nodes of Ranvier

n. The regularly spaced gaps in the myelin sheath surrounding axon fibers in which the axon is exposed to ambient liquids. Nerve impulses in myelinated fibers pass very quickly because depolarization at one end of a myelinated section causes nearly instantaneous depolarization throughout the section so impulses jump from one node to another in a process called saltatory transmission. It is only at the nodes of Ranvier that there is actually an exchange of ions across the cell wall, which takes a small but significant amount of time.