Nerve stimuli cause the resting membrane potential to be interrupted. Stimuli of insufficient strength die away, and are not propagated.

If a stimulus reaches a threshold (or firing level), an action potential will be generated. Action potentials in neurones are very short-lived, lasting little more than a millisecond (Fig 1).

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Above threshold, each action potential has the same magnitude and duration irrespective of the strength of the stimulus. This is known as the 'all or none law'.

During the depolarization phase, voltage-gated Na⁺ channels open, and the membrane suddenly becomes more permeable to Na⁺ ions. These rush from the outside to the inside, changing the polarity across the neurone from -70 mV to +40 mV.

Next, K⁺-gated channels open and K⁺ ions rush to the outside. This repolarization positively charges the outside of the membrane relative to the inside once again.

A refractory period follows. This is a recovery time for the Na⁺-K⁺ pump to re-establish the RMP.