When in its inactive state, or resting potential, the neuron is electrically polarized to approximately -60 mV in relation to the extracellular space, due to a difference in concentration of charged particles and a selective permeability of the cell membrane. When a neuron is depolarized and reaches a potential of about -45mV, a rapid depolarization takes place, due to a decrease in membrane permeability, which allows for a rapid influx of positively charged particles into the axon of the cell. This is the action potential.

The action potential will propagate along the neuron in one direction, which is ensured by the existence of a refractory period, a period in which each area along the axon will not be depolarizable until a given amount of time has passed. The action potential propagates either by contiguous conduction, or in the case of myelinated axons, by saltatory conduction. In contiguous conduction, depolarization spreads from each area of the axon to the next, and in saltatory conduction, polarization travels from one gap (or node of Ranvier) to the next. Saltatory conduction is more rapid, due to a lower number of charged particles travelling across the axon membrane.

Source:

Sherwood, L. (2011). Fundamentals of Human Physiology (4th ed.). (pp. 74-82). Belmont, CA: Brooks/Cole Cengage Learning.