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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.
On Serendip. Myelin is the protective sheath around axons in the nervous system, and is often referred to as 'white matter.' It acts as an insulator to the electrical signal that is conducted down the axon as a neuron fires, and can well be compared to the insulation around an electrical wire.