15.1.5—Action potential sequence

A receptor stimulus first produces a graded receptor potential. If depolarisation reaches threshold, voltage-gated channels produce an all-or-nothing action potential in the sensory neurone; a weaker stimulus does not trigger the impulse.

1. Depolarisation: voltage-gated Na⁺ channels open and Na⁺ enters the axon, making the inside less negative. Positive feedback opens more Na⁺ channels.
2. Action-potential peak: once threshold is crossed, depolarisation continues and the membrane briefly becomes positive.
3. Repolarisation: Na⁺ channels close and voltage-gated K⁺ channels open; K⁺ leaves the axon, returning the membrane towards its resting state.
4. Recovery: K⁺ channels close and the membrane passes through a brief hyperpolarised phase before resting conditions are restored.

In the salt-taste example, sodium ions enter a chemoreceptor and depolarise it. A sufficiently large receptor potential opens voltage-gated Ca²⁺ channels, causing neurotransmitter release that stimulates an action potential in the sensory neurone. Stimulus strength is represented by impulse frequency rather than a larger action-potential amplitude.

Threshold is a boundary: below it there is no action potential, while crossing it produces a stereotyped impulse. This card explains generation and recovery at one membrane region; detailed synaptic transmission and long-distance conduction are covered separately.