a distinct
pathway to spinal cord (cortico-spinal, described with
Figure 42); the cortico-bulbar fibers are those that end
in the brainstem and include several functional components. These, along with the cortico-spinal tract, occupy
the middle one-third of the cerebral peduncle of the
midbrain (see also Figure 42). A subgroup of fibers —
cortico-pontine — go to the nuclei of the pons
(described fully with Figure 53; also discussed with
Figure 55). These occupy the remainder of the cerebral
peduncle.
The cortico-bulbar fibers include those to brainstem
motor control nuclei (including the reticular formation)
and cranial nerve nuclei:
• Brainstem motor control nuclei — Cortical fibers
likely influence all the brainstem motor nuclei, particularly the reticular formation and including the red
nucleus, with the exception of the lateral vestibular
nucleus (see Figure 45). The cortico-reticular fibers
are extremely important for some voluntary movements (indirect pathway) and for muscle tone (discussed with Figure 47).
• Cranial nerve nuclei — The motor neurons of the
cranial nerves of the brainstem (see Figures 5 and 6)
are functionally lower motor neurons; the cortical
motor cells are the upper motor neuron. These motor
nuclei are generally innervated by fibers from both
sides, i.e., each nucleus receives input from both hemispheres. Therefore, loss of cortical innervation to the
cranial nerve motor nuclei is usually associated with a
The cortical input to the sensory nuclei of the brainstem, including the somatosensory nuclei, nuclei
cuneatus and gracilis (see Figure 38), is similar to the
cortical input to the dorsal horn of the spinal cord
(discussed with Figure 42).
• Cortico-pontine fibers — The cortical input to the
pontine nuclei, located in the outer and inner thirds of
the cerebral peduncle (see also Figures 45 and 64), is
discussed with the cerebellum (see Figures 53 and 55).
Temporo-pontine fibers
Occipito-pontine fibers
Parieto-pontine fibers
Cortico-bulbar (and
cortico-spinal) fibers
Fronto-pontine fibers
FIGURE 43: Cortico-bulbar (and Cortico-pontine) Fibers — Brainstem Motor System
FIGURE 44
RUBRO-SPINAL TRACT
NON-PYRAMIDAL MOTOR SYSTEM
The red nucleus is a prominent nucleus of the midbrain.
It gets its name from a reddish color seen in fresh
dissections of the brain, presumably due to its high
vascularity. The nucleus (see Figure 64) has two portions: a small-celled upper division and a lower portion
with large neurons, more ventrally located. The rubrospinal pathway originates, at least in humans, from the
larger cells.
The red nucleus receives its input from the motor areas
of the cerebral cortex and from the cerebellum. The cortical input is directly onto the projecting cells, thus
forming a potential two-step pathway from motor cortex
to spinal cord.
The rubro-spinal tract is also a crossed pathway, with
the decussation occurring in the ventral part of the midbrain (see also Figure 45). The tract descends within the
tegmentum (the central part of the brainstem) and is
not clearly distinguishable from other fiber systems.
The fibers then course in the lateral portion of the
white matter of the spinal cord, just anterior to and
intermingled with the lateral cortico-spinal tract (see
Figure 72).
The rubro-spinal tract is a well-developed pathway in
some animals. In monkeys it seems to be involved in
flexion movements of the limbs. Stimulation of this
tract in cats produces an increase in tone of the flexor
muscles.
The functional significance of this pathway in humans is
not well known. The number of large cells in the red
nucleus in the human is significantly less than in the
monkey. Motor deficits associated with a lesion involving only the red nucleus or only the rubro-spinal tract
have not been adequately described. Although the
rubro-spinal pathway may play a role in some flexion
movements, it seems that the cortico-spinal tract predominates in the human.
The location of this tract within the brainstem is shown
at the cross-sectional levels B1 (upper midbrain), B4
(mid-pons), B7 (mid-medulla), and spinal cord levels
C8 and L3. The tract is said to continue throughout the
length of the spinal cord in primates but probably
extends only into the cervical spinal cord in humans.
The fibers of CN III (oculomotor) exit through the
medial aspect of this nucleus at the level of the upper
midbrain (see Figure 64).
FIGURE 44: Rubro-spinal Tract — Non-Pyramidal Motor System
FIGURE 45
DESCENDING TRACTS AND
MOTOR NUCLEI
MOTOR PATHWAYS AND NUCLEI
Various descending pathways are shown in Figure 45,
along with those cranial nerve nuclei that have a motor
component, using the somewhat oblique posterior view
of the brainstem (see Figure 7). The explanations presented below are in summary form, as most have been
discussed with previous figures.
The major motor pathways include the following:
• Cortico-spinal tract courses in the middle third of the
cerebral peduncle. The tract fibers disperse in the
pontine region between the pontine nuclei, and
regroup as a compact bundle in the medulla where
they are frequently called the pyramidal tract. At the
lowermost part of the medulla (Figure 4), most of the
fibers decussate to form the lateral cortico-spinal tract
of the spinal cord. A small portion of the tract continues ipsilaterally, mostly into the cervical spinal cord
region, as the anterior (ventral) cortico-spinal tract.
• Cortico-bulbar fibers that project to the cranial nerve
nuclei of the brainstem are shown in this diagram.
Cortico-bulbar fibers also include those cortical fibers
that project to the reticular formation and other
brainstem nuclei. These fibers are also located in the
middle third of the cerebral peduncle and are given
off at various levels within the brainstem.
• Cortico-pontine fibers — The descending cortical
fibers from various parts of the cerebral cortex to the
pontine nuclei are found in the outer and inner thirds
of the cerebral peduncles. After synapsing in the
pontine nuclei, the fibers cross and project to the cerebellum via the middle cerebellar peduncle.
• Rubro-spinal tract fibers, which originate from the
lower portion of the red nucleus, decussate in the
midbrain region. The tract descends through the
brainstem. In the spinal cord, the fibers are located
anterior to the lateral cortico-spinal tract.
The cranial nerve nuclei include (see also Figure 5) the
following:
• Oculomotor (to most extra-ocular muscles and
parasympathetic) — This large nucleus, located at the
level of the superior colliculus, sends its fibers to most
of the extraocular muscles. These fibers traverse
through the medial portion of the red nucleus, before
exiting in the fossa between the cerebral peduncles,
the interpeduncular fossa (see Figure 64). The
parasympathetic fibers associated with CN III originate from the Edinger-Westphal nucleus.
• Trochlear (to the superior oblique muscle) — fibers
cross before exiting from the midbrain posteriorly (see
Figure 7). They then wrap around the cerebral peduncles in their course anteriorly.
• Trigeminal (to muscles of mastication) — The motor
fibers pierce the middle cerebellar peduncle as they
exit in the pontine region. (The major part of this
nerve is sensory.)
• Abducens (to the lateral rectus muscle) — The
anterior course of the exiting fibers could not be
depicted from the perspective used in Figure 45.
• Facial (to muscles of facial expression) — The fibers
to the muscles of facial expression have an internal
loop before exiting. The nerve loops over the abducens nucleus, forming a bump called the facial colliculus in the floor of the fourth ventricle (see Figure
7). It should be noted that the nerve of only one side
is shown in this illustration.
• Glossopharyngeal and vagus (branchiomotor and
parasympathetic) — The fibers exit behind the
inferior olive, on the lateral aspect of the medulla.
These fibers include those from the nucleus ambiguus
(branchiomotor to muscles of the pharynx and
larynx) and the parasympathetic fibers from the
dorsal motor nucleus of the vagus that supply the
structures in the neck, thorax, and abdomen. The
fibers of CN XI, the spinal accessory, that originate
from the nucleus ambiguus, will join CN X immediately after exiting. This joining is not shown in the illustration.
• Spinal accessory (to neck muscles) — The fibers that
supply the large muscles of the neck (sternomastoid
and trapezius) originate in the upper spinal cord and
ascend into the skull before exiting. When referring to
the spinal accessory nerve, one usually has in mind
only this component.
• Hypoglossal (to muscles of the tongue) — These
fibers actually course anteriorly, exiting from the
medulla between the inferior olive and the corticospinal (pyramidal) tract.
Fronto-pontine fibers
Temporoparieto-pontine fibers
Cerebral peduncle
CN III
Red n.
Oculomotor n.
Cortico-spinal fibers
Trochelar n.
CN IV
Pontine nuclei
Middle cerebellar
peduncle
Rubro-spinal tract
Trigeminal n. (motor)
CN V (motor)
Abducens n.
CN VII
Facial n.
CN IX (motor)
Hypoglossal n.
CN X (motor)
CN XI
Ambiguus n. (IX, X, XI)
CN XII
Anterior cortico-spinal tract
Pyramidal decussation
Spinal cord
Lateral cortico-spinal tract
FIGURE 45: Descending Tracts and Motor Nuclei — Motor Pathways and Nuclei
FIGURES 46 AND 47
RETICULO-SPINAL TRACTS
INDIRECT VOLUNTARY PATHWAY
As noted with Figures 40A and 40B, the reticular formation is a collection of nuclei that participates in a
number of functions, some quite general (e.g., “arousal”)
and others more specific (e.g., respiratory control).
These nuclei are also part of the indirect voluntary
motor pathway (see Introduction to Motor Systems).
The indirect voluntary pathway — the cortico-reticulospinal pathway — is apparently an older pathway for the
control of movements, particularly of proximal joints
and the axial musculature. Therefore, some voluntary
movements can still be performed after destruction of
the cortico-spinal pathway (discussed with Figure 42).
The reticular formation receives input from many
sources, including most sensory pathways (anterolateral,
trigeminal, auditory, and visual). At this point, the focus
is on the input from the cerebral cortex. These axons
form part of the so-called cortico-bulbar system of
fibers (discussed with Figure 43). Muscle tone is greatly
influenced by activity in the reticular formation, and
cortical input to the reticular formation is part of this
regulation.
There are two pathways (on each side) from the reticular
formation to the spinal cord: one originates in the
pontine region (Figure 46) and one in the medullary
region (Figure 47).
FIGURE 46
PONTINE (MEDIAL)
RETICULO-SPINAL TRACT
The pontine (medial) reticulo-spinal tract originates in
the pontine reticular formation from two nuclei: the
upper one is the oral portion of the pontine reticular
nuclei (nucleus reticularis pontis oralis), and the lower
part is the caudal portion (see Figure 40B). The tract
descends to the spinal cord and is located in the medial
region of the white matter (see Figure 72); therefore,
this pathway is called the medial reticulo-spinal tract.
In terms of function, this pathway exerts its action on
the extensor muscles, in both movements and tone. The
area in the pons is known as the reticular extensor facilitatory area. The fibers terminate on the anterior horn
cells controlling the axial muscles, likely via interneurons
— not directly. This system is complementary to that of
the lateral vestibular nucleus (see Figure 48).
Lesions involving the cortico-bulbar fibers are discussed
with the medullary reticular formation (Figure 47).
The location of the tract in the brainstem is shown at
cross-sectional levels B4 (mid-pons), B5 (lower pons),
B7 (mid-medulla), and spinal cord levels C8 and L3.
FIGURE 46: Pontine (Medial) Reticulo-spinal Tract — Indirect Voluntary Pathway
FIGURE 47
MEDULLARY (LATERAL)
RETICULO-SPINAL TRACT
in tone is velocity dependent and involves the antigravity muscles. In humans, for reasons difficult to
explain, these are the flexors of the upper limb and the
extensors of the lower limb.
This tract originates in the medullary reticular formation, mainly from the nucleus known as the nucleus
gigantocellularis (see Figures 40B and 70). The tract
descends more laterally in the spinal cord than the
pontine pathway and is named the lateral reticulo-spinal
tract (Figure 72). Some of the fibers are crossed. The
tract lies beside the (lateral) vestibulo-spinal pathway.
The increase in reflex responsiveness — hyperreflexia
— is tested with the monosynaptic stretch reflex (deep
tendon reflex, DTR; discussed with Figure 2A). Another
feature of hyperreflexia is clonus, usually elicited at the
ankle by a rapid jerking of the ankle upwards.
This tract also has its greatest influence on axial musculature, and its functional contribution has been classified
as the reticular extensor inhibitory area. In this way, its
influence is opposite to that of the pontine reticular formation. This area depends for its normal activity on influences coming from the cortex.
Hyperreflexia also develops over a period of a few days
or weeks. This development period is thought to be due
to the decrease in the descending influences to the spinal
cord. There are two hypotheses for the increase in reflex
responsiveness:
A.
One possibility is a change of responsiveness of
neurotransmitter receptors of the motor neurons
or the interneurons. This is called denervation
supersensitivity, leading to an increase in the responsiveness of the lower motor neuron.
B.
An alternate possibility is sprouting of the incoming (1A) muscle afferents, from the muscle spindles to “vacated” synaptic sites (due to the loss of
descending fibers). This is called collateral
sprouting. Hence, this increased input to the
motor neurons causes the increased responsiveness of the motor neurons or interneurons.
The location of the tract in the brainstem is shown at
cross-sectional levels B4 (mid-pons), B5 (lower pons),
B7 (mid-medulla), and spinal cord levels C8 and L3.
Clinical Aspects: Spasticity
Lesions involving the motor system, particularly affecting the function of the reticular formation, are often
very confusing, mostly because of the lack of agreed
upon terminology and the lack of a clear understanding
of what is commonly seen clinically. The activity of the
reticular formation has a profound effect on muscle reactivity to passive stretch and on deep tendon reflexes. It
is extremely important for the clinician or neurologist to
be able to detect changes in muscle tone and reflex activity and to differentiate between spasticity and rigidity.
Destruction of the cortical input to the reticular formation results in a disruption of their descending influences by disturbing the balance between the two parts of
the reticular formation. The result is an increase in the
tone of the anti-gravity muscles, which develops over a
period of several days. This condition is called an upper
motor neuron lesion and any injury of the motor cortex
or cortico-bulbar system above the level of the pons may
give rise to this syndrome.
The increase in tone — spasticity — is tested by passive
flexion and extension of a limb. In spasticity, the change
Lesions involving large parts of the motor areas of the
cerebral cortex and lesions of the descending fibers (particularly in the internal capsule) may lead to this clinical
state in which a patient is paralyzed or has marked
weakness, with spasticity and hyperreflexia (with or
without clonus) on the contralateral side. With a lesion
of the spinal cord which involves all the descending
motor pathways of one half of the spinal cord, the clinical deficit would be ipsilateral to the lesion.
In a Parkinsonian patient, the change of muscle tone is
called rigidity (discussed with Figure 50). In contrast, a
lower motor neuron lesion of the anterior horn cell
(e.g., polio) leads to weakness, a decrease in muscle tone,
and hyporeflexia.
FIGURE 47: Medullary (Lateral) Reticulo-Spinal Tract — Indirect Voluntary Pathway
FIGURE 48
LATERAL VESTIBULOSPINAL TRACT
NON-PYRAMIDAL MOTOR SYSTEM
This pathway is very important in that it provides a link
between the vestibular influences (i.e., gravity and
balance) and the control of axial musculature, via the
spinal cord. The main function is to provide corrective
muscle activity when the body (and head) tilt or change
orientation in space (activation of the vestibular system,
CN VIII).
This tract originates in the lateral vestibular nucleus,
which is located in the lower pontine region (see Figures
6 and 49). The nucleus is found at the lateral edge of the
fourth ventricle (see Figure 68) and is characterized by
extremely large neurons. (This nucleus is also called
Deiter’s nucleus in some texts and the large neurons are
often called by the same name.)
The lateral vestibular nucleus receives its major inputs
from the vestibular system and from the cerebellum;
there is no cortical input. This tract descends through
the medulla and traverses the entire spinal cord (see
Figure 72). It does not decussate. The fibers terminate in
the medial portion of the anterior horn, namely on
those motor cells that control the axial musculature (see
Figure 2A).
In terms of function, this pathway increases extensor
muscle tone and activates extensor muscles. It is easier to
think of these muscles as anti-gravity muscles in a fourlegged animal; in humans, one must translate these
muscles as functionally the extensors of the lower extremity and the flexors of the upper extremity.
A lesion of this pathway would occur with spinal cord
injuries and would involve one of the “upper motor
neuron” pathways, leading to spasticity and
hyperreflexia.
The same cross-sectional levels have been used as with
the reticular formation, starting at B4 (mid-pons). The
vestibular nuclei are found at the B5 (the lower pontine
level) and are seen through B7 (mid-medulla); the tract
descends through the spinal cord, as seen at C8 and L3.
In the spinal cord the tract is positioned anteriorly, just
in front of the ventral horn (see Figure 72).
Other Vestibular Connections
The other vestibular nuclei — inferior and medial — contribute to the MLF (medial longitudinal fasciculus, discussed with Figures 49A and 49B). Some of these fibers
descend to the cervical spinal cord and are named the
medial vestibulo-spinal tract (see Figure 72), and others
ascend to the midbrain. They form part of this interconnecting fiber system that coordinates movements of the
eyes and the head and neck with vestibular input. These
descending fibers also influence mainly the “axial” muscles.
FIGURE 48: Lateral Vestibulo-spinal Tract — Non-Pyramidal Motor System
FIGURE 49A
VESTIBULAR SYSTEM
VESTIBULAR NUCLEI
The vestibular system carries information about our position and changes in that position in relation to gravity.
The sensory system is located in the inner ear and consists of three semicircular canals and other sensory
organs in a bony and membranous labyrinth. There is a
peripheral ganglion and the central processes of these
cells, CN VIII, enter the brainstem at the cerebellarmedullary angle, just above the cerebellar flocculus (see
Figures 3 and 4).
The vestibular information is carried to four vestibular
nuclei which are located in the upper medulla and lower
pons: superior, lateral, medial, and inferior (see also
Figure 6). The lateral vestibular nucleus gives rise to the
lateral vestibulo-spinal tract (as described with Figure
48; see also Figure 49B). It serves to adjust the posture to
changes in position in relation to gravity.
The medial and inferior vestibular nuclei give rise to
both ascending and descending fibers which join a conglomerate bundle called the medial longitudinal fasciculus (MLF, described more fully with Figure 49B). The
descending fibers from the medial vestibular nucleus, if
considered separately, could be named the medial
vestibulo-spinal tract and likely participate in postural
adjustments to positional changes.
The ascending fibers adjust the position of the eyes and
coordinate movements of the two eyes by interconnecting the three cranial nerve nuclei involved in the control
of eye movements, CN III (oculomotor), CN IV
(trochlear), and CN VI (abducens), all at different levels
of the brainstem (see also Figure 5). Lateral gaze, a
movement of the eyes to the side (in the horizontal
plane), requires the coordination of the lateral rectus
muscle (abducens nucleus) of one side and the medial
rectus (oculomotor nucleus) of the other side. These
fibers for coordinating the eye movements are carried in
the MLF.
Clinical Aspects
A lesion of the MLF interferes with the normal conjugate movements of the eyes. When a person is asked to
follow an object (e.g., the tip of a pencil) with the head
steady, the two eyes move together, usually in the horizontal plane. With a lesion of the MLF (such as demyelination in multiple sclerosis), the abducting eye moves
normally (intact abducens nucleus) but the adducting
eye fails to follow; yet, adduction is preserved on convergence. This condition is known as internuclear ophthalmoplegia. Sometimes there is monocular horizontal
nystagmus of the abducting eye.
There is a “gaze center” within the pontine reticular formation for saccadic eye movements; these are extremely
rapid (ballistic) movements of both eyes, yoked together,
usually in the horizontal plane. The cortical fibers originate from the frontal eye field (see Figure 12) and also
likely course in the MLF.
There is a small nucleus in the periaqueductal gray
region of the midbrain which is associated with the
visual system and is involved in the coordination of eye
and neck movements. This nucleus is called the interstitial nucleus (of Cajal). It is located near the oculomotor
nucleus. This nucleus (see IN in Figure 49B) receives
input from various sources and contributes fibers to
the MLF. Some may actually name this the interstitiospinal tract.
Interstitial n. of Cajal
Oculomotor n.
Trochlear n.
Abducens n.
Pontine reticular
formation
VESTIBULAR NUCLEI
S = Superior
L = Lateral
M = Medial
I = Inferior
FIGURE 49A: Vestibular System — Vestibular Nuclei
FIGURE 49B
MEDIAL LONGITUDINAL
FASCICULUS
MLF AND ASSOCIATED TRACTS
The MLF (medial longitudinal fasciculus) is a tract
within the brainstem and upper spinal cord which links
the visual world and vestibular events with the movements of the eyes and the neck, as well as links the
nuclei that are responsible for eye movements. The tract
runs from the midbrain level to the upper thoracic level
of the spinal cord. It has a rather constant location near
the midline, dorsally, just anterior to the cerebral aqueduct and the fourth ventricle (see brainstem cross sections, e.g., Figures 65 and 69). Several tracts together
form the actual MLF.
Each of the component parts of the system can be considered separately:
• Vestibular fibers — Of the four vestibular nuclei (see
Figure 49A), descending fibers originate from the
medial vestibular nuclei and become part of the MLF;
this can be named the medial vestibulo-spinal tract.
There are also ascending fibers which come from the
medial, inferior, and superior vestibular nuclei that
also are carried in the MLF. Therefore, the MLF
carries both ascending and descending vestibular
fibers.
• Visuomotor fibers — The interconnections between
the various nuclei concerned with eye movements are
carried in the MLF.
• Vision-related fibers — Visual information is received
by various brainstem nuclei.
• The superior colliculus is a nucleus for the coordination of visual-related reflexes, including eye
movements. It also receives input from the visual
association cortical areas (areas 18 and 19, see
Figure 39A). The descending fibers from the superior colliculus, called the tecto-spinal tract, are very
closely associated with the MLF and can be considered part of this system (although in most books it
is discussed separately). The superior colliculus coordinates the movements of the eyes and the
turning of the neck, in response to visual information. As shown in the upper inset of Figure 49B,
these fibers cross in the midbrain. (Note that the superior colliculus (SC) of only one side is shown in
order not to obscure the crossing fiber systems at
that level.)
• The small interstitial nucleus and its contribution
were noted in Figure 49A. The lower inset of Figure
49B shows the MLF in the ventral funiculus of the
spinal cord at the cervical level (see also Figure 72).
The three components of the tract are identified —
those coming from the medial vestibular nucleus,
the fibers from the interstitial nucleus, and the
tecto-spinal tract. These fibers are mingled together
in the MLF.
In summary, the MLF is a complex fiber bundle which is
necessary for the proper functioning of the visual apparatus. The MLF interconnects the three cranial nerve
nuclei responsible for movements of the eyes with the
motor cells controlling the movements of the head and
neck. It allows the visual movements to be influenced by
vestibular, visual, and other information and carries
fibers (upwards and downwards) that coordinate the eye
movements with the turning of the neck.
The diagram also shows the posterior commissure. This
small commissure carries fibers connecting the superior
colliculi. In addition, it carries the important fibers for
the consensual pupillary light reflex (discussed with
Figure 39B). The role of the commissural nuclei is not
known.
Red n. (RN)
Posterior commissure
Commissural n.
Oculomotor n.
Interstitial n. (IN)
Superior colliculus
(SC)
Spino-tectal tract
Medial longitudinal
fasciculus (MLF)
IN
RN
SC
Trochlear n.
Abducens n.
CN VIII
Interstitio-spinal tract
Rubro-spinal tract
Tecto-spinal tract
Spino-cerebellar
tracts
Dorsal
Ventral
MLF
Vestibular nuclei
Vestibulo-spinal tracts
{
MLF
{
Lateral
Medial
Tecto-spinal tract
Tecto-spinal tract
Interstitio-spinal tract
Medial vestibulo-spinal tract
Lateral vestibulo-spinal tract
Spinal cord
FIGURE 49B: Medial Longitudinal Fasciculus — MLF and Associated Tracts
FIGURE 50
BASAL GANGLIA: CIRCUITRY
MOTOR REGULATORY SYSTEMS
Many years ago it was commonplace to refer to the basal
ganglia as part of the extrapyramidal motor system (in
contrast to the pyramidal motor system - discussed with
Figure 42, the cortico-spinal tract). It is now known that
the basal ganglia exert their influence through the appropriate parts of the cerebral cortex, which then acts
either directly, i.e. using the cortico-spinal (pyramidal)
tract, or indirectly, via certain brainstem nuclei, to alter
motor activity. The term extrapyramidal should probably be abandoned, but it is still frequently encountered
in a clinical setting. Other terms could be used, such as
“non-pyramidal” or simply basal ganglia. At best one
could perhaps consider this system in the same way as
one is accustomed to viewing the influence of the cerebellum on motor control (to be discussed as part of the
motor regulatory systems).
The basal ganglia are introduced in Section A (see
Figures 22–25). In this illustration, the removal of the
head of the caudate nucleus, and some of its body,
exposes the putamen more completely. The two parts of
the globus pallidus are also seen. The illustration also includes the two other parts of the functional basal ganglia
— the subthalamic nucleus and the substantia nigra.
• The subthalamic nucleus is situated in a small region
below the level of the diencephalon. This nucleus is
connected with the globus pallidus, both receiving
fibers from and sending fibers to different parts of
that nucleus. The motor abnormality associated with a
lesion of the subthalamic nucleus is called hemiballismus. The person (or animal) with this abnormality
has sudden flinging movements of one or both limbs,
on the opposite side of the body.
• The substantia nigra is located in the midbrain
region, as a sheet-like nucleus (see also Figure 24). It is
composed of two parts (see Figure 64):
1. The pars reticulata is situated more ventrally. It
receives fibers from the basal ganglia and is
involved in the output from the basal ganglia to
the thalamus.
2. The pars compacta has the pigment-containing
cells. These neurons project their fibers to the
caudate and putamen (the striatum or neostriatum). This is called the nigro-striatal “pathway”; the
neurotransmitter involved is dopamine.
Clinical Aspects
It is the degeneration of these dopamine-containing
neurons, with the consequent loss of their dopamine
input to the basal ganglia, the striatum, that leads to the
clinical entity Parkinson’s disease. Those afflicted with
this disease have slowness of movement (bradykinesia),
reduced facial expressiveness (“mask-like” facies), and a
tremor at rest (typically a “pill-rolling” type of tremor).
On examination, there is rigidity, which is an increased
resistance to passive movement of both flexors and extensors (contrast with spasticity, discussed with Figure
47) which is not velocity-dependent, and there is no
change in reflexes.
Basal Ganglia Circuitry
Information flows into the caudate and putamen from
all areas of the cerebral cortex (in a topographic
manner), from the substantia nigra (pars compacta),
and from the centromedian nucleus of the thalamus (see
Figure 51). This information is processed and passed to
the globus pallidus (internal segment) and the pars
reticulata of the substantia nigra of the midbrain. (In
addition, there is a sub-circuit involving the external
segment of the globus pallidus and the subthalamic
nucleus.)
The most important output from the basal ganglia is
from the internal segment of the globus pallidus. Most
of this information is relayed to the thalamus, to the
specific relay nuclei, ventral anterior (VA) and ventral
lateral (VL) nuclei (see Figure 10). These project to the
supplementary motor and premotor cortical areas (see
Figures 12 and 51). These are the cortical areas concerned with motor planning and motor regulation.
(Contrast this with the projection of the cerebellum to
the cortex — to be discussed subsequently.)
Caudate nucleus
Putamen
Globus pallidus
(external segment)
Subthalamic nucleus
Substantia nigra
Red nucleus
Globus pallidus
(internal segment)
Midbrain
Anterior commissure
Amygdala
FIGURE 50: Basal Ganglia: Circuitry — Motor Regulatory Systems
FIGURE 51
THALAMUS: MOTOR CIRCUITS
MOTOR REGULATORY SYSTEMS
The specific relay nuclei of the thalamus that are linked
with the motor system are the ventral lateral and the
ventral anterior nuclei. These project to different cortical
areas involved in motor control. These nuclei also
receive input from these cortical areas, in line with the
reciprocal connections of the thalamus and cortex. One
of the intralaminar nuclei, the centromedian nucleus, is
also linked with the motor system.
Basal Ganglia
Input to the basal ganglia, the striatal parts, comes from
all parts of the cortex — the cortico-striatal fibers. After
processing, the striatum sends its information to the
globus pallidus (striato-pallidal fibers).
The major outflow from the basal ganglia is from the internal (medial) segment of the globus pallidus. Two
slightly different pathways project to the thalamus —
pallido-thalamic fibers — one passing around and the
other passing through the fibers of the internal capsule
(represented on the diagram by large stippled arrows).
These merge and end in the ventral anterior (VA) and
ventral lateral (VL) nuclei of the thalamus. (The ventral
anterior nucleus is not seen on this section through the
thalamus; see Figure 10.) These nuclei also receive input
from the substantia nigra, pars reticulata (not shown).
The cortical projection is to the premotor and supplementary motor areas, as shown in the small insets.
