An action potential is a transient, electrical signal, which is caused by a rapid change in resting membrane potential (-70mV). This occurs when the threshold potential (-55mV) is reached, this causes a rapid opening in the voltage gated sodium channels leading to a influx of sodium into the cell. The threshold potential also causes a slow opening of voltage gated potassium channels leading to the eflux of potassium out of the cell. This causes the cell to depolarise, meaning the inside of the cell is now positive compared to the outside.
The action potenial starts in the axon hilock as there is a high density of voltage gated sodium channels here, it is also where graded potentials need to reach the threshold potential to cause a action potential.