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myelin sheath. noun. 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 membrane of the Schwann cell, and in the central nervous system it is formed from oligodendrocytes.
axolemma-outer membrane covering an axon. medullated nerve fiber, myelinated nerve fiber-a nerve fiber encased in a sheath of myelin. nodes of Ranvier, Ranvier's nodes-small gaps in the myelin sheath of medullated axons. oligodendria, oligodendroglia-tissue consisting of glial cells with sheetlike processes that form the myelin sheath of nerve fibers.