2nd Lec on Histology of Nervous System by DR. ROOMI

7/31/2019 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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2nd Lec on Histology of Nervous System by DR. ROOMI

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