Explain how the refractory period of a neuron prevents action potentials from travelling in reverse along an axon.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
During an action potential the voltage‑gated Na⁺ channels open, allowing Na⁺ to rush into the axon and depolarise the membrane.
Immediately after depolarisation the same channels become inactivated, creating an absolute refractory period. In this period the channels cannot reopen, no matter how strong the stimulus.
Because the membrane behind the wave is in the refractory period, it cannot depolarise again, so the action potential cannot travel backwards. Only the membrane ahead of the wave, which is still resting, can depolarise and open its Na⁺ channels, allowing the signal to propagate only forward.
The refractory period also keeps individual action potentials separate, preventing fusion and preserving the frequency‑coded information carried by the nerve.
Immediately after depolarisation the same channels become inactivated, creating an absolute refractory period. In this period the channels cannot reopen, no matter how strong the stimulus.
Because the membrane behind the wave is in the refractory period, it cannot depolarise again, so the action potential cannot travel backwards. Only the membrane ahead of the wave, which is still resting, can depolarise and open its Na⁺ channels, allowing the signal to propagate only forward.
The refractory period also keeps individual action potentials separate, preventing fusion and preserving the frequency‑coded information carried by the nerve.
Examiner tips
- Use the term "absolute refractory period" and explain channel inactivation
- Show that the refractory region cannot depolarise, so reverse propagation is impossible
- Mention that this keeps spikes discrete and preserves frequency coding
Common mistakes
- Confusing relative with absolute refractory period
- Saying the wave can travel both ways instead of only forward
- Omitting the role of Na⁺ channel inactivation
Mark scheme (4 marks)
- During an action potential, voltage-gated sodium channels open and sodium ions rush into the axon, causing depolarisation.
- Immediately after depolarisation, the region enters the absolute refractory period during which voltage-gated sodium channels are inactivated and cannot reopen regardless of the stimulus applied.
- The depolarisation therefore only stimulates adjacent, ahead-lying membrane (which has not yet been depolarised) to open its voltage-gated sodium channels, so the action potential can only propagate forwards.
- The refractory period also ensures that individual action potentials remain discrete/separated, preventing them from fusing and maintaining the frequency-coded information carried by the nerve.
Key terms in this question
refractory period · action potential
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Neural signalling questions
- Explain how changes in membrane permeability to sodium and potassium ions bring …
- Explain how the structure of a myelinated motor neurone allows for rapid transmi…
- Explain how the release of a neurotransmitter at a cholinergic synapse leads to …
- Explain how spatial and temporal summation at a postsynaptic neurone determine w…