The functional contribution of the basal ganglia to the
motor system is discussed in Section A. It is still not
known what role these thalamic nuclei play in this
pathway. Clinically, in a person with Parkinson’s disease,
a lesion in the thalamic region that interrupts this
pathway and/or destroys part of these nuclei has been
shown to alleviate some of the symptoms. To date, the
theory has been that the surgical removal of impulses restores the balance between the various thalamic influences to the cortical areas involved in motor control.
The pathway involving the centromedian nucleus (CM)
is rather distinct. The afferents come from the medial
segment of the globus pallidus. Efferents from CM go to
the caudate-putamen, hence forming a feedback loop
within the basal ganglia.
Cerebellum
Note to student: Review after study of the cerebellum.
The other part of the motor regulatory system, the cerebellum, also projects to the cortex via the thalamus. The
dentate nucleus, the largest of the deep cerebellar nuclei
and the one that receives input from the neocerebellum,
projects its fibers via the superior cerebellar peduncle
(see Figures 38 and 55). The major projection is to the
ventral lateral (VL) nucleus, but a different portion of it
than receives from the basal ganglia. From here, the
fibers project to the motor areas of the cerebral cortex,
predominantly the motor area of the precentral gyrus as
well as the premotor area (areas 4 and 6, respectively).
With regard to function, the neocerebellum seems to be
involved in motor planning (discussed with the cerebellum, Figure 55). Again, it is not easy to understand what
role the thalamus plays in integrating this information,
prior to the projection to the cerebral cortex.
The motor areas of the cerebral cortex that receive input
from these two subsystems of the motor system are
shown diagrammatically in Figure 51 — both on the
dorsolateral surface and on the medial surface of the
hemispheres.
Supplementary
motor area
Thalamo-cortical fibers
Premotor area
(area 6)
Cortico-striatal
fibers
Precentral gyrus (area 4)
Ventral lateral n.
Intralaminar n.
Supplementary
motor area
Centromedian n.
Globus
pallidus
Putamen
Red nucleus
Striato-pallidal fibers
Internal capsule (fibers)
Pallido-thalamic fibers
Cerebello-thalamic fibers
Substantia nigra
FIGURE 51: Thalamus: Motor Circuits — Motor Regulatory Systems
FIGURE 52
CEREBELLUM I
FUNCTIONAL LOBES
The anatomical approach to the cerebellum is introduced in the Orientation section (see Figure 8). In order
to understand the functional anatomy of the cerebellum
and its contribution to the regulation of motor control,
it is necessary to subdivide the cerebellum into operational units. The three functional lobes of the cerebellum are the vestibulocerebellum, the spinocerebellum,
and the neo- or cerebrocerebellum. These lobes of the
cerebellum are defined by the areas of the cerebellar
cortex involved, the related deep cerebellar nucleus, and
the connections (afferents and efferents) with the rest of
the brain.
There is a convention of portraying the surface of the
cerebellum as if it were found in a single plane. The best
analogy to use is a book, with the binding towards you.
The binding is the hinge around which the book will
open, and the landmark for this on the cerebellum is the
horizontal fissure, which demarcates the superior and
inferior surfaces. If you place the fingers of your right
hand on the edge of the front cover (the superior surface
of the cerebellum) and the fingers of your left hand on
the edges of the back cover (the inferior surface of the
cerebellum), you can gently open the book to expose
both the front and back covers simultaneously. Both are
now laid out in a single plane. Using the lingula and the
nodulus of the vermis as fixed points (see Figure 16), the
lingula is at the “top” of the cerebellum and the nodulus
is at the bottom of the diagram.
This same portrayal can be done with an isolated cerebellum and attached brainstem in the following way: the
brain knife is introduced at the ponto-medullary junction and directed towards the horizontal fissure. Before
reaching there, the superior and inferior surfaces are
pulled very gently apart, thereby making it possible to
place the cerebellum on a flat surface. Having done this,
it is possible to discuss the three functional lobes of the
cerebellum.
The vestibulocerebellum is composed of two cortical
components, the flocculus and the nodulus; hence it is
also called the flocculonodular lobe. This is the functional part of the cerebellum responsible for balance and
gait. The flocculus is a small lobule of the cerebellum
located on its inferior surface and oriented in a transverse direction, below the middle cerebellar peduncle
(see Figures 3 and 4). The flocculus is connected to the
nodulus of the vermis (see Figure 16), the two together
forming the flocculonodular lobe. The vestibulocerebellum sends its fibers to the fastigial nucleus, one of the
deep cerebellar nuclei which connect the cerebellum
with other parts of the brain (discussed with Figures
54A and 54B).
The functional lobe called the spinocerebellum is concerned with coordinating the activities of the limb musculature. Part of its role is to act as a comparator between
the intended and the actual movements. It is made up of
three areas. The anterior lobe of the cerebellum is an
anatomical division of the cerebellum found on the superior surface, in front of the primary fissure (see Figure
8). Most of the vermis (other than the parts mentioned
above — see Figure 16) comprises the second part of the
spinocerebellum. The third portion is a strip of tissue on
either side of the vermis called the intermediate zone
(or paravermis) — there is no anatomical fissure demarcating this functional area. The output deep cerebellar
nuclei for this functional part of the cerebellum is in
part the fastigial nucleus and mostly the intermediate
nuclei, the globose and emboliform nuclei (see Figures
54A and 54B).
With the exception of the vermis and the adjacent strip
(the intermediate zone), the tissue behind the primary
fissure comprises the neocerebellum. This continues
onto the inferior surface of the cerebellum, until the
dorsal aspect of the medulla is reached. This is the
largest part of the cerebellum and the newest from an
evolutionary point of view. It is usually called the neocerebellum, and also the cerebrocerebellum, since most
of its connections are with the cerebral cortex. This part
of the cerebellum is involved with the overall coordination of voluntary motor activities and is also involved in
motor planning. The output nucleus of this part of the
cerebellum is the dentate nucleus.
L
F
N
F
L = Lingula
N = Nodulus
F = Flocculus
Spinocerebellum
L
Anterior lobe
Primary fissure
Cerebrocerebellum
Horizontal fissure
F
N
F
Vermis
Intermediate
FIGURE 52: Cerebellum I — Functional Lobes
Flocculonodular lobe
FIGURE 53
CEREBELLUM II
AFFERENTS
Information relevant to the role of the cerebellum in
motor regulation comes from the cerebral cortex, the
brainstem, and the muscle receptors in the periphery.
The information is conveyed to the cerebellum mainly
via the middle and inferior cerebellar peduncles. Only
one afferent tract enters via the superior cerebellar
peduncle.
Inferior Cerebellar Peduncle
The inferior cerebellar peduncle goes from the medulla
to the cerebellum. It lies behind the inferior olivary
nucleus and can sometimes be seen on the ventral view
of the brainstem (as in Figure 4). This peduncle conveys
a number of fiber systems to the cerebellum. These are
shown schematically in Figure 53 with the ventral view
of the brainstem and cerebellum. The fiber systems
include the posterior (dorsal) spino-cerebellar pathway,
the cuneo-cerebellar tract, olivo cerebellar tract, and
other cerebellar afferents:
• The posterior (dorsal) spino-cerebellar pathway is
conveying proprioceptive information from most of
the body. This is one of the major tracts of the inferior
peduncle. These fibers, carrying information from the
muscle spindles as well as from cutaneous sources,
relay in the dorsal nucleus of Clarke in the spinal cord
(see Figure 2A). They ascend ipsilaterally in a tract
which is found at the edge of the spinal cord (see
Figure 72). The dorsal spino-cerebellar fibers terminate ipsilaterally. These fibers are distributed to the
spino-cerebellar areas of the cerebellum.
• The homologous tract for the upper limb is the
cuneo-cerebellar tract. These fibers relay in the accessory (external) cuneate nucleus in the lower medulla
(see Figures 70 and 71). This pathway is not shown in
the diagram.
• The olivo-cerebellar tract is also carried in this peduncle. The fibers originate from the inferior olivary
nucleus (see Figures 4 and 70), cross in the medulla,
and are distributed to all parts of the cerebellum.
These axons have been shown to be the climbing
fibers to the main dendritic branches of the Purkinje
neuron.
• Other cerebellar afferents from other nuclei of the
brainstem, including the reticular formation, are conveyed to the cerebellum via this peduncle. Most important are those from the vestibular nuclei to the
vestibulocerebellum. Afferents from the visual and auditory system are also known to be conveyed to the
cerebellum.
Middle Cerebellar Peduncle
All parts of the cerebral cortex contribute to the massive
cortico-pontine system of fibers (described with Figures
43 and 45). These descend via the anterior and posterior
limbs of the internal capsule, then the inner and outer
parts of the cerebral peduncle and terminate in the
pontine nuclei (shown in Figure 67). The fibers synapse,
cross, and go to all parts of the cerebellum via the
middle cerebellar peduncle (not labeled in this diagram;
see Figure 3). This input provides the cerebellum with
the cortical information relevant to motor commands
and the intended motor activities.
Superior Cerebellar Peduncle
One group of cerebellar afferents, those carried in the
ventral (anterior) spino-cerebellar tract, enters the cerebellum via the superior cerebellar peduncle. These fibers
cross in the spinal cord, ascend (see Figure 72), enter the
cerebellum, and cross again, thus terminating on the
same side from which they originated.
All afferent fibers provide excitatory influences to the
deep cerebellar nuclei via collaterals and end in the cerebellar cortex.
Inferior olivary nucleus
Olivo-cerebellar fibers
Inferior cerebellar peduncle
Dorsal spino-cerebellar tract
FIGURE 53: Cerebellum II — Afferents
FIGURE 54A
CEREBELLUM III
CIRCUITRY
The cerebellum is organized with cortical tissue on the
outside, the cerebellar cortex. The cortex consists of
three layers and all areas of the cerebellum are histologically alike. The most important cell of the cortex is the
Purkinje neuron, which forms a single layer; their
massive dendrites receive the various cerebellar afferents.
Various interneurons are also located in the cortex. The
axon of the Purkinje neuron is the only axonal system to
leave the cerebellar cortex.
Deep within the cerebellum are masses of gray matter
which are part of the cerebellar circuitry. These are
called the intracerebellar nuclei or the deep cerebellar
nuclei; both names are used. There are four pair of deep
cerebellar nuclei — the fastigial nucleus (most medially), the intermediate group (named the globose and emboliform), and the lateral or dentate nucleus. Each
belongs to a different functional part of the cerebellum.
These nuclei are the output nuclei of the cerebellum to
other parts of the central nervous system.
The position of the deep cerebellar (intracerebellar)
nuclei, which are located within the cerebellum, is indicated in Figure 54A. Their location is superimposed
upon the ventral view of the cerebellum. (The intracerebellar nuclei are shown from a posterior perspective in
Figure 54B.) The nuclei are arranged in the following
manner:
1.
The fastigial (medial) nucleus is located next to
the midline.
2.
The globose and emboliform nuclei are slightly
more lateral; often these are grouped together
and called the intermediate or interposed nucleus.
3.
The dentate nucleus, with its irregular margin, is
most lateral. This nucleus is sometimes called the
lateral nucleus and is by far the largest.
The nuclei are located within the cerebellum at the level
of the junction of the medulla and the pons. Therefore,
the cross sections shown at this level (see Figure 68) may
include these deep cerebellar nuclei. Usually, only the
dentate nucleus can be identified in the real sections.
The same holds true for sections of the gross brainstem
and cerebellum done at this level.
Overall, the circuitry is as follows: input (excitatory) to
the cerebellum goes to both the deep cerebellar nuclei
and the cerebellar cortex. After processing in the cortex,
the Purkinje neuron influences (inhibitory) the activity
of the neurons of the deep cerebellar nuclei. Their
output (excitatory) exits the cerebellum to the brainstem
and to the cerebral cortex via the thalamus, which modulates motor activity.
Details of Cerebellar Circuitry
The cerebellum receives information from many parts
of the nervous system, including the spinal cord,
vestibular system, brainstem, and cerebral cortex. Most
of this input is related to motor function, but some is
also sensory. These afferents are excitatory in nature
and influence the ongoing activity of the neurons in
the intracerebellar nuclei, as well as projecting to the
cerebellar cortex.
The incoming information to the cerebellar cortex is
processed by various interneurons of the cerebellar
cortex and eventually influences the Purkinje neuron,
which leads to either increased or decreased firing of this
neuron. Its axon is the only one to leave the cerebellar
cortex and these axons project (in an organized manner)
to the deep cerebellar nuclei.
The Purkinje neurons are an inhibitory neuron and their
influence modulates the activity of the deep cerebellar
nuclei. Increased firing of the Purkinje neuron increases
the ongoing inhibition onto these deep cerebellar nuclei,
while decreased Purkinje cell firing results in a decrease
in the inhibitory effect on the deep cerebellar cells, i.e., it
results in the increased firing of the deep cerebellar
neurons (called disinhibition).
Axons from the deep nuclei neurons project from the
cerebellum to many areas of the CNS, including brainstem motor nuclei (e.g., vestibular, reticular formation)
and thalamus (to motor cortex). In this way the cerebellum exerts its influence on motor performance. This
topic is discussed with Figure 54B.
Fastigal n.
Emboliform n.
Globose n.
Dentate n.
FIGURE 54A: Cerebellum III — Circuitry
FIGURE 54B
CEREBELLUM IV
cent to the inferior cerebellar peduncle. The name of
this particular bundle is the juxtarestiform body*, a
rather awkward name for a bundle of fibers.
EFFERENTS
It is interesting to note that the cerebellar cortex projects
fibers directly to the lateral vestibular nucleus. As would
be anticipated, these are inhibitory. The lateral vestibular
nucleus (see Figure 49A) could therefore, in some sense,
be considered one of the intracerebellar nuclei. This
nucleus also receives input from the vestibular system
then projects to the spinal cord (review with Figure 48).
Figure 54B is a dorsal view of the diencephalon, midbrain, and cerebellum. The superior surface of the cerebellum is visualized, and the position of the deep
cerebellar (intracerebellar) nuclei within the cerebellum
has been added. This view of the cerebellum is similar to
the photograph of the brain in Figure 8. The third ventricle is situated between the two diencephala, with the
interconnecting massa intermedia. The pineal gland is
seen attached to the posterior aspect of the thalamus.
Below are the colliculi, superior, and inferior.
The vermis includes a group of midline folia which are
elevated. In front of the primary fissure is the anterior
lobe, part of the spinocerebellum. The location of the
horizontal fissure is also indicated, separating the superior surface from the inferior one. The perspective of
this diagram is such as to include several folia belonging
to the inferior surface of the cerebellum (see Figure 52).
The intracerebellar nuclei are depicted within the cerebellum, as if they could be seen from the outside
(similar to the view presented from the ventral perspective, in Figure 54A). Their location within the cerebellum will be correlated with the projection they receive
from the cerebellar cortex (refer also to Figure 52).
The fastigial nuclei are centrally located and receive
fibers from the vermis. These nuclei are connected with
the vestibulocerebellum and, to some degree, with the
spinocerebellum. From the fastigial nuclei, efferent fibers
go to brainstem motor nuclei (e.g., vestibular nuclei and
reticular formation), influencing balance and gait. They
exit from the cerebellum in a bundle that is found adja-
The emboliform and globose are the intermediate nuclei
and they receive fibers from the intermediate zone of the
cerebellum and the anterior lobe. These nuclei are functionally part of the spinocerebellum. These fibers also
project to brainstem nuclei, including the red nucleus of
the midbrain and to the appropriate limb areas of the
motor cortex. The cortical fibers project via the thalamus (see below) to the motor areas of the cerebral
cortex and are involved in the comparator function of
this part of the cerebellum.
The dentate nucleus is most lateral of the intracerebellar
nuclei and it receives input from the neocerebellum. Its
connections are described with Figure 55.
*The name is derived from an older terminology — the inferior cerebellar peduncle used to be called the restiform body. It is unlikely that
a student will be exposed to this terminology in a clinical setting
except, perhaps, in neuroradiology.
Third ventricle
Diencephalon
Fastigial n.
Pineal
Superior colliculus
Inferior colliculus
Dentate n.
Primary fissure
Globose n.
Horizontal fissure
Emboliform n.
Vermis
FIGURE 54B: Cerebellum IV — Efferents
FIGURE 55
CEREBELLUM V
SUPERIOR CEREBELLAR PEDUNCLE
The superior cerebellar peduncle is the major outflow
tract of the cerebellum. This peduncle connects the cerebellum with the midbrain; the fibers are on their way to
the thalamus and cortex. It is difficult to visualize this
pathway and even more difficult to locate it on the
actual specimen (see Figures 7, 8, and 38).
The outflow fibers originate mainly from the dentate
nucleus, with some from the intermediate nucleus (not
shown). The axons start laterally and converge towards
the midline. In this part of their course they are located
in the roof of the upper half of the fourth ventricle (see
Figure 7). Some fibers that form a bridge between the
superior cerebellar peduncles in this area are named the
superior medullary velum (discussed with Figure 7; see
also Figure 38). From this dorsal perspective it is possible to visualize the superior cerebellar peduncles in a
gross brain specimen.
The superior cerebellar peduncles continue to “ascend”
and enter the upper part of the pons (see the cross
section in Figure 66). In the lower midbrain (see Figure
65) there is a complete decussation of fibers. Some of
the fibers may terminate in the red nucleus of the midbrain, particularly those from the interposed nucleus.
The majority of the fibers, particularly those from the
dentate nucleus, terminate in the ventral lateral nucleus
(VL) of the thalamus (see Figure 51).
Note to student: At this point it is important to return
to Figure 10 and review the specific thalamic relay —
ventral lateral nucleus of the thalamus to the motor
cortex.
From here they are relayed to the motor cortex, predominantly area 4, and also to the premotor cortex, area 6. It
is thought the neocerebellum is also involved in motor
planning.
Therefore, the neocerebellum is linked to the cerebral
cortex by a circuit which forms a loop. Fibers are relayed
from the cerebral cortex via the pons (the pontine
nuclei) to the cerebellum. The ponto-cerebellar fibers
cross and go to the neocerebellum of the opposite side.
After cortical processing, the neocerebellar fibers project
to the dentate nucleus. The efferents project to the thalamus, after crossing (decussating) in the lower midbrain.
From the thalamus, fibers are relayed mainly to the
motor areas of the cerebral cortex. Because of the two
crossings, the messages are returned to the same side of
the cerebral cortex from which the circuit began.
Clinical Aspects
Lesions of the neocerebellum of one side cause motor
deficits to occur on the same side of the body, that is
ipsilaterally for the cerebellum. The explanation for this
lies in the fact that the cortico-spinal tract is also a
crossed pathway (see Figure 42). For example, the errant
messages from the left cerebellum which are delivered to
the right cerebral cortex cause the symptoms to appear
on the left side — contralateral for the cerebral cortex
but ipsilaterally from the point of view of the cerebellum.
The cerebellar symptoms associated with lesions of the
neocerebellum (or the superior cerebellar peduncle) are
collectively called dyssynergia, in which the range, direction, and amplitude of voluntary muscle activity are disturbed. The specific symptoms include the following:
• distances are improperly gauged when pointing, called
dysmetria, and include pastpointing;
• rapid alternating movements are poorly performed,
called dysdiadochokinesis;
• complex movements are performed as a series of
successive movements, called decomposition of
movement;
• tremor is seen during voluntary movement, an
intention tremor (in contrast to Parkinsonian tremor
which is present at rest and disappears during voluntary movement);
• disturbances occur in the normally smooth production of words, resulting in slurred and explosive
speech.
In addition, cerebellar lesions in humans are often associated with hypotonia and sluggish deep tendon reflexes.
Red nucleus
Fibers to thalamus
Superior cerebellar
peduncle
FIGURE 55: Cerebellum V — Superior Cerebellar Peduncle
Section C
NEUROLOGICAL
NEUROANATOMY
Clinical neurology is based upon a detailed understanding of the structure and function of the nervous system,
in health and disease. The task of the neurologist is to
establish
• whether the disease process is neurological;
• where the nervous system is involved, i.e., localization;
and
• what the pathophysiological mechanisms are, i.e.,
the disease.
The clinical decision-making process involves taking
a detailed history, doing a complete neurological
examination, and establishing a differential diagnosis.
Select investigations are usually necessary to confirm the
diagnosis. Appropriate therapy can then be instituted
and the patient (and family) can be counseled regarding
the likely prognosis.
Skilled and knowledgeable clinicians can recognize
diseases based upon their presentation (for example,
vascular lesions have a sudden onset versus slow onset
for tumors), the patient’s age, the parts of the nervous
system involved, and the evolution of the disease
process.
The learning objective of this section is to enable
the student to localize the disease process within the
nervous system. The emphasis is on the brainstem. In
addition, the vascular supply of the brain and spinal
cord need to be studied at this point.
Vascular Supply
The CNS is totally dependent upon a continuous supply
of blood; viability of the neurons depends upon the immediate and constant availability of both oxygen and
glucose. Interruption of this lifeline causes sudden loss
of function. Study of the nervous system must include a
complete knowledge of the blood supply and structures
(nuclei and tracts) situated in the vascular territory of
the various arteries. Failure of the blood supply to a
region, either because of occlusion or hemorrhage, will
lead to death of the neurons and axons, resulting in
functional deficits.
Areas of gray matter, where the neurons are located,
have a greater blood supply than white matter. Loss of
oxygen and glucose supply to these neurons will lead to
loss of electrical activity after a few minutes (in the
adult) and, if continued, to neuronal death. The white
matter requires less blood supply; loss leads to destruction of the axons in the area of the infarct and an interruption of pathways. The axonal portions (and the
synapses) which are separated from the cell body will
degenerate, leading to a loss of function.
Visualization of the arterial (and venous) branches can
be accomplished using MR angiogram and arteriogram:
• MR Angiogram — Using neuroradiology imaging
with MRI (discussed with Figure 17), the major blood
vessels (such as the Circle of Willis) can be visualized;
this is called a magnetic resonance angiogram, or
MRA (see Figure 57).
• Arteriogram — By injecting a radiopaque substance
into the arteries (a procedure done by a neuroradiologist) and following its course through a rapid series of
x-rays (called an arteriogram), a detailed view of the
vasculature of the brain is obtained; either the carotid
or vertebral artery is usually injected, according to
which arterial tree is under investigation.
Histological Neuroanatomy
In addition to a detailed look at the blood supply as a
basis for analyzing the clinical consequences of vascular
lesions, this section also presents the detailed neuroanatomy that is needed for localization of lesions in
the brainstem and also the spinal cord. A series of illus-
trations is presented through the brainstem with details
of the tracts, cranial nerve nuclei, and other nuclei of
importance. Accompanying these schematics are photographs of the brainstem from the human brain at the
same levels.
Intracranial Pressure (ICP)
Note to student: Consult other texts for a visual understanding of these structures.
In addition to knowledge of the brain and the function
of the various parts, many disease processes exert their
effect because of a rise in intracranial pressure (ICP)
causing a shifting of structures within the skull. The
adult skull is a rigid container filled with the brain, the
cerebrospinal fluid (CSF), and blood. The interior of the
skull is divided into compartments by folds of dura: the
falx cerebri in the midline between the hemispheres
and the tentorium cerebelli, which separates the
hemispheres from the contents of the posterior cranial
fossa (brainstem + cerebellum).
Any increase in volume inside the skull — due to brain
swelling, tumor, abscess, hemorrhage, abnormal amount
of CSF — causes a rise in pressure inside the skull
(increased ICP). Although brain tissue itself has no pain
fibers, the blood vessels and meninges do, hence any
pulling on the meninges may give rise to a headache. A
rise in ICP can be seen in due course by examining the
optic disc; the margins become blurred and the disc
itself engorged, called papilledema, with a pathological
increase of ICP.
Depending upon the lesion, sooner or later a displacement of brain tissue from one compartment to another
occurs. This pathological displacement itself causes
damage to the brain. This is called a brain herniation
syndrome, and typically occurs
• through the tentorial notch, also called uncal
herniation (discussed with Figure 14);
• through the foramen magnum, also known as tonsillar herniation (discussed with Figure 8); and
• under the falx cerebri itself.
These shifts are life-threatening and require emergency
management.
FIGURE 56
BLOOD SUPPLY I
THE ARTERIAL CIRCLE OF WILLIS
(OVERLAY)
The arterial circle of Willis is a set of interconnecting
arteries of the vertebral and common carotid arteries. It
is located at the base of the brain, surrounding the optic
chiasm and the hypothalamus (the mammillary nuclei;
review Figures 13 and 14). Within the skull it is situated
above the pituitary fossa (and gland). The major arteries
to the hemispheres — the cerebral cortex — are
branches of this arterial circle.
Figure 56 is a photographic view of the inferior aspect of
the brain, including brainstem and cerebral hemispheres
(as in Figure 13), with the blood vessels presented as an
overlay (created with Photoshop) onto this illustration.
The cut end of the internal carotid arteries is a starting
point. Each artery divides into the middle cerebral
artery (MCA) and the anterior cerebral artery (ACA).
The middle cerebral artery courses within the lateral
fissure. (It is shown lightly shaded on the left side, as if it
could be visualized in its course through the lateral
fissure.) Within the fissure, small arteries are given off to
the basal ganglia, called the striate arteries (not labeled;
see Figure 60). The artery continues and further
branches are distributed onto the dorsolateral surface of
the brain (see Figure 58).
By removing the optic chiasm, the anterior cerebral arteries can be followed anteriorly. A very short artery
connects the two of them, the anterior communicating
artery. This anterior cerebral artery supplies the medial
surface of the brain (see Figure 59).
The vertebro-basilar system supplies the brainstem and
cerebellum, and the posterior part of the hemispheres.
The two vertebral arteries unite to form the midline
basilar artery, which courses in front of the pons. The
basilar artery terminates at the midbrain level by dividing into two posterior cerebral arteries. These supply
the inferior aspect of the brain and particularly the
occipital lobe.
The arterial circle is completed by the posterior communicating artery (one on each side), which connects the
internal carotid or middle cerebral artery (the anterior
circulation) with the posterior cerebral artery (from the
posterior circulation).
Small arteries directly from the circle (a few are shown)
provide the blood supply to the diencephalon (thalamus
and hypothalamus), some parts of the internal capsule,
and part of the basal ganglia. The major blood supply to
these regions is from the striate arteries (see Figure 60).
The branches from the vertebral and basilar artery
supply the brainstem. There are three major branches
from this part of the arterial tree to the cerebellum —
the posterior inferior cerebellar artery (PICA), the
anterior inferior cerebellar artery (AICA), and the
superior cerebellar artery. All supply the lateral aspects
of the brainstem en route to the cerebellum as the
circumferential branches. Small branches directly from
the vertebral and basilar arteries (a few are shown)
supply the medial structures of the brainstem, known as
paramedian arteries (further discussed with Figure 62).
The blood supply to the spinal cord is discussed with
Figure 72.
Clinical Aspects
The most common clinical lesion involving the cerebral
blood vessels is occlusion, often due to an embolus originating from the heart or the carotid bifurcation in the
neck. This results in infarction of the nervous tissue supplied by that branch and typically a sudden loss of function; the clinical deficit will depend upon where the
occlusion occurs.
If there were an occlusion of one of the major blood
vessels, it is possible that one of the major branches of
the Circle would be large enough to provide sufficient
blood to the area deprived. Usually this is not the case
when there is a sudden occlusion (e.g., an embolus), but,
given some time, these connecting channels may become
large enough to prevent the death of the part of the
brain deprived of blood.
Middle cerebral a.
Anterior comm. a.
Anterior cerebral a.
Internal carotid a.
Middle cerebral a.
Posterior comm. a.
Posterior cerebral a.
Basilar a.
Superior cerebellar a.
Posterior inferior
cerebellar a.
Vertebral a.
Anterior spinal a.
FIGURE 56: Blood Supply I — The Arterial Circle of Willis (Overlay)
FIGURE 57
BLOOD SUPPLY II
MR ANGIOGRAM (MRA)
Recent advances in technology have allowed for a visualization of the arterial system without injecting a radiopaque substance directly into an artery, a rather
invasive procedure with some risk. The image obtained
is called an MR angiogram (MRA). Although the quality
of such images cannot match the detail seen after an angiogram of select blood vessels, the noninvasive nature
of this procedure and the fact that the patient is not
exposed to any risk clearly establishes this investigation
as desirable to provide some information about the state
of the cerebral vasculature.
Figure 57 shows the arterial Circle of Willis as if looking
at the brain from below (as in Figure 56). The carotid
artery and its branches can be followed. The basilar
artery is seen at its termination, as it divides into the
posterior cerebral arteries. The communicating arteries
— anterior and posterior — can also be seen.
