An action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. In neuronal cells, it is transmitting the excitement along the axon for cell-to-cell communication. An action potential follows the “all or nothing”-law. Over a certain threshold, it is unstoppable released.
Action potentials are generated by voltage-gated ion channels the plasma membrane. These channels are shut when the membrane potential is near the resting potential (~-70 mV) of the cell, but they rapidly begin to open if the membrane potential increases to a precisely defined threshold value. When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient rising the membrane potential further.
The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels are then inactivated. As the sodium channels close, sodium ions can no longer enter the neuron, and they are actively transported out of the plasma membrane. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state. After an action potential has occurred, there is a transient negative shift, called the hyperpolarization or refractory period, due to additional potassium currents. This is the mechanism that prevents an action potential from traveling back the way it just came.