Nodes of Ranvier

Nodes of Ranvier, also known as neurofibril nodes, are regularly spaced gaps in the myelin sheath around an axon or nerve fiber. About one micrometer in length, these gaps expose the axonal membrane to the extracellular fluid. (The myelin sheath is the fatty tissue layer coating the axon.)

Function in action potentials
The myelin sheath helps speed the neural impulse by insulating the electrical current and making it possible for the impulse to jump from node to node, a process known as saltatory conduction, as opposed to traveling down the axon in tiny increments.

An action potential is the sharp electrochemical response of a stimulated neuron, a neuron whose membrane potential has been changed by a nearby cell, cells, or an experimentor. In an action potential, the cell membrane potential changes drastically and quickly as ions flow in or out of the cell. The action potential "travels" from one place in the cell to another, but ion flow occurs only at the nodes of Ranvier. Therefore, the action potential signal "jumps" along the axon, from node to node, rather than propagating smoothly, as they do in axons that lack a myelin sheath. This is due to clustering of voltage-gated Na+ and K+ ion channels at the Nodes of Ranvier. Unmyelinated axons do not have Nodes of Ranvier; voltage gated ion channels in these axons are considerably less ordered and spread over the entire membrane surface.

Nodes of Ranvier can be thought of as a digital electronic amplifier held between insulated conductors - the myelinated axons (real electronic amplifiers function quite differently from this, but work in an analogous fashion, using a small electric potential to control a larger one).

History
The myelin sheath and the nodes were discovered by French pathologist and anatomist Louis-Antoine Ranvier (1835-1922).