What is an action potential and how does it work?

An action potential is the brief electrical charge that travels down the axon of a neuron when the cell is stimulated beyond its threshold. At rest, the neuron has a negative internal charge of about negative 70 millivolts called the resting potential. When stimulated, sodium channels open and positively charged sodium ions rush in, depolarizing the membrane. This triggers a chain reaction down the axon. After firing, potassium channels open to restore the negative charge in a process called repolarization.