8.4—Action potential conduction

When membrane depolarisation reaches threshold, voltage-gated sodium channels open and sodium ions enter. Potassium ions then leave to repolarise the membrane; the signal propagates as neighbouring sections reach threshold.

The all-or-nothing action potential is regenerated along the axon, so its amplitude does not fade with distance. Refractory periods help maintain one-way transmission.

A stronger stimulus is represented by more frequent action potentials, not a larger action potential, once threshold has been crossed.

Threshold is not a graded “bigger signal”, and ion movement is not the same as the whole impulse physically travelling as one sodium wave.