Action Potential
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State
An action potential is a brief, all-or-nothing reversal of membrane potential: once a stimulus depolarizes the membrane to a threshold near −55 millivolts, sodium channels open and the neuron fully depolarizes.
Show
After peak depolarization, voltage-gated potassium channels open, repolarizing the membrane and briefly overshooting into hyperpolarization.
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An action potential is "all-or-nothing" — a neuron reaching threshold always fires fully, never partially.