part of derives from/develops from brain myelin myelin sheath BTO:0000900 BrendaTissueOBO The cylindrical covering on the axons of some neurons; it consists of concentric layers of myelin, formed in the peripheral nervous system by the plasma membrane of Schwann cells, and in the central nervous system by oligodendrocytes. It is interrupted at intervals along its length by gaps known as nodes of Ranvier. Myelin is an electrical insulator that serves to speed the conduction of nerve impulses. medullary sheath oligodendrocyte myelinating Schwann cell non-myelinating Schwann cell