Nerve fibre node. Transmission electron micrograph (TEM) of a section through a nerve fibre (axon) at a node of Ranvier. The axon (purple, centre) is surrounded by a fatty insulating sheath of myelin (red), which 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: x12,000 when printed 10 centimetres wide.