Definition of myelin sheath
: a layer of myelin surrounding the axons of some neurons —called also medullary sheath
First Known Use of myelin sheath
Medical Definition of myelin sheath
: the insulating covering that surrounds an axon with multiple spiral layers of myelin, that is discontinuous at the nodes of Ranvier, and that increases the speed at which a nerve impulse can travel along an axon—called also medullary sheath
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