Action potential

The action potential is the neuron's electrical signal. The neuron is wrapped in a membrane, and there is a voltage between its inside and its outside, a difference in charge much like the one between the two ends of a battery. The action potential is a rapid sequence of changes in that voltage.

It all starts when the voltage reaches a trigger point, the threshold. At that moment the sodium channels, tiny gates in the membrane, open and sodium flows into the cell: that is depolarization. This rise is all-or-none, like a light switch: either it fires completely or nothing happens. Next the potassium channels open, potassium flows out, and the voltage returns toward its resting level: that is repolarization.

The signal travels along the axon, the long projection of the neuron. When the axon is wrapped in myelin, an insulating layer, the signal is regenerated at each node of Ranvier, a short stretch that the myelin leaves exposed. It moves forward like someone crossing a stream by hopping from stone to stone. This is called saltatory conduction, and it can make the signal travel more than ten times faster than in an unmyelinated axon.