myelin sheath

n.
The insulating envelope of myelin that surrounds the core of a nerve fiber or axon and facilitates the transmission of nerve impulses. In the peripheral nervous system, the sheath is formed from the cell membrane of the Schwann cell and, in the central nervous system, from oligodendrocytes. Also called medullary sheath.

A whitish, fatty sheath covering many nerve fibres. The sheath is produced by Schwann cells. Myelin consists of phospholipids and protein, which protect and electrically insulate the fibres. Myelinated fibres (i.e. nerve fibres with myelin sheaths) transmit nerve impulses faster than fibres lacking a sheath. The velocity of nerve impulse transmission in a large myelinated fibre can reach about 120 m s⁻¹ (about 250 mph). See also saltatory conduction.

n.

The insulating envelope of myelin that surrounds the core of a nerve fiber or axon and that facilitates the transmission of nerve impulses, formed from the cell membrane of the Schwann cell in the peripheral nervous system and from oligodendroglia cells. Also called medullary sheath.

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The noun has one meaning:

Meaning #1: a layer of myelin encasing (and insulating) the axons of medullated nerve fibers
  Synonym: medullary sheath

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