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, in neurons of the peripheral nervous system, and from oligodendrocytes in the central nervous system. The sheath is interrupted at intervals along the axon by nodes of Ranvier, where there are voltage-gated ion channels and the axon is exposed to the extracellular fluid. Myelinated sections of axon are called internodes. During conduction of a nerve impulse the action potential effectively ‘jumps’ from node to node, a mechanism called saltatory conduction, which vastly increases the speed of conduction compared to unmyelinated axons.