In certain classes of vertebrate neurons, the axon is covered by layers of membranes from Schwann cells. These layers of membrane are known as the myelin sheath. When a signal is being conducted through the axon, these myelin sheaths serve as an insulator from the outside environment. However, these sheaths are periodically interrupted (about every millimeter) by a space that is exposed to the outside. These spaces are called the nodes of Ranvier. They allow membrane depolarization, thereby amplifing the signal and allowing it to propogate to the next node.

With axons often being many centimeters to meters in length in certain organisms, myelin helps conserve energy of signal propogation and increases the speed with which it moves across the axon.