How Neurons Fire: The Action Potential

Imagine a neuron at rest, sitting inside your brain with its interior more negative than its surroundings. This resting state is maintained by a pump in the cell membrane that exchanges sodium and potassium ions at a metabolic cost. But when the neuron receives a strong enough signal from its neighbors, something dramatic happens—a wave of electrical and chemical change sweeps down the axon in roughly a millisecond. This wave is the action potential, and it is how neurons fire.

The action potential begins when the neuron receives input that pushes its membrane voltage toward a critical threshold, usually around −55 millivolts. At rest, the neuron sits at about −70 millivolts. As long as the voltage stays below threshold, nothing special occurs. But if the signal is strong enough to cross that threshold, a cascade of ion-channel openings is triggered. The threshold concept is not incidental—it is the fundamental basis of the neuron's all-or-nothing character. A strong signal and a slightly-stronger-than-threshold signal produce identical action potentials. The neuron does not have a dial; it has a switch.

Once threshold is crossed, voltage-gated sodium channels open rapidly. Sodium ions, which are far more concentrated outside the cell, rush inward. This inflow of positive charge depolarizes the membrane, driving the voltage positive—to peak values around +30 millivolts. This is the rising phase, and it is driven entirely by sodium. The cell membrane's voltage at any moment is governed by the relative conductance of sodium and potassium ions; when sodium channels are open and potassium channels are mostly closed, sodium wins and the voltage rises.

But the rising phase contains the seed of its own reversal. Voltage-gated sodium channels do not stay open indefinitely; they inactivate after about 1 millisecond. Inactivation is different from closing—an inactivated channel is locked shut and cannot open again immediately, even if the voltage drops below threshold. Meanwhile, voltage-gated potassium channels open more slowly than sodium channels, and they stay open longer. By the time potassium channels are fully conducting, sodium has already inactivated. Now potassium ions, concentrated inside the cell, rush outward, repolarizing the membrane back toward its negative resting potential. The falling phase is driven by potassium.

During the action potential and for a brief window after, the neuron enters its refractory period. The absolute refractory period—when sodium channels remain inactivated—makes it impossible to fire another action potential, no matter how strong the input signal. This has an immediate consequence: action potentials move in only one direction down the axon, because the region behind the action potential is in its absolute refractory period and cannot fire again. Relative refractoriness follows, during which sodium channels have recovered but the membrane is more negative than rest; a stronger-than-normal signal is required to reach threshold.

The refractory period solves a crucial problem. If neurons could fire continuously without pause, they would lack temporal resolution—the ability to distinguish the timing of separate signals. It also determines the maximum firing frequency of a neuron: typically 100 to 1000 action potentials per second, set by how long inactivation lasts. Longer refractory period means slower maximum firing. Some neurons have longer refractory periods and fire more slowly; others have shorter ones and fire faster. This variation is not accidental. Neurons are tuned by their ion-channel properties to operate at different timescales, allowing different brain circuits to process information at different speeds.

The action potential is thus not a single event but a choreography of openings and closings. Sodium enters, potassium exits, channels inactivate, recovery begins. The outcome is a stereotyped electrical impulse that travels the length of the axon and triggers the release of neurotransmitters at the synapse. This same mechanism, refined in detail but recognizable in form, fires in neurons throughout the animal kingdom—from sea slugs to humans. Understanding the action potential is understanding how the neuron code works.