The Action Potential

The nerve impulse step by step: threshold, the rush of sodium, the overshoot, potassium repolarisation, the refractory period — and why every spike is all-or-nothing.

The action potential — also called a spike or nerve impulse — is the basic electrical message of the nervous system. It is a brief reversal of the membrane voltage that travels along the axon without fading.

1. Reaching threshold

At rest the membrane sits around −70 mV. When inputs depolarise the axon's initial segment to around −55 mV — the threshold — enough voltage-gated sodium channels open that the process becomes self-reinforcing.

2. The rising phase

Sodium ions rush into the cell, making the inside more positive, which opens more sodium channels, which lets in more sodium. In a fraction of a millisecond the voltage shoots up past zero to about +30 to +40 mV — the overshoot Hodgkin and Huxley first measured.

3. The falling phase

Two things end the spike. Sodium channels inactivate — a part of the protein plugs the pore — and slower voltage-gated potassium channels open. Potassium flows out, driving the voltage back down.

4. Undershoot and refractory period

Potassium channels close slowly, so the voltage briefly dips below rest — the afterhyperpolarisation. For about a millisecond, inactivated sodium channels can't reopen at all (the absolute refractory period), and for a few more milliseconds a stronger stimulus is needed (the relative refractory period). This sets a maximum firing rate of a few hundred spikes per second, and ensures spikes travel in one direction: the patch just behind the spike is still refractory.

All or nothing

Below threshold there is no spike; above it, a full-sized one. Information is carried by the timing and rate of spikes, not their size — a firm touch makes touch neurons fire faster, not bigger. Surprisingly little ion movement is needed: a single spike changes ion concentrations only very slightly, which is why a neuron can fire thousands of times before its pumps fall behind.