Myelin is the fatty substance that provides AXON insulation in the nervous system. In the CENTRAL NERVOUS SYSTEM myelin is provided by OLIGODENDROGLIA; in the PERIPHERAL NERVOUS SYSTEM it is supplied by SCHWANN CELLS. In both cases, cell processes wrap around axons forming a multilayer insulating sheath (see PROCESS). Gaps in the myelin sheath (NODE OF RANVIER) permit ionic flow across the axon MEMBRANE. Since such activity is only present at limited points on myelinated axons, ACTION POTENTIAL speed is greatly accelerated compared to the speed along UNMYELINATED axons. Myelin has a complex structure. It is composed of LIPIDS (70–85%), PROTEINS (15–30%) with a small GANGLIOSIDE component. Most of the protein—some 80%—is made either PROTEOLIPID or MYELIN BASIC PROTEIN (though note that myelin basic protein is a term that actually covers a complex group of proteins, all derived from a single GENE).
Many fibres in the nervous system are normally unmyelinated (also called non-myelinated). Obviously, this presents no problem for normal neural activity. However, loss of myelin (DEMYELINATION) or the failure to form myelin properly (DYSMYELINATION) has important functional consequences. MULTIPLE SCLEROSIS is the best known of the demyelinating disorders.
Newman S., Saito M. & Yu R.K. (1995) Biochemistry of myelin proteins and enzymes. In Neuroglia, ed. H.Kettenmann & B.R.Ransom, pp. 535–554, Oxford University Press: Oxford.
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