6.2.3Organisms Respond to Changes in their Environments (A Level only)

Action Potentials — AQA A Level Biology 7402 Revision Notes & Topical Questions

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6.2.3 Action Potentials — AQA A Level Biology 7402 notes: Describe the sequence of changes in membrane permeability and potential that produce an action potential.An action potential is a rapid, temporary reversal of the potential difference across the axon membrane, produced whe…

all-or-nothing
voltage-gated channel
Resting state
Stimulus and depolarisation
positive feedback
Repolarisation

Section 1 of 9Definition

An action potential is a rapid, temporary reversal of the potential difference across the axon membrane, produced when a stimulus is large enough to reach the threshold (about -55 mV).

It is the electrical signal of a nerve impulse.

All-or-nothing

Action potentials are all-or-nothing: below threshold none occurs,

and above it every action potential has the same size, about 100 mV from trough to peak.

The changes depend on two kinds of voltage-gated channel, for Na⁺ and K⁺, which open and close in response to the potential across the membrane.

Section 2 of 9Stages of an action potential

1Resting state

Resting state polarised at -70 mV; voltage-gated Na⁺ and K⁺ channels are closed.

2Stimulus and depolarisation

Stimulus and depolarisation a stimulus causes some voltage-gated Na⁺ channels to open.
Na⁺ diffuses in down its electrochemical gradient, making the inside less negative.
If the potential reaches the threshold, more voltage-gated Na⁺ channels open (positive feedback), and large numbers of Na⁺ enter rapidly.
The potential reaches about +40 mV.

3Repolarisation

Repolarisation at about +40 mV the Na⁺ channels close and voltage-gated K⁺ channels open.
K⁺ diffuses out, so the inside becomes negative again.

4Hyperpolarisation

Hyperpolarisation the K⁺ channels close slowly, so more K⁺ leaves than necessary and the potential dips below -70 mV briefly.

5Return to resting potential

K⁺ channels close and the sodium–potassium pump, with the leak channels, restores the resting ion distribution.

Section 3 of 9The refractory period

After an action potential there is a refractory period during which the membrane cannot be stimulated, or needs a much larger stimulus. Na⁺ channels are inactivated during repolarisation and the start of hyperpolarisation and cannot open. The refractory period has three important consequences:

Discrete events

Action potentials are discrete events, separated from each other.

One direction

Impulses travel in one direction only, because the region behind is refractory.

Maximum frequency

It limits the maximum frequency of impulses along an axon.

Section 4 of 9Strength of stimulus

Because of the all-or-nothing principle, a stronger stimulus does not make a larger action potential. It produces a higher frequency of action potentials. A larger stimulus may also stimulate more neurones, or neurones with a higher threshold, so the brain interprets the strength of a stimulus from the frequency of impulses and from the number of neurones firing.

Section 5 of 9Reading graphs

On a graph of potential against time, the rising section is depolarisation, the peak around +40 mV marks the closing of Na⁺ channels, the falling section is repolarisation, and the dip below -70 mV is hyperpolarisation.

Reading graphs

If a sub-threshold stimulus is shown, the potential rises a little and falls back with no spike.

The time between the starts of two peaks gives the minimum interval between action potentials, from which the maximum frequency can be calculated.

When describing a graph, quote values from the axes and name the ion movements responsible at each stage.

Section 6 of 9Explaining a data question

Questions about action potentials often give a graph or a table of channel behaviour and ask for an explanation.

A reliable approach is to work through the trace in order.

Name the stage, name the channels that are open at that stage, state the direction of ion movement and say what that does to the charge on the inside.

For example, on the rising phase between -55 mV and +40 mV, voltage-gated Na⁺ channels are open, Na⁺ enters by diffusion,

and the inside becomes positive.

On the falling phase, Na⁺ channels have closed, K⁺ channels are open and K⁺ leaves.

Experiments with drugs fit this pattern.

Tetrodotoxin, from the puffer fish, blocks voltage-gated Na⁺ channels, so action potentials cannot be generated and nerve conduction fails.

Tetraethylammonium blocks voltage-gated K⁺ channels, so repolarisation is delayed and the action potential lasts longer.

Using such evidence, you can explain why each ion is needed for its particular part of the cycle.

A further check is to compare conduction when the external Na⁺ concentration is lowered: the peak of the action potential is smaller because less Na⁺ enters,

which confirms that the inward movement of this ion produces the rising phase.

Always link the evidence to the ion and the channel, not just to the shape of the graph.

Section 7 of 9Exam-style questions

Describe the changes in membrane permeability that lead to depolarisation. [3]3 marks
Explain the importance of the refractory period. [3]3 marks
Explain why a larger stimulus produces a larger response in the brain although each action potential is the same size. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Action potential	Brief reversal of membrane potential from negative to positive inside
Depolarisation	Inside of membrane becoming less negative as Na⁺ enters
Repolarisation	Return of the inside to negative as K⁺ leaves
Threshold	Potential that must be reached to trigger an action potential
All-or-nothing	Response that occurs fully at threshold or not at all
Refractory period	Period after an action potential when the membrane cannot be restimulated
Hyperpolarisation	Potential temporarily more negative than the resting potential

Section 9 of 9Exam technique

Saying that K⁺ enters during repolarisation; K⁺ leaves.

Stating that the sodium–potassium pump causes depolarisation; depolarisation is passive diffusion of Na⁺ through channels.

Saying a stronger stimulus makes a bigger action potential; size is fixed, only frequency changes.

Confusing hyperpolarisation with depolarisation.

Forgetting that the threshold must be reached before the all-or-nothing response occurs.

Last updated: 18 October 2026

Section 1 of 9Definition

Section 2 of 9Stages of an action potential

Section 3 of 9The refractory period

Section 4 of 9Strength of stimulus

Section 5 of 9Reading graphs

Section 6 of 9Explaining a data question

Section 7 of 9Exam-style questions

Describe the changes in membrane permeability that lead to depolarisation. [3]3 marks

Explain the importance of the refractory period. [3]3 marks

Explain why a larger stimulus produces a larger response in the brain although each action potential is the same size. [2]2 marks

Section 8 of 9Key terms

Section 9 of 9Exam technique

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