What are nerve impulses?
A nerve impulse is an electrical signal that travels along an axon. There is an electrical difference between the inside of the axon and its surroundings, like a tiny battery. When the nerve is activated, there is a sudden change in the voltage across the wall of the axon, caused by the movement of ions in and out of the neuron. This triggers a wave of electrical activity that passes from the cell body along the length of the axon to the synapse.
Got the need for speed?
The speed of nerve impulses varies enormously in different types of neuron. The fastest travel at about 250 mph, faster than a Formula 1 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 neurons 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.
How do nerve impulses code information?
What is myelin?
Neurons that need to transmit electrical signals quickly are sheathed by a fatty substance called myelin. Myelin acts as an electrical insulator, and signals travel 20 times faster when it is present. In the disease multiple sclerosis, the myelin around the axons of some nerves gradually breaks down, so that the nerves can no longer efficiently carry electric signals between the brain and body.