From Threshold to Spike
If enough excitatory input pushes a neuron's membrane from its mV resting potential up to about mV, it crosses threshold and fires an action potential — a rapid, stereotyped voltage spike that travels down the axon. Crucially, the action potential obeys the all-or-none law: once threshold is crossed the spike always reaches the same peak (about mV), regardless of how much the threshold was exceeded. A stimulus that never reaches threshold produces no action potential at all.
| Phase | What happens |
|---|---|
| Depolarization | Voltage-gated Na+ channels open; Na+ rushes in, driving the membrane toward mV |
| Repolarization | Na+ channels inactivate; voltage-gated K+ channels open, and K+ rushes out |
| Hyperpolarization | K+ channels close slowly, so the membrane briefly overshoots below mV |
| Return to rest | The Na+/K+ pump and leak channels restore the resting potential |
During the absolute refractory period (while Na+ channels are inactivated), no stimulus, however strong, can trigger a second spike — this enforces one-way propagation down the axon. During the following relative refractory period, only an unusually strong stimulus can fire a second spike. Myelin speeds conduction dramatically: the impulse jumps between unmyelinated gaps called Nodes of Ranvier in a process called saltatory conduction, reaching up to 120 meters per second in large myelinated axons versus about 1 meter per second in unmyelinated ones.
Common pitfall: thinking a stronger stimulus produces a "bigger" action potential. It does not — intensity is coded by how often a neuron fires (firing rate), not by the size of any single spike, which is fixed once threshold is crossed.
A single accent trace sweeping left to right on a voltage-vs-time axis: flat at rest, rising through threshold, peaking at overshoot, falling through repolarization, dipping through hyperpolarization.