# Explain how spatial and temporal summation at a postsynaptic neurone determine whether an action potential is generated.

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

## Mark scheme (4 marks)

1. Spatial summation occurs when multiple presynaptic neurones release neurotransmitter onto the same postsynaptic neurone simultaneously, with their individual EPSPs combining.
2. Temporal summation occurs when a single presynaptic neurone fires repeatedly in rapid succession, so successive EPSPs arrive before the previous one has decayed and accumulate.
3. Both forms of summation produce a graded change in membrane potential at the axon hillock; if the combined depolarisation reaches the threshold potential, voltage-gated sodium channels open.
4. If the combined postsynaptic potential does not reach threshold, no action potential is generated, illustrating that summation acts as an integrating mechanism allowing the neurone to process the relative strength and frequency of incoming signals.

## Key terms

- [temporal summation](https://www.gradenine.co.uk/glossary/temporal-summation)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-spatial-and-temporal-summation-ad58c787) · Published by Druglandscape Ltd.