Clinical Aspects
One of the characteristic vascular lesions in the vicinity
of the arterial circle of Willis is a Berry aneurysm. It is
caused by a weakness of part of the artery wall, causing a
local ballooning of the artery. Often these rupture spontaneously, particularly if there is accompanying hypertension. This sudden rupture occurs into the
subarachnoid space and may also involve nervous tissue
of the base of the brain. The whole event is known as a
subarachnoid hemorrhage and must be considered
when one is faced clinically with an acute major cerebrovascular accident, CVA, without trauma.
An MRA will provide sufficient information if an
aneurysm is suspected, or when a screening examination
is necessary because of a positive family history of
cerebral aneurysm.
1. Carotid siphon
2. Middle cerebral a.
3. Anterior cerebral a.
4. Anterior communicating a.
5. Posterior communicating a.
6. Basilar a.
7. Posterior cerebral a.
FIGURE 57: Blood Supply II — MR Angiogram (MRA)
FIGURE 58
BLOOD SUPPLY III
CORTICAL: DORSOLATERAL
(OVERLAY)
Figure 58 shows the blood supply to the dorsolateral
aspect of the hemispheres; it has been created (with
Photoshop) by superimposing blood vessels on this view
of the brain (see Figure 12).
After coursing through the depths of the lateral fissure
(see Figure 56), the middle cerebral artery emerges and
breaks into a number of branches that supply different
parts of the dorsolateral cortex — the frontal, parietal,
and temporal areas of cortex. Each branch supplies a
different territory.
The branches of the middle cerebral artery extend
towards the midline sagittal fissure, where branches
from the other cerebral vessels are found, coming from
the medial aspect of the hemispheres (see Figure 59). A
zone, the arterial borderzone region (a watershed area),
remains between the various arterial territories. This
area is poorly perfused and prone to infarction, particularly if there is a loss of blood pressure (e.g., with cardiac
arrest or after a major hemorrhage).
Clinical Aspects
The most common clinical lesion involving these cerebral blood vessels is occlusion, often caused by an
embolus originating from the heart or the carotid bifurcation in the neck. This results in infarction of the
nervous tissue supplied by that branch; the clinical
deficit will depend upon which branches are involved.
For example, loss of sensory and/or motor function to
the arm and face region is seen after the blood vessel to
the central region is occluded. The type of language loss
depends upon the branch affected in the dominant
hemisphere: a motor deficit with problems in expression
is seen with a lesion affecting Broca’s area, whereas a
comprehension deficit is found with a lesion affecting
Wernicke’s area.
Recent studies indicate that the core of tissue that has
lost its blood supply is surrounded by a region that has a
marginal blood supply — the penumbra, as it is called.
In this penumbra, the blood supply is reduced below the
level of nervous tissue functionality and the area is
therefore “silent,” but the neurons are still viable and
might be rescued!
These studies have led to a rethinking of the therapy of
strokes:
• In the acute stage, if the patient can be seen quickly
and investigated immediately, the site of the lesion
might be identified. It is then possible for a neuroradiologist (with specialized training) to insert a catheter
into the artery and to inject a drug that could dissolve
the clot. If done soon enough after the “stroke,” it
might be possible to avert any clinical deficit!
• There may be an additional period beyond this initial
timeframe when damaged neurons in the penumbra
can be rescued through the use of neuroprotective
agents, specific pharmacological agents that protect
the neurons from the damaging consequences of loss
of blood supply.
Because strokes cause a loss of function and diminished
quality of life, and since our population is aging,
research on stroke and on how neurons die (e.g., by
apoptosis), and how this process could be arrested is one
of the most active areas of neuroscience research.
FIGURE 58: Blood Supply III — Cortical: Dorsolateral (Overlay)
FIGURE 59
BLOOD SUPPLY IV
CORTICAL: MEDIAL (OVERLAY)
In Figure 59, the blood supply to the medial aspect of
the hemispheres has been superimposed (using
Photoshop) onto this view of the brain (see Figure 16).
Two arteries supply this part: the anterior cerebral artery
and the posterior cerebral artery.
Anterior Cerebral Artery (ACA)
The anterior cerebral artery (ACA) is a branch of the
internal carotid artery (see Figures 56 and 57) from the
Circle of Willis. It runs in the interhemispheric fissure,
above the corpus callosum, and supplies the medial
aspects of the frontal lobe and the parietal lobe; this includes the cortical areas responsible for sensory-motor
function of the lower limb.
Clinical Aspects
The deficit most characteristic of an occlusion of the
ACA is selective loss of function of the lower limb.
Clinically, the control of micturition seems to be located
on this medial area of the brain, perhaps in the supplementary motor area (see Figure 51), and symptoms
related to bladder control may also occur.
Posterior Cerebral Artery (PCA)
The posterior cerebral artery (PCA), a branch of the
vertebro-basilar system, supplies the occipital lobe and
the visual areas of the cortex (area 17).
Clinical Aspects
The clinical deficit found after occlusion of this blood
vessel on one side is a loss of one-half of the visual field
of both eyes — a contralateral homonymous hemianopsia. (Note to student: This is an opportune time to
review the optic pathway and to review the visual field
deficits that are found after a lesion in different parts of
the system.)
Both sets of arteries have branches that spill over to the
dorsolateral surface. As noted in Figure 58, there is a gap
between these and the territory supplied by the middle
cerebral artery, known as the arterial borderzone region.
The visual cortex is supplied by the posterior cerebral
artery (from the vertebro-basilar system). Part of the
occipital pole, with the representation of the macular
area of vision, is supplied by the middle cerebral artery
(from the internal carotid system, see Figure 58). In
some fortunate cases, macular sparing is found after
occlusion of the posterior cerebral artery, presumably
because of the other blood supply to this area.
FIGURE 59: Blood Supply IV — Cortical: Medial (Overlay)
FIGURE 60
BLOOD SUPPLY V
INTERNAL CAPSULE AND
BASAL GANGLIA
In Figure 60, a coronal section of the brain (see Figures
29 and 78), the middle cerebral artery, is shown
schematically traversing the area of the lenticular
nucleus and the internal capsule. The artery begins as a
branch of the Circle of Willis (see Figure 56; also Figure
57). It then emerges after passing through the lateral
fissure to supply the dorsolateral cortex (see Figure 58).
One of the most important sets of branches of the
middle cerebral artery — within the lateral fissure —
is the group of arteries that supply much of the internal
structures of the hemispheres. These are known as the
striate arteries, also called lenticulo-striate arteries
(discussed with Figure 26; see also Figures 27 and 29).
These small blood vessels are the major source of blood
supply to the internal capsule and the adjacent portions
of the basal ganglia (head of caudate nucleus and
putamen), as well as the thalamus. Additional blood
supply to these structures comes directly from small
branches of the Circle of Willis (discussed with Figure
56) and from other blood vessels.
Clinical Aspects
These small caliber arteries are functionally different
from the cortical (cerebral) vessels. Firstly, they are endarteries and do not anastomose. Secondly, they react to a
chronic increase of blood pressure (hypertension) by a
necrosis of the muscular wall of the blood vessels, called
fibrinoid necrosis. Two possibilities follow:
• These blood vessels may occlude, causing small infarcts in the region of the internal capsule. As these
small infarcts resolve, they leave small “holes” called
lacunes (lakes), which can be visualized radiographically. Hence, they are known as lacunar infarcts, a
common form of a stroke. The extent of the clinical
deficit with this type of infarct depends upon its location and size in the internal capsule (see Figure 26). A
relatively small lesion may cause major motor and/or
sensory deficits on the contralateral side, resulting in a
devastating incapacity of the person, with contralateral paralysis. (Note: The student should review the
major ascending and descending tracts at this time
and their course through the internal capsule.)
• The other possibility is that these weakened blood
vessels can rupture, leading to hemorrhage deep in the
hemispheres. Although the blood supply to the white
matter of the brain is significantly less (because of the
lower metabolic demand), this nervous tissue is also
dependent upon a continuous supply of oxygen and
glucose. A loss of blood supply to the white matter
will result in the loss of the axons (and myelin) and
hence interruption of the transmission of information. This type of stroke may result in a more extensive clinical deficit, because the hemorrhage itself
causes a loss of brain tissue, as well as a loss of the
blood supply to areas distal to the site of the
hemorrhage.
Brain hemorrhage can be visualized by CT (computed
tomography; reviewed with Figure 17).
FIGURE 60: Blood Supply V — Internal Capsule and Basal Ganglia
FIGURE 61
THALAMUS
NUCLEI : HISTOLOGICAL
The thalamus is introduced initially in Section A
(Orientation) with a schematic perspective (see Figures
9 and 10). As noted in that section, there are two ways of
classifying the nuclei of the thalamus: functionally and
topographically (review text with Figure 10).
In the schematic diagram, the various nuclei of the
thalamus are arranged in a somewhat orderly manner.
In reality, however, sections taken at different planes
through the thalamus will show different nuclei of
varying size in a continuously changing configuration.
In the present diagram, the thalamus is being shown
from the histological perspective at three different planes
of section.
In the main diagram, the thalamus is opened in its
middle to show some of the nuclei, as well as the internal medullary lamina with its intralaminar nuclei, including the important centromedian nucleus (see Figure
51). This view is at the level of the ventral lateral nucleus
and the ventral posterolateral nucleus; a portion of the
ventral posteromedial nucleus is also seen (all specific
relay nuclei). The dorsomedial nucleus (an association
nucleus) is also visible. Exterior to the main thalamus is
the reticular nucleus, one of the nonspecific thalamic
nuclei that is part of the ascending reticular activating
system (discussed with Figure 40A).
Two other cuts have been made, one more anteriorly
and the other posteriorly, showing the configuration of
the different nuclei. The anterior cut includes the mammillary nucleus of the hypothalamus and the mammillothalamic tract that goes to the anterior nucleus of the
thalamus; this is an association nucleus (actually a group
of nuclei) which belongs to the limbic system. It is discussed in that section (see Figure 81A).
In the posterior cut, the section goes through the lateral
geniculate nucleus (the latter is a laminated nucleus; see
also Figures 39A and 39B), the medial geniculate
nucleus, and the pulvinar.
In addition, the diagram shows the relationship of the
thalamus with adjacent structures. Above the thalamus is
the body of the lateral ventricle (see Figure 20B), with
the choroid plexus intervening. At the lateral edge of the
ventricle is the body of the caudate nucleus and the stria
terminalis (see also Figure 36). Near the midline is the
fornix, just below the corpus callosum (see Figures 77A
and 80). Lateral to the thalamus is the internal capsule;
the posterior limb is seen in the anterior cut (see Figures
26 and 27) with the lentiform nucleus and the so-called
inferior limb, which is actually the auditory radiation
(see Figure 36) in the posterior cut.
Both cuts include also the temporal lobe, with the
inferior horn of the lateral ventricle and the tail of the
caudate nucleus in its upper aspect. Protruding into the
ventricle is the hippocampus proper and the dentate
gyrus (reviewed with the limbic system in Section D).
Stria terminalis
Anterior n.
Choroid plexus
Stria terminalis
Septum pellucidum
Reticular n.
Fornix
Lentiform nucleus
Corpus callosum
Internal capsule
(posterior limb)
Mammillo-thalamic tract
Ventral lateral n.
Fornix
Cistern
Third ventricle
Pulvinar
Mammillary n.
Ventral anterior n.
Internal medullary lamina
Lateral dorsal n.
Caudate nucleus
(tail)
Internal capsule
(inferior limb)
Dorsomedial n.
Hippocampus proper
Ventral lateral n.
Lateral ventricle (inferior horn)
Reticular n.
Ventral anterior n.
Intralaminar nuclei
Pulvinar
Ventral posterolateral n.
Ventral posteromedial n.
Centromedian n.
Medial geniculate body
Lateral geniculate body
FIGURE 61: Thalamus — Nuclei: Histological
FIGURE 62
BRAINSTEM: HISTOLOGY
B5 — CN VI, VII, and part of VIII, the
lowermost pons
• three through the medulla
B6 — CN VIII (some parts), the uppermost
medulla
VENTRAL VIEW: SCHEMATIC
Figure 62 uses the diagrammatic of the brainstem (see
Figure 3) which is similar to the photographic view of
the brainstem (see Figure 4). Study of the brainstem is
continued here by examining its histological neuroanatomy through a series of cross sections. Since it is
well beyond the scope of the nonspecialist to know all
the details, salient points have been selected:
• cranial nerve nuclei,
• ascending and descending tracts,
• specific brainstem nuclei that belong to the reticular
formation,
• and other select special nuclei.
As has been indicated, comprehending the attachment of
the cranial nerves to the brainstem is one of the keys to
understanding this part of the brain (see Figures 5 and
6). Wherever one sees a cranial nerve attached to the
brainstem, one knows that its nucleus, or some of its
nuclei, will be located at that level. Therefore, if one visually recalls or memorizes the attachment of the cranial
nerves to the brainstem, one has a key to its understanding. Conversely, in the clinical setting, knowledge of the
cranial nerve(s) involved is the main clue for localizing a
lesion in the brainstem.
Since the focus here is on the cranial nerves, only a
limited number of cross sections are studied. This
diagram shows the ventral view of the brainstem, with
the attached cranial nerves, and indicates the sections
that will be depicted in the subsequent series. The letter
“B” refers to the brainstem level.
Eight cross sections are taken at the following levels:
• two through the midbrain
B1 — CN III, upper midbrain
B2 — CN IV, lower midbrain
• three through the pons
B3 — uppermost pons (level for a special
nucleus; there is no cranial nerve attachment at
this level)
B4 — CN V (through the principal and motor
nuclei), mid-pons
B7 — CN IX, X, and XII, the mid-medullary
level
B8 — lowermost medulla, with some special
nuclei.
The information presented in this series should be sufficient for a student to recognize the clinical signs that
would accompany a lesion at a particular level, espcially
as they involve the cranial nerves. Such lesions would interrupt ascending and/or descending tracts, and this information would assist in localizing the lesion.
Note: The student using this Atlas should note the
following points:
1.
A small figurine of this view of the brainstem will
be shown with each cross-sectional level, with the
plane of section indicated.
2.
Reference to these cross sections is by either the
figure number or the cross-sectional level (e.g.,
B2).
3.
These cross-sectional levels are the ones shown
alongside the pathways in Section B – Functional
Systems.
Blood Supply
The vertebro-basilar system supplies the brainstem in
the following pattern (see Figures 13 and 56).
Penetrating branches from the basilar artery supply
nuclei and tracts which are adjacent to the midline.
These are known as the paramedian branches. The
lateral aspects of the brainstem, both tracts and nuclei,
are supplied by one of the cerebellar circumferential
arteries (posterior inferior, anterior inferior, superior).
Specific lesions are discussed with the cross-sectional
levels.
B1
B2
B3
B4
B5
B6
B7
B8
FIGURE 62: Brainstem: Histology — Ventral View: Schematic
FIGURE 63
BRAINSTEM: HISTOLOGY
SAGITTAL VIEW: SCHEMATIC
Figure 63 is a schematic of the brainstem from a midsagittal view (see Figure 16 for a photograph of the brain
from this view). This view is presented because it is one
that is commonly used to portray the brainstem. It
should be correlated with the ventral view shown in
Figure 62. This schematic is also shown in each of the
cross section diagrams, with the exact level indicated, in
order to orient the student to the plane of section
through the brainstem.
The location of some nuclei of the brainstem can be
easily visualized using this sagittal view, including the
red nucleus in the midbrain, the pontine nuclei which
form the bulging “pot-belly” of the pons, and the inferior olivary nucleus of the medulla. Some of the cranial
nerve attachments are shown as well, but not labeled.
other nuclei, including the red nucleus and the inferior olive, as well as the remaining tracts.
• The ventricular system (which has been represented
by stippling) is found throughout the brainstem (see
Figures 20A and 20B). Sections can be oriented according to the parts of the ventricular system that pass
through this region, namely the aqueduct in the midbrain region and the fourth ventricle (note shape)
lower down.
• The tectum — with the four colliculi — is located
behind (dorsal to) the aqueduct of the midbrain; likewise, the fourth ventricle separates the pons and
medulla from the cerebellum. The upper part of the
roof of the fourth ventricle is called the superior
medullary velum (see Figure 7). The location of the
choroid plexus in the inferior aspect of the roof of the
fourth ventricle is also shown (see also Figure 20A).
Note: The brainstem description starts from the midbrain and goes downwards to the medulla for two
reasons:
Using this orientation, one can approach the description
of the brainstem cross sections systematically:
1.
This order follows the numbering of the cranial
nerves, from above downwards.
• The most anterior portion of each area of the brainstem contains some representation of the descending
cortical fibers, specifically the cortico-bulbar, corticopontine, and cortico-spinal pathways (see Figures 42
and 43). In the midbrain, the cerebral peduncles
include all these axon systems. The cortico-bulbar
fibers are distributed to the various brainstem and
cranial nerve nuclei. In the pons, the cortico-pontine
fibers terminate in the pontine nuclei, which form the
bulge known as the pons proper; the cortico-spinal
fibers are dispersed amongst the pontine nuclei. In the
medulla, the cortico-spinal fibers form the pyramids.
The medulla ends at the point where these fibers decussate (see Figure 4).
2.
This sequence has been described for the fibers
descending from the cortex.
• The central portion of the brainstem is the tegmentum. The reticular formation occupies the core region
of the tegmentum (see Figures 40A and 40B). This
area contains virtually all the cranial nerve nuclei and
Others may prefer to start the description of the cross
sections from the medulla upwards.
Note to the student:
The various nuclei of the brainstem have been shaded
differently, so that sensory and motor nuclei can be distinguished. Cerebellar-related and vestibular-related
nuclei and tracts have been assigned other shadings.
This visual cataloging is maintained uniformly throughout the brainstem cross sections. The numbers beside
the various nuclei are there as guides to color-coding;
those students who wish to color the various nuclei
should assign a different color to each (as suggested in
the COLOR CODE, found on p. xviii ). The nuclei are
shown in color on the accompanying CD-ROM.
B1
B2
B3
B4
Choroid plexus
B5
B6
Foramen of Magendie
B7
B8
R = Red nucleus
P = Pontine nuclei
IO = Inferior olivary n.
FIGURE 63: Brainstem: Histology — Sagittal View: Schematic
THE MIDBRAIN
FIGURES 64 AND 65
The midbrain is the smallest of the three parts of the
brainstem. Often, it is not actually seen on an inferior
view of the brain because the temporal lobes of the
hemispheres tend to obscure its presence (see photograph of the inferior view of the brain, Figure 13).
The midbrain area is easily recognizable from the
anterior view in a dissected specimen of the brainstem
(see photograph in Figure 4). Most anteriorly is the
massive cerebral peduncles. The peduncles contain
axons that are a direct continuation of the fiber systems
of the internal capsule (see Figure 26). Within them are
found the pathways descending from the cerebral cortex
to the brainstem (cortico-bulbar), the cerebellum via the
pons (cortico-pontine), and the spinal cord (corticospinal tracts) (see also Figures 42 and 43).
When viewing a mid-sagittal section of the brain and
brainstem (Figure 16), one can easily identify the midbrain area as the part containing the cerebral aqueduct.
Posterior to the aqueduct are the two pair of colliculi,
which can also be seen on the dorsal view of the isolated
brainstem (Figure 7). The four nuclei together form the
tectal plate, or tectum. The superior colliculus is a subcortical center for certain visual reflexes. These nuclei
give rise to a fiber tract — the tecto-spinal tract — that
descends to the cervical spinal cord as part of the medial
longitudinal fasciculus (see Figure 49B). The system is
involved in the coordination of the movements of the
eyes with those of the head and neck, both of these responding to the visual image. The inferior colliculus is a
relay nucleus in the auditory pathway and is discussed
with this system (see Figures 35 and 36).
There are two special nuclei in the midbrain region —
the substantia nigra and the red nucleus, as well as the
superior colliculus and the pretectal region.
The substantia nigra is found throughout the midbrain
and is located behind the cerebral peduncles (see also
Figure 50). It derives its name from the dark melaninlike pigment found within its neurons in a freshly dissected specimen (see Figure 14; the nucleus has been
color-coded black). This nucleus is important in the regulation of movements. The substantia nigra is functionally part of the basal ganglia, with which it has
interconnections. In fact, it consists of two parts: the
pars compacta and the pars reticulata. The neurons of
the pars compacta produce dopamine, a distinct neurotransmitter. Loss of these neurons results in the clinical
entity, Parkinson’s disease (discussed with Figure 50).
The pars reticulata is one of the output nuclei of the
basal ganglia to the thalamus (the other portion is the
internal segment of the globus pallidus).
It is important to realize that this nuclear area, despite
its name, is clear (white) in most photographs in atlases
because the pigment is not retained when the tissue is
processed for sectioning. With myelin-type stains, the
area will appear “empty”; with cell stains, the neurons
will be visible. The pigment is not present in all species.
The red nucleus is located in the tegmentum, the inner
region of the brainstem. The label “red” is derived from
the fact that this nucleus has a reddish color in a freshly
dissected specimen, presumably due to its marked vascularity. (In the figure, this nucleus has been color-coded
red.) The red nucleus is found at the superior colliculus
level. It also gives rise to a fiber tract that descends to the
spinal cord, the rubro-spinal tract (see Figure 44 ). The
functional role of the red nucleus in humans is not
clearly known.
The substantia nigra, the red nucleus, and the superior
colliculus are all involved with more integrative aspects
of motor control.
The pretectal region is located in front of and somewhat
above the superior colliculus. This region is the center
for the reflex response of the pupil to light, the pupillary
light reflex (discussed with Figure 39B). The reflex requires input from receptors in the retina, and the output
occurs via the parasympathetic neurons in the EdingerWestphal nucleus (see Figure 5) and the oculomotor
nerve (CN III), with a synapse in the ciliary ganglion in
the orbit. Light shone on one eye will normally lead to a
rapid constriction of the pupil on the same side — the
direct response, and a similar response on the other side
— the consensual response. The connection of the responses of the two eyes is via commissural fibers (in the
posterior commissure, see Figure 49B). The pupillary
light reflex is one of the most important reflexes to test
clinically, particularly in head-injured and comatose patients, so knowledge of this pathway and the areas involved is essential.
The reticular formation is found in the core area of the
tegmentum. The reticular formation of the midbrain is
particularly important for the maintenance of consciousness. A rather special part of the reticular formation at this level is the periaqueductal gray surrounding
the aqueduct. This region forms part of the descending
control system for pain modulation (see Figure 41).
Some lesions might destroy much of the brainstem yet
leave the midbrain intact (e.g., a thrombosis of the
basilar artery), which might allow the patient to survive
in a rather tragic state known as locked-in syndrome.
Usually, all voluntary movements are gone, except for
some vertical eye movements; usually there is no loss of
sensation. The patient is left in a state of consciousness
with intellectual functions generally intact.
The midbrain is examined at two levels:
1.
the upper (rostral) one passes through the third
nerve nucleus and the superior colliculus;
2.
the lower (caudal) one is at the level of the IVth
nerve nucleus and the inferior colliculus, and the
decussation of the superior cerebellar peduncles.
FIGURE 64
UPPER MIDBRAIN
CROSS SECTION (B1)
In a cross-section of the midbrain, the most ventral (anterior) structure is the cerebral peduncle. Posterior to it
is the substantia nigra. The superior colliculus is located
dorsally, behind the aqueduct. The region surrounding
the aqueduct of the midbrain (the aqueduct of Sylvius)
is the periaqueductal gray. The remainder of the midbrain is the tegmentum, with nuclei and tracts.
Within the cerebral peduncles, the fiber systems are segregated: the cortico-bulbar and particularly the corticospinal pathways occupy its middle one-third area; the
outer and inner portions carry the cortico-pontine fibers
(see Figures 26, 42, 43, and 45).
The substantia nigra has two parts which are functionally quite distinct: the pars compacta and pars reticulata.
The pars reticulata lies adjacent to the cerebral peduncle
and contains some widely dispersed neurons; these
neurons connect the basal ganglia to the thalamus as
one of the output nuclei of the basal ganglia (similar to
the globus pallidus). The pars compacta is a cell-rich
region, located more dorsally, whose neurons contain
the melanin-like pigment (see Figure 14). These are the
dopaminergic neurons which project to the neostriatum
(discussed with Figure 50).
The oculomotor nucleus (CN III) is quite large and occupies the region in front of the periaqueductal gray,
near the midline; this is the typical location for all nuclei
that are efferent to somatic muscles. These motor
neurons are quite large in size and easily recognizable.
The parasympathetic portion of this nucleus is incorporated within it and is known as the Edinger-Westphal
(EW) nucleus. The fibers of CN III exit anteriorly
between the cerebral peduncles, in the interpeduncular
fossa.
The red nucleus is located within the tegmentum. With
a cell-type stain, one can discern the outline of the
nucleus; large neurons are typical of the ventral part of
the nucleus. With a section that has been stained for
myelin, the nucleus is seen as a clear zone. The fibers of
CN III exit through the medial portion of this nucleus.
The red nucleus gives origin to a descending pathway,
the rubro-spinal tract (see Figures 44 and 45) which is
involved in motor control. This pathway is not thought
to play as important a role in the overall functioning of
the motor system in humans, compared to other
mammals and higher apes.
The superior colliculus gives rise to a descending
pathway that is involved in the control of eye and neck
movements. Functionally these can be considered part of
the medial longitudinal fasciculus — the MLF (discussed with Figure 49B). In fact, this descending
pathway travels with the MLF throughout the brainstem
and upper spinal cord. The MLF stains heavily with a
myelin-type stain and is found anterior to the somatic
motor nucleus, next to the midline, at this level as well
as the other levels of the brainstem.
The ascending (sensory) tracts present in the midbrain
are a continuation of those present throughout the
brainstem. The medial lemniscus, the ascending trigeminal pathways, and the fibers of the anterolateral system
incorporated with them are on their way to the thalamus
(see Figure 34). These ascending pathways are located in
the outer part of the tegmentum, on their way to the
ventral posterolateral (VPL) and ventral posteromedial
(VPM) nuclei of the thalamus.
Also to be noted at this level is the brachium of the inferior colliculus, a part of the auditory pathway. This fiber
bundle connects the inferior colliculus to the medial
geniculate nucleus of the thalamus. It is situated close to
the surface on the dorsal aspect (see Figures 7 and 36).
The nuclei of the reticular formation are found in the
central region of the brainstem (the tegmentum); they
are functionally part of the ascending reticular activating
system and play a significant role in consciousness (discussed also with Figure 40A). The periaqueductal gray
surrounding the cerebral aqueduct is involved with pain
and also with a descending pathway for the modulation
of pain (see Figure 41).
FIGURE 64: Upper Midbrain — Cross Section (B1)
FIGURE 65
LOWER MIDBRAIN
CROSS SECTION (B2)
The cerebral peduncles are still located anteriorly. Often,
the section is done at the level of the lowermost midbrain and includes the beginning of the pontine nuclei.
Therefore, one may see a somewhat confusing mixture
of structures. The substantia nigra is located immediately behind the fibers of the cerebral peduncle. The
unique feature in the lower midbrain is the decussation
(crossing) of the superior cerebellar peduncles. Dorsal to
the cerebral aqueduct is the inferior colliculus.
The nucleus of CN IV, the trochlear nucleus, is located
in front of the periaqueductal gray, next to the midline.
Because it supplies only one extra-ocular muscle, it is a
smaller nucleus than the oculomotor. CN IV heads dorsally and exits from the brainstem below the inferior colliculus (see Figure 7), on the posterior aspect of the
brainstem. The MLF lies just anterior to the trochlear
nucleus.
The medial lemniscus, the trigeminal fibers, and the anterolateral fibers (system) are situated at the lateral edge
of the tegmentum (see Figures 34 and 38). In fact, they
are found at this level at the surface of the midbrain. In
very select cases, particularly with cancer patients who
are suffering from intractable pain, it is possible to surgically sever the sensory ascending pathways at this level.
Obviously, this is a rather dangerous and difficult neurosurgical procedure; today it would be considered only as
a measure of last resort. Pain control is usually accomplished through the use of drugs, often in a pain clinic.
However, infarcts of this area would interrupt all the ascending sensory fibers from the head and body of the
opposite side.
In sections through the lower levels of the midbrain,
there is a brief appearance of a massive fiber system (as
seen with a myelin-type stain) occupying the central
region of the midbrain, the decussation of the superior
cerebellar peduncles. These fibers are the continuation
of the superior cerebellar peduncles found in the pons,
which are crossing (decussating) at this level. The fibers
are coming from the deep cerebellar nuclei (the intracerebellar nuclei), mainly the dentate nucleus, and are
headed for the ventral lateral (VL) nucleus of the thalamus, and then to the motor cortex (discussed with
Figures 38 and 55 ). Some of the fibers that come from
the intermediate nucleus synapse in the red nucleus.
The nuclei of the reticular formation of this level are in
the central region of the brainstem (the tegmentum);
they are functionally part of the ascending reticular activating system and play a significant role in consciousness. The periaqueductal gray surrounding the cerebral
aqueduct is involved with pain and also with a descending pathway for the modulation of pain (see Figure 41).
