myelin sheath (medullary sheath) A fatty sheath, composed of lipids, proteins, and polysaccharides (myelin), that surrounds and insulates the axons of neurons and permits increased current flow of nerve impulses. The myelin is produced by Schwann cells. Constrictions which occur along the sheath (nodes of Ranvier) delineate adjacent Schwann cells.

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.