AP® Psychology · Unit 1 · Topic 1.3
The Action Potential
An action potential is the brief electrical spike a neuron fires once its membrane voltage crosses threshold. It always fires at full strength or not at all, never partway.
A resting neuron sits at about -70 mV, more negative inside than outside, held there by ions parked across its membrane. Incoming signals nudge that voltage up or down. If enough of them push the inside voltage up to around -55 mV, the threshold, the neuron hits a tipping point it cannot walk back from. Sodium channels swing open, sodium rushes in, and the voltage rockets upward past zero in a spike called the action potential. Potassium channels then open, potassium rushes out, and the voltage crashes back down, even dipping briefly below -70 mV before settling back to rest.
Threshold acts like a switch, not a dial. Depolarization below -55 mV just fades out, no spike, nothing travels down the axon. Cross it, and the neuron fires the exact same spike every time, regardless of whether the triggering stimulus was barely enough or far more than enough. This is the all-or-none law: a stronger stimulus cannot make one action potential bigger, it can only make the neuron fire more often, with more neurons joining in. Size is fixed; frequency and recruitment carry the information about intensity.
Right after a spike, the neuron enters a refractory period, during which sodium channels are locked shut and cannot reopen no matter how strong the next stimulus is. That forces the action potential to move in one direction only, down the axon away from the cell body, since the patch of membrane that just fired cannot fire again immediately. This refractory period is also what caps how fast a neuron can fire repeatedly, setting up the next topic: how that spike crosses the synapse to reach the next neuron.
Key terms
- Resting potential
- The steady voltage of a neuron at rest, about -70 mV, with the inside more negative than the outside.
- Threshold
- The voltage level, around -55 mV, that depolarization must reach to trigger an action potential.
- Depolarization
- A shift in membrane voltage toward less negative, moving the neuron closer to firing.
- All-or-none law
- The rule that a neuron fires a full-strength action potential or none at all, with no in-between sizes.
- Refractory period
- The brief span after a spike when sodium channels cannot reopen, preventing the action potential from reversing direction.
Watch it explained
- How it worksWalk me through the voltage trace of one action potential from resting to spike and back.
- How it worksHow does the threshold decide whether incoming signals turn into a spike or fade out?
- Worked exampleShow me the voltage trace for a neuron sitting at -70 mV that gets pushed up to -55 mV.
- Common mistakeWhy doesn't a stronger stimulus make a neuron fire a bigger action potential?
- How it worksHow does the refractory period stop a spike from running backwards down the axon?
Check yourself
End of topic.