# Explain the sequence of ionic events that occur at a synapse when an action potential arrives at a presynaptic terminal, leading to the generation of a new action potential in the postsynaptic neuron.

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

## Mark scheme (4 marks)

1. Arrival of the action potential causes voltage-gated calcium ion channels in the presynaptic membrane to open, allowing calcium ions to enter the presynaptic terminal.
2. Influx of calcium ions causes synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter molecules into the synaptic cleft by exocytosis.
3. Neurotransmitter molecules diffuse across the synaptic cleft and bind to specific receptor proteins on the postsynaptic membrane.
4. Binding of neurotransmitter causes ligand-gated ion channels (e.g. sodium ion channels) to open in the postsynaptic membrane, causing depolarisation that, if sufficient, generates a new action potential.

## Key terms

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

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-the-sequence-of-ionic-events-82b0d908) · Published by Druglandscape Ltd.