from The American Heritage® Dictionary of the English Language, 4th Edition

  • 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. An envelope of myelin that surrounds nerve fibres.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. a layer of myelin encasing (and insulating) the axons of medullated nerve fibers


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