Myelin is an electrical insulator; however, its function of facilitating conduction in axons has no exact analogy in electrical circuitry. In unmyelinated fibers, impulse conduction is propagated by local circuits of ion current that flow into the active region of the axonal membrane, through the axon and out through adjacent sections of the membrane (Fig. 4-1). These local circuits depolarize the adjacent piece of membrane in a continuous, sequential fashion. In myelinated axons, the excitable axonal membrane is exposed to the extracellular space only at the nodes of Ranvier; this is the location of sodium channels [2].