6.2.6Organisms Respond to Changes in their Environments (A Level only)

Calculating Maximum Impulse Frequency — AQA A Level Biology 7402 Revision Notes & Topical Questions

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6.2.6 Calculating Maximum Impulse Frequency — AQA A Level Biology 7402 notes: Use the refractory period to calculate the maximum frequency of impulses along an axon.After each action potential an axon has a refractory period, during which voltage-gated Na+ channels are inactivate…

Exam-style question 1
Exam-style question 2
Hertz (Hz)
Frequency coding
Reciprocal
Only when the channels recover can the axon fire again

Section 1 of 9Why is frequency limited?

After each action potential an axon has a refractory period, during which voltage-gated Na⁺ channels are inactivated and a new action potential cannot be generated.

Only when the channels recover can the axon fire again.

The refractory period therefore sets a limit on how many impulses can be sent each second.

The impulses are discrete events separated by at least this time.

Without the refractory period, impulses could merge into one continuous depolarisation and no information could be carried by the number of impulses.

Section 2 of 9The calculation

The maximum frequency of impulses is the reciprocal of the minimum time between the starts of successive action potentials: frequency = 1 / minimum interval. If the time is in milliseconds, divide 1000 by the time to give the number of impulses per second (hertz).

The calculation

Identify the minimum interval between two action potentials.

This is the duration of one action potential plus the refractory period if they are given separately,
- or it can be read from a graph as the time from the start of one peak to the start of the next.

Convert the time to seconds.

Take the reciprocal.

For example, if one action potential plus recovery takes 5 ms, the maximum frequency is 1000 / 5 = 200 impulses per second.

Section 3 of 9Reading data from graphs

You may be given a trace of potential difference against time.

Measure the time between two corresponding points, such as two peaks.

If the x-axis is in milliseconds and the peaks are 4 ms apart, the frequency is 250 s^-1.

If several impulses occur in a given period, divide the number of impulses by the time interval.

When a trace shows the refractory period explicitly, remember that the minimum interval includes the action potential itself as well as the period after it.

A common type of question gives a trace with a short gap and asks whether the second impulse could be real; compare the gap with the minimum interval.

Section 4 of 9Why frequency matters

The nervous system uses the frequency of impulses to code for stimulus intensity: a stronger stimulus produces more impulses per second.

Why frequency matters

A neurone with a short refractory period can signal across a greater range of intensities.

Frequency cannot exceed the maximum,

so a stimulus beyond that intensity is signalled by recruiting additional neurones,
which also have different thresholds.

The refractory period is related to the time needed for Na⁺ channels to recover from inactivation and for the resting ion distribution to be restored.

Section 5 of 9Linking to other topics

The same type of reasoning applies to rates generally: always state units, convert to a consistent time unit before calculating, and give an answer to appropriate significant figures.

Linking to other topics

In questions about drugs or temperature, ask how the change would alter the refractory period and therefore the maximum frequency: a longer refractory period lowers the maximum frequency.

Lowering the temperature slows channel recovery and so increases the refractory period.

Compare this with the heart, where a long refractory period of cardiac muscle prevents tetanus and allows the chambers to relax and refill.

Section 6 of 9Worked method for graph questions

When a question gives a trace, a good method is: (1) read two equivalent points, such as the start of two successive rising phases; (2) subtract to find the interval in ms; (3) convert to seconds; (4) take the reciprocal.

For a trace showing several impulses, count the number of complete cycles in a measured time and divide.

If the question asks you to decide whether an impulse is possible, compare the gap with the refractory period, not with the total time of the graph.

Be careful that the refractory period begins once the action potential starts,

so it overlaps with part of the spike, and only the sum of the two phases is needed if the question gives them separately.

Finally, if asked about the effect of a drug or toxin that prolongs the opening of channels, explain that the interval increases and so the maximum frequency falls.

Remember to give the final answer with a unit and, if asked, to the stated number of significant figures.

Worked checkan interval of 8 ms gives 125 impulses per second, and halving the interval to 4 ms doubles the maximum to 250.

Section 7 of 9Exam-style questions

The refractory period of a neurone is 2.5 ms and an action potential lasts 1.5 ms. Calculate the maximum frequency of impulses. [3]3 marks
A trace shows 12 action potentials in 60 ms. Calculate the mean frequency per second and state whether this is possible if the minimum interval is 6 ms. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Maximum frequency	Greatest number of action potentials per second an axon can conduct
Refractory period	Time during which a new action potential cannot be generated
Hertz (Hz)	Unit of frequency, events per second
Frequency coding	Representing stimulus strength by the rate of action potentials
Reciprocal	One divided by a number

Section 9 of 9Exam technique

Forgetting to convert ms to s, producing an answer 1000 times too small.

Using only the refractory period and leaving out the duration of the action potential when it is given as part of the interval.

Giving frequency as a time (ms) instead of a rate per second.

Saying a larger stimulus increases the size of impulses rather than their frequency.

Dividing time by 1 instead of 1 by time.

Last updated: 13 October 2026