Explain how the refractory period of a neuron limits the direction and frequency of action potential propagation.

IB DP Biology Standard Level (2023 syllabus) — C2.2 Neural signalling · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

During the refractory period the voltage‑gated Na⁺ channels are inactivated (or closed and unable to open), so a new action potential cannot be generated in that region.
This inactivation prevents the action potential from travelling backwards, ensuring unidirectional propagation along the axon.
Because no new action potential can begin until the refractory period is over, there is a minimum time interval between successive action potentials, limiting the maximum frequency of firing.
Stronger stimuli can increase firing frequency only up to this maximum set by the refractory period, which is how stimulus intensity is coded (rate coding) rather than by changing the size of individual action potentials (all‑or‑nothing principle).

Examiner tips

  • Use the word ‘inactivation’ for Na⁺ channels; this shows understanding of the refractory period. Show the two effects: prevention of back‑propagation and a minimum inter‑spike interval. Link the minimum interval to maximum firing frequency. Mention rate coding to explain intensity coding.
  • common_mistakes
  • :
  • Confusing the refractory period with the resting potential. Saying the action potential can travel in both directions. Forgetting to mention that the refractory period limits frequency, not amplitude.

Mark scheme (4 marks)

  1. During the refractory period, voltage-gated sodium channels are inactivated (or closed and unable to open), so a new action potential cannot be generated in that region.
  2. This prevents the action potential from travelling backwards (in the direction it came from), ensuring unidirectional propagation along the axon.
  3. Because no new action potential can begin until the refractory period is over, there is a minimum time interval between successive action potentials, limiting the maximum frequency of firing.
  4. Stronger stimuli can increase firing frequency only up to this maximum set by the refractory period, which is how stimulus intensity is coded (rate coding) rather than by changing the size of individual action potentials (all-or-nothing principle).

Key terms in this question

refractory period · action potential

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