What are the cells that produce the myelin sheath? Of which substance is the myelin sheath formed?
In the central nervous system (CNS) the myelin sheath is made by apposition of oligodendrocyte membranes. Every oligodendrocyte can cover portions of axons of several dissimilar neurons. In the peripheral nervous system (PNS) the myelin sheath is made by consecutive Schwann cell membranes covering segments of a one axon. The Ranviers' nodes appear in the intercellular space between these cells.