Node of Ranvier

axon part that is a gap in the myelin where voltage-gated sodium channels cluster and saltatory conduction takes place

Nodes of Ranvier are gaps in the myelin sheath of a nerve cell's axon. They have multiple ion channels, allowing ions to move in or out of the axon at those points.

Nodes of Ranvier
Gray's Anatomy diagram
Diagram of a neuron, showing the Nodes of Ranvier

This means the gaps are not insulated from the spaces which surround them (the intracellular space). When impulses move along an axon, they will seem to "jump" from one node to the next along the axon. This is known as saltatory conduction (Latin saltare "to hop or leap").