myelin sheath

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myelin sheath (medullary sheath) A fatty sheath, composed of lipids, proteins, and polysaccharides (myelin), that surrounds and insulates the axons of neurons and permits increased current flow of nerve impulses. The myelin is produced by Schwann cells. Constrictions which occur along the sheath (nodes of Ranvier) delineate adjacent Schwann cells.

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