NEUROANATOMY 1 – General features of the spinal cord, scalp

NEUROANATOMY 1 – General features of the spinal cord,
scalp, skull and brain
Spinal Cord
Dura mater:
• The outer covering of the spinal cord
• Separated from the periosteum by the epidural space
• Epidural space contains extensive venous plexus and the small arteries supplying the contents of the
vertebral canal
• Separated from the arachnoid by the subdural space
Denticulate ligament:
• A narrow sheet of fibrous tissue on either side of the spinal cord
• Between the ventral and dorsal roots, crosses the subarachnoid space to interconnect the pia and dura
mater
• Medially: derived in a continuous line from the pia mater of the lateral margin of the cord
• Laterally: forms a series of 21 tooth-like processes piercing the arachnoid and attaching to the dura mater
In addition, the arachnoid mater is linked to the pia in the median plane posteriorly by an interrupted sheet of
fibrous tissue, the subarachnoid septum:
• There are fewer trabeculae in the spinal cord than in the cranial cavity
• Usually found posteriorly
The spinal cord is therefore suspended in CSF within the subarachnoid space by:
• Dorsal and ventral nerve roots
• Denticulate ligaments
• Posteriorly: subarachnoid septum and a few trabeculae
Epidural anaesthesia – Local anaesthetic introduced into the epidural space acts on whole spinal segments
without the problems of access to some peripheral nerves
Spinal anaesthesia – Anaesthetic is introduced into the CSF in the lumbar cistern
Caudal to the lumbar enlargement, the cord tapers abruptly to form a conical termination, the conus medullaris:
• Situated at the level of L1/L2
• Nerve roots pass across the subarachnoid space at increasingly oblique angles from cervical to sacral
From the tip of the conus medullaris runs a fine connective tissue filament, the filum terminale:
• A continuation of the pia mater
• Continues down to the body of S2, where it pierces the arachnoid and dura
• Forms the coccygeal ligament, which continues on to the dorsum of the first coccygeal segment
An enlargement of the subarachnoid space between the conus medullaris of spinal cord (about vertebral level L2)
and inferior end of subarachnoid space and dura mater (about vertebral level S2) is called the lumbar cistern:
• The cauda equina occupies the subarachnoid space
There are 31 pairs of nerve roots:
• 8 cervical
• 12 thoracic
• 5 lumbar
• 5 sacral
• 1 coccygeal
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They are formed from a series of small rootlets, 6-8 in number
Every dorsal root has a round swelling, the dorsal root ganglion
SENSORY rootlets emerge from the POSTEROLATERAL sulcus
MOTOR rootlets emerge from the ANTEROLATERAL sulcus
The anterior (ventral) median fissure:
• Deepens caudally
• Contains pia mater
• Within the linear splendens at the mouth of the fissure, runs the anterior spinal artery
The posterior (dorsal) median sulcus:
• Overlies the posterior median septum of neuroglia
The cord tapers gently from rostral to caudal, but there are two regions that are swollen:
• The cervical enlargement extends from C3 to T2 (with a maximum circumference at C6)
• The lumbosacral enlargement extends between nerves L1 and S3 (with a maximum circumference at
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spinal level L3, the lower border of the 12 thoracic vertebrae)
The transverse diameter of the cord is always greater than the anteroposterior diameter and this is most marked in
the cervical segments.
Scalp
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S – Skin; similar structure to skin throughout the body
C – Connective tissure; dense, containing arteries, nerves and veins supplying the scalp
A – Aponeurotic layer; of the occipitofrontalis muscle, moving the scalp, raising the eyebrows
L – Loose connective tissue; facilitates movement of the scalp proper over the calvaria
P – Pericranium; the periosteum on the outer surface of the calvaria
Identify on the scalp:
• The superficial temporal artery, its branches and accompanying vein
• Auriculotemporal nerve (Mandibular div. CN V)
• Supratrochlear nerve (Opthalmic div. CN V) and vessels
• Supraorbital nerve (Opthalmic div. CN V) and vessels
• Greater occipital nerve (dorsal ramus of C1)
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Frontalis muscle
Occipitalis muscle
Skull
The upper part of the skull is called the calvarium. Anteriorly and posteriorly the skull is thicker than it is laterally,
which means the side of the skull is more likely to fracture following an impact.