Some unusually large round cells are often seen at the
edges of the periaqueductal gray; these cells are part of
the mesencephalic nucleus of the trigeminal (CN V)
nerve (see Figure 6). Between the cerebral peduncles is a
small nucleus, the interpeduncular nucleus, which
belongs with the limbic system.
The inferior colliculus is a relay nucleus in the auditory
pathway (see Figures 35 and 36). The ascending auditory
fibers, the lateral lemniscus, are still present at this level
and are often seen terminating in this nucleus. After
synapsing here, the fibers relay to the medial geniculate
nucleus via the brachium of the inferior colliculus, seen
at the upper midbrain level (Figure 64).
FIGURE 65: Lower Midbrain — Cross Section (B2)
THE PONS
FIGURES 66, 67, AND 68
The pons, medulla, and cerebellum form the so-called
hindbrain. These structures are located in the posterior
cranial fossa of the skull, along with the fourth ventricle.
This ventricle separates the pons and medulla anteriorly
from the cerebellum posteriorly (see Figures 16 and 20A).
The pons is characterized by its protruding anterior
portion, the pons proper. This consists of a large group
of nuclei, the pontine nuclei. The cortico-pontine fibers,
which have descended through the cerebral peduncles,
terminate in these nuclei. From here, they are relayed to
the cerebellum (see Figure 53) via the middle cerebellar
peduncle. The pons (proper) forms a bridge-like
structure between these two prominent middle
cerebellar peduncles.
Also found intermingled with the pontine nuclei are the
fiber bundles that belong to the cortico-spinal system.
These continue through this region (without synapsing)
and emerge at the medullary level to form the pyramids
(see Figure 42).
Behind the pons proper is the tegmentum, the region of
the brainstem that contains the cranial nerve nuclei,
most of the ascending and descending tracts, and the
nuclei of the reticular formation. The cranial nerves attached to the pons include the trigeminal (CN V), the
abducens (CN VI), the facial (CN VII), and part of CN
VIII (the vestibulo-cochlear). The various nuclei of
these cranial nerves are located within the pontine
tegmentum.
CN V:
Parts of the trigeminal nerve nuclei are found
at all levels of the pons (see Figures 6 and 33).
The mid-pontine section is taken at the level of
the attachment of CN V, and both the principal
sensory nucleus and the motor nucleus are
found at this cross-sectional level.
CN VI: CN VI is a typical somatic motor nucleus, innervating the lateral rectus muscle of the eye.
The nucleus is located in the lowermost pons
and its fibers exit anteriorly and at a slightly
lower level, at the junction between the pons
and medulla (see Figures 4 and 5).
CN VII: CN VII has a most unusual course within the
brainstem. The fibers of VII form an internal
loop over the abducens nucleus (see Figure 45;
also note the facial colliculus in Figure 7); they
ascend, loop, and then descend to exit laterally
at the junction between the pons and the cerebellum, the cerebello-pontine angle.
CN VIII: The fibers of the cochlear and vestibular divisions of CN VIII enter the brainstem adjacent
to CN VII, at the cerebello-pontine angle (see
Figure 3).
• Cochlear portion — The auditory fibers synapse in
the dorsal and ventral cochlear nuclei, which can be
seen in the medulla in a section just below this level
(review with the auditory system, Figures 35 and
36). After this, there will be a synapse in the nuclear
group called the superior olivary complex, which is
found in the lowermost pons. Some of the fibers
cross the midline before synapsing, and some cross
after synapsing. The crossing fibers form a structure
known as the trapezoid body. After one or more
synapses, the fibers then ascend and, in so doing,
form a new tract, the lateral lemniscus which actually commences at this level.
• Vestibular portion — The vestibular nuclei are
found in the lowermost pontine region and at the
upper levels of the medulla (see Figures 6 and 49A).
The lateral vestibular nucleus gives rise to the lateral
vestibulo-spinal tract (see Figure 48). The medial
and superior vestibular nuclei contribute fibers to
the medial longitudinal fasciculus (MLF, discussed
with Figure 49B), relating the vestibular sensory information to eye movements.
A not uncommon tumor, called an acoustic neuroma,
can occur along the course of the acoustic nerve, usually
at the cerebello-pontine angle. This is a slow-growing
benign tumor, composed of Schwann cells, the cell responsible for myelin in the peripheral nervous system.
Initially, there will be a complaint of hearing loss, and/or
perhaps a ringing noise in the ear (called tinnitus).
Because of its location, as the tumor grows it begins to
compress the adjacent nerves (including VII). Eventually,
if left unattended, there are additional symptoms due to
further compression of the brainstem. Detection of this
tumor has been made much easier with modern
imaging techniques. Surgical removal, though, still requires considerable skill so as not to damage adjacent
structures, including CN VII.
The ascending tracts present in the tegmentum are those
conveying sensory information from the body and face.
These include the medial lemniscus and the anterolateral
fibers (system). The medial lemniscus shifts its position
in its course through the brainstem (see Figure 38),
moving from a central to a lateral position. The fibers of
the trigeminal system that have crossed in the pons (discriminative touch from the principal nucleus of V) and
those of pain and temperature (from the descending
nucleus of V) that crossed in the medulla join together
in the upper pons with the medial lemniscus. The anterolateral system, which is too small to be identified,
also eventually becomes incorporated with the medial
lemniscus (see Figure 38) in the mid and upper pons.
The lateral lemniscus (auditory) also is located in the
tegmentum.
One of the distinctive nuclei of the pons is the locus
ceruleus, a pigment-containing nucleus located in the
upper pontine region and discussed with Figure 66.
The nuclei of the reticular formation of the pons have
their typical location in the tegmentum (see Figures 40A
and 40B). These nuclei play a particular role in the
voluntary control of movements of the proximal joints
and axial musculature, as well as the regulation of
muscle tone (discussed with Figures 46 and 47).
The fourth ventricle begins in the pontine region (see
Figure 20A). It starts as a widening of the aqueduct then
continues to enlarge so that it is widest at about the level
of the junction between the pons and medulla. There is
no pontine nucleus dorsal to the fourth ventricle. The
cerebellum is located above (posterior to) the roof of the
ventricle. It is at the level of the mid- and lower pontine
cross sections that one finds the deep cerebellar nuclei
(presented in Figures 54A and 54B).
The pons is represented and discussed in three sections:
• Uppermost pons — This has been taken at the level of
a distinctive nucleus, the locus ceruleus. There are features here that are important in making the transition
between the pons and the midbrain.
• Mid (middle) pons — This is at the level of the attachment of the trigeminal nerve. It includes the
massive middle cerebellar peduncles.
• Lowermost pons — This cross section is taken just
above the junction with the medulla. This lowermost
level is one of the most complex sections of the brainstem, because it has the nuclei of cranial nerves VI,
VII, and parts of both divisions of CN VIII.
FIGURE 66
UPPER PONS
CROSS SECTION (B3)
The upper pons is presented mainly to allow an understanding of the transition of midbrain to pons. This particular section is taken at the uppermost pontine level,
where the trochlear nerve, CN IV, exits (below the
inferior colliculus; see Figure 7). It is the only cranial
nerve that exits posteriorly; its fibers cross (decussate)
before exiting.
Anteriorly, the pontine nuclei are beginning to be
found. Cortico-pontine fibers terminate in the pontine
nuclei. From these cells, a new tract is formed which
crosses and projects to the cerebellum, the middle cerebellar peduncle. The cortico-spinal fibers become dispersed between these nuclei and course in bundles
between them (without synapsing).
Centrally, the cerebral aqueduct is beginning to enlarge,
becoming, by definition, the fourth ventricle. The MLF
is found in its typical location ventral to the fourth ventricle, next to the midline. Nuclei of the reticular formation are present as they are throughout the brainstem.
The ascending tracts include the lateral lemniscus (auditory), the medial lemniscus and anterolateral system (somatosensory from the body), and the ascending
trigeminal fibers (see Figure 34). The auditory fibers are
located dorsally, just before terminating in the inferior
colliculus (in the lower midbrain, which is just above
this level).
The medial lemniscus, with the inclusion of fibers of the
anterolateral system, as well as most of the ascending
trigeminal fibers, is located midway between its more
central position inferiorly and the lateral position found
in the midbrain (see Figure 38). In sections stained for
myelin, it has a typical “comma-shaped” configuration.
There is a rather special nucleus, the locus ceruleus,
located at this level. The nucleus derives its name from
its bluish color in fresh specimens. (It has therefore been
color-coded blue.) The locus ceruleus is considered part
of the reticular formation (as discussed with Figure
40B). This nucleus is unique because of its widespread
connections with virtually all parts of the brain and
because it has noradrenaline as its catecholamine neurotransmitter substance. The nucleus is located in the
dorsal part of the tegmentum, not too far from the edges
of the fourth ventricle.
Nearby are some of the very large neurons belonging to
the mesencephalic nucleus of the trigeminal (see Figure
6). This small cluster of cells might not be found in
every cross section of this particular region.
The superior cerebellar peduncle is found within the
tegmentum of the pons. These fibers carry information
from the cerebellum to the red nucleus and the thalamus. The fibers, which are the axons from the deep cerebellar nuclei, leave the cerebellum and course in the roof
of the fourth ventricle (superior medullary velum; see
Figures 7, 38, and 55). They then enter the pontine
region and move towards the midline, finally decussating
in the lower midbrain (see cross section B2, Figure 65).
The uppermost part of the cerebellum is found at this
level. One of the parts of the vermis, the midline portion
of the cerebellum, is identified — the lingula. This particular lobule is a useful landmark in the study of the
cerebellum and is identified in the discussion of
anatomy of the cerebellum in Figure 52.
FIGURE 66: Upper Pons — Cross Section (B3)
FIGURE 67
MID PONS
CROSS SECTION (B4)
The cross section presented in Figure 67 is taken
through the level of the attachment of the trigeminal
nerve. Anteriorly, the pontine nuclei and the bundles of
cortico-spinal fibers are easily recognized. The pontine
cells (nuclei), whose axons cross and then become the
middle cerebellar peduncle, are particularly numerous at
this level. The cortico-spinal fibers are also very dispersed at this level.
The trigeminal nerve is attached along the course of the
middle cerebellar peduncle. The CN V nerve has several
nuclei with different functions (see Figures 5, 6, and 33).
This level contains only two of its four nuclei: the principal (or main) sensory nucleus and the motor nucleus.
The principal sensory nucleus subserves discriminative
(e.g., two-point) touch sensation and accounts for the
majority of fibers. The motor nucleus supplies the
muscles of mastication and sometimes exits as a separate
nerve, alongside the major sensory root. Within the
pons, these nuclei are separated by the fibers of CN V;
the sensory nucleus (with smaller cells) is found more
laterally, and the motor nucleus (with larger cells) more
medially.
The ascending fiber systems are easily located at this
cross-sectional level. The medial lemniscus has moved
away from the midline, as it ascends (see Figure 38).
The anterolateral fiber system becomes associated with it
by this level. The lateral lemniscus is seen as a distinct
tract, lying just lateral to the medial lemniscus. The MLF
is found in its typical location anterior to the fourth
ventricle.
The core area of the tegmentum is occupied by the
nuclei of the reticular formation. Some of the nuclei
here are called the oral portion of the pontine reticular
formation (see Figure 46). This “nucleus” contributes
fibers to a descending reticulo-spinal tract that is involved in motor control and plays a major role in the
regulation of muscle tone (discussed with Figures 46
and 47).
The fourth ventricle is quite wide at this level. At its
edges are found the superior cerebellar peduncles,
exiting from the cerebellum and heading towards the
midbrain (red nucleus) and thalamus. The thin sheet of
white matter that connects these peduncles is called the
superior medullary velum. These peduncles and the superior medullary velum can be located in a specimen
(such as the one shown in Figure 7), a dorsal view of the
isolated brainstem. These structures are found dorsally
just below the exiting fibers of CN IV.
The cerebellum, which is quite large at this level, is situated behind the ventricle. The lingula of the cerebellum
is again labeled and is seen sometimes actually intruding
into the ventricular space.
FIGURE 67: Mid Pons — Cross Section (B4)
FIGURE 68
LOWER PONS
CROSS SECTION (B5)
This cross section of the lowermost pons is very
complex because of the number of nuclei related to the
cranial nerves located in the tegmental portion, including CN V, VI, VII, and VIII. In this section some of the
tracts are shifting in position or forming. Anteriorly, the
pontine nuclei have all but disappeared, and the fibers of
the cortico-spinal tract are regrouping into a more
compact bundle that will become the pyramids in the
medulla (below).
Cranial Nerve V
The fibers of the trigeminal nerve carrying pain and
temperature, which entered at the mid-pontine level,
descend into the lower pons and continue through the
medulla into the upper level of the spinal cord (see
Figure 33). This pathway is the descending trigeminal
tract, also called the spinal tract of V because it reaches
to the level of the spinal cord.
Cranial Nerve VI
The abducens nucleus is a somatic motor nucleus to the
lateral rectus muscle of the eye. It is located (as expected) in front of the ventricular system. The MLF, also as
usual, is found just anteriorly. Some of the exiting fibers
of CN VI may be seen at this level. The nerve emerges
anteriorly at the junction of pons and medulla (see
Figures 4 and 5).
Cranial Nerve VII
The facial nerve nucleus is located in the ventrolateral
portion of the tegmentum, where the branchiomotor
nucleus is supposed to be located. The motor cells (to
the muscles of facial expression) are large. (The
parasympathetic portion of this nucleus is rarely identifiable.) As explained previously, the fibers of CN VII
form an internal loop (see Figure 45). It is common to
see only parts of the course of this nerve on any one
section through this level of the pons, and one must
know the course of the nerve to be able to identify it.
The diagram is drawn as if the whole course of this
nerve were present in a single section.
Cranial Nerve VIII: Cochlear Division
CN VIII enters the brainstem slightly lower, at the
ponto-cerebellar angle (see Figures 3 and 4). The two
distinctive parts of this nerve at this level are the crossing fibers, which form the so-called trapezoid body, and
the superior olivary complex. The superior olivary
complex includes a group of nuclei that subserve the
function of sound localization; they also give rise to a
unique bundle of fibers, the olivo-cochlear bundle,
which goes from the CNS to the outer hair cells of the
cochlea (discussed with Figure 35).
Cranial Nerve VIII: Vestibular Division
Of the four vestibular nuclei (see Figures 6 and 49A),
three are found at this level. The lateral vestibular
nucleus, with its giant-sized cells, is located at the lateral
edge of the fourth ventricle. The large neurons are dispersed through the nuclear area. This nucleus gives rise
to the lateral vestibulo-spinal tract (see Figure 48). The
medial vestibular nucleus is also present at this level, an
extension from the medullary region. There is also a
small superior vestibular nucleus in this region. The
latter two nuclei contribute fibers to the MLF, relating
the vestibular sensory information to eye movements
(discussed with Figure 49B).
The tegmentum of the pons also includes the ascending
sensory tracts and the reticular formation. The medial
lemniscus is often somewhat obscured by the fibers of
the trapezoid body. It is situated close to the midline but
here has changed its orientation from that seen in the
medullary region (see Figure 38 and cross sections of the
medulla). The anterolateral system is too small to be
identified. The nuclei of the reticular formation include
the caudal portion of the pontine reticular formation
which also contributes to the pontine reticulo-spinal
tract (see Figure 46).
The fourth ventricle is very large but often seems smaller
because the lobule of the cerebellar vermis, called the
nodulus (part of the flocculonodular lobe; refer to
Figure 52), impinges upon its space.
The intracerebellar (deep cerebellar) nuclei are also
found at this cross-sectional level. They are located
within the white matter of the cerebellum (discussed
with Figures 54A and 54B). Usually only the most lateral
and largest nucleus, the dentate, can be identified.
FIGURE 68: Lower Pons — Cross Section (B5)
THE MEDULLA
FIGURES 69, 70, AND 71
This part of the brainstem has a different appearance
from the midbrain and pons because of the presence of
two new distinct structures — the pyramids and the
inferior olivary nucleus:
• The pyramids are an elevated pair of structures
located on either side of the midline (see Figures 3
and 4). They contain the cortico-spinal fibers that
have descended from the motor (and sensory) areas of
the cortex, funneled via the internal capsule (posterior
limb), and then continued through the cerebral peduncles of the midbrain and the pontine region, and
now emerge as a distinct bundle (see Figure 42). The
cortico-spinal tract is in fact often called the pyramidal tract because its fibers form the pyramids. Most of
its fibers cross (decussate) at the lowermost part of the
medulla (see Figure 4).
• The inferior olive is a prominent nuclear structure
that has a distinct scalloped profile when seen in cross
section. It is so large that it forms a prominent bulge
on the lateral surface of the medulla. Its fibers are distributed to the cerebellum (discussed with Figure 53).
The tegmentum is the area of the medulla that contains
the inferior olivary nucleus, the cranial nerve nuclei, and
the nuclei of the reticular formation. Cranial nerves
VIII, IX, X (including cranial part of XI), and XII are attached to the medulla and have their nuclei here. The
most prominent nucleus of the reticular formation in
this region has very large cells and is called the nucleus
gigantocellularis (see Figure 40B).
Also included in the tegmentum are the various tracts.
The ascending fibers include the large bundle, the
medial lemniscus, carrying discriminative touch sensation, joint position, and the “sense” of vibration from
the body (see Figure 32). This tract is formed in the lowermost medulla from the dorsal column nuclei (cuneate
and gracile; see Figure 38), and the fibers cross and
ascend. The anterolateral system, a smaller tract carrying
pain and temperature information from the body, is not
usually identifiable in the cross sections, but knowledge
of its location is important for vascular lesions that
occur at this level. The spinal trigeminal tract (and
nucleus), conveying the modalities of pain and temperature from the face and teeth, is also found throughout
the medulla (see Figure 33). One of the most important
sensory systems in the medulla is the solitary nucleus
and tract, which subserves both taste and visceral afferents (discussed with Figure 6). The MLF is still a distinct
tract in its usual location (see Figure 49B).
The fourth ventricle lies behind the tegmentum, separating the medulla from the cerebellum. The ventricle
tapers and becomes quite narrow in the lowest part of
the medulla (see Figures 20A and 20B); eventually it is
continuous with the central canal of the spinal cord. The
roof of this lower part of the ventricle has choroid
plexus (see Figure 63). CSF escapes from the fourth ventricle via the various foramina located here, and flows
into the subarachnoid space (discussed with Figure 21).
The medulla is represented by three sections:
• the uppermost section typically includes the CN VIII
nerve (both parts) and its nuclei;
• the section through the middle of the medulla is at the
mid-olivary level and includes the nuclei of cranial
nerves IX, X, and XII;
• the lowermost section is at the level of the dorsal
column nuclei, the nuclei gracilis and cuneatus, and
the decussating fibers of the medial lemniscus.
Clinical Aspects
Vascular lesions in this area of the brainstem are not uncommon. The midline area is supplied by the paramedian branches from the vertebral artery (see Figure 56).
The structures included in this territory are the corticospinal fibers, the medial lemniscus, and the hypoglossal
nerve and its nucleus. The lateral portion is supplied by
the posterior inferior cerebellar artery, a branch of the
vertebral artery (see Figure 56), often called PICA by the
neuroradiologists. This artery is apparently quite prone
to infarction, for some unknown reason. Included in its
territory are the cranial nerve nuclei and fibers of IX and
X, the descending trigeminal nucleus and tract, fibers of
the anterolateral system, and the solitary nucleus and
tract, as well as descending autonomic fibers. The whole
clinical picture is called the lateral medullary syndrome
(Wallenberg syndrome).
Interruption of the descending autonomic fibers gives
rise to a clinical condition called Horner’s syndrome in
which there is loss of the autonomic sympathetic supply
to one side of the face, ipsilaterally. This leads to drooping of the (upper) eyelid, dry skin, and constriction of
the pupil. The pupillary change is due to the competing
influences of the parasympathetic fibers, which are still
intact. Lesions elsewhere can also give rise to a Horner’s
syndrome.
It is instructive for a student to work out the clinical
symptomatology that would be seen following a vascular lesion affecting each of these branches (medial vs.
lateral).
FIGURE 69
UPPER MEDULLA
CROSS SECTION (B6)
This cross section has the characteristic features of the
medullary region, namely the pyramids anteriorly with
some remaining parts of the inferior olivary nucleus
situated just behind.
The medial lemniscus is the most prominent ascending
(sensory) tract throughout the medulla. The tracts are
located next to the midline, oriented in the anteroposterior (ventrodorsal) direction, just behind the pyramids
(see Figure 38). Dorsal to them, also along the midline,
are the paired tracts of the MLF, situated in front of the
fourth ventricle. The anterolateral tract lies dorsal to the
olive, although it is not of sufficient size to be clearly
identified. Both the medial lemniscus and the anterolateral system are carrying fibers from the other side of the
body at this level.
The descending tract of CN V is present more laterally,
carrying fibers (pain and temperature) from the ipsilateral face (see Figure 33). Medial to this tract, along its
full extent, is a corresponding nucleus which is called by
the same name. The descending fibers synapse in this
nucleus, cross, and then ascend (see Figure 38) eventually joining the medial lemniscus in the upper pons.
The eighth nerve enters the medulla at its uppermost
level, at the cerebello-pontine angle, passing over the inferior cerebellar peduncle. The nerve has two nuclei
along its course, the ventral and dorsal cochlear nuclei.
The auditory fibers synapse in these nuclei and then go
on to the superior olivary complex in the lower pons
region. The crossing fibers are seen in the lowermost
pontine region as the trapezoid body (see Figures 35
and 36, also Figure 68).
The vestibular part of the eighth nerve is represented at
this level by two nuclei, the medial and inferior vestibular nuclei. Both these nuclei lie in the same position as
the vestibular nuclei in the pontine section, adjacent to
the lateral edge of the fourth ventricle. The inferior
vestibular nucleus is rather distinct because of the many
axon bundles that course through it. The vestibular
nuclei contribute fibers to the MLF (discussed with
Figure 49B).
The solitary nucleus is found at this level, surrounding a
tract of the same name. This nucleus is the synaptic
station for incoming taste fibers and for visceral afferents entering with CN IX and X. The solitary nucleus
and tract are situated just beside (anterior to) the
vestibular nuclei.
The core area is occupied by the cells of the reticular formation. The most prominent of its nuclei at this level is
the gigantocellular nucleus, which gives rise to the
medullary (lateral) reticulo-spinal tract (see Figure 47).
The other functional aspects of the reticular formation
should be reviewed (discussed with Figures 40A and
40B).
The other prominent tract in the medullary region is the
inferior cerebellar peduncle (see Figure 4). This tract is
conveying fibers to the cerebellum, both from the spinal
cord and the medulla, including the inferior olivary
nucleus (discussed with Figure 53).
The fourth ventricle is still quite large at this level. Its
roof (lower portion) has choroid plexus (see Figure 63).
Behind the ventricle is the cerebellum, with the vermis
(midline) portion; the lateral lobule that is present at
this juncture is the cerebellar tonsil (see Figures 4 and 13).
FIGURE 69: Upper Medulla: Cross Section (B6)
FIGURE 70
MID MEDULLA
CROSS SECTION (B7)
This is a classic level for descriptive purposes. The inferior olive and pyramids are easily recognized. Cranial
nerves IX, X, and XII are attached to the medulla at this
level with the many nuclei associated with these nerves.
The hypoglossal nucleus (CN XII) is a somatic motor
nucleus occupying the same location — near the
midline, and in front of the ventricle — as the nuclei of
CN III, IV, and VI. The fibers of CN XII exit anteriorly,
between the pyramid and the olive (see Figures 3 and 4).
The MLF lies in front of the nucleus of CN XII, and the
medial lemniscus lies in front of that, both situated adjacent to the midline. Vascular lesions of the brainstem in
this region (supplied by the small paramedian arteries
— see Figure 56) might involve all of these structures,
namely the pyramids, the medial lemniscus, and CN XII.
(The student should try to work out the clinical deficits
that would be found following a lesion of this nature.)
The other ascending sensory systems are found in the
lateral aspect of the medulla. The fibers of the anterolateral system are situated dorsal to the olive. The descending nucleus and tract of the trigeminal system have the
same location in the lateral aspect of the tegmentum.
Therefore, a lesion here (e.g., occlusion of the posterior
inferior cerebellar artery) will produce a different
pattern of sensory loss (as discussed above).
CN IX (glossopharyngeal) and CN X (vagus) are attached at the lateral aspect of the medulla (see Figures 3
and 7). Their efferent fibers are derived from two nuclei:
the dorsal motor nucleus, which is parasympathetic, and
the nucleus ambiguus, which is branchiomotor (see
Figure 5). The dorsal motor nucleus lies adjacent to the
fourth ventricle just lateral to the nucleus of XII. The
nucleus ambiguus lies dorsal to the olivary nucleus; in a
single cross section only a few cells of this nucleus are
usually seen, making the identification of this nucleus
difficult (i.e., “ambiguous”) in actual sections. The taste
and visceral afferents that are carried in these nerves
synapse in the solitary nucleus which is located in the
posterior aspect of the tegmentum.
The other nuclei in this lateral region of the medulla are
sometimes difficult to sort out. Sometimes the vestibular
nuclei are present at this level. In some sections the accessory cuneate nucleus may be found. This nucleus is a
relay for some of the cerebellar afferents from the upper
extremity (see Figure 53). The fibers then go to the cerebellum via the inferior cerebellar peduncle, which is
found along the dorsal margin of the medulla (see
Figure 4).
The reticular formation occupies the central core of the
tegmentum, as usual. Some large cells are present at this
level, and these are said to form a “nucleus,” the nucleus
gigantocellularis, in this part of the reticular formation
(see Figure 40B). These cells give rise to a descending
tract, the lateral reticulo-spinal tract (see Figure 47) and
are involved in the voluntary control of proximal joints
as well as with the regulation of muscle tone.
The fourth ventricle is still a rather large space, behind
the tegmentum, with the choroid plexus attached to its
roof (in this area). Very often these structures are absent
in sections taken at this level and the ventricle appears
“open.” There is no cerebellar tissue posteriorly since the
section is below the level of the cerebellum (see the
schematic accompanying this figure).
FIGURE 70: Mid Medulla — Cross Section (B7)
FIGURE 71
LOWER MEDULLA
CROSS SECTION (B8)
The medulla seems significantly smaller in size at this
level, approaching the size of the spinal cord below. The
section is still easily recognized as medullary because of
the presence of the pyramids and the inferior olivary
nucleus.
The dorsal aspect of the medullary tegmentum is occupied by two large nuclei: the nucleus cuneatus (cuneate
nucleus, lateral) and the nucleus gracilis (gracile
nucleus, more medial). These are found on the dorsal
aspect of the medulla (see Figures 7 and 38). These
nuclei are the synaptic terminations of the tracts of the
same name that have ascended the spinal cord (see
Figure 32). The fibers relay in these nuclei and then
move anteriorly to form the medial lemniscus. In so
doing, they pass through the tegmentum and are seen as
a stream of axons called the internal arcuate fibers.
These axons actually cross the midline (decussate) to
form the medial lemniscus of the other side. At this
level, the medial lemniscus is situated between the
olivary nuclei and dorsal to the pyramids and is oriented
anteroposteriorly.
The nuclei of CN X as well as CN XII are present as
before, as is the descending nucleus and tract of V (CN
XI is discussed with Figure 3). The MLF and anterolateral fibers are in the same position. The solitary tract and
nucleus are also still found in the same location. The internal arcuate fibers may obscure the exact location of
the nucleus ambiguus. Finally, the reticular formation is
still present.
The accessory cuneate nucleus would definitely be found
at this level (as discussed with Figure 70). The inferior
cerebellar peduncle has not yet been formed.
Cross sections through the lowermost part of the
medulla may include the decussating cortico-spinal
fibers, i.e., the pyramidal decussation (seen in Figure 4).
This would therefore alter significantly the appearance
of the structures in the actual section.
Posteriorly, the fourth ventricle is tapering in size, giving
a “V-shaped” appearance to the dorsal aspect of the
medulla (see Figure 7). It is usual for the ventricle roof
to be absent at this level. This is likely accounted for by
the presence of the foramen of Magendie, where the CSF
escapes from the ventricular system into the subarachnoid space (see Figure 20A). Posterior to this area is the
cerebello-medullary cistern, otherwise known as the
cisterna magna (see Figure 20A; also Figure 63 — not
labeled).
One special nucleus is found in the “floor” of the ventricle at this level, the area postrema. This forms a little
bulge that can be appreciated on some sections. The
nucleus is part of the system that controls vomiting, and
it is often referred to as the vomiting “center.” It is interesting to note that this region lacks a blood-brain
barrier, allowing this particular nucleus to be “exposed”
directly to whatever is circulating in the blood stream.
