Expansion
Depolarisation by sodium influx, repolarisation by potassium efflux, then hyperpolarisation
Expansion
- Resting: about minus 70 mV
- Threshold: about minus 55 mV, at which voltage-gated sodium channels open regeneratively
- Depolarisation: rapid sodium influx towards plus 30 mV
- Repolarisation: sodium channels inactivate and voltage-gated potassium channels open
- Hyperpolarisation (undershoot): potassium channels close slowly, so the potential briefly falls below rest
- Return to rest
The response is all-or-none: reaching threshold produces a full action potential, and stimulus strength is coded by frequency, not amplitude.
Refractory periods: the absolute period lasts while sodium channels are inactivated and no stimulus can excite; the relative period follows, when a stronger stimulus can. Together they ensure unidirectional propagation and limit maximum firing rate.
Local anaesthetics block voltage-gated sodium channels from the inside, preferentially in rapidly firing (use-dependent) fibres, which is why small unmyelinated pain fibres are blocked before large motor fibres.