Anterior cranial fossae:
• Limited anteriorly and laterally by the orbital part of the frontal bone and lesser wing of the sphenoid
• Identify the crista galli and the cribiform plate
• Contains the frontal lobe of the brain
Middle cranial fossae:
• Floor is formed by the body of the sphenoid
• Contains the temporal lobe of the brain
• Identify the greater and lesser wings of the sphenoid bone, separated by the superior orbital fissure
• Identify the posterior and anterior clinoid processes, and the internal carotid arteries
• Identify the dorsum sellae
• The infundibulum (aka pituitary stalk) of the pituitary gland passes through the centre of the diapragma
sellae, a layer of dura mater stretched between the four clinoid processes forming the roof of the sella
turica (aka pituitary fossa)
• The foramen spinosum admits the middle menengeal artery
Posterior cranial fossae:
• Formed by the clivus, representing the fusion of the sphenoid with the occipital bone
• Note the jugular foramen at the end of a substantial groove formed by the sigmoid venous sinus
• Contains the cerebellum, pons and medulla oblongata
Dura Mater
This is the outer layer and takes the form of a fibrous sheet:
• Within the skull, fused with another fibrous sheet, the periosteum of the internal cranial bones
• Therefore consists of 2 layers:
o Outer endosteal layer (periosteum)
o Inner meningeal layer (dura mater proper)
• The dura is continuous with the perineurium of the cranial and peripheral nerves
• Supplied by the middle meningeal artery
Identify:
• The falx cerebri
o Attaches anteriorly to the crista galli of the ethmoid bone
o Posteriorly, blends with the upper surface of the tentorium cerebella
o Encloses between its layers the superior and inferior sagittal sinuses
• The tentorium cerebelli
o Covers the cerebellum, forming a thick fibrous roof over the posterior cranial fossa
o Attached to the falx cerebri along the mindline, enclosing the straight sinus
o Anterior border encloses a horseshoe-shaped space between it and the dorsum sellae called the
tentorial inscisure, occupied by the midbrain and vermis of the cerebellum
o Anteriorly, margins are fixed to the anterior and posterior clinoid processess
o Encloses the transverse sinus posteriorly
o Encloses the superior petrosal sinus laterally
o Forms the roof and lateral wall of the cavernous sinsus anteriorly
The two cavernous sinuses are separated by the sella turica and run between the anterior and posterior clinoid
processes. They contain cranial nerves III, IV and VI
Arachnoid Mater
Separated from the dura by the subdural space, the arachnoid mater bridges over the contours of the brain
creating subarachnoid spaces that fill with CSF called subarachnoid cisterns:
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Cerebellomedullary cistern (cistern magna) – Formed by the dorsal surface of the medulla and the
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inferior surface of the cerebellum. Communicates with the 4 ventricle via the foramen of magendie
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Pontine cistern – Found on the ventral surface of the pons. On each side, the lateral foramen of luschka
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allows communication between the 4 ventricle and the pontine cistern
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Interpenduncular cistern – Overlies the ventral surface of the midbrain, formed by arachnoid bridging the
gap between the two temporal lobes. Contains the circle of willis
Superior cistern
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Cistern of the lateral fissure – Formed by the arachnoid mater bridging the lateral sulcus. Contains the
middle cerebral artery
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Superior cistern – Occupies the space between the posterior end of the CC and the superior surface of
the cerebellum. Contains the great cerebral vein (of Galen)
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Cisterna ambiens – Encircles the midbrain. Clearly visible on a normal CT scan
Pia Mater
Covers the surface of the CNS and follows its contours. The pia is separated from the arachnoid by the
subarachnoid space, which contains the CSF. There are fine strands of connective tissue called trabeculae that
connect the pia to the arachnoid.
