An action potential is a rapid, temporary shift in electrical charge across a cell membrane, fundamental for communication in nerve and muscle cells. Its peak marks a crucial turning point, where the cell’s internal charge reaches its maximum positive value before returning to its resting state.
Reaching the Peak
The action potential’s peak begins with rapid depolarization, triggered when a stimulus causes the membrane potential to reach a threshold, typically around -55 mV. Once attained, voltage-gated sodium (Na+) channels, specialized protein structures in the cell membrane, open quickly. These channels allow positively charged sodium ions to rush into the cell.
The rapid influx of sodium ions occurs because sodium concentration is higher outside the cell, and the cell’s interior is negatively charged, creating a strong electrochemical gradient. This inward movement of positive charge causes the membrane potential to swing rapidly from its negative resting state to a positive value, often reaching approximately +30 mV. This swift change defines the action potential’s peak.
The Moment of Transition
At the peak of the action potential, ion channel activity marks the transition to the repolarization phase. Voltage-gated sodium channels quickly enter an “inactivated” state. This inactivation, distinct from simple closing, physically blocks the channel pore, stopping sodium ion flow even while the membrane is depolarized. This rapid inactivation prevents further sodium influx and ensures unidirectional action potential propagation.
Simultaneously, or with a slight delay, voltage-gated potassium (K+) channels fully open. These channels respond to membrane potential changes, but their opening is slower than sodium channels. As the membrane potential becomes positive during depolarization, these channels activate, and by the peak, they are fully open. This allows potassium ions to flow out of the cell, moving down their electrochemical gradient.
Initiating Repolarization
The events at the action potential’s peak initiate the swift decrease in membrane potential that characterizes repolarization. With sodium channel inactivation, the inward flow of positive charge into the cell abruptly ceases. Simultaneously, fully open potassium channels facilitate outward movement of positive potassium ions from the cell. This shift from inward sodium to outward potassium current drives the membrane potential’s rapid decline.
This net efflux of positive charge quickly reverses the membrane potential, causing it to fall sharply from its positive peak back towards its negative resting state. The combined effect of sodium channel inactivation and delayed potassium channel opening ensures the cell rapidly transitions away from its depolarized state. This swift return towards the resting potential marks the beginning of the repolarization phase.