Periodic gaps, about 1 micrometer wide, between myelin sheath (Schwann) cells on axons of particular neurons (see figure below). Fat being a good insulator means that the myelin sheaths facilitate the speed of an electrical impulse travelling along an axon. The impulse jumps from node to the next as a speed that can attain 120 m/s, something referred to as saltatory conduction (a speed much faster than obtains for non-myelinated axons). The impulse travels along the axon by a process of de-polarization and re-polarization of the nerve membrane; thus at the unmyelinated gaps or nodes, electrical activity is generated. The nodes of Ranvier also function to allow nutrients to enter and leave a neuron. The nodes were first described by the histologist Louis-Antoinine Ranvier (1835-1922) in 1878 who considered them to be constrictions.
