To prevent the loss of linear signal propagation, glial cells form a phospholipid-based layer of insulation called the myelin sheath along the length of the axon (eFig. 9.7). The myelin sheath is formed by oligodendrocytes in the CNS and Schwann cells in the PNS. Interspersed between the segments of myelin are exposed segments of the axon called nodes of Ranvier, which have a large population of voltage-gated ion channels. Presence of the ion channels facilitates rapid conduction of the action potential (a transient voltage change in the axonal membrane) from node to node in a process called saltatory conduction (eFig. 9.7).