An action potential is a travelling membrane change

By UK A Level Biology · independent study · myAudience

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An action potential is a travelling membrane change

Ion gradients and selective membrane permeability support a resting potential. Once a stimulus reaches threshold, voltage-gated channels generate an action potential through changing sodium and potassium movement. The sodium–potassium pump maintains gradients over time; it is not the direct cause of every rapid voltage change.

Action potentials are all-or-none events. Stimulus intensity can be represented by firing frequency, while refractory periods help limit frequency and prevent immediate backward re-excitation. Myelin supports saltatory conduction between nodes rather than a signal passing continuously through every covered membrane segment.

During depolarisation, sodium entry makes the interior less negative; subsequent potassium movement supports repolarisation. Sodium-channel inactivation contributes to the absolute refractory period. A simplified 2 ms minimum interval corresponds to 1 / 0.002 = 500 impulses/s, but actual firing can be slower. Conduction speed in metres per second is a different measurement from firing frequency in impulses per second.

Worked example: A signal travels 1.2 metres in 0.02 seconds. Calculate average conduction speed.

Speed = distance/time.

1.2/0.02 = 60.

Answer: 60 metres per second.

Check your understanding: Does a stronger above-threshold stimulus normally double the amplitude of every action potential?

Answer: No.

Why: Intensity can be encoded by frequency; individual action potentials follow an all-or-none pattern.