The axons of motor neurones are enveloped in a myelin sheath (Fig 1).

Myelin is a protein-lipid complex, and is formed by the layers of plasma membrane derived from Schwann cells. The myelin sheath extends along the length of the axon, but is interrupted at regular intervals by gaps known as the Nodes of Ranvier.

The axon membrane is insulated from the extracellular fluid by these layers of myelin, except at the Nodes of Ranvier.

Action potentials jump from one node to the next, a process known as saltatory conduction. This process enables a faster conduction velocity and conserves energy at the same time.