FIGURE 71: Lower Medulla — Cross Section (B8)
FIGURE 72
SPINAL CORD TRACTS
C8 LEVEL
The major tracts of the spinal cord are shown on this
diagram, the descending tracts on the left side and the
ascending ones on the right side. (Note the different
shading intensity of the two systems; the cerebellarrelated tracts have a different shading.) In fact, both sets
are present on both sides. The functional aspects of each
of these tracts should be reviewed by noting the loss of
function that would be found following a lesion of the
various pathways.
tive touch sensation, joint position and vibration.
The somatotopic arrangement of the fibers should be
reviewed.
• Anterolateral system, consisting of the anterior
(ventral) spino-thalamic and lateral spino-thalamic
tracts (see Figure 31) — These pathways carry pain
and temperature, as well as crude touch information.
There is also a somatotopic arrangement of fibers in
this system.
• Spino-cerebellar tracts, anterior (ventral) and posterior (dorsal); (see Figure 49B, and Figure 53) — These
tracts convey information from the muscle spindles as
part of the “comparator” function of the cerebellum.
Descending tracts:
Special tract:
• Lateral cortico-spinal, from the cerebral (motor)
cortex — This pathway crosses in the lowermost
medulla (see Figure 42). These fibers supply mainly
the lateral motor neurons that control fine motor
movements of the hand and fingers.
The dorsolateral fasciculus, better known as the tract of
Lissauer, carries intersegmental information, particularly
relating to pain afferents.
• Anterior (ventral) cortico-spinal, also from the motor
cortex — These fibers, which do not cross in the pyramidal decussation (see Figure 42), go to the motor
neurons that supply the proximal and axial
musculature.
• Rubro-spinal, from the red nucleus — This tract
crosses at the level of the midbrain (see Figures 44 and
45). Its role in motor function in humans is not certain.
• Lateral and medial reticulo-spinal tracts, from the
medullary and pontine reticular formation, respectively (see Figures 46 and 47) — These pathways are
the additional ones for voluntary control of the proximal joints and for posture, as well as being important
for the control of muscle tone.
• Lateral vestibulo-spinal, from the lateral vestibular
nucleus — Its important function is participating in
the response of the axial muscles to changes in gravity.
This pathway remains ipsilateral (see Figure 48).
• Medial longitudinal fasciculus (MLF) — This pathway
is involved in the response of the eye and muscles of
the neck to vestibular and visual input (see Figure 49A).
It likely descends only to the cervical spinal cord level.
Ascending tracts:
• Dorsal column tracts, consisting at this level of both
the fasciculus cuneatus and fasciculus gracilis (see
Figure 32) — These are the pathways for discrimina©2000 CRC Press LLC
Clinical Aspects
Traumatic lesions of the spinal cord occur following car
and bicycle accidents and still occur because of diving
accidents in shallow water (swimming pools!).
Protruding discs can impinge upon the spinal cord.
Other traumatic lesions involve gunshot and knife
wounds. A classic lesion of the spinal cord is the BrownSequard syndrome, a lesion of one-half of the spinal
cord on one side. Although rare, review of the various
deficits helps a student learn which side of the body is
affected because of the various crossing of the pathways
(ascending and descending) at different levels.
The vascular supply of the spinal cord is notoriously
poor. Clinically, it is known that the blood supply to the
mid-thoracic region is marginally adequate. The main
blood supply to the spinal cord comes from two branches, one from each vertebral artery (see Figure 56). These
two small arteries join and form the anterior spinal
artery, which descends in the midline. The anterior
spinal artery supplies the ventral horn and the anterior
and lateral group of tracts, including the lateral corticospinal pathway. This artery receives supplementary
branches from the aorta along its way, which follow the
nerve roots and are called radicular arteries. There are
two very small posterior spinal arteries; the posterior
spinal artery supplies the dorsal horn and the dorsal
columns.
Lateral
cortico-spinal
tract
Dorsolateral
fasciculus
Fasciculus
gracilis
Fasciculus
cuneatus
Posterior (dorsal)
spino-cerebellar
tract
Rubro-spinal
tract
Medullary
(lateral)
reticulo-spinal
tract
Anterior (ventral)
spino-cerebellar
tract
Lateral
vestibulo-spinal
tract
Pontine (medial)
reticulo-spinal
tract
Anterior
cortico-spinal
tract
Lateral
Medial
spino-thalamic
longitudinal
tract
fasciculus
Anterior
(MLF)
spino-thalamic
tract
MOTOR
CEREBELLAR
SENSORY
FIGURE 72: Spinal Cord Tracts — C8 level
FIGURE 73
SPINAL CORD
CROSS-SECTIONAL VIEWS
The spinal cord is introduced in Section A; see Figures
1A, 1B, 2A, and 2B. The tracts that ascend from the
spinal cord (sensory and cerebellar) and the various
motor pathways that descend to the spinal cord have
likewise been reviewed in Section B.
The white matter surrounds the gray matter and is
divided by it into three areas: the dorsal, lateral, and anterior areas. These zones are sometimes referred to as
funiculi (single is funiculus). Various tracts are located
in each of these three zones, some ascending and some
descending (see Figure 72).
Cross Section of Spinal Cord:
Cervical Level — C8
This cross section of the spinal cord has been made at
the cervical enlargement (C8). This level is shown in
many of the illustrations of the various pathways in
Section B.
amount of gray matter. There are fewer muscles and less
dense innervation of the skin in the thoracic region.
The gray matter also has a lateral horn, which represents
the sympathetic neurons. This cell group is the pre-ganglionic sympathetic portion of the autonomic nervous
system. The lateral horn is present throughout the thoracic region of the spinal cord and also the upper segments of the lumbar region (T1–L2).
Lumbar Level — L3
This cross-sectional level of the spinal cord is shown in
the various illustrations of the pathways in Section B.
At this level the spinal cord is similar in appearance to
the cervical section because both are innervating the
limbs. There is, however, proportionately less white
matter at the lumbar level. The descending tracts are
smaller because many of the fibers have terminated at
higher levels. The ascending tracts are smaller because
they are conveying information only from the lower
regions of the body.
This level contributes to the formation of the brachial
plexus to the upper limb. The gray matter ventrally is
very large because of the number of neurons involved in
the innervation of the upper limb, particularly the
muscles of the hand. The dorsal horn is likewise large,
because of the number of afferents coming from the
skin of the fingers and hand.
By this level of the spinal cord, in the adult human, the
spinal cord segments do not match the vertebral level.
This is because the spinal cord in the human usually
ends, in fact, at the vertebral level L2 in the adult. Below
this level the vertebral canal is filled with nerve roots —
the cauda equina (like a horse’s tail; see Figure 1A). The
CSF space in that vertebral region is the lumbar cistern.
The white matter is very large in comparison with lower
regions because
Sacral Level — S3
1.
all the ascending tracts are present and are carrying information from the lower parts of the body
as well as the upper limb; and
2.
the descending tracts are fully represented, as
many of the fibers will terminate in the cervical
region of the spinal cord. In fact, some of them
do not descend to lower levels.
Thoracic Level — T6
The thoracic region of the spinal cord presents an
altered morphology because of the decrease in the
The sacral region of the spinal cord is the smallest in
size. The white matter is quite reduced in size. There is
still a fair amount of gray matter because of the innervation of the pelvic musculature. The cord tapers and ends
as the conus medullaris (see Figure 1B).
This region of the spinal cord also contains the pre-ganglionic parasympathetic neurons of the autonomic
nervous system , which innervate the bowel and the
bladder. Injury to this region of the spinal cord results in
a serious problem in terms of regulation of bowel and
bladder function.
Cervical
Thoracic
Lumbar
Sacral
FIGURE 73: Spinal Cord — Cross-Sectional Views
Section D
THE LIMBIC SYSTEM
INTRODUCTION
The term limbic is almost synonymous with the term
emotional brain — the parts of the brain involved with
our emotional state. In 1937, Dr. James Papez initiated
the limbic era by proposing that a number of limbic
structures in our brain formed the anatomical substratum for emotion.
EVOLUTIONARY PERSPECTIVE
This section begins with a brief overview of theories of
brain function in the lower animal kingdom, then
extends the discussion to mammals and finally to
humans.
Dr. Paul MacLean has postulated that there are in fact
three separable “brains” that have evolved. The premammalian (reptilian) brain has the capacity to look
after the basic life functions and, behaviorally, has
organized ritualistic stylized patterns of behavior. In
higher species, including mammals, forebrain structures
have evolved which relate to the external world (e.g.,
visual input). These are adaptive, allowing for a modification of behavior depending upon the situation.
MacLean has also suggested that the limbic system arises
in early mammals to link these two brain functions.
According to this scheme, the limbic system relates the
reptilian brain, which monitors the internal milieu, with
the newer forebrain areas of mammals, which are responsible for analyzing the external environment.
Hence, we now view the limbic system as those parts of
the brain that are involved in regulating the internal
state of the animal in relation to the external world.
DEFINITION
Most of us are quite aware or have a general sense of
what we mean when we use the terms emotion or feelings, yet it is somewhat difficult to explain or define precisely. A medical dictionary (Stedman’s) defines emotion
as “a strong feeling, aroused mental state, or intense state
of drive or unrest directed toward a definite object and
evidenced in both behavior and in psychologic changes.”
Thus, emotions involve the following:
• Physiologic changes — These changes include basic
drives involving thirst, sexual behavior, and appetite.
They are often manifested as involving the autonomic
nervous system and/or endocrine system.
• Behavior — The animal or human performs some
type of motor activity, for example fighting, fleeing,
displaying anger, mating behavior; in humans, it
might include facial expression.
• Alterations in the mental state — This alteration can
be understood as a subjective change in the way the
organism “feels” or reacts to the events occurring in
the outside world. In humans, we use the term
psychological reaction.
It is clear, at least in humans, that some of these functions and behaviors must involve the cerebral cortex. In
addition, many of these alterations are conscious and
involve association areas. In fact, humans are sometimes
able to describe and verbalize their reactions or the way
they feel. Both cortical and subcortical areas (e.g., basal
ganglia) might be involved in the behavioral reactions
associated with emotional responses. The hypothalamus,
along with brainstem nuclei, controls the autonomic
changes as well as the activity of the pituitary gland underlying the endocrine responses.
Therefore, we can finally arrive at a definition of the
limbic system as an inter-related group of cortical and
subcortical (noncortical) structures that are involved in
the regulation of the internal/emotional state, with the
accompanying physiological, behavioral, and psychological responses. In summary, limbic functions can be
summarized by the use of a rather simple mnemonic
using Fs — feeding, fighting, fleeing, fornicating, feeling.
NEURAL STRUCTURES
In neuroanatomical terms, the limbic system now is
thought to include cortical and noncortical (subcortical,
diencephalic, and brainstem) structures. The following
is a listing of the structures. Core structures are those
definitely associated with the limbic system; those listed
as extended are closely connected with limbic functions:
Cortical
core — parahippocampal gyrus, cingulate gyrus,
hippocampal formation (three subparts which
are “buried” in the medial temporal lobe in
humans)
extended — parts of the prefrontal and orbitofrontal cortex (the limbic forebrain)
Noncortical
forebrain
core — amygdala, septal region, ventral portions
of the basal ganglia (including the nucleus
accumbens)
extended — the basal forebrain
diencephalic and brainstem
core — certain nuclei of the thalamus, the
hypothalamus
extended — parts of the midbrain (the limbic
midbrain)
These structures are collectively called the limbic
system. The particular role of the olfactory system and
its connections are discussed in the context of the limbic
system.
OVERVIEW OF “KEY” LIMBIC
STRUCTURES
Key structures of the limbic system integrate information and relate the external and internal worlds — the
parahippocampal gyrus, the hippocampal formation,
the amygdala, and the hypothalamus.
The parahippocampal gyrus has widespread connections
with many cortical (particularly sensory) areas and is
probably the source of the most significant afferents to
the hippocampal information. The hippocampal formation is an older cortical region that is involved with integrating information (its role in the formation of
memory for facts and events is discussed below). The
amygdala is in part a subcortical nucleus involved with
internal (visceral afferent) information as well as
sensory input about olfaction (our sense of smell). The
hypothalamus oversees autonomic and hormonal regulation. Both areas control motor and autonomic activities of the organism, (the amygdala in part via the
hypothalamus), and both are involved in generating
emotional reactions.
Certain basic drives (as they are known in the field of
psychology), such as hunger, thirst, sex, and temperature
regulation, are regulated through the limbic system. For
example, the reaction to a cold environment or dehydration leads to a complex series of motor activities, autonomic adjustments, as well as hormonal changes. It is
not difficult to show how the hypothalamus regulates
the pituitary gland and that it is the master comptroller
of autonomic functions, but clearly additional connections are required for the behavioral (motor) activities.
In humans, there is also an internal state of discomfort
to being cold, which we call an emotional reaction —
where is this “feeling” generated in the brain? It is generally assumed that other mammals also have “feelings,”
which can be assessed in other ways, when faced with
similar situations. Certainly, it is hard to deny that
higher apes behave and react to these events in terms
that can only be described as emotional.
It has also been suggested that some mammalian behavior associated with caring for its young is associated
with the limbic structures, such as recognizing and responding to the vocalizations of the “pups” in mice and
rats, and the particular tone of a baby’s crying. Some of
this may involve the cingulate gyrus. This notion of
rearing and “family” would add another “F” to our list of
limbic functions.
It is also interesting to speculate that the elaboration of
limbic functions is closely associated with the development of self-awareness, consciousness of the self (not an
“F” word).
Limbic Connections
The limbic system has internal circuits involving the key
structures; these link the amygdala, the hippocampal
formation, and the hypothalamus, as well as other struc-
tures of the limbic system. There are multiple interconnections within and between these structures, and
knowledge of the circuits of the limbic system, which are
quite complex, allows one to trace pathways within it.
Only some of these pathways are presented here. The
best known of these in terms of function (and for historical reasons) is the Papez Circuit (discussed with
Figure 81). Additional pathways connect the limbic
structures to the remainder of the nervous system and
through which the limbic system influences the activity
of the nervous system.
Memory
Unfortunately, the definition and description of the
limbic system does not include one aspect of brain function that seems to have evolved in conjunction with the
limbic system — memory. Memory systems are now
thought of as comprising two types:
• memory for facts and events — declarative or episodic
memory, and
• memory for skills and procedures — procedural
memory.
Some parts of the hippocampal formation are specifically necessary for the initial formation of declarative
memories. It is critical to understand that this initial
step is an absolute prerequisite to the formation of any
memory trace. It is interesting to speculate (as we see
below) that forgetting may be theoretically more appropriate for this unique aspect of limbic function. Once
encoded by the hippocampal formation, the memory
trace is then transferred to other parts of the brain for
short and long-term storage. The limbic system is not
involved in the storage and retrieval of long-term
memories.
It is interesting to speculate that part of the function of
the limbic system is to undo or unlock the fixed behavioral patterns of the old reptilian brain. In order to do
this, one needs to remember what happened the last
time when faced with a similar situation. Hence, the development of memory functions of the brain in association with the evolution of the limbic system. The
availability of stored memories makes it possible for
mammals to override or overrule the stereotypical behaviors of the reptile, allowing for more flexibility and
adaptiveness when faced with a changing environment
or altered circumstances. Therefore, we suggest that
another “F” mnemonic — forgetting — may be applicable for this “memory” function.
FIGURE 74
LIMBIC LOBE
The limbic lobe refers to cortical areas of the limbic
system that form a border (limbus) around the inner
structures of the diencephalon and midbrain. These cortical areas include the cingulate gyrus, parahippocampal
gyrus, and cortical components in the hippocampal
formation.
One of the distinguishing features of the limbic cortical
areas is the fact that, for the most part, these are older
cortical areas consisting of three to five layers (allocortex), in contrast to the six-layered neocortex typical of
the dorsolateral areas of the cerebral cortex.
The cingulate gyrus lies above the corpus callosum.
Some parts of this gyrus consist of a five-layered cortex,
as well as neocortex. The cingulate gyrus is connected
reciprocally with the parahippocampal gyrus via by a
bundle of fibers in the white matter, known as the cingulum bundle. This connection unites the various portions of the limbic lobe and is discussed as part of the
limbic circuit known as the Papez circuit (discussed with
Figure 81). It also has widespread connections with the
frontal lobe.
MacLean’s studies have indicated that the development
of this gyrus is correlated with the evolution of the
mammalian species. He has postulated that this gyrus is
important for nursing and play behavior, characteristics
associated with the rearing of young in mammals. It is
this cluster of behavioral patterns that forms the basis
for including “family” in the list of functions of the
limbic system. The cingulate gyrus also seems to have an
important role in attention.
The parahippocampal gyrus, situated on the inferior
aspect of the brain (see Figures 13 and 14), is a critical
area of the limbic lobe. It is also composed of five- and
six-layered cortex. This gyrus has widespread connections with many areas of the cerebral cortex, including
all the sensory cortical regions, as well as the cingulate
gyrus. It is also heavily connected (reciprocally) with the
hippocampal formation. It is thought to play a key role
in limbic function and memory.
A number of cortical areas located in the most medial
aspects of the temporal lobe, deep to the parahippocampal gyrus, form part of this limbus — these are collectively called the hippocampal formation. One of its
three major components, the hippocampus proper, is
not at the surface of the hemisphere, as would be expected for a cortical area. Understanding the development of the region explains this fact (discussed with
Figure 77B). The other parts of the hippocampal formation are the dentate gyrus, part of which can be found at
the surface, and the subicular region (a cortical area).
The hippocampus proper and the dentate cortex are
three-layered cortical areas while the subicular region
has four to five layers (see Figures 77A and 77B).
Of the many tracts of the limbic system, two major
tracts have been included in this diagram: the fornix and
the anterior commissure. The fornix is one of the more
visible tracts and is often seen during dissections of the
brain (e.g., see Figure 16). This fiber bundle connects
the hippocampal formation with other areas (discussed
with Figure 77B). The anterior commissure is an older
commissure than the corpus callosum and connects
several structures of the limbic system on the two sides
of the brain. These include the amygdala, the hippocampal formation, and parts of the parahippocampal gyrus.
The anterior commissure will be seen on many of the
limbic diagrams and is also a convenient reference point.
The details of these various limbic structures, their important connections, and the functional aspects of these
cortical components of the limbic system are discussed
with the relevant diagram. Other areas of the brain are
now known to be involved in limbic functions, and they
are included in the limbic system. This includes large
parts of the so-called prefrontal cortex, particularly cortical areas lying above the orbit, the orbitofrontal cortex
(not labeled in the figure). A small cortical region under
the anterior part (the rostrum) of the corpus callosum is
discussed with the septal region.
Corpus callosum “area”
Fornix
Cingulate gyrus
Diencephalon
Midbrain
Septal region
Anterior commissure
Cerebellum
Mammillary n.
Pons
Amygdala
Medulla
FIGURE 74: Limbic Lobe
Hippocampal formation
FIGURE 75
LIMBIC STRUCTURES
The diagram in Figure 75 focuses on the noncortical
components of the limbic system — forebrain, diencephalon, and midbrain. Each of the structures, including the connections, are discussed in greater detail in
subsequent illustrations. In those discussions, this
diagram will sometimes be used as a locator map, where
the parts of the limbic system being described will be indicated appropriately.
The noncortical areas shown in this diagram include
• amygdala
• septal region
• thalamus
• hypothalamus
• limbic midbrain
The olfactory system is also discussed in this section.
The amygdala (amygdaloid nucleus), as discussed (with
Figure 23), is anatomically one of the basal ganglia.
Functionally, and through its connections, it is part of
the limbic system. Therefore, it is considered in this
section. Parts of the basal ganglia, namely the ventral
portions of the putamen and globus pallidus (not shown
on this diagram), might also have limbic functions and
these are discussed as well (with Figure 85B).
The septal region includes two components, the cortical
gyri below the rostrum of the corpus callosum and some
nuclei deep to them. These nuclei are not located within
the septum pellucidum in the human.
The nucleus accumbens (see Figures 24 and 85B) is a
specific nucleus adjacent to these septal nuclei which has
recently been found to have important functions in
reward and punishment. It might be the critical area of
the brain involved in addiction.
Two of the nuclei of the thalamus, the anterior group of
nuclei and the dorsomedial nucleus (see Figures 81 and
82), are part of the pathways of the limbic system, relaying information from subcortical nuclei to limbic parts
of the cortex (the cingulate gyrus and areas of the prefrontal cortex).
The hypothalamus lies below and somewhat anterior to
the thalamus. Only a few of the nuclei are shown, and
amongst these is the prominent mammillary nucleus,
visible on the inferior view of the brain (seen in Figure
14). The connection of the hypothalamus to the pituitary gland is not shown.
The limbic system also includes nuclei of the midbrain,
the limbic midbrain. Some of the descending limbic
pathways terminate in this region and it is important to
consider the role of this area in limbic functions. An important limbic pathway, the medial forebrain bundle
(see Figure 83B) interconnects the septal region, the hypothalamus, and the limbic midbrain.
The olfactory system is described with the limbic
system, because many of its connections are directly
with limbic areas. Years ago, it was commonplace to
think of various limbic structures as part of the “smell
brain,” the rhinencephalon. We now know that this is
not correct, and that the limbic system has many other
functional capabilities.
Not represented in this diagram is the region known as
the basal forebrain. This is a subcortical region, which is
composed of a group of structures, located beside the
hypothalamus and below the anterior commissure (see
Figures 85A and 85B). This somewhat obscure region
has connections with several limbic areas and the prefrontal cortex. It may play a major role in memory.
The various pathways shown in Figure 75 — fornix, stria
terminalis, ventral amygdalofugal — are discussed with
the relevant structures.
Fornix
Cingulate gyrus
a
rpus c llosum
Co
Stria terminalis
Thalamus
Septal nuclei
Midbrain
Pons
Hypothalamic nuclei
Mammillary n.
Medulla
Lateral olfactory stria
Olfactory bulb
Amygdala
Ventral amygdalofugal pathway
Olfactory tract
FIGURE 75: Limbic Structures
FIGURE 76
HIPPOCAMPUS
(PHOTOGRAPHIC VIEW)
Figure 76 shows the brain from the dorsolateral aspect.
The left hemisphere is partially obscured by the (gloved)
hand holding the structures. The right hemisphere has
been dissected by cutting away all of the hemisphere
above the corpus callosum, and then removing some
cortical tissue of the temporal lobe, thereby exposing the
inferior horn of the lateral ventricle (see Figure 20A).
This dissection exposes a large mass of tissue which is
actually protruding into this part of the ventricle, the
hippocampus, as it is often called as a gross brain structure. In fact, the correct term is hippocampal formation,
as it is composed of various parts (discussed below). The
choroid plexus tissue has been removed from this part of
the ventricle in order to improve visualization of the
structures. The protrusion of the hippocampus into the
inferior horn of the lateral ventricle can also be seen in
coronal sections through this region (see Figures 29, 36,
78, and 80).
The hippocampal formation is composed of three distinct regions — the hippocampus proper (Ammon’s
horn), the dentate gyrus and the subicular region; these
are explained in the next diagrams. The fiber bundle
which arises from the visible hippocampus, the fornix,
can be seen along its innermost aspect. The fornix receives fibers from some of the parts of the hippocampal
formation. Its course and connections are considered
with Figure 77.
Memory
We now know that the hippocampal formation is one of
the critical structures for memory. This function of the
hippocampal formation is known because of a patient
called H.M. in the literature, who has been extensively
studied by neuropsychologists. H.M. had surgery several
decades ago for sound therapeutic reasons — removal of
an epileptic area in the temporal lobe of one side — but
unfortunately before the functional contribution of this
area was known. Most importantly, the surgeons did not
know (and could not have known according to the
methods available at that time) that the contralateral
hippocampal area was also severely damaged.
We now know that bilateral damage or removal of the
anterior temporal lobe structures, including the amygdala and the hippocampal formation, leads to a unique
condition in which the person can no longer form new
declarative or episodic memories, although older memories are intact. The individual cannot remember what
occurred moments before. Therefore, the individual is
unable to learn — to acquire new information — and is
not able to function independently.
Apparently, the full-blown syndrome results from
damage to the hippocampus on both sides. Today, if
surgery is to be performed in this region, special testing
is done to ascertain that the side contralateral to the
surgery is intact and functioning.
This area is prone to damage for a variety of reasons, including trauma and vascular conditions. The key
neurons for this memory function are located in the
hippocampus proper and these neurons are extremely
sensitive to anoxic states. An acute hypoxic event, such
as occurs in a cardiac arrest, is thought to trigger a
delayed death of these neurons, several days later. This is
termed apoptosis, programmed cell death. Much research
is now in progress to try to understand this cellular phenomenon and to devise methods to stop this reaction of
these neurons.
Currently, studies indicate that in certain forms of dementia, particularly Alzheimer’s, there is a loss of
neurons in this same region of the hippocampus proper.
This loss is due to involvement of these neurons in the
disease process. Again, this correlates with the memory
deficit seen in this condition, although the disease
clearly involves other areas which correlate with the
other deficits typical of the early stages of this disease.
Fornix
Lateral ventricle
(inferior horn)
Hippocampus
Temporal lobe
FIGURE 76: Hippocampus (Photographic View)
FIGURE 77A
HIPPOCAMPAL FORMATION
The diagram in Figure 77 (which is the same one as
Figure 75) highlights the “hippocampus,” i.e. the hippocampal formation, and the pathway known as the
fornix.
The hippocampal formation refers to older cortical
regions, all consisting of less than six layers, which are
located in the medial aspect of the temporal lobe in
humans. Much of the difficulty of understanding these
structures is their anatomical location deep within the
medial portions of the temporal lobe.
In the rat, the hippocampal formation is located dorsally, above the thalamus. During the evolution of the temporal lobe, these structures have migrated into the
temporal lobe, leaving behind a fiber pathway, the
fornix, which is located above the thalamus. In fact, a
vestigial part of the hippocampal formation is still
located above the corpus callosum, as shown in this
illustration (not labeled).
The hippocampal formation includes:
• the hippocampus proper, a three-layered cortical area
which during development becomes “rolled-up” and is
no longer found at the surface of the hemispheres (as
is the case for all other cortical regions);
• the dentate gyrus, a three-layered cortical area which
is partly found on the surface of the brain, although
its location is so deep as to present a challenge to nonexperts to actually locate and visualize this thin ridge
of cortex;
• the subicular region, a transitional cortical area of
three to five layers which becomes continuous with
the parahippocampal gyrus (located on the inferior
aspect of the brain).
Clinical Aspect
The term medial temporal (mesiotemporal) sclerosis is a
general term for damage to the hippocampal region
located in this part of the brain. It is now possible to
view this area in detail on MRI and to assess the volume
of tissue. Bilateral damage here apparently correlates
with the loss of memory function in humans, for the
formation of new memories for facts or events.
Cingulate gyrus
Fornix
Septal region
Co
s callosum
u
p
r
Mammillary n.
FIGURE 77A: Hippocampal Formation
FIGURE 77B
HIPPOCAMPAL FORMATION
3 PARTS
The hippocampal formation is one of the most important components of the limbic system in humans. It is
certainly the most complex.
One expects a cortical area to be found at the surface of
the brain, even if this surface is located deep within a
fissure. During the evolution and development of the
hippocampal formation, these areas became rolled up
within the brain. (The student is advised to consult
Williams and Warwick for a detailed visualization and
understanding of this phenomenon.) Of the three parts,
the hippocampus proper is found completely “within
the brain.”
Hippocampus Proper — The hippocampus proper consists of a three-layered cortical area forming a large
mass, which actually intrudes into the ventricular space
of the inferior horn of the lateral ventricle (see Figures
76 and 78). In a coronal section through this region,
there is a resemblance of the hippocampal structures to
the shape of a seahorse. It is from this shape that the
name “hippocampus” is derived, from the French word
for seahorse. The other name for this area is Ammon’s
horn or cornu ammonis (abbreviated as CA), named
after an Egyptian deity with ram’s horns because of the
curvature of the placement of the hippocampus in the
brain. This cortical region has been divided into a
number of subportions (CA 1–4, usually studied in
more advanced courses).
Dentate Gyrus — The dentate gyrus is also a phylogenetically older cortical area consisting of only three
layers. During the formation discussed above, the
leading edge of the cortex detaches itself and becomes
the dentate gyrus. Parts of it remain visible at the surface
of the brain. Since this small surface is buried on the
most medial aspect of the temporal lobe and is located
deep within a fissure, it is rarely located in studies of the
gross brain. Once visualized, it is seen to have ridges or a
serrated surface, which seemed to suggest tooth marks,
giving it the name dentate (referring to teeth; see also
Figure 80).
The appearance of the dentate gyrus is shown on the
view of the medial aspect of the temporal lobe (on the
far side of Figure 77B). A coronal section through the
temporal lobe, as seen in the lower part of the diagram
(see also Figure 36), indicates that the dentate gyrus is
more extensive than its visible medial portion.
