Nerve fibre. Transmission electron micrograph (TEM) of a section through a sciatic nerve, showing a node of Ranvier and myelinated and unmyelinated nerve fibres (axons). Myelin (brown) is a fatty insulating sheath composed of schwann cells that surrounds the axon and increases the speed of the nerve signal. Nodes of Ranvier are areas of exposed axon, with a high concentration of sodium channels. Instead of travelling continuously along the length of the axon, the impulse jumps between these nodes in a process called saltatory conduction. This allows the impulse to travel over long distances. Magnification: x10,000 when printed 10 centimetres wide.