Figure 1. Myelination (a) and morphology of the node of Ranvier (b) in the peripheral nervous system (PNS) and central nervous system (CNS). (a) Cartoon of myelination shows that Schwann cells ensheathe unmyelinated axons in the PNS, but form myelin only after a one-to-one association has occurred. Oligodendrocytes myelinate numerous axons in the CNS, and form shorter internodal lengths than in the PNS. The box at the bottom of the axon contains a node of Ranvier and is shown at higher magnification in (b). (b) The myelin sheath is interrupted at regularly spaced nodes of Ranvier, where  Na⁺ channels are clustered in high density, and other molecules are discretely localized in the paranodal and juxtaparanodal subcellular zones. The top half of the figure shows a node of Ranvier in the PNS; the lower shows a node in the CNS.

Figure 2. The discrete molecular organization of ion channels in the rat peripheral and central nervous systems. (a, b) Nodes of Ranvier in the peripheral and central nervous systems, respectively, labelled for  Na⁺ channels (green), Caspr (red) and Kv1.2  K⁺ channel subunits (blue). (c) Four myelinated axons from the peripheral nervous system, visualized using Hoffman optics and immunofluorescence, two of which have  Na⁺ channels clustered in the nodal gap (green). Bars, 10 m.

Figure 3. Saltatory conduction in myelinated axons. Transmembrane currents appear to jump from node to node, as each depolarizing action potential causes the next node to be excited (top). (+, –) show the relative charges at each point along the axon during action potential propagation. Further, were transmembrane currents to be recorded at each of the sites indicated by the green arrows, they would be detected only at nodes of Ranvier (bottom).