Explain how the release of a neurotransmitter at a cholinergic synapse leads to the generation of a new action potential in the postsynaptic neurone.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
An action potential arriving at the presynaptic terminal depolarises the membrane, opening voltage‑gated Ca²⁺ channels.
Ca²⁺ influx triggers synaptic vesicles containing acetylcholine to fuse with the presynaptic membrane and release ACh into the synaptic cleft by exocytosis.
ACh diffuses across the cleft and binds to nicotinic acetylcholine receptors on the postsynaptic membrane.
Binding opens ligand‑gated Na⁺ channels, allowing Na⁺ influx, depolarising the postsynaptic membrane; if the depolarisation reaches threshold, a new action potential is generated.
Ca²⁺ influx triggers synaptic vesicles containing acetylcholine to fuse with the presynaptic membrane and release ACh into the synaptic cleft by exocytosis.
ACh diffuses across the cleft and binds to nicotinic acetylcholine receptors on the postsynaptic membrane.
Binding opens ligand‑gated Na⁺ channels, allowing Na⁺ influx, depolarising the postsynaptic membrane; if the depolarisation reaches threshold, a new action potential is generated.
Examiner tips
- Use the exact sequence of events: depolarisation → Ca²⁺ influx → vesicle fusion → ACh release → receptor binding → Na⁺ influx → threshold reached.
- Mention the specific receptor type (nicotinic) and ion (Na⁺) to show depth of knowledge.
- Keep the answer concise – 4 points, one sentence per point.
Common mistakes
- Confusing the presynaptic and postsynaptic sides; e.g., saying ACh is released from the postsynaptic neuron.
- Omitting the role of voltage‑gated Ca²⁺ channels or ligand‑gated Na⁺ channels.
- Using the wrong receptor type (muscarinic instead of nicotinic) or wrong ion (K⁺ instead of Na⁺).
Mark scheme (4 marks)
- An action potential arriving at the presynaptic terminal causes depolarisation, triggering voltage-gated calcium ion channels to open.
- Calcium ions cause synaptic vesicles containing acetylcholine to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft by exocytosis.
- Acetylcholine diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane.
- Receptor binding causes ligand-gated sodium ion channels to open, allowing Na⁺ influx that depolarises the postsynaptic membrane; if threshold is reached, a new action potential is generated.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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