The medullated nerve fibres of the peripheral nervous system consist of the axon, myelin or medullary sheath and neurolemma. In the central nervous system, instead of the neurolemma, there is an irregular covering made of glial cells.
Myelin forming the myelin sheath is glistening while when fresh, but with osmic acid fixation becomes grey or blacks in ordinary preparations the myelin is dissolved by fat solvents and in its place a clear space is left. This space is bridged by fine trabeculae- neurokeratin network.
Myelin sheath is not continuous but interrupted at intervals or 80 to 600 microns by constrictions called the nodes of Ranvier.
Myelin sheath is absent where the fibre branches and also at the beginning and at termination of the fibre.
In certain preparations, the myelin between the node of Ranvier is broken up by oblique clefts or fissures extending from the surface called clefts of Schemidt-Lantermann and the segments between them are the Schemidt Lantermann segments.
At the nodes of Ranvier the neurolemma sheath dips into come in contact with the axon.
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