How Neurons Fire: The Action Potential

Press your finger to a hot pan and your hand jerks back before you have time to think. The message that yanks your arm away travels along nerve cells called neurons, and it travels as a brief electrical spike called an action potential. To see how that spike works, you have to picture what a resting neuron is doing before anything happens.

A neuron at rest is not actually at rest. It is constantly pumping sodium ions out of itself and potassium ions in, like a bouncer keeping certain people outside and others inside. Because the inside ends up with fewer positive charges than the outside, the inside of the cell sits at about negative seventy millivolts compared to the outside. This difference in charge across the membrane is called the resting membrane potential. The neuron is loaded like a spring, waiting for a trigger.

The trigger arrives when something nudges the voltage upward — usually a signal from another neuron. If the nudge is strong enough to push the voltage past a value called threshold, tiny gates in the membrane snap open. These are voltage-gated sodium channels: doors that stay shut until the voltage gets high enough, and then fly open. Sodium ions, which have been crowded outside, rush in. Each one carries a positive charge, so the inside of the cell becomes rapidly more positive. This sudden flip is called depolarization.

Here is the step that the cartoon version of this story usually gets wrong. Sodium does not stay flowing. The sodium channels are not simple on-off switches; they snap open, and then a fraction of a millisecond later they automatically jam themselves shut. This jammed state is called inactivation, and it matters enormously, as we will see.

While the sodium channels are inactivating, a second set of gates opens more slowly: voltage-gated potassium channels. Potassium ions, crowded on the inside, now rush out. Each one that leaves takes a positive charge with it, so the inside swings back toward negative. This return is called repolarization. The potassium channels are a little sluggish about closing, so the voltage actually overshoots, dipping briefly below the resting value before settling back.

Now, why does the signal travel down the axon in one direction instead of sloshing back and forth? Because of inactivation. The patch of membrane that just fired cannot fire again immediately — its sodium channels are jammed shut and need time to reset. So when the depolarization spreads to neighboring membrane and opens their sodium channels, the wave can only move forward, into fresh territory. The brief window when a patch of membrane refuses to fire again is called the refractory period, and it is what turns the action potential into a one-way pulse rather than a ripple in a pond.

One more feature is worth naming. An action potential is all-or-nothing. A stronger stimulus does not produce a bigger spike; it produces more spikes per second. The neuron speaks in a code of frequency, not volume, and that code is what lets a single jerk of the hand outrun a thought.