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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Arrival of the action potential opens voltage‑gated Ca²⁺ channels in the presynaptic membrane, allowing Ca²⁺ to enter the terminal.
Ca²⁺ influx triggers synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft by exocytosis.
Neurotransmitter diffuses across the cleft and binds to specific receptors on the postsynaptic membrane.
Binding opens ligand‑gated ion channels (e.g. Na⁺ channels), depolarising the postsynaptic membrane; if the depolarisation reaches threshold, a new action potential is generated.
Ca²⁺ influx triggers synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft by exocytosis.
Neurotransmitter diffuses across the cleft and binds to specific receptors on the postsynaptic membrane.
Binding opens ligand‑gated ion channels (e.g. Na⁺ channels), depolarising the postsynaptic membrane; if the depolarisation reaches threshold, a new action potential is generated.
Examiner tips
- Use the exact sequence: Ca²⁺ entry → vesicle fusion → neurotransmitter release → receptor binding → ion channel opening → depolarisation → new AP.
- Mention voltage‑gated Ca²⁺ channels and ligand‑gated Na⁺ channels explicitly to hit all marks.
Common mistakes
- Confusing the order of events (e.g. stating neurotransmitter release before Ca²⁺ influx).
- Using generic terms like ‘ion channels’ without specifying Ca²⁺ or Na⁺.
- Omitting the requirement that depolarisation must reach threshold to generate a new AP.
Mark scheme (4 marks)
- 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.
- Influx of calcium ions causes synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter molecules into the synaptic cleft by exocytosis.
- Neurotransmitter molecules diffuse across the synaptic cleft and bind to specific receptor proteins on the postsynaptic membrane.
- 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 in this question
presynaptic terminal · action potential
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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