Subicular Region — The next part of the cortically
rolled-in structures that makes up the hippocampal formation is the subicular region (see also Figure 80). The
cortical thickness is transitional, starting from the threelayered hippocampal formation to the six-layered
parahippocampal gyrus. (Again, there are a number of
subparts of this area which are rarely studied in an introductory course.)
In the temporal lobe, the hippocampal formation is adjacent to the six-layered parahippocampal gyrus, with
which it has extensive connections.
Connections and Function
The hippocampal formation receives its major input
from the adjacent parahippocampal gyrus, as well as the
amygdala. There are extensive interconnections within
the component parts of the hippocampal formation itself.
Part of the output of this cortical region is directed back
towards the parahippocampal gyrus, which itself has extensive connections with other cortical areas of the
brain, particularly sensory areas. This is analogous to
cortical association pathways described earlier.
The other major output of the hippocampal formation
is through the fornix. Only the hippocampus proper and
the subicular region project fibers into the fornix. This
can be regarded as a subcortical pathway which terminates in the septal region (via the precommissural fibers,
discussed with Figure 83B) and in the mammillary
nucleus of the hypothalamus (via the post-commissural
fibers, discussed with Figure 83A). There are also reciprocal connections in the fornix. The dentate gyrus connects only with other parts of the hippocampal
formation and does not project beyond.
Corpus callsoum
(splenium)
Dentate gyrus
Fornix
Precommisural
fibers
Hippocampus
proper
Mammillary n.
Dentate gyrus
Subicular region
Parahippocampal
gyrus
Collateral fissure
FIGURE 77B: Hippocampal Formation — 3 Parts
Temporal lobe
FIGURE 78
CORONAL BRAIN SECTION
PHOTOGRAPHIC VIEW
The section shown in Figure 78 is taken posterior to the
one shown in Figure 29 and includes part of the lateral
ventricle as it begins to curve into the temporal lobe (see
Figures 20A and 20B). The basal ganglia are no longer
present. The most posterior portion of the thalamus, the
pulvinar, is seen (see Figures 10 and 61). Between the
thalamus and the corpus callosum is the fornix.
The space between the thalamic areas is not the third
ventricle (it is also too large to be the normal third ventricle) because this coronal section has been taken so
posteriorly; it is, in fact, outside the brain, posterior to
the diencephalic region (see Figure 16). It is located
behind the pineal and the colliculi, and in front of the
cerebellum (see also Figure 8). The quadrigeminal plate
cistern is found in this location (see Figure 28A). It also
contains some important cerebral veins which drain the
interior of the brain (these have been removed from this
specimen). The section also includes the brainstem.
The inferior horn of the lateral ventricles is found in the
temporal lobes on both sides and is seen as only a small
crescent-shaped cavity (shown also in Figure 36). The
inferior horn of the lateral ventricle is reduced to a
narrow slit because a mass of tissue protrudes into this
part of the ventricle from its medial-inferior aspect.
Closer inspection of this tissue reveals that it is gray
matter; this gray matter is the hippocampus proper.
This plane of viewing allows one to follow the gray
matter from the hippocampus proper medially and
through an intermediate zone, known as the subiculum
or subicular region (see Figure 77B), until it becomes
continuous with the gray matter of the parahippocampal gyrus. The hippocampus proper has only three cortical layers. The subicular region consists of four to five
layers; the parahippocampal gyrus is mostly a six-layered
cortex.
The subicular region, as part of the hippocampal formation, both receives from and sends fibers to the other
parts of the hippocampal formation, as well as contributing the majority of fibers to the fornix.
This view allows us to see that the parahippocampal
gyrus is so named because it lies beside the hippocampus. The collateral sulcus (fissure), seen previously on
the inferior aspect of the temporal lobe (see Figures 13
and 14), is indicated.
Memory
Recent studies in humans have indicated that the
neurons located in one portion of the hippocampus
proper (CA3 region) are critical for the formation of
new memories — declarative or episodic types of memories (not procedural). This means that in order for the
brain to remember some new fact or event, the new information must be registered within the hippocampal
formation. This information is processed through some
complex circuitry in these structures and is retained for
a brief period. In order for it to be remembered for
longer periods, some process not yet understood occurs
so that the transient memory trace is transferred to
other parts of the brain and stored in working memory
or as a long-term memory. (An analogy to computers
might be useful here.)
In the study of the function of the hippocampus in
animals, there is considerable evidence that the hippocampal formation is involved in constructing a spatial
map. According to this literature, the hippocampal formation is needed for orientation in a complex environment (such as a maze). It is not quite clear whether this
is a memory function, or whether this spatial representation depends upon the connections of the hippocampal
formation and parahippocampal gyrus with other parts
of the brain.
Corpus callosum
Lateral ventricle
Fornix
Thalamus
Lateral ventricle
(inferior horn)
Hippocampal
formation
Collateral sulcus/fissure
Subicular
region
Brainstem
FIGURE 78: Coronal Brain Section — (Photographic View)
FIGURE 79A
AMYGDALA
The diagram in Figure 79A (which is the same one as
Figure 75) highlights a functional portion of the limbic
system: the amygdala and its pathways (the stria terminalis and the ventral amygdalofugal pathway), the septal
region, as well as functionally connected portions of the
midbrain and medulla.
The amygdala (amygdaloid nucleus) is a subcortical
nuclear structure located in the temporal lobe (in
humans). As a subcortical nucleus of the forebrain, it
belongs by definition with the basal ganglia, but is now
usually described with the limbic system.
The amygdala is located between the temporal pole and
the “end” of the inferior horn of the lateral ventricle (in
the temporal lobe; see Figures 25 and 80). Its mass is responsible for the elevation known as the uncus, which is
seen on the inferior aspect of the brain (see Figures 13
and 14) as a large medial protrusion of the anterior
aspect of the temporal lobe.
The amygdala receives input from the olfactory system
as well as from visceral structures. Two fiber tracts are
shown connecting the amygdala to other limbic structures, a dorsal one (the stria terminalis) and a ventral
one (the ventral amygdalofugal pathway, consisting of
two parts). These are described in detail with the following diagram.
Stimulation of the amygdaloid nucleus produces a
variety of vegetative responses, including licking and
chewing movements. Functionally, in animal experimentation stimulation of the amygdala might produce a
“rage” response, whereas removal of the amygdala (bilaterally) results in docility. These responses are also seen
with stimulation or lesions in the hypothalamus.
In monkeys, bilateral removal of the anterior parts of
the temporal lobe (including the amygdala) produces a
number of effects which are collectively called the
Kluver-Bucy syndrome. The monkeys evidently become
tamer after the surgery, put everything into their mouths,
and display inappropriate sexual behavior. In rather
unusual circumstances, bilateral destruction of the amygdala is recommended in humans for individuals whose
violent behavior cannot be controlled by other means.
This type of treatment is called psychosurgery.
The amygdala is known to have a low threshold for electrical discharges, which may make it prone to be the
focus for seizure development. This seems to occur in
kindling, an experimental model of epilepsy. In humans,
epilepsy from this part of the brain (anterior and medial
temporal regions) usually gives rise to complex partial
seizures in which oral and licking movements are often
seen, along with a loss of conscious activity.
The amygdala is also known to contain a high amount
of enkephalins. It is not clear why this is so and what is
the functional significance.
The role of the amygdala in the formation of memory is
not clear. Bilateral removal of the anterior portions of
the temporal lobe in humans for the treatment of severe
cases of epilepsy, results in a memory disorder, which
has been described with the hippocampal formation
(discussed with Figure 76). It is possible that the role of
the amygdala in the formation of memories is mediated
either through the connections of this nuclear complex
with the hippocampus or with the dorsomedial nucleus
of the thalamus.
callosum
pus
r
Co
Stria terminalis
Septal region
Midbrain
Medulla
Ventral
amygdalofugal
pathway
Amygdala
FIGURE 79A: Amygdala
FIGURE 79B
AMYGDALA — CONNECTIONS
One of the other major differences between the amygdala and the other parts of the basal ganglia is that the
amygdala is not a homogeneous nuclear structure but is
in fact composed of different parts. (These parts are not
usually studied in an introductory course.)
The amygdala receives a variety of inputs from other
parts of the brain, including the adjacent parahippocampal gyrus (not illustrated). It receives olfactory input directly (via the lateral olfactory stria; see also Figure 84)
and indirectly from the cortex of the uncal region (as
shown on the left side of the diagram).
The amygdaloid nuclei are connected to the hypothalamus, thalamus (mainly the dorsomedial nucleus), and
the septal region. These connections, which are reciprocal, travel through two routes:
• a dorsal route, known as the stria terminalis, which
follows the ventricular curve and is found on the
upper aspect of the thalamus (see Figures 61 and 75).
The stria terminalis lies adjacent to the body of the
caudate nucleus in this location (see Figure 80) and
connects the amygdala with the hypothalamus and the
septal region.
• a ventral route, known as the ventral pathway or the
ventral amygdalofugal pathway. This pathway, which
goes through the basal forebrain region (see Figure
85B), connects to the hypothalamus (as shown) and to
the thalamus (the fibers are shown “en route”), particularly the dorsomedial nucleus (see Figure 82).
Further possible connections of the amygdala with other
limbic structures and other parts of the brain can occur
via the hypothalamus (discussed with that structure,
Figure 83A), via the septal region (see Figure 83B), and
via the dorsomedial nucleus of the thalamus (see Figure
82).
The amygdala also has connections with autonomicrelated nuclei in the midbrain (the limbic midbrain) and
possibly also in the medulla. These influences may be
direct and indirect (as shown).
The anterior commissure conveys connections between
the nuclei of the two sides (discussed also with Figure 74).
Stria terminalis
Thalamus
Hypothalamic-midbrain
fibers
Periaqueductal gray
“Limbic” midbrain
Midbrain
Septal
region
Descending
autonomic fibers
Anterior
commissure
Medulla
Uncal cortex
Parasympathetic nuclei
Optic tract
Hypothalamic nuclei – preoptic
– medial
– lateral
Lateral olfactory
stria
Amygdala
FIGURE 79B: Amygdala — Connections
– to thalamus
– to hypothalamus
FIGURE 80
LIMBIC STRUCTURES AND
THE LATERAL VENTRICLE
The temporal lobe is a more recent addition in the
evolution of the hemispheres and develops later in the
formation of the brain. During the development of
the temporal lobe, a number of structures migrate into
it — the lateral ventricle, hippocampal formation,
caudate nucleus, as well as various tracts, the fornix and
stria terminalis. It is easiest to visualize these structures
in the temporal lobe in relation to the lateral ventricle
(refer to Figures 20A, 20B, and 25).
Caudate Nucleus (see Figure 23) — The various parts of
the caudate nucleus are shown in Figure 80. The large
head is found in relation to the anterior horn of the
lateral ventricle, where it in fact bulges into this part. It
is also seen in a horizontal section through the brain
(see Figure 27). The body of the caudate nucleus is coincident with the body of the lateral ventricle, being found
on its lateral aspect (see Figures 25 and 61). In this location it is situated above the thalamus (see Figure 79B).
The fibers of the fornix pass in front of the interventricular foramen of Monro (see medial view of brain in
Figure 16). At this point some of the fibers are given off
to the septal region and pass in front of the anterior
commissure. These are the precommissural fibers (see
Figure 77B). Others continue (behind the anterior commissure; postcommissural fibers) through the hypothalamus and terminate in the mammillary nucleus (which
is not portrayed in this diagram; see Figure 77B).
Stria Terminalis — The stria terminalis (connecting
amygdala with septal region and hypothalamus) follows
essentially the same course as the fornix (see Figure
79B). Its fibers lie slightly more medially and are found
on the dorsal aspect of the thalamus, in the floor of the
body of the lateral ventricle. In the temporal lobe, the
stria is found in the roof of the inferior horn of the
lateral ventricle (see also Figure 36).
This view of the relationship of these various structures
is augmented by a number of sections at various points:
• the first section is through the anterior horn of the
ventricle, in front of the interventricular foramen (of
Monro);
As the caudate nucleus curves into the temporal lobe, it
becomes the tail of the caudate nucleus. In the temporal
lobe it is found on the upper aspect of the inferior
horn of the ventricle, its roof (see Figure 61).
• the following section is over the dorsal aspect of the
thalamus and above the third ventricle;
Amygdala
• the last section is through the temporal lobe, including the hippocampal formation.
The amygdala is clearly seen to be situated anterior to
the temporal horn of the lateral ventricle and in front of
the hippocampal formation.
Fornix — The fornix is easily found in studies of the
gross brain (e.g., Figure 16). Its fibers can be seen as a
continuation of the hippocampal formation (see Figures
76 , 77A, and 77B), and these fibers course on the inner
aspect of the ventricle as they sweep forward above the
thalamus. In the area above the thalamus and below the
corpus callosum (see coronal section, Figure 29), the
fornix is found at the lower edge of the septum pellucidum. Here, the fornix of one side is in fact adjacent to
that of the other side (see also Figure 74). There are
some interconnections between the two sides in this area
(see Figure 75).
• the next section shows the ventricle at its curvature
(the atrium) into the temporal lobe;
Note: The initials used in these sections to identify
structures are found in brackets after the labeled structure in the main part of the diagram.
LV
LV
CB
CT
Caudate nucleus – body (CB)
ST
ST
F
Lateral ventricle (LV)
F
Stria terminalis (ST)
Fornix (F)
ST
CH
Occipital
horn
LV
Caudate nucleus – head (CH)
Temporal horn
Dentate gyrus
Amygdala
Hippocampus proper
Caudate nucleus –
tail (CT)
FIGURE 80: Limbic Structures and the Lateral Ventricle
F
ST
CT
LV
FIGURE 81
LIMBIC DIENCEPHALON
ANTERIOR NUCLEUS
LIMBIC DIAGRAM (INSET)
The inset diagram of the structures of the limbic system
(see Figure 75) has certain parts accentuated, as these are
the areas shown in the detailed diagram. The thalamus
on the far side is the one seen in the detailed diagram, as
are the two mammillary nuclei, and a small portion of
the cingulate gyrus. (Note: The student is advised to
refer to the diagram and classification of the thalamic
nuclei, Figure 10.)
ANTERIOR NUCLEUS —
CINGULATE GYRUS
The detailed diagram shows some of the major connections of the limbic system. A major tract leaves the
mammillary nuclei — the mammillo-thalamic tract —
and its fibers are headed for a group of association
nuclei of the thalamus called the anterior nuclei. The
fibers from this nuclear group project to the cingulate
gyrus.
Axons leave the anterior nuclei of the thalamus and
course through the anterior limb of the internal capsule.
These fibers course between the caudate nucleus (head
and body) and the lentiform nucleus (which is just
visible in the background). The axons terminate in the
cortex of the cingulate gyrus after passing through the
corpus callosum.
PAPEZ CIRCUIT
About 60 years ago James Papez described a circuit
involving some limbic and cortical structures and
pathways. He postulated that this was the anatomical
substrate for emotional experiences. This pathway forms
a loop and is known as the Papez circuit.
If one commences with the hippocampal formation and
proceeds through the fornix, some of the fibers terminate in the mammillary nuclei of the hypothalamus.
From there, the mammillo-thalamic tract ascends to the
anterior group of thalamic nuclei. This group of nuclei
project to the cingulate gyrus (discussed with Figure 74).
From the cingulate gyrus there is an association bundle,
the cingulum, that connects the cingulate gyrus (reciprocally) with the parahippocampal gyrus as part of the
limbic lobe (refer to Figure 74). The parahippocampal
gyrus projects to the hippocampal formation, which
processes the information and sends it via the fornix to
the septal region and mammillary nuclei of the hypothalamus. Hence the circuit is formed.
We now have a broader view of the limbic system and
the precise functional role of the Papez circuit is not
completely understood. It should be realized that although the circuitry forms a loop, the various structures
have connections with other parts of the limbic system
and other areas of the brain and can thus influence
other neuronal functions.
Clinical Aspect
In the days of psychosurgery, bilateral removal of the
cingulate gyrus was done for certain psychiatric disorders. One of these was extreme obsessive-compulsive behavior. This type of surgery is rarely performed now
because new and powerful drugs have been found to
help treat these disorders.
Cingulate projections
Cingulate gyrus
Corpus callosum
Thalamus
Caudate nucleus
Mammillo-thalamic tract
Internal capsule
(anterior limb)
Anterior n.
Fornix
FIGURE 81: Limbic Diencephalon — Anterior Nucleus
Mammillary n.
FIGURE 82
LIMBIC DIENCEPHALON
DORSOMEDIAL NUCLEUS
anterior limb of the internal capsule (between the head
of the caudate nucleus and the lentiform nucleus) is
seen on the left side of the diagram. The fibers course in
the white matter of the frontal lobes.
LIMBIC DIAGRAM (INSET)
Psychosurgery
The areas of the thalamus indicated in the inset of
Figure 82 are portrayed somewhat differently and are
those shown in the more detailed diagram. These areas
include the thalamus on the distal side, as well as the
thalamus situated nearer the viewer. The focus here is on
the medial nuclear mass of the thalamus, the dorsomedial nucleus.
DORSOMEDIAL NUCLEUS —
PREFRONTAL CORTEX
The cortical area known as the prefrontal cortex, anterior to the motor regions (discussed with Figure 11), is
thought to be the most recently evolved portion of the
cerebral cortex. This area becomes progressively larger as
one ascends through the mammalian kingdom and the
higher apes. Our expanded view of the limbic system
now includes its extension to the prefrontal cortex,
specifically the infraorbital and medial portions of the
frontal lobe — the limbic forebrain.
The thalamic nucleus that connects to this region of
cortex is the dorsomedial nucleus, an association
nucleus (see Figure 10). It collects information from a
variety of sources. Some information comes from the
amygdala, via the ventral pathway (see also Figure 79B),
and other information comes from other thalamic
nuclei. It also collects information from various hypothalamic nuclei, as well as the ventral parts of the
basal ganglia that are connected with the limbic system
(see Figure 85B).
The dorsomedial nucleus projects heavily to the prefrontal cortex. The course of the fibers through the
This connection between the dorsomedial nucleus of the
thalamus and the prefrontal cortex has been extensively
studied in humans, because surgical interruption of
these fibers (bilaterally) used to be done as psychosurgery for some psychiatric disorders, including
schizophrenia. This operation, called a frontal lobotomy,
was frequently performed throughout North America
prior to the availability of drugs to treat psychiatric disorders. The surgical approach was probably abandoned
in the 1950s.
Long-term studies of individuals who have had frontal
lobotomies have shown profound personality changes in
these individuals. These people become emotionally
“flat” and lose some hard-to-define human quality in
their interpersonal interactions. In addition, such an individual might perform socially inappropriate acts not
in keeping with his or her personality prior to surgery.
Because the long-term effects of this surgery, which
eventually became clear, and because powerful and selective drugs are now widely available for various psychiatric conditions, this surgery is never performed now.
This surgical procedure had also been recommended for
the treatment of pain in terminal cancer patients, as part
of the palliative care of an individual. After the surgery,
the individual is said still to have the pain but no longer
to suffer from it; that is, the psychic aspect of the pain
has been removed. There may even be a reduced
demand for pain medication such as morphine. Again,
other approaches to pain management are now used.
Caudate nucleus (head)
Internal medullary lamina
Lentiform nucleus
Dorsomedial n.
Thalamus
Amygdala
Internal capsule
(anterior limb)
Ventral amygdalofugal
pathway
Prefrontal
projections
Prefrontal cortex
FIGURE 82: Limbic Diencephalon — Dorsomedial Nucleus
FIGURE 83A
HYPOTHALAMUS AND
LIMBIC MIDBRAIN
One of the core structures of the limbic system is the hypothalamus. The (prominent) mammillary nuclei are
part of the hypothalamus (see Figures 13 and 14). The
hypothalamus is closely connected to the septal region
(labeled but not marked in Figure 83A). It is also connected directly with nuclei of the midbrain which are
collectively called the limbic midbrain. There are also
some indirect connections to nuclei of the medulla via
descending autonomic fibers. Both parts of the brainstem are therefore highlighted in this illustration.
HYPOTHALAMUS
The hypothalamus is primarily responsible for the
control of homeostatic mechanisms, including water
balance, temperature regulation, and food intake (also
discussed as part of the introduction to the limbic
system). It accomplishes these tasks in two ways — as a
neural structure linked into the limbic system, and as a
neuroendocrine structure controlling the activities of
the pituitary gland. In its neural role, it acts as the head
ganglion of the autonomic nervous system, influencing
both sympathetic and parasympathetic activities.
Some of the major inputs to the hypothalamus come
from limbic structures, including the amygdala (via the
stria terminalis and the ventral pathway; see Figure 79B)
and the hippocampal formation (via the fornix ; see
Figure 77B). Stimulation of particular small areas of the
hypothalamus can lead to a variety of behaviors (e.g.,
sham rage) similar to that which occurs following stimulation of other parts of the limbic system (e.g., the
amygdala).
The hypothalamus is usually divided (see Figure 83B)
into a medial and lateral group of nuclei, with the third
ventricle between the two sides. A number of nuclei that
control the anterior pituitary gland are located in the
medial group. This occurs via the median eminence (see
Figure 14) and the portal system of veins along the pituitary stalk. Other nuclei in the supraoptic region connect
directly with the posterior pituitary via the pituitary stalk.
Running through the lateral mass of the hypothalamus
is a prominent fiber tract — the medial forebrain
bundle (see Figure 83B). This tract interconnects the hypothalamus with two areas — the septal region of the
forebrain (discussed with Figure 83B), and certain midbrain nuclei associated with the limbic system, the
limbic midbrain. All of the connections are reciprocal in
this system. Other fiber bundles connect the hypothalamus with the limbic midbrain.
The mammillary nuclei, large nuclei of the hypothalamus, are of special importance as part of the limbic
system. They receive a direct input from the hippocampal formation (via the fornix) and give rise to fibers that
connect directly to the limbic midbrain. In addition, a
major fiber tract, the mammillo-thalamic tract, connects
the mammillary nuclei with the thalamic anterior group
of nuclei as part of the Papez circuit (discussed with
Figure 81).
LIMBIC MIDBRAIN
A number of limbic pathways terminate within the reticular formation of the midbrain (see Figures 40B, 64, and
65), including the periaqueductal gray, leading to the
notion that this area be incorporated in the structures
that comprise the limbic system – thus use of the term
limbic midbrain.
Two limbic pathways — the medial forebrain bundle,
and the descending tract from the mammillary nuclei —
terminate in the midbrain reticular formation. From
there, apparently, descending pathways convey the “commands” to the parasympathetic and other nuclei of the
pons and medulla (e.g., the dorsal motor nucleus of the
vagus, the facial nucleus for emotional facial responses)
and areas of the reticular formation of the medulla concerned with cardiovascular and respiratory control
mechanisms (discussed with Figure 40A). Other connections are apparently made with autonomic neurons in
the spinal cord (e.g., for sympathetic-type
responses).
callosum
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Septal
region
Midbrain
Hypothalamic
nuclei
Medulla
Mammillary n.
FIGURE 83A: Hypothalamus and Limbic Midbrain
FIGURE 83B
SEPTAL REGION AND
MEDIAL
FOREBRAIN BUNDLE
SEPTAL REGION
The septal region includes both cortical and subcortical
areas that belong to the forebrain. The cortical areas are
found under the rostrum of the corpus callosum and
include the subcallosal gyrus (see Figures 74 and 75).
Nuclei lying deep in this region are called the septal
nuclei and in some species (not humans) are in fact
located within the septum pellucidum (the septum that
separates the anterior horns of the lateral ventricles; see
Figure 28A).
Several decades ago experiments were done in rats with
a small electrode implanted in the septal region; pressing
a bar completed an electrical circuit that resulted in a
tiny (harmless) electric current going through the brain
tissue. It was shown that rats will quickly learn to press a
bar to deliver a small electric current to the septal
region. In fact, the animals will continue pressing the
bar, virtually nonstop, even in preference to food! From
this it has been inferred that the animals derive some
type of pleasant sensation from stimulation of this
region. Some call this septal region the “pleasure center”;
it has since been shown that other areas can produce
similar behavior. However, this type of positive effect is
not seen in all parts of the brain, and, in fact, in some
areas an opposite (negative) reaction is seen.
The septal region receives input from the amygdala (via
the stria terminalis; see Figure 79B) and the hippocampal formation (via the fornix; see Figure 77B). The major
connection of the septal region with the hypothalamus
and the limbic midbrain occurs via the medial forebrain
bundle.
MEDIAL FOREBRAIN BUNDLE
Knowledge of the medial forebrain bundle of fibers is
necessary if one is to understand the circuitry of the
limbic system and how the limbic system influences the
activity of the nervous system. The medial forebrain
bundle (MFB) interconnects the septal region, the hy-
pothalamus, and the limbic midbrain; it is a two-way
pathway. Part of its course is through the lateral part of
the hypothalamus where the fibers become somewhat
dispersed (as illustrated). The MFB connects the septal
region with the hypothalamus and extends into the
limbic midbrain. It is relatively easy to understand how
the septal region and the hypothalamus can influence
the behavior of the animal (e.g., autonomic activity via
descending autonomic fibers).
Note to student: Visualization of the location of this
pathway is a definite challenge.
The Habenula (not illustrated)
In many texts, the habenular region of the diencephalon
is labeled and supplementary information is provided
about these structures.
The habenular nuclei are a group of small nuclei which
are part of the diencephalon. They are situated at the
posterior end of the thalamus, on its upper surface. The
pineal gland is attached to the brain in this region.
There is another circuit whereby septal influences are
conveyed to the midbrain. The first part of the pathway
is the stria medullaris (note that confusion of terminology is definitely a possibility) which connects the septal
nuclei (region) with the habenular nuclei. The stria
medullaris is found on the medial surface of the thalamus. The tract is seen in Figure 16, a view of the medial
aspect of the brain. It is not labeled but can be located
above the letter “T” on the diagram.
From the habenular nuclei, a tract descends to the midbrain reticular formation, mainly to a nucleus located
between the cerebral peduncles, the interpeduncular
nucleus (see cross section B2, Figure 65). This tract is
best called the habenulo-interpeduncular tract. In some
texts it is labeled the fasciculus retroflexus. The further
connections of the interpeduncular nucleus are unclear
but are thought to be similar to those of other midbrain
reticular formation nuclei which have a limbic function.
Fornix
Stria terminalis
Third ventricle
Dorsal longitudinal bundle
Septal region
“Limbic” midbrain
Anterior commissure
Mammillo-tegmental tract
Descending
autonomic
fibers
Medial
forebrain
bundle
Medulla
Midbrain
Temporal lobe
Parasympathetic nuclei
Medial forebrain bundle
Mammillary n.
Pituitary stalk
Hypothalamic nuclei
– medial
– lateral
Amygdala
FIGURE 83B: Septal Region and Medial Forebrain Bundle
FIGURE 84
OLFACTORY SYSTEM
The olfactory system, our sense of smell, is a sensory
system that inputs directly to the limbic system, and
does not have a thalamic nucleus. The olfactory system
is a phylogenetically older sensory system. Its size
depends somewhat on the species, being large in animals
which have a highly developed sense of smell; in
humans, the olfactory system is small. Its component
parts are the olfactory bulb and tract, and various areas
where the primary olfactory fibers terminate, including
the amygdala and the cortex over the uncal region.
Olfactory Nerve, Bulb, and Tract
The sensory cells in the nasal mucosa project their axons
into the CNS. These tiny fibers, which constitute the
actual peripheral olfactory nerve (CN I), pierce the cribriform plate in the roof of the nose and terminate in the
olfactory bulb, part of the CNS. There is a complex
series of interactions in the olfactory bulb, and one cell
type then projects its axon into the olfactory tract, a
CNS pathway.
The olfactory tract runs posteriorly along the inferior
surface of the frontal lobe (see Figures 13 and 14) and
terminates by dividing into lateral and medial tracts,
called stria. At this dividing point there are a number of
small holes for the entry of several blood vessels to the
interior of the brain, the anterolateral striate arteries
(discussed with Figure 60; see also Figures 85A and
85B). This triangular area is known as the anterior perforated space.
It is best to remember only the lateral tract as the principal tract of the olfactory system. It is said to have cortical tissue along its course for the termination of some
olfactory fibers. The lateral tract ends in the cortex of
the uncal area (see Figures 13 and 14), with some of the
fibers terminating in an adjacent part of the amygdaloid
nucleus (see also Figure 79B). It is interesting to note
that the olfactory system terminates directly in primary
olfactory areas of the cortex without a thalamic relay.
Olfactory Connections
The connections of the olfactory system involve limbic
cortex. These are called secondary olfactory areas and
include the cortex in the anterior portion of the
parahippocampal gyrus, an area that has been referred
to as entorhinal cortex. The term rhinencephalon refers
to the olfactory parts of the CNS, the “smell brain.” This
input of olfactory information into the limbic system
makes sense if one remembers that one of the functions
of the limbic system is procreation of the species. Smell
is important in many species for mating behavior and
for identification of the nest and territory.
