the lipoproteinaceous envelope in vertebrates surrounding most axons larger than 0.5-mcm diameter; it consists of a double plasma membrane wound tightly around the axon in a variable number of turns and supplied by oligodendroglia cells (in the brain and spinal cord) or Schwann cells (in peripheral nerves); unwound, the double membrane would appear as a sheetlike cell expansion devoid of cytoplasm but for a few narrow cytoplasmic strands corresponding to apparent interruptions of the regular myelin structure, the incisures of Schmidt-Lanterman. The myelin sheath of each axon is composed of a fairly regular longitudinal sequence of segments, each corresponding to the length of sheath supplied by a single oligodendroglia or Schwann cell; in the short interval between each two neighboring segments, the nodes of Ranvier, the axon is unmyelinated even though it is enclosed by complex, fingerlike, membranous expansions of the neighboring oligodendroglia or Schwann cells.
Synonym(s): medullary sheath
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