Parts of the Brain
Hindbrain (Rhombenecephalon)
• Medulla oblongata (derived from the myelencephlon)
• Pons (or metencephlon)
• Cerebellum
Midbrain (Mesencephlon)
• Tectum – Dorsal to the cerebral aqueduct, comprises of the superior and inferior colliculi
• Cerebral peduncle – Ventral to the cerebral aqueduct
o Dorsal part called the tegmentum
o Ventral part called the crus cerebri
Together, the hindbrain and midbrain form the brainstem.
Forebrain (Prosencephalon)
• Diencephlon – Comprises of the thalamus, hypothalamus, epithalamus (includes the habenula and pineal
body) and sub-thalamus (lies behind and to the sides of the hypothalamus)
• Telencephlon – Consists of an outer layer called the cerebral cortex. Found deep within the tissue are the
basal ganglia
The two hemispheres are separated by the deep longitudinal fissure, the floor of which is formed by a bundle of
white matter called the corpus callosum.
The lateral sulcus:
• Begins on the inferior surface at the anterior perforated substance
• Extends laterally between the frontal and temporal lobes
• Divides into the short anterior and descending and the long posterior rami (floor formed by the insula)
• Cortex forming the walls of the sulcus are called the frontal, parietal and temporal opercula
The central sulcus:
• Begins a little behind the midpoint of the hemisphere and runs forwards and downwards
• On either side lie the precentral (motor) and postcentral (sensory) gyri
• Medially, it is surrounded by a U-shaped gyrus known as the paracentral lobule
The parieto-occipital sulcus:
• Separates the parietal and occipital lobes on the medial surface
• Inferiorly, forms a Y-shaped arrangement with the calcarine sulcus
• Laterally and inferiorly, the border is marked by the preoccipital notch
Preoccipital notch
The cerebral cortex is divided into lobes: frontal, parietal, occipital and temporal
The space between the cerebellum and the occipital lobes is occupied by the tenorium cerebelli
On the base of the brain, identify:
• The olfactory bulbs and the olfactory tract
• The medial and lateral olfactory striae on the anterior border of the anterior perforated substance
• The optic nerves converging in the midline to form the optic chiasm, then passing backwards and laterally
as the optic tracts
• The mammillary bodies
• The tissue behind the optic chiasm up to and including the mammillary bodies is part of the hypothalamus
• Behind the mammillary bodies, the cerebral peduncles
• Covered by arachnoid, the space between the cerebral penduncles is the interpenduncular fossa
• The pons
• The cerebellum
• The medulla oblongata
Ventricular System and CSF
CSF is produced in the choroid plexus, which is particularly prominent in the lateral ventricle. It then circulates via
the ventricular system before entering the subarachnoid space from where it is reabsorbed into the saggital venous
sinous. Rebasorbtion occurs via tufts of arachnoid mater, called arachnoid villi, which pierce the dura to lie within
the superior sagittal sinus. With advancing age, these tend to calcify, forming arachnoid granulations. Their
presence tends to cause reabsorption of the bone leading to the presence of small pits in the inner surface of the
skull in the midline.
Movement of CSF:
• Choroid plexus in the lateral ventricles
• Exits through the interventricular foramen (of monro)
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• 3 ventricle, surrounded by the thalamus
• Exits through the cerebral aqueduct (of sylvius)
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• 4 ventricle, at the back of the medulla
• Exits through two lateral apertures (foramina of Luschka) and one median aperture (foramen of Magendie)
• Flows through the cerebromedullary cistern down the spinal cord and over the cerebral hemispheres
Hydrocephalus is the abnormal accumulation of CSF in the ventricles, usually due to blockage of CSF outflow in
the ventricles or in the subarachnoid space over the brain. This may cause increased intracranial pressure inside
the skull and progressive enlargement of the head, convulsion, and mental disability.
