Myelin Sheath

Also found in: Dictionary, Thesaurus, Medical, Legal, Wikipedia.

myelin sheath

[′mī·ə·lən ′shēth]
An investing cover of myelin around the axis cylinder of certain nerve fibers.

Myelin Sheath


(medullary substance; medullary sheath), the sheath enclosing the medullated nerve fibers. The outside of the myelin sheath is covered with a plasma membrane of the Schwann cell; inside, it borders the surface membrane of the axon—the axolemma.

It is believed that the myelin sheath consists of myelin, which includes biomolecular layers of lipides (a phosphatide-cholesterol complex and other chemical components) and coaxially distributed monomolecular layers of protein. The myelin sheath forms as a result of the envelopment of the nerve-cell process by Schwann cells. In the vicinity of the nodes of Ranvier, the myelin sheath is interrupted; the area between any two nodes is formed by a single Schwann cell. The myelin sheath prevents the scattering of nerve impulses and their transfer to other nerve fibers. Impulses are conducted more rapidly in medullated nerve fibers than in nonmedullated ones.

References in periodicals archive ?
Quantification of the fiber diameter axon diameter and myelin sheath thickness
Early work in the field focused on looking for a cause, but much of the work today seems to center on rebuilding the myelin sheath with drugs, with the hope of helping those with long term deterioration reverse the damage.
In MS, the patients' immune systems turn on their own central nervous systems, stripping the protective myelin sheaths from nerve fibers and leading to problems with vision, balance, and coordination.
The aim of this study was to identify invertebrate homologs of vertebrate glial proteins adapted to shape myelin sheath formation.
Nerve fibers were enveloped by cytoplasmic lamellae of perineural cells and adjacent collagen bundles, their diameter ranged from 1 to 6 [micro]m, and the myelin sheath ranged from 0.
In this disorder the antibodies destroy the myelin sheath that surrounds the peripheral motor neurons and cause inflammation of the axons of the affected nerve cells.
In particular, differences were marked in genes involved in compaction and maintenance of the myelin sheath, and in aptosis and survival of oligodendrocytes.
In some instances, attempted remyelination was suggested by the formation of a secondary thin myelin sheath around an axon that had abnormal myelin (figure 2, D).
Waxman's early research showed that when the axon's protective myelin sheath is damaged, as happens in MS, the exposed parts of the axon don't contain enough sodium channels to power nerve conduction.
In the cases of MS, Guillain-Barre and myelitis, it is said to cause the body to attack the protective myelin sheath that covers the nervous system, slowly stripping it away.
In contrast to mutated GFAP, healthy GFAP is responsible for maintaining nerves and the myelin sheath.
About two-thirds of people with CMT have type 1, which affects the myelin sheath, the insulating covering that surrounds nerve fibers (see illustration, right).