Action Potentials
The electrical pulse neurons use to send signals: resting potential, ion channels, and the all-or-none wave that travels down axons.
An action potential is a rapid, stereotyped change in the electrical potential across a neuron's membrane that propagates along the axon and carries information. Neurons signal not with graded voltages but with all-or-none pulses: either the membrane depolarizes past threshold and a full action potential fires, or it does not. The nervous system encodes information in the timing and frequency of these pulses.
The resting state is set by ion gradients. The sodium–potassium pump continuously moves three sodium ions out and two potassium ions in, building concentration gradients, while the membrane at rest is far more permeable to potassium than to sodium, so the resting potential — about −70 millivolts — sits close to the potassium equilibrium potential. A stimulus that depolarizes the membrane to threshold, roughly −55 millivolts, triggers the sequence:
| Phase | Ion channels | Membrane potential |
|---|---|---|
| Depolarization | Voltage-gated Na⁺ channels open | Rises toward +40 mV |
| Repolarization | Na⁺ channels inactivate; K⁺ channels open | Falls back toward resting |
| Afterhyperpolarization | K⁺ channels remain open briefly | Dips below resting |
| Resting | Pump and leak channels reestablish gradients | −70 mV |
The voltage-gated sodium channel is the workhorse: it opens fast, allowing sodium to rush in and drive the membrane positive, then inactivates within a millisecond, ending the sodium current. The slower-opening voltage-gated potassium channel repolarizes the membrane and briefly overshoots. Two refractory periods follow: the absolute refractory period, during which sodium channels are inactivated and no stimulus can fire a second pulse, and the relative refractory period, during which a stronger-than-usual stimulus can. Refractoriness sets the maximum firing rate and, crucially, ensures the pulse travels in one direction.
Propagation works by local currents: the depolarized region acts as a battery that depolarizes the adjacent membrane, which fires in turn, so the pulse travels as a wave. Myelinated axons conduct much faster — up to about 100 meters per second versus roughly 1 meter per second in unmyelinated fibers — because the myelin sheath insulates the membrane and the pulse jumps between the gaps, the nodes of Ranvier, in saltatory conduction. This is why demyelinating diseases such as multiple sclerosis slow or block neural transmission.
At the axon terminal the action potential triggers voltage-gated calcium channels, neurotransmitter release, and a postsynaptic response in the next cell — the synapse. Beyond the nervous system, action potentials drive muscle contraction and are the signals measured by electrocardiography and electroencephalography. Local anesthetics work by blocking voltage-gated sodium channels, preventing action potentials in sensory fibers. This article is an educational reference; medical decisions should be made with qualified professionals.
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neurons neuroscience physiology