Olfactory influences may spread to other parts of the
limbic system, including the amygdala and the septal
region. Through these various connections, information
can reach the dorsomedial nucleus of the thalamus.
Smell is an interesting sensory system. We have all had
the experience of a particular smell evoking a flood of
memories, often associated with strong emotional overtones. This simply demonstrates the extensive connections that the olfactory system has with components of
the limbic system and therefore with other parts of the
brain.
One form of epilepsy often has a significant olfactory
aura that precedes the seizure itself. In such cases, the
“trigger” area is often orbitofrontal cortex. This particular form of epilepsy has unfortunately been known as
uncinate fits. The name is derived from a significant association bundle which interconnects this part of the
frontal lobe and the anterior parts of the temporal lobe
— the uncinate bundle.
Diagonal Band
This obscure fiber bundle and an associated nucleus are
additional olfactory connections, some of which interconnect the amygdala with the septal region (see also
Figure 85B).
callosum
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Diagonal
band
Olfactory
bulb
Olfactory
tract
Lateral
olfactory
stria
FIGURE 84: Olfactory System
FIGURE 85A
BASAL FOREBRAIN
BASAL NUCLEUS
Figure 85A shows the basal forebrain using the same
diagram of the limbic system as in Figure 75. This area,
previously called the substantia innominata, contains a
variety of neurons. It is located below the anterior commissure and lateral to the hypothalamus. On the gross
brain, this region can be found by viewing the inferior
surface of the brain where the olfactory tract ends and
divides into medial and lateral stria (see Figures 13, 14,
and 84). This particular spot is the location where a
number of blood vessels, the striate arteries, penetrate
the brain substance, hence, it is called the anterior perforated space. The basal forebrain region is found above
this area.
The basal forebrain contains a group of diverse structures:
• clusters of large cells that are cholinergic, and which
have been collectively called the basal nucleus (of
Meynert);
• groups of cells that are continuous with the
amygdala, now called the extended amygdala;
• the ventral portions of the putamen and globus
pallidus, namely the ventral striatum and ventral
pallidum; and
• the nucleus accumbens, which may include a number
of diverse neurons within its boundaries.
Cholinergic Neurons
These are rather large neurons found in clusters
throughout this region. The clusters of cells are collectively called the basal nucleus (of Meynert).These cells
project to widespread areas of the prefrontal cortex, providing that area with cholinergic innervation.
Several years ago it was reported that there was a depletion of acetylcholine in the frontal lobe areas in
Alzheimer patients. Subsequent reports indicated that
this depletion was accompanied by a loss of these
cholinergic cells in the basal forebrain. Many thought
that the “cause” of Alzheimer’s disease had been uncovered: a cellular degeneration of a unique group of cells
and a neurotransmitter deficit. (The model for this way
of thinking is Parkinson’s disease.) These reports were
followed immediately by several therapeutic trials using
medication to boost the acetylcholine levels of the brain.
It is currently thought that cortical degeneration is the
primary event in Alzheimer’s disease, starting often in
the parietal areas of the brain. We now know that several
other neurotransmitters are depleted in the cortex in
Alzheimer’s disease. This information would lead us to
postulate that the loss of the target neurons in the prefrontal cortex, the site of termination for the cholinergic
neurons, would be followed, or accompanied, by the degeneration of the cholinergic cells of the basal forebrain.
Notwithstanding this current state of our knowledge,
therapeutic intervention to boost the cholinergic levels
of the brain is currently considered a valid therapeutic
approach, particularly in the early stages of this tragic
human disease. New drugs that maintain or boost the
level of acetylcholine in the brain are currently undergoing evaluation and apparently some improvement in
memory function has been reported, which may last for
a short while.
Extended Amygdala
A group of cells extends medially from the amygdaloid
nucleus and follows the ventral pathway (the ventral
amygdalofugal pathway; Figures 75, 79A, 79B, 82, and
83B) through this basal forebrain region. These neurons
receive a variety of inputs from the limbic cortical areas
and from other parts of the amygdala. Its output projects to the hypothalamus and to autonomic-related
areas of the brainstem, thereby influencing neuroendocrine, autonomic, and somatomotor activities.
callosum
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Basal forebrain
FIGURE 85A: Basal Forebrain — Basal Nucleus
FIGURE 85B
BASAL FOREBRAIN
BASAL GANGLIA
Figure 85B presents a somewhat schematic view of the
various nuclei located in the basal forebrain area. The
hypothalamus is shown in the midline, with the third
ventricle. The penetrating striate arteries are seen in the
anterior perforated area. This view shows the ventral
pathway emerging from the amygdala with some of the
fibers going to the hypothalamus and the others on their
way to the dorsomedial nucleus of the thalamus. The anterior commissure demarcates the upper boundary of
this area. The cell clusters that form the basal (cholinergic) nucleus are contained within this area but are not
portrayed.
Ventral Striatum and Pallidum
The lowermost portion of the putamen and globus pallidus are found in the basal forebrain area; they are referred to as the ventral striatum and ventral pallidum.
The ventral part of the striatum (the putamen) receives
input from limbic cortical areas, as well as a dopaminergic pathway from a group of dopamine-containing cells
in the midbrain. The information is then relayed to the
ventral pallidum (both parts of the globus pallidus are
seen on the left side of the diagram). This area has a significant projection to the dorsomedial nucleus of the
thalamus, hence to prefrontal cortex.
The overall organization is therefore quite similar to that
of the dorsal parts of the basal ganglia, although the sites
of relay and termination are different. Just as the amygdala is now considered a limbic nucleus, many now
argue that the ventral striatum and pallidum should be
included with the limbic system.
Nucleus Accumbens
This group of cells contains neurons that are part of the
basal ganglia and in part other possibly limbic neurons
(see also Figure 24). It has many of the connections of
the ventral striatum as well as those of the extended
amygdala. Its functional contribution is still unknown,
although it might be the neural area that becomes activated in situations that involve reward and punishment,
integrating certain cognitive aspects of the situation with
the emotional component. There is strong evidence that
this area is involved in addiction behavior in animals
and likely in humans.
In summary, the region of the basal forebrain has important links with other parts of the limbic system.
There is a major output to the prefrontal cortex, which
is considered by some to be the forebrain component of
the limbic system. The basal forebrain is thus thought to
have a strong influence on drives and emotions, as well
as higher cognitive functions that have an emotional
component. The cholinergic neurons in this area might
have a critical role in memory.
Septal region
Diagonal band
Nuclei of
diagonal band
Globus pallidus
Putamen
Ventral pallidum
Ventral putamen
Amygdala
Nucleus accumbens
(cut)
Anterior commissure
Hypothalamus
Optic tract
Third ventricle
Striate branches
Middle cerebral artery
FIGURE 85B: Basal Forebrain — Basal Ganglia
LIMBIC SYSTEM: SYNTHESIS
After studying the structures and connections of the
limbic system in some detail, a synthesis of the anatomical information with the notion of an “emotional” part
of the brain seems appropriate. It is not easy to understand how the limbic system is responsible for the reactions required by the definition of “emotion” proposed
in the introduction to this section.
The key structures of the limbic system are the limbic
lobe (the cortical regions, including the hippocampal
formation), the amygdala, the hypothalamus, and the
septal region. The limbic pathways interconnect these
limbic areas (e.g., the Papez circuit). In many ways it
seems that the limbic structures communicate with each
other. What is not clear is how activity in these structures influences the rest of the brain. How does the
limbic system influence changes in the physiological
systems, motor activity (behavior), and mental state
(psychological reactions)? The following discussion is
presented as a way of understanding the outcome or
output of limbic function.
Hormonal and Homeostatic Responses
Hormonal changes, as regulated by the hypothalamus,
are part of the physiological responses to emotional
states, both acute and chronic. The work of Dr. Hans
Selye, for example, has shown how chronic stress influences our body and mind. Complex motor actions are
often associated with responses to homeostatic changes.
Consider, for example, the motor activities associated
with thirst, temperature regulation, and satisfying other
basic drives. The amygdala and septal region are likely
involved in the motor patterns associated with these
basic drives.
Physiological Responses
Some of the responses to the emotional states involve
the autonomic nervous system which is regulated in the
hypothalamus. Limbic activity involves areas of the midbrain reticular formation and other brainstem nuclei in
specific ways. The best examples are perhaps the facial
expressions associated with emotions, the responses to
pain that are generated in part in the brainstem, and the
various parasympathetic nuclei controlling the pupil,
salivation, respiration, blood pressure, pulse, and various
gastrointestinal functions. The basic “fight or flight” response to emergency situations involves a considerable
number of physiological reactions.
Motor and Psychological Responses
The ventral parts of the basal ganglia and various cortical areas are likely the areas of the CNS involved with
the motor activities associated with emotional reactions.
Neocortical areas that are involved in limbic function
include portions of the prefrontal cortex, the cingulate
gyrus, and the parahippocampal gyrus. Activity in these
limbic cortices (and the associated thalamic nuclei) are
clearly candidates for the psychological reactions of
emotion.
In summary, the limbic system has many connections
outside itself through which it influences the hormonal,
autonomic, motor, and psychological functions of the
brain. The older cortical regions of the hippocampal formation seem to have an additional function related to
the formation of new episodic memories, specifically
related to events and factual information. Why this is so
and how this evolved is a matter of speculation.
One hopes that the basic activities of the limbic system
that are involved in preservation of the self and species
can be controlled and tamed by higher-order cortical influences, leading humankind to a more human and
hopefully a more humane millennium.
BRAINSTEM (HUMAN)
CROSS SECTIONS
The following figures are photographs of sections of the
human brainstem. Because of the complexity of this
part of the brain, including them seemed the best way to
assist the student to visualize the structures labeled in
the diagrams.
Each section is indentified with
• the part of the brainstem (e.g., upper midbrain),
• the section level (e.g., B1), as in Figures 62 and 63,
and
• the corresponding illustration in Section C (e.g.,
Figure 64).
The eight cross sections were matched as closely as possible with the diagrammatic illustrations of the brainstem shown in Figures 64–71 (from upper midbrain to
lower medulla).
Brains are removed at autopsy, usually for medical or
legal reasons. This specimen was selected because the
brain itself was not the primary reason for the death.
Preservation of the brain tissue is invariably less than
perfect in the following figures because of the delay
between the time of death and the autopsy. The brains
are usually stored in 20% formalin.
The brainstem was subdivided at approximately the
levels indicated in Figures 62 and 63, embedded in
paraffin, and sectioned at 15 µm. The sections were then
stained with the Kluver-Barrera stain which used luxolfast blue to stain the myelinated fibers blue. The sections
were counterstained with cresyl violet, resulting in a
violet-reddish color of the neuronal cell bodies, the
perikaryon. (The Nissl bodies are violet at higher magnification, with the cell cytoplasm taking on a reddish
hue). In a black and white illustration, the areas with
myelinated tracts are black, whereas the areas with cells
and virtually no myelin are “clear” or white.
These sections are not labeled here, and the student
should refer to the corresponding level in the Atlas to
identify the structures. These same sections are on the
accompanying CD, in color, and are labeled there.
Human brainstem: Upper Midbrain — B1 level; see Figure 64
Human brainstem: Lower Midbrain — B2 level; see Figure 65
Human brainstem: Upper Pons — B3 level; see Figure 66
Human brainstem: Mid Pons — B4 level; see Figure 67
Human brainstem: Lower Pons — B5 level; see Figure 68
Human brainstem: Upper Medulla — B6 level; see Figure 69
Human brainstem: Mid Medulla — B7 level; see Figure 70
Human brainstem: Lower Medulla — B8 level; see Figure 71
ANNOTATED BIBLIOGRAPHY
This select list of references with commentary is designed to help the student learn about the structure,
function, and diseases of the human brain. Recent publications have usually been preferentially selected. The
listing includes texts, atlases, and videotapes, as well as
CD-ROMs and web sites.
This edited large text, with many color illustrations, is
an excellent reference book, mainly for neuroanatomical
detail.
TEXTS
A thorough presentation of the functional (physiological) aspects of the nervous system, which is suitable as a
reference book and for graduate students.
Adams, R. D., Victor, M., and Ropper, A. H., Principles of
Neurology, 6th ed., McGraw-Hill, New York, 1997.
A comprehensive neurology text — part devoted to cardinal manifestations of neurologic diseases and part to
major categories of diseases.
Asbury, A. K., McKhann, G. M., and McDonald, W. I.,
Diseases of the Nervous System: Clinical Neurobiology,
2nd ed., W. B. Saunders, Philadelphia, 1992.
A complete neurology text, in two volumes, on all
aspects of basic and clinical neurology and the therapeutic approach to diseases of the nervous system.
Carpenter, M. B., Core Text of Neuroanatomy, 4th ed.,
Williams and Wilkins, Baltimore, 1991.
A detailed presentation of the subject matter by a highly
respected author. Also available is the full reference text
[Carpenter’s Human Neuroanatomy (1995), now with A.
Parent as the author].
Crossman, A. R. and Neary, D., Neuroanatomy, Churchill
Livingstone, Edinburgh, 1995.
A concise, well-illustrated (with color) presentation of
the nervous system with many clinical correlations. This
slim book contains much essential information.
Guyton, A. C., Basic Neuroscience: Anatomy and
Physiology, 2nd ed., W. B. Saunders, Philadelphia, 1991.
This is a sufficient explanation of the physiological
aspects of the nervous system, with handy diagrams.
Haines, D. E., Fundamental Neuroscience, Churchill
Livingstone, New York, 1997.
Kandel, E. R., Schwartz, J. H., and Jessell, T. M.,
Principles of Neural Science, 3rd ed., Elsevier, New
York, 1991.
Kiernan, J. A., The Human Nervous System, 7th ed.,
Lippincott-Raven, Philadelphia, 1998.
This is the book previously authored by Dr. M. Barr.
Clearly written and clearly presented neuroanatomical
information, with a glossary and accompanying diskette.
Kolb, B. and Whishaw, I. Q., Fundamentals of Human
Neuropsychology, 4th ed., W. H. Freeman and Co., New
York, 1996.
A classic in the field and highly recommended for a
good understanding of the human brain in action.
Topics discussed include memory, attention, language,
and the limbic system.
Martin, J. H., Neuroanatomy: Text and Atlas, 2nd ed.,
Appleton & Lange, Stamford, CT, 1996.
A very complete text with some fine illustrations,
written as the neuroanatomical companion to Kandel et
al. Includes an atlas section.
Merritt, H. H., Merritt’s Textbook of Neurology, 9th ed.,
Williams and Wilkins, Baltimore, 1997.
A well-known, complete and trustworthy neurology
textbook, now edited by L. P. Rowland.
Nolte, J., The Human Brain, 4th ed., Mosby, St. Louis,
1999.
An excellent neuroscience text with anatomical and
functional (physiological) information on the nervous
system. Includes several hundred illustrations in full
color, along with three-dimensional brain reconstructions by John Sundsten.
Williams, P. and Warwick, R., Functional Neuroanatomy
of Man, W. B. Saunders, Philadelphia, 1975.
This is the “neuro” section from Gray’s Anatomy.
Although somewhat dated, there is excellent reference
material on the central nervous system, as well as on the
nerves and autonomic parts of the peripheral nervous
system.
Wilson-Pauwels, L., Akesson, E. J., and Stewart, P.A.,
Cranial Nerves: Anatomy and Clinical Comments, B. C.
Decker, Toronto, 1988.
A handy resource on the cranial nerves, with some very
nice illustrations. Relatively complete and easy to follow.
Young, P.A. and Young, P. H., Basic Clinical
Neuroanatomy, Williams and Wilkins, Baltimore, 1997.
A clearly presented and well-illustrated (no color) text
that integrates structure, function, and clinical aspects.
Includes questions and answers, an appendix on cranial
nerves, and a glossary, as well as a small atlas.
ATLASES
DeArmond, S. J., Fusco, M. M. and Dewey, M. M.,
Structure of the Human Brain: A Photographic Atlas,
2nd ed., Oxford University Press, New York, 1976.
An excellent and classic reference to the neuroanatomy
of the human CNS. No explanatory text and no color.
England, M. A. and Wakely, J., Color Atlas of the Brain
and Spinal Cord, Mosby Year Book, St. Louis, 1991.
A very well-illustrated atlas, with most photographs and
sections in color. Little in the way of explanatory text.
Haines, D., Neuroanatomy: An Atlas of Structures,
Sections and Systems, 4th ed., Williams and Wilkins,
Baltimore, 1995.
A popular atlas that has some fine photographs of the
brain, some color illustrations of the vascular supply, a
radiologic section, a very detailed atlas portion, and
limited (schematic) presentation with text of functional
systems.
Hanaway, J., Woolsey, T. A., Gado, M. H., and Roberts,
M. P., The Brain: A Visual Guide to the Human Central
Nervous System, Fitzgerald Science Press, Maryland,
1998.
A complete pictorial atlas of the human brain, with
some color illustrations and radiographic material.
Netter, F. H., The CIBA Collection of Medical Illustrations,
Volume 1: Nervous System, Part 1, Anatomy and
Physiology, CIBA, Summit, NJ, 1983.
A classic. Excellent illustrations of the nervous system, as
well as of the skull, autonomic and peripheral nervous
systems, and embryology. The text is interesting but
might be dated.
Nolte, J. and Angevine, J. B., The Human Brain in
Photographs and Diagrams, Mosby, St. Louis, 1995.
A well-illustrated, color atlas, with text, and neuroradiology. Functional systems are drawn both onto the sections and separately. Quite detailed.
VIDEOTAPES BY THE AUTHOR
These are edited video presentations on the skull and the
brain as the material would be shown to students in the
anatomy laboratory.
They have been prepared with the same teaching orientation as the Atlas and are particularly useful for selfstudy or small groups.
These videotapes of actual specimens are particularly
useful for students who have limited or no access to
actual brain specimens. The videotapes are fully narrated, and each lasts approximately 20–25 minutes.
Interior of the Skull — A detailed look at the bones of
the skull, the cranial fossa, and the various foramina for
the cranial nerves and other structures. Includes views of
the meninges and venous sinuses.
The Gross Anatomy of the Human Brain
Part I: The Hemispheres — A presentation on the hemispheres, the functional areas of the cerebral cortex, including the basal ganglia.
Part II: Diencephalon, Brainstem and Cerebellum — A
detailed look at the brainstem, with a focus on the
cranial nerves, and a functional presentation of the
cerebellum.
Part III: Cerebrovascular System and Cerebrospinal Fluid
— A presentation of these two subjects.
Part IV: The Limbic System — A quite detailed presenta-
tion on the various aspects of the limbic system, with
much explanation and special dissections.
Note: It is suggested that these videotapes be purchased
by the library or by an institutional (or departmental)
media or instructional resource center. Information regarding the purchase of these and other videotapes may
be obtained from Health Sciences Consortium (a nonprofit publishing cooperative for instructional media),
201 Silver Cedar Ct., Chapel Hill, NC, USA, 27514-1517.
Phone: (919) 942-8731. Fax: (919) 942-3689.
CD-ROMS
Numerous CDs are appearing on the market and their
evaluation by the teaching faculty is critical before recommending them to the students. It is a difficult task
indeed to review all the CDs now available and perhaps
one that can be shared with the students after they have
completed their program of study on the nervous
system.
Sundsten, John W., The Digital Anatomist - Interactive
Brain Atlas, University of Washington.
An excellent visual resource to the nervous system, using
computer graphic reconstructions of the brain.
Sundsten, John W. and Mulligan, K. A., The Digital
Anatomist — Neuroanatomy Interactive Syllabus,
University of Washington.
A more functional presentation of the nervous system,
with some explanatory text, using many of the same
images.
Both CDs are available from University of Washington,
Health Sciences Center for Educational Resources
Box 357161, Seattle, WA 98195-7161
Phone: (206) 685-1186. Fax: (206) 543-8051.
e-mail: [email protected]
Coppa, Gary and Tancred, Elizabeth, Brainstorm:
Interactive Neuroanatomy, Stanford University.
A highly interactive and well-integrated cross-linked
presentation of the anatomy and some functional
aspects of the nervous system. Published by Mosby,
11830 Westline Industrial Drive, P.O. Box 46908, St.
Louis, MO., USA, 63146-9934
WEB SITES
Web sites are recommended to students only after the
sites have been critically evaluated by the teaching
faculty. If keeping up with various teaching texts and
CD-ROMs is difficult enough, a critical evaluation of the
various web sites is an impossible task for any single
person. This is indeed a task to be shared with colleagues
and students, and perhaps by a consortium of teachers.
At this point, only a limited number of sites can be
suggested. For the time being, it is still an open playing
field.
General Neuroscience
http://faculty.washington.edu/chudler/ehc.html
This site is maintained by Eric H. Chudler, Ph.D.,
Research Associate Professor, Dept. of Anesthesiology,
University of Washington, Box 356540, Seattle, WA
98195-6540, [email protected].
This site focuses on systems neuroscience and includes
numerous excellent Internet resources concerning all
other aspects of neuroscience. Links on this page are
limited to those that the author has found to be the
most interesting and useful.
Examples of links:
• Neuroscience Education
• Neuroscience Journals
• Neuroscience, Psychology
• Neurological Disorders (Alzheimer’s Disease,
Huntington’s)
• Searching the Web
• Search Engines
• Miscellaneous Links
• Reference, Science
• Neuroscience for Kids
Neurological Disorders
http://www.dana.org/brainweb/
This site has links to information on brain diseases and
disorders maintained by the Dana Foundation. The
Dana Alliance for Brain Initiatives, an independent nonprofit organization of more than 175 pre-eminent neuroscientists, including six Nobel laureates, recommends
the Internet sites reviewed below as helpful resources for
people concerned about brain diseases and disorders.
Send your comments and questions to
[email protected].
The links include the following diseases:
• Alcohol and drug abuse
• Alzheimer’s disease
• ALS (Lou Gehrig’s disease)
• Anxiety
• Autism
• Blindness/vision impairment
• Brain tumors
• Cerebral palsy
• Deafness/hearing impairment
• Depression/manic depression
• Epilepsy
Biological Structure, University of Washington, Seattle,
WA.
The Atlas is available on CD-ROM and there are similar
movie materials on the videodisc from the University of
Washington Health Sciences Center for Educational
Resources (see CD-ROMS above).
Neuroanatomy Interactive Syllabus
Authors: John W. Sundsten and Kathleen A. Mulligan
Content: This Syllabus uses the images in the
Neuroanatomy Atlas above, and many others. It is organized into functional chapters suitable as a laboratory
guide, with an instructive text accompanying each
image. It contains 3-D computer graphic reconstructions of brain material, MRI scans, tissue sections (some
enhanced with pathways) gross brain specimens and dissections; and summary drawings.
• Headache
• Head injury
• Huntington’s disease
• Learning disabilities
• Multiple sclerosis
• Pain (chronic)
• Parkinson’s disease
• Schizophrenia
• Sleep disorders
• Spinal cord injury
• Stroke
• Tourette syndrome
• General health and neuroscience sites
Inclusion of any particular organization does not imply
endorsement by the Charles A. Dana Foundation or the
Dana Alliance for Brain Initiatives. The information
provided is not a substitute for medical advice; be sure
to consult your doctor for diagnosis and treatment.
Digital Anatomist Project
http://www9.biostr.washington.edu/
Interactive Brain Atlas
Author: John W. Sundsten
Content: 2-D and 3-D views of the brain from cadaver
sections, MRI scans, and computer reconstructions.
Institution: Digital Anatomist Program, Dept. of
Chapters include:
• Topography and development
• Vessels and ventricles
• Spinal cord, brainstem, and cranial nerves
• Sensory and motor systems
• Cerebellum and basal ganglia
• Eye movements
• Hypothalamus and limbic system
• Cortical connections and forebrain
• MRI scan serial sections
Institution: Digital Anatomist Program, Department of
Biological Structure, University of Washington,
Seattle,WA.
Note: The Neuroanatomy Interactive Syllabus is available
on CD-ROM (Java program running on Mac and PC
platforms) from the University of Washington Health
Sciences Center for Educational Resources. See above
under CD-ROMs.
E-mail: [email protected]
The Whole Brain Atlas
http://www.med.harvard.edu:80/AANLIB/home.html
Authors: K. Johnson (Harvard) and J. Becker (MIT)
This presents a gallery of images on the normal and diseased brain(cerebrovascular, tumor, degenerative
conditions).
GLOSSARY
Abducens nerve
Sixth cranial nerve (CN VI); to
lateral rectus muscle of the eye.
Accessory nerve
Eleventh cranial nerve (CN XI);
see Spinal accessory nerve.
Afferent
Toward (sensory if toward the
CNS).
Agnosia
Lack of ability to recognize the
significance of sensory stimuli
(auditory, visual, tactile).
Agraphia
Inability to express thoughts in
writing because of a central
lesion.
Akinesia
Absence, loss, or weakness of
motor function; lack of spontaneous movement (as in
Parkinson’s disease).
Alexia
Word blindness; inability to read
due to a central lesion.
Allocortex
The phylogenetically older cerebral cortex, consisting of less than
six layers. Includes paleocortex
(e.g., subicular region = 3–5
layers) and archicortex (e.g., hippocampus proper = 3 layers).
Ammon’s horn
Amygdala
Angiogram
Anopsia
The hippocampus, which has an
outline in cross section suggestive
of a ram’s horn. Also known as
the cornu ammonis (CA).
Amygdaloid nucleus or body in
the temporal lobe of the cerebral
hemisphere. It is a nucleus of the
limbic system.
Display of blood vessels, for diagnostic purposes, by using contrast
medium injected into the vascular
system and x-rays or using MRI.
A defect of vision (e.g., hemianopsia — loss of one half of
visual field).
Antidromic
Relating to the propagation of an
impulse along an axon in a direction that is the reverse of the
normal or usual direction.
Aphasia
A disruption or disorder of language; specifically a deficit of
expression by speech or of comprehending spoken or written
language.
Apraxia
Inability to carry out purposeful
or skilled movements despite the
preservation of power, sensation,
and coordination.
Arachnoid
The middle meningeal layer,
forming the outer boundary of
the subarachnoid space.
Archicerebellum
A phylogenetically old part of the
cerebellum, functioning in the
maintenance of equilibrium.
Includes flocculonodular lobe.
Archicortex
Three-layered cortex included in
the limbic system; located mainly
in the hippocampus proper and
dentate gyrus of the temporal
lobe.
Area postrema
An area in the caudal part of the
floor of the fourth ventricle, with
no blood-brain-barrier, involved
in vomiting.
Areflexia
Loss of reflexes (usually tested
using the stretch/deep tendon
reflex).
Ascending tract
Central sensory pathway, usually
from spinal cord to brainstem,
cerebellum, or thalamus.
Association fibers
Fibers connecting parts of the
cerebral hemisphere, on the same
side.
Astereognosis
Loss of ability to recognize objects
or to appreciate their form by
touching or feeling them.
Astrocyte
A type of neuroglial cell.
Asynergy
Disturbance of the proper sequencing in the contraction of
muscles, at the proper moment,
and of the proper degree, so that
the act is not executed accurately
or smoothly.
Ataxia
Athetosis
Autonomic
A loss of coordination of
voluntary movements.
An affliction of the nervous
system, caused by degenerative
changes in the striatum, characterized by bizarre, writhing movements of the fingers and toes.
Autonomic system; usually taken
to mean the efferent or motor
innervation of viscera (smooth
muscle and glands).
the subthalamus, and the substantia nigra.
Basilar artery
The major artery supplying the
brainstem and cerebellum,
formed by the two vertebral
arteries.
Brachium
With regard to the CNS, denotes a
large bundle of fibers connecting
one part with another (e.g.,
brachia associated with the colliculi of the midbrain).
Bradykinesia
Abnormal slowness of movements
(seen usually in Parkinson’s
disease).
Brainstem
In the mature human brain,
usually denotes the medulla, pons,
and midbrain.
Brodmann areas
Numerical subdivisions of the
cerebral cortex on the basis of histological differences between different functional areas (e.g., area 4
is the motor cortex; area 17 is the
primary visual area).
Bulb
Referred at one time to the
medulla oblongata but, in the
context of “cortico-bulbar tract,”
refers to the brainstem, in which
motor nuclei of cranial nerves are
located.
CAT or CT scan
Computerized axial tomography
— a diagnostic imaging technique
that uses x-rays and computer
reconstruction of the brain.
Carotid siphon
Hairpin bend of the internal
carotid artery within the skull.
Cauda equina
Translates as horse’s tail; the lower
lumbar, sacral, and coccygeal
spinal nerves as they lie in the
subarachnoid space within the
lumbar (CSF) cistern.
Caudal
Towards the tail, or hindmost
part, of neuraxis.
