Next page Anatomy Myelin is formed within the central nervous system by the oligodendrocyte and in the peripheral nervous system by the Schwann cell. It is the major component of normal white matter and a wide variety of stains are available to examine it.
Oligodendrocytes are responsible for the formation of myelin sheaths surrounding axons.
They are not confined to the white matter and within grey matter they are often closely associated with nerve cells. They have relatively small amounts of cytoplasm around the nucleus but have several processes which wrap themselves around axons to form an insulating sheath composed of a variable number of layers of cell membrane, through which very little current leeks. Nodes of Ranvier are composed of gaps between myelin sheets formed by different oligodendrocytes and at these points almost all of the sodium channels responsible for nerve conduction exist. The formation of a myelin sheath is the result of a process from an oligodendrocyte spiralling around an axon so that the cytoplasm is extruded until the opposite membranes meet, thus forming a multi-layered lipoprotein coat with a node of Ranvier at each end. An internode is the section between two nodes of Ranvier.