What is an action potential, and what steps are involved?

Action potentials are changes of electrical potential in a cell. The normal resting membrane potential inside cells is around -70mV. This membrane potential is maintained through the action of the Na⁺K⁺ATPase pump (which moves 3Na⁺ out and 2 K⁺ in), leaky Na channels (that allow influx of Na⁺) and leaky K⁺ channels (that allow efflux of K⁺). The net movement of Na⁺ and K⁺ is balanced at this point.An action potential starts when Na⁺ channels open and Na⁺ starts moving into the cell. This starts depolarisation of the cell, where the membrane potential starts increasing. At around -30mV a threshold is met. This is an all or nothing response, if enough stimulus is present all the voltage-gated Na⁺ channels open causing a rapid depolarisation of the cell. When the membrane potential reaches around +30/40mV the Na⁺ channels close, and the voltage-gated K⁺ channels open at the same time. As K⁺ starts moving into the cell, it starts repolarisation of the cell - the membrane potential starts moving down. The K⁺ channel remain open past -70mV and eventually close when the potential reaches -90mV - this is known as hyperpolarisation. The action of the Na⁺K⁺ATPase pump then brings the cell back to its resting membrane potential (-70mV). At this point a second action potential can occur.