Fig. 1.

Early model of saltatory conduction in frogs (A), recent version attributed to frogs and mammals (B), and diagram of the major axonal regions described in this report (C). A: original model of saltatory conduction in frog sciatic nerve, illustrating inward current at nodes of Ranvier (red arrow), induced outward currents (blue arrows), also at nodes of Ranvier, and “nerve impulse” (green arrows) conducted in cytoplasm. [Modified from Tasaki (1939).] B: recent diagram showing colocalization of voltage-gated Na⁺ channels and voltage-gated K⁺ channels within nodes of Ranvier, with temporal but not spatial separation of Na⁺ and K⁺ currents. [Modified from Purves (2012)]. C: diagram delineating functional domains of mammalian myelinated axons described in this report. Node of Ranvier, red; paranode, green (para = both sides of); juxtaparanodal axolemma, purple (juxtaparanode = adjacent to both paranodes); internodal axolemma, light blue; inner mesaxon, dark blue line (mesaxon = mesentery-like support for axon). SLI, Schmidt-Lanterman incisure.