Node of Ranvier

A Node of Ranvier is a tiny bare gap in the insulation that wraps a nerve fiber. Many of the long output cables of neurons — called axons, the wire-like tails that carry signals away from a cell — are sheathed in a fatty white wrapping called myelin, which works just like the plastic coating on an electrical wire. But that coating is not one continuous tube. It comes in segments laid end to end, and between each segment there is a short, uncovered pinch of the axon. Each of those regularly spaced gaps is a Node of Ranvier, named after the French scientist who first described them.

These little gaps are where the real action happens, and they make nerve signals travel astonishingly fast. The electrical pulse that races along an axon can only be freshly regenerated at the bare nodes, because that is where the channels that pump charge across the membrane are concentrated; under the fatty myelin, the fiber is sealed off and the signal simply coasts. So instead of crawling smoothly down the whole length of the axon, the pulse effectively leaps from one node to the next, skipping over the insulated stretches in between. This jumping shortcut — called saltatory conduction, from the Latin for ‘to leap’ — lets signals move many times faster than they would in a bare fiber, while using far less energy.