The need for speed
The speed of nerve impulses varies enormously in different types of neurone. The fastest travel at about 250 mph, faster than a Formula One racing car. For the impulse to travel quickly, the axon needs to be thick and well insulated. This uses a lot of space and energy, however, and is found only in neurones that need to transfer information urgently. For example, if you burn your fingers it is important that your brain gets the message to withdraw your hand very quickly.
