# Explain how the refractory period of a neuron prevents action potentials from travelling in reverse along an axon.

> IB DP Biology Higher Level (2023 syllabus) — C2.2 Neural signalling · Explain · 4 marks

## Mark scheme (4 marks)

1. During an action potential, voltage-gated sodium channels open and sodium ions rush into the axon, causing depolarisation.
2. 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.
3. 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.
4. 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

- [refractory period](https://www.gradenine.co.uk/glossary/refractory-period)
- [action potential](https://www.gradenine.co.uk/glossary/action-potential)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-refractory-period-of-7ceba55c) · Published by Druglandscape Ltd.