myelin sheath

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myelin sheath (medullary sheath) The layer of fatty material that surrounds and electrically insulates the axons of most vertebrate and some invertebrate neurons. The myelin sheath enables a more rapid transmission of nerve impulses (at speeds up to 120 m s–1). It consists of layers of membrane derived from Schwann cells. The sheath is interrupted at intervals along the axon by nodes of Ranvier; myelinated sections of axon are called internodes.

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