Caudate nucleus
Part of the neostriatum, consists
of a head, body, and tail (which
extends into the temporal lobe).
Autonomic nervous Visceral innervation; sympathetic
system
and parasympathetic divisions.
Axon
Efferent process of a neuron conducting impulses to other neurons
or to muscle fibers (striated and
smooth) and gland cells.
Babinski reflex
Actually an incorrect term —
should be a Babinski response.
Stroking the outer border of the
sole of the foot in an adult normally results in a plantar (downgoing) of the toes. The Babinski
response indicates a lesion of the
Pyramidal tract and consists of an
upgoing of the first toe and a
fanning of the other toes.
Ballismus
Basal ganglia
(nuclei)
Violent jerking or flinging movements of a limb, usually on one
side (hemiballismus) or of one
limb, due to a lesion of the
subthalamic nucleus.
Nuclei involved in motor control,
the caudate, putamen and globuspallidus (the lentiform nucleus),
Central nervous
system
Cerebellar peduncle Inferior, middle, and superiorfiber tracts linking cerebellum and
brainstem.
Cerebellum
Cerebral aqueduct
(of Sylvius)
Cerebral peduncle
(The little brain) An older part of
the brain with motor functions,
dorsal to the brainstem, situated
in the posterior cranial fossa.
Passage carrying CSF through
midbrain; part of ventricular
system.
Descending cortical fibers in the
basal portion of the midbrain;
sometimes includes the substantia
nigra (located immediately
behind).
Cerebrospinal fluid
CSF; fluid in ventricles and in
subarachnoid space (and cisterns).
Cerebrum
The principal portion of the
brain, including the diencephalon
and cerebral hemispheres, but not
the brainstem and cerebellum.
Cervical
Referring to the neck region; the
part of the spinal cord that supplies the structures of the neck.
Chorda tympani
Part of the seventh cranial nerve
(CN VII) (see Facial nerve) ;
carrying taste from anterior two
thirds of tongue and parasympathetic innervation.
Chordotomy
Cutting of the spinothalamic tract
for intractable pain (tractotomy).
Also spelled cordotomy.
Chorea
A disorder characterized by irregular, spasmodic, involuntary
movements of the limbs or facial
muscles. Attributed to degenerative changes in the neostriatum.
Choroid
A delicate membrane; choroid
plexuses are found in the
ventricles of the brain.
Choroid plexus
Vascular structures “secreting”
CSF into ventricles.
Cingulum
A bundle of association fibers in
the white matter under cingulate
gyrus; part of Papez (limbic)
circuit.
CNS; brain and spinal cord.
Circle of Willis
Anastomosis between internal
carotid and basilar arteries
around pituitary.
Cistern(a)
Expanded portion of subarachnoid space containing CSF, e.g.,
cisterna magna (cerebellomedullary cistern); lumbar
cistern.
Claustrum
A thin sheet of gray matter, of
unknown function, situated
between the lentiform nucleus
and the insula.
CNS
Abbreviation for central nervous
system.
Colliculus
A small elevation or mound.
Superior and inferior colliculi
comprising the tectum of the
midbrain; facial colliculus in the
floor of the fourth ventricle.
Commissure
A bundle of nerve fibers connecting structures on one side to the
other in the hemispheres (e.g.,
corpus callosum).
Conjugate eye
movements
Movement of both eyes together
(so that image falls on the corresponding points of both retinas).
Contralateral
On the opposite side.
Corona radiata
Fibers radiating from the internal
capsule to various parts of the
cerebral cortex. A term often used
by neuroradiologists.
Corpus callosum
The main (largest) neocortical
commissure of the cerebral
hemispheres.
Corpus striatum
Caudate, putamen, and globus
pallidus nuclei inside cerebral
hemisphere, with motor function;
part of the basal ganglia.
Cortex
Outer layer of gray matter
(neurons and neuropil) of the
cerebral hemispheres (mostly six
layers) and cerebellum (three
layers).
Cortico-bulbar tract Descending tract connecting
motor cortex with motor cranial
nerve nuclei and other nuclei of
brainstem (including reticular
formation).
Cortico-fugal fibers
Axons carrying impulses away
from the cerebral cortex.
Cortico-petal fibers
Axons carrying impulses towards
the cerebral cortex.
Cortico-spinal tract Descending tract, from motor
cortex to anterior (ventral) horn
cells of the spinal cord (sometimes direct); also called
Pyramidal tract.
Cranial nerve nuclei Collections of cells in brainstem
giving rise to or receiving fibers
from cranial nerves (CN III–XII);
may be sensory, motor, or
autonomic.
Cranial nerves
Twelve pairs of nerves arising
from the brain and innervating
structures of the head and neck
(CN I and II are actually CNS
tracts).
CSF
Cerebrospinal fluid in ventricles,
subarachnoid space, and cisterns.
Cuneatus
(Cuneate) Sensory tract (fasciculus cuneatus) of the dorsal
column of spinal cord, from the
upper limbs and body; cuneate
nucleus of medulla.
Decerebrate
posturing
Characterized by extension of the
upper and lower limbs; lesion at
the brainstem level between the
vestibular nuclei and the red
nucleus.
Decorticate
posturing
Characterized by extension of the
lower limbs and flexion of the
upper; lesion above the level of
the red nucleus.
Decussation
The point of crossing of paired
tracts. Decussations of the
pyramids, medial lemnisci, and
superior cerebellar peduncles are
examples.
Dendrite
Receptive process of a neuron.
Dendritic spine
Cytoplasmic excrescence of a
dendrite and site of an excitatory
synapse (can be visualized using
special stains at the light
microscopic level).
Dentate
(Toothed) Dentate nucleus of the
cerebellum (intracerebellar
nucleus); dentate gyrus of the
hippocampal formation.
Descending tract
Central motor pathway (e.g., from
cortex to brainstem or spinal
cord).
Diencephalon
Consisting of the thalamus, epithalamus (pineal), subthalamus,
and hypothalamus.
Diplopia
Double vision.
Dominant
hemisphere
The hemisphere responsible for
language; this is the left hemisphere in about 85–90% of
people.
Dorsal column
Fasciculus (tract) gracilis and fasciculus cuneatus of spinal cord,
pathways for fine touch and
conscious proprioception and
vibration.
Dorsal root
Afferent sensory component of
spinal nerve.
Dorsal root
ganglion
A group of peripheral neurons
whose axons carry afferent information from the periphery; their
central process enters the spinal
cord.
Dura
Dura mater, the thick external
layer of the meninges (brain and
spinal cord).
Dural venous
sinuses
Large venous channels for draining blood from the brain; run
within dura mater of the skull.
Dyskinesia
Abnormality of motor function,
characterized by involuntary,
purposeless movements.
Fasciculus
A large tract or bundle of nerve
fibers.
Disturbance of the ability to
control the range of movement in
muscular action.
Fasciculus cuneatus Part of dorsal column; ascending
tract for conscious proprioception
and discriminative touch (from
upper body and upper limb).
Dysphagia
Difficulty in swallowing.
Fasciculus gracilis
Efferent
Away from the central nervous
system; usually means motor to
muscles.
Part of dorsal column; ascending
tract for conscious proprioception
and discriminative touch (from
lower body and lower limb).
Emboliform
Emboliform nucleus of the cerebellum, one of the intracerebellar
(deep cerebellar) nuclei.
Fastigial nucleus
One of the deep cerebellar
(intracerebellar) nuclei.
Fiber
Synonymous with an axon (either
peripheral or central).
Fimbria
A band of nerve fibers along the
medial edge of the hippocampus,
continuing as the fornix.
Flaccid paralysis
Muscle paralysis with hypotonia
due to a lower motor neuron
lesion.
Flocculus
Lateral part of flocculonodular
lobe (vestibulocerebellum).
Folium/folia
A flat, leaflike fold of the cerebellar cortex. Plural is folia.
Foramen
An opening between spaces (e.g.,
Monro and Magendie).
Foramen of
Luschka
Lateral foramen of fourth
ventricle.
Foramen of
Magendie
Median foramen of fourth
ventricle.
Foramen of
Monro
Between each lateral ventricle and
third ventricle.
Forebrain
Anterior division of embryonic
brain; cerebrum and
diencephalon.
Fornix
The efferent (noncortical) tract of
hippocampal formation, arching
over the thalamus and terminating in the mammillary nucleus of
the hypothalamus and in the
septal region.
Fourth ventricle
Cavity between brainstem and
cerebellum, containing CSF.
Dysmetria
Entorhinal
Ependyma
The entorhinal area is the anterior
part of the parahippocampal
gyrus of the temporal lobe adjacent to the uncus. It is involved
with olfaction (smell).
Lining epithelium of the ventricles of brain, choroid plexus (with
specialized junctions here), and
also central canal of spinal cord.
Epithalamus
A region of the diencephalon
above the thalamus; includes the
habenula and pineal body.
Extrapyramidal
system
In broadest terms, consists of all
motor parts of the central nervous
system except the Pyramidal
motor system. “extrapyramidal
system” is subject to various interpretations and is most often used
clinically to mean basal ganglia.
Facial nerve
Falx
Seventh cranial nerve (CN VII);
Motor to muscles of facial expression; carries taste from anterior
two thirds of tongue (see also
chorda tympani); also parasympathetic to two salivary glands,
lacrimal, and nasal glands.
Two of the dural partitions in the
cranial cavity: the large falx
cerebri between the cerebral
hemispheres, and the small falx
cerebelli.
Frontal lobe
Part of cerebral hemisphere, in
front of central fissure.
Funiculus
A large aggregation of white
matter in the spinal cord, can
contain several tracts.
Ganglion/ganglia
Geniculate bodies
Genu
Glial cell
Globus pallidus
A swelling composed of nerve
cells, as in dorsal root and sympathetic ganglion. Also used inappropriately for certain regions of
gray matter in the brain (e.g.,
basal ganglia of the cerebral hemisphere). Plural is ganglia.
Medial and lateral, specific relay
nuclei of thalamus, for auditory
(medial) and visual (lateral)
pathways.
Knee or bend; middle part of internal capsule; genu of facial
nerve. Also geniculate nuclei of
thalamus, and geniculate ganglion
of facial nerve.
Supporting cell in central nervous
system (astrocyte and oligodendrocyte); also called neuroglial
cell.
Medial part of lentiform nucleus
of corpus striatum; efferent part
of basal ganglia.
Glossopharyngeal
nerve
Ninth cranial nerve (CN IX); to
muscles of swallowing and carries
taste from posterior one third of
tongue. Needed for gag reflex.
Gracilis
(Gracile) Sensory tract (fasciculus
gracilis) of the dorsal column of
spinal cord; nucleus gracilis of
medulla.
Granule
Used to denote small neurons,
such as granule cells of cerebellar
cortex and stellate cells of cerebral
cortex. Hence granular cell layers
of both cortices.
Gray matter
Nerve tissue, mainly nerve cell
bodies and adjacent neuropil;
looks grayish after fixation in
formalin.
Gyrus/gyri
A convoluted fold of a cerebral
hemisphere; includes cortex and
white matter. Plural is gyri.
Habenula
A small swelling in the epithalamus, adjacent to the posterior end
of the roof of the third ventricle;
part of the limbic system.
Hemiballismus
A violent form of motor restlessness involving one side of the
body, caused by a destructive
lesion involving the subthalamic
nucleus.
Hemiparesis
Muscular weakness affecting one
side of the body.
Hemiplegia
Paralysis of one side of the body.
Hindbrain
Posterior division of the embryonic brain; pons, medulla, and
cerebellum of adult.
Hippocampus/
hippocampus
proper
Specialized area of phylogenetically old (three-layered) cortex;
located in medial part of
temporal lobe; part of
hippocampal formation of limbic
system.
Hydrocephalus
Excessive accumulation of cerebrospinal fluid.
Hyperreflexia
Abnormal increase in muscle
(deep tendon/stretch) reflexes;
usually seen with spasticity as a
result of upper motor neuron
lesion.
Hypertonia
Increased tone of muscles manifested by increased resistance to
passive stretch or movements.
Hypoglossal nerve
Twelfth cranial nerve (CN XII); to
muscles of the tongue.
Hypothalamus
A region of the diencephalon that
serves as the main controlling
center of the autonomic nervous
system and is involved in several
limbic circuits. Also involved in
regulation of the pituitary gland.
Infarction
Local death of tissue because of
loss of blood supply.
Infundibulum
(Funnel) Infundibular stem of
the neurohypophysis (posterior
pituitary).
Innervation
Nerve supply, sensory or motor.
Insula
(Island) Cerebral cortex concealed
from surface view and lying at the
bottom of the lateral fissure (also
called the island of Reil).
Internal capsule
White matter between lentiform
nucleus and head of caudate
nucleus, and thalamus; consists of
anterior limb, genu, and posterior
limb.
Internal carotid
artery
One of the pair of arteries supplying the brain.
Interventricular
foramen
(of Monro); two openings from
each lateral ventricle into third
ventricle.
Ipsilateral
On the same side.
Isocortex
Cerebral cortex having six layers
(neocortex).
Kinesthesia
The sense of perception of
movement.
Lacune
Irregularly shaped venous “lake”
or channel draining into the
superior sagittal sinus; also the
pathological “hole” after an
infarct in the internal capsule.
Lateral foramen
(Foramen of Luschka) Openings
in lateral edges of fourth ventricle
for escape of CSF into subarachnoid space (cerebello-medullary
cistern).
Lateral ventricle
Cavity, one in each cerebral hemisphere, containing CSF; consists
of anterior horn, body, atrium
(or trigone), posterior horn, and
inferior horn (in temporal lobe).
Lemniscus
Used to designate a bundle of
nerve fibers (pathway) in the
central nervous system (e.g.,
medial lemniscus and lateral
lemniscus).
Lentiform
Lens-shaped; lentiform nucleus, a
component of the basal ganglia.
Also called lenticular nucleus;
composed of putamen and globus
pallidus.
Leptomeninges
Arachnoid and pia mater, part of
meninges.
Lesion
Any injury or damage — vascular,
tumor, traumatic, etc.
Limbic system
Part of brain associated with
emotional behavior.
Limbus
(Border) Limbic lobe; a C-shaped
configuration of cortex on the
medial surface of the cerebral
hemisphere, consisting of the cingulate and parahippocampal gyri,
and the hippocampal formation.
Locus ceruleus
A small nucleus of the uppermost
pons on each side of the floor of
the fourth ventricle; contains
pigment in freshly sectioned brain.
Lower motor
neurons
Anterior horn cells (and their
axons) of spinal cord, or cells in
motor cranial nerve nucleus; also
alpha motor neuron.
Lumbar
Referring to the lower back
region.
Mammillary
Mammillary bodies; nuclei of the
hypothalamus which are seen as
small swellings on the ventral
surface of diencephalon (also
spelled mamillary).
Massa intermedia
A bridge of gray matter connecting the thalami of the two sides
across the third ventricle; present
in 70% of human brains. Also
called the inter-thalamic
adhesion.
Medial lemniscus
Brainstem portion of sensory
pathway for fine touch and conscious proprioception, after
synapse in nucleus gracilis and
nucleus cuneatus.
Medial longitudinal A tract throughout the brainstem
fasciculus (MLF) and cervical spinal cord which interconnects visual and vestibular
input with movements of the eyes
and the head and neck.
Neocortex
Six-layered cortex, characteristic
of mammals and constituting
most of the cerebral cortex in
humans.
Neostriatum
The phylogenetically newer part
of the basal ganglia consisting of
the caudate nucleus and putamen;
the striatum.
Medulla
Caudal portion of the brainstem;
in current usage, “medulla” refers
to the medulla oblongata.
Meninges
Covering layers of the CNS —
dura, arachnoid, and pia.
Nerve fiber
Axonal cell process, plus sheathing
cells, plus myelin if present.
Mesencephalon
The midbrain (upper part of the
brainstem).
Neuraxis
Metathalamus
The medial and lateral geniculate
bodies (nuclei).
The straight longitudinal axis of
the embryonic or primitive neural
tube, bent in later evolution and
development.
Midbrain
The middle division of the embryonic brain, part of the adult
brainstem. Also known as
mesencephalon.
Neuroglia
Accessory or interstitial cells of
the CNS; includes astrocytes,
oligodendrocytes, microglial cells,
and ependymal cells.
Mnemonic
Pertaining to memory.
Neuron
Motor
Having to do with movement or
response.
MRI/NMR
Magnetic resonance imaging, a
diagnostic imaging technique that
does not use x-rays (uses extremely strong magnet).
The morphological unit of the
nervous system, consisting of the
nerve cell body and its processes
(dendrites and axon).
Neuropil
A complex net of nerve cell processes — axon terminals and dendrites and synapses — occupying
the intervals between cell bodies
in gray matter.
NMR/MRI
Nuclear magnetic resonance, also
known as magnetic resonance
image (MRI); a diagnostic
imaging method for brain tissue
and other organs.
Nociceptive
Refers to an injurious stimulation.
Node of Ranvier
Gap in myelin sheath between
two successive Schwann cells or
oligodendrocytes; necessary for
saltatory (rapid) conduction.
Nucleus/nuclei
An aggregation of nerve cells
within the CNS. In histology, the
nucleus of a cell. Plural is nuclei .
Nystagmus
An involuntary oscillation of the
eye(s).
Occipital lobe
Part of cerebral hemisphere,
mostly related to vision.
Oculomotor nerve
Third cranial nerve (CN III); to
most muscles of the eye, and to
iris and lens.
Myelin
Myelin sheath
Myotatic reflex
Neocerebellum
The layers of lipid and protein
substances forming a sheath
around nerve fibers which are
important for rapid nerve
conduction.
Covering of nerve fiber, formed
and maintained by Schwann cell
(PNS) or oligodendrocyte (CNS).
A deep tendon reflex which causes
a reflex contraction of the same
muscle; monosynaptic from
muscle spindle afferents to
anterior horn cell. Also spelled
myotactic reflex.
The phylogenetically newest part
of the cerebellum, present in
mammals and especially well
developed in humans. Ensures
smooth muscle action in the finer
voluntary movements and
involved in motor planning.
Olfactory nerve
First cranial nerve (CN I); special
sense of smell.
Oligodendrocyte
A neuroglial cell; forms and maintains myelin sheath in the CNS in
the same manner as the Schwann
cell in peripheral nerves.
PET
Positron emission tomography; a
neuroimaging technique used to
visualize areas of the living brain
which become “activated” under
certain task conditions.
Pia
Pia mater; the thin innermost
layer of the meninges, attached to
the surface of the brain and spinal
cord. Forms the inner boundary
of the subarachnoid space.
Optic chiasm(a)
Partial crossing of optic nerves
(nasal half of retina representing
the temporal visual fields), after
which the optic tracts are formed.
Optic nerve
Second cranial nerve (CN II); for
special sense of vision (actually a
tract of the CNS).
Pineal
Pertaining to the pineal body; also
called the pineal gland (part of
epithalamus).
Paleocortex
Phylogenetically older cerebral
cortex consisting of three to five
layers (e.g., subicular region).
Plexus
An arrangement of interwoven
vessels or nerves that form a
network.
Papilledema
Edema of the optic disc, visualized with an ophthalmoscope
(also called a choked disc); usually
a sign of abnormal increased
intracranial pressure.
PNS
Peripheral nervous system.
Pons
(Bridge) That part of the brainstem that lies between the
medulla and the midbrain;
appears to constitute a bridge
between the right and left halves
of the cerebellum.
Proprioception
The sense of body position (conscious or unconscious).
Proprioceptor
One of the specialized sensory
endings in muscles, tendons, and
joints; provides information concerning movement and position
of body parts (proprioception).
Ptosis
Drooping of the upper eyelid.
Pulvinar
The posterior nucleus of the
thalamus; involved with vision.
Putamen
The larger and lateral part of the
lentiform nucleus; part of the
neostriatum (with the caudate
nucleus) of the basal gangla.
Pyramidal system
Called such because the corticospinal tracts occupy the pyramidshaped areas on the ventral
surface of the medulla. Pyramidal
tract refers specifically to the
cortico-spinal tract.
Quadrigeminal
(plate)
Referring to the tectum of the
Paralysis
Loss of voluntary action.
Paraplegia
Paralysis of both legs and lower
part of trunk.
Paresis
Muscle weakness.
Paresthesia
Abnormal sensation, tingling
(pins and needles).
Pathway
A chain of functionally interconnected neurons (nuclei) and their
axons, making a connection
between one region of CNS and
another; a tract, e.g., visual
pathway, sensory pathway (dorsal
column — medial lemniscus).
Peduncle
A thick stalk or stem; bundle of
nerve fibers. (Note cerebral, from
the cerebral cortex, in midbrain;
also three cerebellar).
Perikaryon
The cytoplasm surrounding the
nucleus. Sometimes refers to the
cell body of a neuron.
Peripheral nervous
system
Nerve roots, peripheral nerves,
and ganglia (motor, sensory, and
autonomic) outside the CNS.
midbrain with the four colliculi
(also called the tectum).
Quadriplegia
Paralysis affecting the four limbs
(also called tetraplegia).
Raphe
An anatomical structure in the
midline; in the brainstem, several
nuclei of the reticular formation
are in the midline of the medulla,
pons, and midbrain (these
nuclei use serotonin as the
neurotransmitter).
Septum pellucidum A triangular double membrane
separating the frontal horns of
the lateral ventricles. Situated in
the median plane, it fills in the
interval between the corpus
callosum and the fornix.
Somatic
Nucleus in the midbrain (reddish
color in fresh material).
Used in neurology to denote the
body, exclusive of the viscera (as
in somatic efferent neurons supplying the skeletal musculature).
Somatic senses
Reticular formation of brainstem;
pertaining to or resembling a net.
Touch, pain, temperature, pressure, proprioception, vibration.
Somatotopic
Reticular formation Diffuse nervous tissue nuclei and
connections in brainstem; quite
old phylogenetically.
The orderly representation of the
body parts in CNS pathways,
nuclei, and cortex
Somesthetic
The consciousness of having a
body. Somesthetic senses are the
general senses of pain, temperature, touch, pressure, position,
movement, and vibration.
Spasticity
Increased resistance to passive
stretch of the antigravity muscles,
usually flexors of the upper limb
and extensors of the lower limb in
humans.
Special senses
Sight, hearing, balance, taste (gustatory), and smell (olfactory).
Spinal accessory
nerve
Eleventh cranial nerve
(CNXI);usually refers to the part
of the nerve that originates in the
upper spinal cord (C1–5) and in
nervates the sternomastoid and
trapezius.
Spinal shock
Complete “shut down” of all
spinal cord activity (in humans)
below a lesion, following a sudden
interruption of cortical input
(e.g., severed cord after a diving or
motor vehicle accident).
Spino-cerebellar
tracts
Ascending tracts, anterior and
posterior, for “unconscious” proprioception to cerebellum.
Spino-thalamic
tracts
Ascending tracts for pain and
temperature (lateral), and
nondiscriminative or light touch
and pressure (anterior).
Red nucleus
Reticular
Rhinencephalon
Refers in humans to structures
related to the olfactory system.
Rigidity
Stiffness; usually applied to
muscles in Parkinson’s disease in
which there is increased resistance
to passive movement of both
flexors and extensors.
Rostral
Towards the nose, or the most anterior end of the neuraxis.
Rubro-
Red; pertaining to the red
nucleus, as in rubro-spinal tract
and cortico-rubral.
Saccadic
To jerk; extremely quick movements of both eyes together (conjugate movement) in altering
direction of gaze.
Sacral
Referring to the pelvic region.
Schwann cell
Sheathing cell of peripheral nerve
fibers; responsible for formation
and maintenance of myelin.
Secretomotor
Motor nerve supply to a gland.
Sensory
Having to do with receiving
information, usually from the
environment.
Septal region
A cortical area ventral to the anterior end of the corpus callosum
on the medial aspect of the frontal
lobe that includes the septal
nuclei.
Split brain
A brain in which the corpus callosum has been severed in the
midline, usually as a therapeutic
measure for intractable epilepsy.
Subthalamus
Region of the diencephalon
beneath the thalamus, containing
fiber tracts and the subthalamic
nucleus; part of basal ganglia.
Strabismus
A squint; deviation of the eyes;
lack of parallelism of the visual
axes of the eyes.
Sulcus/sulci
Stria
A slender strand of fibers (e.g.,
stria terminalis from amygdala).
Groove between adjacent gyri of
the cerebral cortex (a deep sulcus
may be called a fissure). Plural is
sulci.
Synapse
Area of structural and functional
specialization between neurons
where transmission occurs (excitatory, inhibitory, or modulation)
using neurotramsmitter substances (e.g., glutamate, GABA).
Syringomyelia
A condition characterized by
central cavitation of the spinal
cord and gliosis around the cavity.
Tectum
Roof of the midbrain (behind the
aqueduct) consisting of the paired
superior and inferior colliculi;
also called the quadrigeminal
plate.
Tegmentum
The “core area” of the brainstem,
between the ventricle (or aqueduct) and the cortico-spinal tract.
Contains the reticular formation,
cranial nerve and other nuclei,
and various tracts.
Telencephalon
Rostral part of embryonic forebrain; primarily cerebral hemisphere of adult brain.
Tentorium
The tentorium cerebelli is a dural
partition between the occipital
lobes of the cerebral hemispheres
and the cerebellum; a hiatus or
notch allows passage of brainstem
(midbrain).
Thalamus
A major portion of the diencephalon with sensory, motor, and
integrative functions; consists of
several nuclei with connections to
areas of the cerebral cortex.
Third ventricle
Cavity (midline) in diencephalon,
containing CSF.
Striatum
The phylogenetically more recent
part of the basal ganglia (neostriatum) consisting of the caudate
nucleus and the putamen (lateral
portion of the lentiform nucleus).
Stroke
A sudden severe attack; usually
refers to a sudden loss of neurologic function. Mostly this is
due to a vascular lesion, either
infarct (embolus, occlusion) or
hemorrhage.
Subarachnoid space Space between arachnoid and pia
mater, containing CSF.
Subcortical
Not in the cerebral cortex, i.e., at a
functionally or evolutionary lower
level in the central nervous
system; also refers to white matter
of the cerebral hemispheres.
Subicular region
Transitional cortex (3–5 layers)
between that of the parahippocampal gyrus and the hippocampus proper; part of limbic
system.
Substantia gelatinosa A cluster (nucleus) of small
neurons at the apex of the dorsal
gray horn throughout the spinal
cord; receives pain and temperature afferents.
Substantia nigra
A large nucleus with motor functions in the midbrain consisting
of two parts; many of the constituent cells in the pars compacta
contain dark melanin pigment.
These neurons degenerate in
Parkinson’s disease; the pars reticulata is an output nucleus of the
basal ganglia.
Vagus
Tenth cranial nerve (CN X); supplies motor fibers to larynx; the
major parasympathetic nerve to
organs of the thorax and
abdomen.
Velum
A membranous structure; The superior medullary velum forming
the roof of the fourth ventricle.
Transverse crossing fibers of the
auditory pathway situated in the
ventral portion of the tegmentum
of the lower pons.
Ventral root
Efferent (motor) component of
mixed spinal nerve.
Ventricles
CSF-filled cavities inside the
brain.
Fifth cranial nerve (CN V). The
major sensory nerve of the head
(face, eye, tongue, nose, sinuses);
also supplies muscles of
mastication.
Vermis
Unpaired midline portion of cerebellum between hemispheres.
Vertebral artery
An artery (one of a pair) supplying spinal cord and brainstem.
Vestibulocochlear
Eighth cranial nerve (CN VIII);
special senses of hearing and
balance (acoustic nerve is not
correct).
Visceral
Referring to internal organs.
White matter
Nerve tissue made up of nerve
fibers (axons), some of which are
myelinated; appears whitish after
fixation in formalin.
Tomography
Sectional roentgenography.
Computerized tomography (CT
scan) is a valuable diagnostic
technique.
Tract
A bundle of nerve fibers within
the CNS, with a common origin
and termination, e.g., optic tract,
cortico-spinal tract.
Trapezoid body
Trigeminal nerve
Trochlear nerve
Fourth cranial nerve (CN IV); to
the superior oblique muscle of the
eye.
Uncus
The hooked-back portion of the
rostral end of the parahippocampal gyrus of the temporal
lobe, constituting a landmark
(e.g., uncal herniation) the amygdaloid nucleus lies deep to this
area.
Upper motor
neuron
Cell in motor cortex or other
motor areas in the brain or brainstem connected by descending
tract to lower motor neurons in
brainstem (for cranial nerves) or
spinal cord (for body and limbs).
Upper motor
neuron lesion
Disorder characterized by spasticity and hyperreflexia seen a few
weeks following a lesion of the
brain (cortex, white matter of
hemisphere, or spinal cord) involving descending motor influences to lower motor neuron (of
the spinal cord).

ATLAS OF FUNCTIONAL NEUROANATOMY

Transcription

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