Nervous system

NERVOUS TISSUE
• The nervous sys. is classified into:
• Central nervous system (C.N.S):
brain and spinal cord.
• Peripheral nervous system (P.N.S):
a) Peripheral nerves
Cranial
Spinal
b) Ganglia Cerebro–spinal
Autonomic Sympathetic
Parasympathetic
Parasympathetic
THE
NEURON
Definition:
• Is the structural and functional unit
of the nervous tissue.
Structure:
• The neuron or the nerve cell
consists of:
• Cell body
• It contains the nucleus and
surrounding cytoplasm.
• Processes: axon and dendrites.
dendrites
(1) THE CELL BODY:
• The Cell
membrane: very
thin.
• The nucleus:
Large, rounded,
central and (open
– face type) with a
prominent
nucleolus.
The cytoplasm:
•Nissl granules
•Neurofibrils
•Golgi apparatus
•Mitochondria
(1) THE CELL BODY:
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Inclusions
Melanin pigments
Lipofuscin pigments:
Glycogen granules
Fat droplets.
droplets
Centrosomes
• Are absent in
• mature nerve
• cells as they
• can not divide.
• Are present in
• young nerve
• cells of certain
• vertebrates.
Number:
The axon
The dendrites
Single.
Multiple.
Thickness
Thin &
and Length: long.
Thick &
short.
Diameter:
It has a constant
diameter along its
length.
Their diameter decrease
gradually towards their
ends i.e. tapering.
Contour:
Smooth.
Irregular.
The axon
The dendrites
Branch. It branches at its
terminal end only i.e.
“terminal arborization”
but it may give collateral
branches which arise at
right angles.
They give extensive
branching along their
course. The branches
arise at acute angles and
may show spines.
Conte
:nt
Contain neurofibrils and
Nissl granules.
Contains
mitochondria and
neurofibrils
Conductio
n of nerve
impulses:
The axon
The dendrites
carries impulses away
from the cell body
centrifugal conduction
carry the impulses to the
cell body)
Centripetal conduction
Classification of the neurons (nerve cells)
According to the number of processes
(Polarity):
Unipolar nerve cells:
Nerve cells are rounded with only one process.
Site: mesencephalic nucleus of the trigeminal
nerve.
Pseudounipolar nerve cells:
Nerve cells are rounded.
These cells are at first bipolar, but as development
proceeds the two processes of the cell meet at one
side of the body forming a single process which
divides in inverted T- shaped manner into 2
branches, a dendrite and an axon.
The single process of each cell is convoluted at its
beginning in the nerve cell forming a glomerulus in
its cytoplasm.
Site: spinal ganglia.
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Bipolar nerve cells:
The cells are fusiform or spindle-shaped, with 2 processes, a
dendrite and an axon.
Site:
Retina of the eye.
Spiral ganglia and vestibular ganglia in the ear.
Olfactory epithelium of the nose.
Multipolar nerve cells:
Have many dendrites and one axon.
Polygonal or stellate shaped: star-shaped.
Site:
- Sympathetic ganglia.
- Anterior horn cells of the spinal cord.
Pyramidal:
Site:
cerebral cortex.
Pyriform: flask –shaped cells.
Site:
Purkinje cells of the cerebellum.
THE NERVE FIBER
• Definition: It is the axon of a nerve
cell.
• The axon may be naked or
,with myelin sheath( ensheathed
)or both neurolemma.
• The axon (Axis Cylinder)
• Structure :
• The axon is surrounded by a
membrane called “axolemma”
and its cytoplasm is known as
“axoplasm”. It contains
neurofilaments, neurotubules
and mitochondria but no Nissl
granules.
• Length:
• May be very long (Golgi type I) or
•
very short (Golgi type II).
• Diameter:
- thick … conducting impulses rapidly
- Thin….. conducting impulses slowly.
The myelin Sheath
Histological appearance:
• It is a white tubular covering for the
axon.
• It is interrupted at intervals by
constrictions called “nodes of
Ranvier”. The segment between
two successive nodes is called
“internodal segment”.
Chemical Structure:
• It is a lipoprotein sheath
• It appears in the form of concentric
layers derived mainly from the lipid
middle layer of the cell membrane of
Schwann cell
Staining:
• In H & E preparation: the fat of
myelin dissolves in fat solvent (xylol)
and appears as an empty space.
• It is stained black by osmic acid.
acid
Function:
Insulation of the nerve impulses.
The Neurolemmal
Sheath (Schwann Cells)
• Histological appearance:
• It is formed of a chain of cells
forming a tube around myelin
sheath.
• Each Schwann cell has a flat oval
nucleus and corresponds to an
internodal segment.
• The cells come in contact with the
axon at the nodes of Ranvier.
• Function:
-Insulation of nerve impulses in
nonmyelinated nerve fibers.
-Important role in regeneration of
peripheral nerve after injury. (No
regeneration of nerve fibers
occurs in the CNS where the
fibers are myelinated without
neurolemma).
Types of Nerve Fibers:
Naked Fibers: (non myelinated
without neurolemma) e.g.
- Termination of peripheral nerves.
- Fibers in the gray matter.
Ensheathed Fibers:
-Myelinated with neurolemma:
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e.g. peripheral nerves.
-Myelinated without neurolemma:
•
e. g. nerve fibers in the white
•
matter and optic nerve.
-Non- myelinated with neurolemma:
•
e.g. sympathetic nerve fibers.
Structure of a peripheral Nerve Trunk:
Nervous component:
Groups of nerve fibers (myelinated axons with
neurolemma) arranged in bundles.
-In Hx. & E. stained section:
Each fiber shows a centrally stained acidophilic
axon, surrounded by an empty space of the
dissolved myelin and a thin outer pink rim of
The nuclei of these .cell cytoplasm Schwann
.cells may be seen if they are cut in the section
-In osmic acid stained section:
The myelin sheath of the nerve fibers appears
.as rounded black circles
C. T. component:
-Epineurium :
It is a dense C. T., which surrounds the whole
nerve trunk.
-Perineurium:
It is a dense C. T., which surrounds the bundles
of nerve fibers.
-Endoneurium: (Henle’s sheath)
It is a delicate loose C. T. between the
individual nerve
GANGLIA
•Definition:
• A nerve ganglion is a collection of nerve cells and
nerve fibers, covered by a C. T. capsule, outside
the C. N. S.
• NB: a collection of nerve cells inside the C. N. S is
called nucleus.
Site :
C.T.
Capsul.
Sympathetic
Spinal
ganglion
ganglion
Nerve cells:
Type:
Size:
Arrangement:
Number:
Nuclei:
Satellite Cs
Glomerulus
:
Spinal
ganglion
Sympathetic
ganglion
Nerve
Fiber
s
Spinal
ganglion
Thickly myelinated.
Widely separated from each
other
Parallel to the capsule.
absent Synapse: (as the fibers
are the peripheral
processes of the nerve
cells whose axons enter
the C.N.S)
Sympathetic
ganglion
Thinly or non myelinated
Close to each other.
.Irregularly distributed
Present Synapse
fibers preganglionic)
synapse with the ganglion
cells whose axons form
.)the postganglionic fibers
Blood
supply:
Function:
Spinal
ganglion
Sympathetic ganglion
Acts as relay for
different
sensations
before their
entrance into
the spinal cord.
Acts as relay for
different motor
functions arising
from the spinal cord.
Ne
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Practical
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