# Explain how an action potential is generated at the axon hillock of a neuron following stimulation.

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

> Neurons integrate incoming signals at the axon hillock, where a decision to 'fire' or not is made. This process depends on changes in membrane permeability to specific ions.

## Mark scheme (4 marks)

1. Depolarising stimuli cause the membrane potential at the axon hillock to rise towards the threshold potential (approximately −55 mV).
2. When threshold is reached, voltage-gated sodium channels open, allowing Na⁺ to rush into the cell down its electrochemical gradient.
3. The rapid influx of Na⁺ causes the inside of the membrane to become positive (depolarisation), reversing the membrane potential to approximately +30 mV.
4. This follows an all-or-nothing principle: if the stimulus reaches threshold, a full-sized action potential is always generated; if it does not reach threshold, no action potential occurs.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-an-action-potential-is-4b40e865) · Published by Druglandscape Ltd.