Saltatory Conduction

Action potentials can travel along axons at speeds of 0.1-100 m/s. This means that nerve impulses can get from one part of a body to another in a few milliseconds, which allows for fast responses to stimuli. (Impulses are much slower than electrical currents in wires, which travel at close to the speed of light, 3x10 8 m/s.) The speed is affected by 3 factors:

• Temperature. The higher the temperature, the faster the speed. So homeothermic (warm-blooded) animals have faster responses than poikilothermic (cold-blooded) ones.

• Axon diameter. The larger the diameter, the faster the speed. So marine invertebrates, who live at temperatures close to 0°C, have developed thick axons to speed up their responses. This explains why squid have their giant axons.

• Myelin sheath. Only vertebrates have a myelin sheath surrounding their neurons. The voltage-gated ion channels are found only at the nodes of Ranvier, and between the nodes the myelin sheath acts as a good electrical insulator. The action potential can therefore jump large distances from node to node (1mm), a process that is called saltatory propagation. This increases the speed of propagation dramatically, so while nerve impulses in unmyelinated neurons have a maximum speed of around 1 m/s, in myelinated neurons they travel at 100 m/s.