Action potential

In physiology, 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. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and endocrine cells.

In neurons, action potentials play a central role in cell-to-cell communication by providing for the propagation of signals along the neuron's axon towards boutons at the axon ends which can then connect with other neurons at synapses, or to cells or glands. In muscle cells, for example, an action potential is the first step in the chain of events leading to contraction.

Action potentials involve both voltage-gated sodium channels and potassium channels embedded in the cell membrane. These ion channels are shut when the membrane potential is near the resting membrane potential of the cell, but they rapidly begin to open if the membrane potential increases to a precisely defined threshold potential.

Depolarization

When the voltage-gated sodium channels open (in response to depolarization or hyperpolarization), they allow an inward flow of sodium ions, which changes the electrochemical gradient and causes more channels to open, increasing the membrane potential and producing a greater electric current across the cell membrane, and so on. The process proceeds explosively until all of the available voltage-gated sodium channels are open, resulting in the large peak of the action potential. The rapid influx of sodium ions depolarizes the cell, causing the sodium channels to inactive (close).

Repolarization

As the voltage-gated sodium channels close, no more sodium ions enter the neuron and are actively transported back out of the cell. Voltage-gated potassium channels are then activated, and there is an outward flow of potassium ions during the repolarization step, returning the electrochemical gradient to the resting state.

Hyperpolarization

After an action potential has occurred, there is a transient negative shift, called the hyperpolarization, due to extra potassium ions leaving the neuron. The whole process is represented in Figure 1.

Figure 1. Action potential steps.