The myelin sheath is a white, cylindrical layer covering the axons of certain neurons in the nervous system. It consists of several concentric layers of myelin, interrupted at intervals by gaps called nodes of Ranvier. Its job is to act as an electrical insulator for the axons which speeds the transmission of nerve impulses. It is formed in the peripheral nervous system by the plasma membrane of Schwann cells, and by oligodendrocytes in the central nervous system.