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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.
Mauthner's sheath axolemma. medullary sheath , myelin sheath the sheath surrounding the axon of myelinated nerve cells, consisting of concentric layers of myelin formed in the peripheral nervous system by the plasma membrane of Schwann cells, and in the central nervous system by oligodendrocytes.