The action potential is the single hardest graph in introductory physiology, and students lose marks not because the idea is impossible but because they memorize the shape of the curve without knowing which ion is moving at each point. An action potential is a brief, all-or-nothing electrical signal that travels down a neuron — and it is fully explained by sodium and potassium ions crossing the membrane through gated channels. Once you can name the channel for each phase, the graph stops being a curve to memorize and becomes a sequence you can reason through.
Start With the Resting Membrane Potential
Before anything fires, a neuron sits at a resting membrane potential of about −70 mV — the inside of the cell is negative relative to the outside. Two things hold it there.
First, the sodium-potassium pump continuously moves 3 sodium ions (Na⁺) out of the cell for every 2 potassium ions (K⁺) it brings in. This costs ATP and builds the concentration gradients: lots of Na⁺ outside, lots of K⁺ inside.
Second, the membrane at rest is far more permeable to K⁺ than to Na⁺, because some K⁺ leak channels are always open. K⁺ drifts out down its gradient, carrying positive charge with it and leaving the inside negative.
The takeaway: at rest, the cell is polarized and primed. The gradients are loaded, like a stretched spring waiting for a trigger.
Reaching Threshold: The All-or-Nothing Trigger
A stimulus — a signal from another neuron — causes a small depolarization, meaning the inside becomes slightly less negative. If that depolarization reaches threshold, about −55 mV, the action potential fires. If it does not reach threshold, nothing happens.
This is the all-or-nothing principle: an action potential is the same size every time. A stronger stimulus does not make a bigger spike; it makes spikes fire more often. The neuron encodes intensity as frequency, not amplitude.
Threshold matters because it is the point where voltage-gated sodium channels begin to open in large numbers, and that opening feeds itself.
Depolarization: Sodium Rushes In
At threshold, voltage-gated Na⁺ channels snap open. Sodium floods into the cell, driven by both its concentration gradient (more Na⁺ outside) and the electrical gradient (the inside is negative, attracting positive ions).
The inflow of positive charge makes the inside rapidly less negative, then positive — the membrane potential shoots from −55 mV up to about +30 mV. This is depolarization, the steep upstroke of the graph.
It is a positive feedback loop: depolarization opens Na⁺ channels, which lets in more Na⁺, which depolarizes the membrane further, which opens still more channels. That runaway loop is why the upstroke is so fast and so steep.
Repolarization: Sodium Closes, Potassium Opens
The spike does not stay up. Two events bring it down.
The voltage-gated Na⁺ channels inactivate — they close automatically a fraction of a millisecond after opening, stopping the sodium inflow. Inactivation is a separate gate from the one that opened, and it is why the cell cannot fire again immediately.
At nearly the same time, the slower voltage-gated K⁺ channels finish opening. Potassium now rushes out of the cell, down its gradient, carrying positive charge away. Losing positive charge makes the inside negative again — this is repolarization, the downstroke back toward −70 mV.
Hyperpolarization and the Refractory Period
The voltage-gated K⁺ channels are slow to close, so K⁺ keeps leaving slightly too long. The membrane briefly overshoots to about −80 mV — more negative than rest. This dip is hyperpolarization. The sodium-potassium pump and the closing of K⁺ channels then restore the −70 mV resting potential.
During this whole sequence the neuron is in a refractory period. In the absolute refractory period, the Na⁺ channels are inactivated and no stimulus, however strong, can trigger another action potential. In the relative refractory period, the membrane is hyperpolarized, so only an unusually strong stimulus will fire it. The refractory period does two useful things: it caps the maximum firing rate, and it forces the signal to travel in one direction — the patch behind the spike cannot re-fire, so the wave only moves forward.
Getting Help
The action potential is the foundation for nearly every later topic in neural and muscle physiology — once a signal reaches a muscle, it triggers the events covered in sliding filament theory simplified. For more step-by-step physiology walkthroughs, see the full set of Anatomy & Physiology study guides.
Conclusion
An action potential explained well is just a sequence of ion movements: a polarized cell at −70 mV, a stimulus that reaches the −55 mV threshold, sodium rushing in to depolarize to +30 mV, sodium inactivating while potassium exits to repolarize, a brief hyperpolarization, and a refractory period that resets the system. Match each phase to its ion and its channel, and the graph explains itself.
