2nd Lec on Histology of Nervous System by DR. ROOMI
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HISTOLOGY OF NERVOUS TISSUE
BY
DR. MUDASSAR ALI ROOMI (MBBS, M. PHIL)
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Peripheral Nervous System
The main components of the peripheral nervous
system are the
nerves,
ganglia, and
nerve endings.
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Nerve Fibers
Nerve fibers consist of axons enclosed
within a special sheath of cells derived
from the embryonic neural crest cells. ***
peripheral nerves contain groups of nerve
fibers.
In peripheral nerve fibers, axons are
sheathed by Schwann cells, also calledneurolemmocytes . ****
The sheath may or may not form myelin
around the axons, depending on their
diameter.
Axons of small diameter are usually
unmyelinated nerve fibers . Progressively thicker axons are generally
sheathed by increasingly numerous
concentric wrappings of the enveloping
cell, forming the myelin sheaths. These
fibers are known as myelinated nerve
fibers.5/30/2012 3
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Myelinated
Fibers
As axons of large diameter grow inthe PNS, they are engulfed alongtheir length by many undifferentiatedneurolemmocytes and becomemyelinated nerve fibers.
The plasma membrane of the
covering neurolemmocyte (Schwanncell) fuses around the axon andbecomes wrapped around the nervefiber as the glial cell body movesaround and around the axon manytimes.
The multiple layers of Schwann cellmembrane unite as a layer myelin, awhitish lipoprotein complex whoseabundant lipid component is partlyremoved by standard histologicprocedures.
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Myelinated Fibers (cont.)
With the TEM the myelin sheath canappear as a thick electron-densecover in which individual membranelayers are seen.
Membranes of Schwann cells have ahigher proportion of lipids (80%) than
do other cell membranes. Between adjacent Schwann cells the
myelin sheath shows small nodalgaps along the axon, also callednodes of Ranvier.
The length of axon covered by oneSchwann cell is called the internodal
segment and may be more than 1millimeter.
Unlike oligodendrocytes of the CNS,Schwann cells only form myelinaround a portion of one axon.
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Process of myelination
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Process of myelination (cont.)
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Myelination of nerve fibers
In PNS In CNS
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Unmyelinated Fibers
The CNS is rich in unmyelinatedaxons which are not sheathed at allbut run free among the otherneuronal and glial processes.
However in the PNS, even all
unmyelinated axons are envelopedwithin simple folds of Schwanncells. In this situation the glial celldoes not form multiple wrapping ofitself as myelin.
Unlike their association with
individual myelinated axons, eachSchwann cell can enclose portionsof many unmyelinated axons withsmall diameters.
Adjacent Schwann cells alongunmyelinated nerve fibers do notform nodes of Ranvier.5/30/2012 9
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Degeneration and regeneration of
nerve fibers Injured fibers in peripheral nerves have a
good capacity for regeneration and return
of function.
In a wounded nerve fiber, it is important
to distinguish changes occurring proximal
to the injury from those in the distal
segment.
The proximal segment maintains its
continuity with the trophic center in the
perikaryon and can regenerate, while the
distal segment, separated from the nerve
cell body, degenerates.
This is called as Wallerian degeneration
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Degeneration and regeneration of
nerve fibers (Cont.) The onset of regeneration is accompanied by
changes in the perikaryon: chromatolysis ordissolution of the RER and a consequentdecrease in cytoplasmic basophilia; anincrease in the volume of the perikaryon;and migration of the nucleus to a peripheralposition in the perikaryon.
The proximal segment of the axon
degenerates close to the wound for a shortdistance, but growth starts as soon as debrisis removed by macrophages.
Macrophages produce cytokines whichstimulate Schwann cells.
In the nerve segment distal to the injury theaxon and myelin, but not the connectivetissue, degenerate completely and are
removed by macrophages. While these regressive changes take place,
Schwann cells proliferate within theconnective tissue sleeve, giving rise to rowsof cells that serve as guides for the sproutingaxons formed during the reparative phase.
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MEDICAL APPLICATION
When there is an extensivegap between the distal andproximal segments of cut orinjured peripheral nerves,
or when the distal segmentdisappears altogether (as inthe case of amputation of alimb), the newly growingaxons may form a swelling,
or neuroma, that can be thesource of spontaneous pain.
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