Explain why the refractory period is essential for the transmission of nerve impulses along a neurone.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When a nerve impulse travels along a myelinated motor neurone, each region of the axon membrane undergoes a sequence of permeability changes. For a brief period after depolarisation, the membrane cannot be depolarised again, regardless of the strength of the stimulus applied.
Model answer (4 marks)
1. The refractory period guarantees that the action potential propagates only in the forward direction, preventing a backward wave.
2. During this period the voltage‑gated Na⁺ channels are inactivated/closed, so the membrane cannot be depolarised again regardless of stimulus strength.
3. Consequently the refractory period limits how quickly successive action potentials can occur, setting a maximum firing frequency.
4. Because the frequency of impulses, not their amplitude, can vary, stimulus intensity is encoded by changes in firing rate rather than by larger depolarisations.
2. During this period the voltage‑gated Na⁺ channels are inactivated/closed, so the membrane cannot be depolarised again regardless of stimulus strength.
3. Consequently the refractory period limits how quickly successive action potentials can occur, setting a maximum firing frequency.
4. Because the frequency of impulses, not their amplitude, can vary, stimulus intensity is encoded by changes in firing rate rather than by larger depolarisations.
Examiner tips
- Use the term ‘refractory period’ and link it to Na⁺ channel inactivation; mention one‑way propagation explicitly.
- Show that the period limits maximum frequency and explain that frequency encodes intensity, not amplitude.
- Keep each point concise and use exam‑style terminology.
- Avoid unnecessary explanation of action potential shape.
Common mistakes
- Confusing the refractory period with the resting potential; not specifying Na⁺ channel inactivation.
- Claiming the membrane can still be depolarised during the refractory period.
- Saying the impulse amplitude changes with stimulus intensity instead of frequency.
Mark scheme (4 marks)
- The refractory period ensures impulses travel in one direction only
- The membrane cannot be re-depolarised during the refractory period because sodium ion channels are inactivated / closed
- The refractory period limits the maximum frequency of impulses
- This means impulse frequency (rather than amplitude) codes for stimulus intensity
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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