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myelin sheath[′mī·ə·lən ′shēth]
(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.