node of Ranvier, periodic gap in the insulating sheath (myelin) on the axon of certain neurons that serves to facilitate the rapid conduction of nerve impulses. These interruptions in the myelin covering were first discovered in 1878 by French histologist and pathologist Louis-Antoine Ranvier, who described the nodes as constrictions.

The myelin sheath consists of concentric layers of lipids, including cholesterol and variable amounts of cerebrosides and phospholipids, separated by thin layers of protein. This arrangement gives rise to a high-resistance, low-capacitance electrical insulator. However, nodes of Ranvier interrupt the insulation at intervals, and this discontinuity enables impulses to jump from node to node in a process known as saltatory conduction.

Nodes of Ranvier are approximately 1 μm wide and expose the neuron membrane to the external environment. These gaps are rich in ion channels, which mediate the exchange of certain ions, including sodium and chloride, that are required to form an action potential—the reversal of electrical polarization of the neuron membrane that initiates or is part of a wave of excitation that travels along the axon. The action potential propagated by one node of Ranvier jumps to and is regenerated at the next node along the axon, thereby enabling the action potential to travel rapidly along the fibre.