That is a long answer. Schwann cells form the myelin sheath along the axon of a neuron. Schwann cells and the myelin sheath have lipids in their membranes (the phospholipid bilayer) and they are poor conductors of electrical signals. As a neuron fires, a wave of depolarization propagates down the axon to the next neuron. If the entire axon membrane had to depolarize the signal would move very slowly. The Schwann cells wrapped around the axon leave gaps between them (nodes of ranvier), exposing the axon membrane. The depolarization is forced to jump from one gap to the next. This is called saltation (jumping) and it speeds up nerve impulse transmission. One important place this is found on the axon of the sciatic nerve which goes from the pelvis to the big toe and is about 1 meter long.