Treatment is surgical. It involves the placement of a ventricular catheter (a tube made of silastic), into the cerebral
ventricles to bypass the flow obstruction/malfunctioning arachnoidal granulations and drain the excess fluid into
other body cavities, from where it can be reabsorbed
Intracranial Bleeds
Exradural haematoma
• A build up of blood between the skull and the dura mater
• Build up of blood may increase pressure in the intracranial space and compress
delicate brain tissue
• Cause is usually traumatic
• The pterion region which overlies the middle meningeal artery is exceptionally
weak and prone to injury
• Bleeding is rapid because it is usually from arteries
• Usually appear convex in shape
Subdural haematoma
• Blood gathers between the dura and the arachnoid
• Usually results from tears in veins that cross the subdural space
• Bleeding often separates the dura and the arachnoid layers
• Much more common than epidural hemorrhages, subdural hemorrhages
generally result from shearing injuries due to various rotational or linear forces
• Slower onset than those of epidural hemorrhages because the lower pressure
veins bleed more slowly
Subarachnoid haematoma
• Occur within the brain tissue itself
• Bleeding into the subarachnoid space surrounding the brain, the area between
the arachnoid membrane and the pia mater
• Bleeding may occur spontaneously, usually from a cerebral aneurysm, or may
result from trauma
• The classic symptom is the thunderclap headache (a headache described as
the "most severe ever" developing over seconds to minutes)
Structure of Neural Tissue
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Neuronal cell bodies and their local processes form the grey matter
Collections of nerve fibres form the white matter
Collections of nerve cells within the brain are called nuclei or ganglia. IN the spinal cord these are called
horns
Flatter sheets of neurones forming the outer surface of the cerebral hemispheres are called the cortex
Transverse sections through the spinal cord look histologically different:
Types of Neurone (functional groups)
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General – pertain to most areas of the body, including the sensing of pain and temperature, movement of
skeletal muscle etc
Special – Much more specific functions such as vision, hearing and balance
Efferent (motor) – Sends information to the periphery
Afferent (sensory) – Relay information from the periphery
Somatic – Derived from the somatic mesoderm, including dermatomes and myotomes
Visceral – Internal organs, glands, smooth muscle and skeletal muscle of brachial arch origin
Neurones concerned woth the input and output of information between the periphery and brain can be categorized
into six or seven functional groups:
General Somatic Afferent
• Arise from cells in the spinal ganglia and are found in all the spinal nerves, except occasionally the first
cervical
• Conduct impulses of pain, touch and temperature from the surface of the body through the posterior roots
to the spinal cord and impulses of muscle sense, tendon sense and joint sense from the deeper structures
• Examples of nerves containing GSA fibers include the trigeminal nerve, the facial nerve, the
glossopharyngeal nerve, and the vagus nerve.
General Somatic Efferent
• Arise from motor neuron cell bodies in the ventral horns of the gray matter within the spinal cord
• They exit the spinal cord through the ventral roots, carrying motor impulses to skeletal muscle
• Examples of nerves that contain GSE fibers include the oculomotor nerve, the trochlear nerve, the
abducens nerve, and the hypoglossal nerve
General Visceral Afferent
• Conduct sensory impulses from the viscera through the rami communicantes and posterior roots to the
spinal cord
• They are probably limited to the white rami connected with the spinal nerves in two groups, the first
thoracic to the second lumbar and the second sacral to the fourth sacral nerves
• Examples of nerves containing GVA fibers include the glossopharyngeal nerve and the vagus nerve
General Visceral Efferent
• Arise from cells in the lateral column or the base of the anterior column and emerge through the anterior
roots and white rami communicantes
• These are preganglionic fibers which end in various sympathetic ganglia from which postganglionic fibers
conduct the motor impulses to the smooth muscles of the viscera and vessels and secretory impulses to
the glands
• Examples of nerves containing GVE fibers include the oculomotor nerve, the facial nerve, the
glossopharyngeal nerve, and the vagus nerve
(Special Visceral Afferent)
• Refers to afferent nerves that develop in association with the gastrointestinal tract
• Examples of nerves containing SVA fibers include the olfactory nerve, the facial nerve, the
glossopharyngeal nerve, and the vagus nerve
Special Visceral Efferent (Branchiomotor)
• Refers to efferent nerves which supply muscles which derived from the branchial arches
• Examples of nerves that contain SVE fibers include the trigeminal nerve, the facial nerve, the
glossopharyngeal nerve, the vagus nerve, and the accessory nerve
Special Somatic Afferent
• Refers to afferent nerves of the special senses
• Examples of nerves containing SSA fibers include the optic nerve and the vestibulocochlear nerve