An action potential is a rapid, temporary change in the electrical voltage across a cell membrane, particularly in excitable cells like neurons and muscle cells. It involves a quick rise and fall in membrane potential that propagates along the cell membrane without losing strength
. Key features of an action potential:
- It begins when a stimulus depolarizes the membrane to a threshold level (around -50 to -55 mV), triggering voltage-gated ion channels to open
- Sodium channels open first, allowing sodium ions (Na⁺) to rush into the cell, causing rapid depolarization and a reversal of membrane polarity
- Then, sodium channels close and potassium channels open, allowing potassium ions (K⁺) to exit the cell, which repolarizes and often hyperpolarizes the membrane back toward its resting potential (around -70 mV)
- The entire event follows an "all-or-none" principle: subthreshold stimuli do not trigger an action potential, while threshold or stronger stimuli produce a full, uniform response
- After an action potential, the neuron enters a refractory period during which it cannot fire another action potential immediately, ensuring unidirectional propagation of the signal
Functionally, action potentials serve as the fundamental signals for communication in the nervous system. They propagate along axons to transmit information to other neurons, muscles, or glands. In muscle cells, they initiate contraction, while in endocrine cells, they can trigger hormone release
. In summary, an action potential is a brief, self-propagating electrical impulse caused by the orchestrated opening and closing of ion channels, enabling rapid communication within and between cells
