A student states that the refractory period of a neurone is simply a time delay before the next action potential can occur. Explain why this statement is incomplete, and describe the importance of the refractory period in ensuring the correct transmission of a nerve impulse.

OCR A-Level Biology A (H420) — 5.3 Neuronal communication · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

During an action potential, voltage-gated sodium ion channels open, causing rapid depolarisation of the axon membrane. After this, the membrane undergoes repolarisation and a brief period during which it cannot be depolarised again, regardless of the size of the stimulus applied.

Model answer (4 marks)

1. The refractory period is not merely a delay; during the absolute refractory period voltage‑gated Na⁺ channels are inactivated, so the membrane cannot be depolarised again regardless of stimulus strength.
2. This inactivation prevents successive action potentials from overlapping, ensuring that each impulse remains a discrete, separate event.
3. Because the region behind the wave‑front cannot be re‑excited, the refractory period enforces unidirectional propagation of the action potential along the axon.
4. By limiting the maximum firing frequency, the refractory period allows the nervous system to encode stimulus intensity as a change in impulse frequency.

Examiner tips

  • Use the term ‘absolute refractory period’ and mention Na⁺ channel inactivation; this shows understanding of the mechanism.
  • Explain how the refractory period prevents summation/fusion of impulses – a key point in the mark scheme.
  • Mention unidirectional transmission and frequency limitation – both are required for full marks.
  • Keep the answer concise and use correct terminology (e.g., ‘discrete events’, ‘maximum frequency’).

Common mistakes

  • Confusing the refractory period with a simple time delay; forgetting to mention Na⁺ channel inactivation.
  • Failing to state that the refractory period prevents summation or fusion of impulses.
  • Omitting the role of the refractory period in enforcing unidirectional propagation or limiting firing frequency.

Mark scheme (4 marks)

  1. During the refractory period, voltage-gated sodium ion channels are inactivated (absolute refractory period), so the membrane cannot be depolarised again regardless of stimulus strength.
  2. The refractory period ensures action potentials are discrete, separate events by preventing summation or fusion of successive impulses.
  3. The refractory period enforces unidirectional transmission of the action potential along the axon, because the region behind the action potential cannot be re-excited.
  4. The refractory period limits the maximum frequency of action potentials, allowing the nervous system to encode stimulus intensity as a change in impulse frequency.

Key terms in this question

refractory period · action potential

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