Trace transmission at a cholinergic synapse
Explain the role of calcium ions in acetylcholine release, receptor activation and synapse reset.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- A control pathway links a stimulus, receptor, coordination system and response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which ion enters the presynaptic terminal to trigger acetylcholine release?
What you will learn
- Describe cholinergic synapse structure and acetylcholine transmission
- Explain how calcium ions trigger vesicle release and how acetylcholine is removed
Cholinergic synaptic transmission
| Feature | Meaning |
|---|---|
| Arrival | Impulse opens voltage-gated calcium channels in the presynaptic terminal |
| Release | Calcium entry triggers vesicle fusion and acetylcholine release |
| Reception | Acetylcholine binds postsynaptic receptors and changes ion permeability |
| Reset | Acetylcholinesterase breaks down acetylcholine; choline is recycled |
Build the mechanism
An action potential reaching a presynaptic terminal opens voltage-gated calcium channels. Calcium ions enter and trigger synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine by exocytosis. Acetylcholine diffuses across the cleft and binds to receptors on the postsynaptic membrane, changing ion-channel activity and depolarising the next cell. Acetylcholinesterase hydrolyses acetylcholine; choline is taken back up and reused.
Keep the sequence precise
Calcium ions trigger transmitter release from the presynaptic terminal; they are not the neurotransmitter crossing the cleft. Breakdown of acetylcholine prevents continuous stimulation and allows the synapse to reset.
Pause and explain
A student says vesicles release acetylcholine because sodium enters the terminal. Correct the causal link.
Worked example
What is the immediate role of calcium entry at a cholinergic terminal?
Show the worked solution
- An impulse opens voltage-gated calcium channels.
- Calcium entry triggers vesicle fusion and transmitter release.
Answer Calcium entry into the presynaptic terminal triggers synaptic vesicles to fuse with the membrane and release acetylcholine.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the sequence from an arriving action potential to postsynaptic depolarisation.
Give me a hint
Name the first event, then follow each link in order.
Compare my reasoning
- Calcium channels open and calcium enters.
- Vesicles release acetylcholine, which binds postsynaptic receptors.
An action potential opens presynaptic voltage-gated calcium channels. Calcium enters and triggers vesicle fusion; acetylcholine diffuses across the cleft and binds postsynaptic receptors, changing ion permeability and depolarising the membrane.
Look for these in your work
- Mechanism stated
- Steps linked in order
A chemical blocks acetylcholinesterase. Predict one effect at a cholinergic synapse.
Give me a hint
Use the terms in the question and state what changes.
Compare my reasoning
- Acetylcholine is not broken down at the normal rate.
- It can remain in the cleft and stimulate receptors for longer.
Acetylcholine may remain in the cleft longer and continue activating postsynaptic receptors, prolonging stimulation.
Look for these in your work
- Evidence interpreted
- Biological direction explained
A student says vesicles release acetylcholine because sodium enters the terminal. Correct the causal link.
Give me a hint
Apply the mechanism to the new context without adding unsupported claims.
Compare my reasoning
- The arriving impulse opens voltage-gated calcium channels.
- Calcium entry triggers exocytosis.
At the presynaptic terminal, depolarisation opens voltage-gated calcium channels. Calcium ions enter and trigger synaptic-vesicle fusion and acetylcholine release.
Look for these in your work
- Mechanism applied
- Limits preserved
Common mistakes
- Calling calcium the synaptic transmitter
- Omitting acetylcholinesterase from synapse recovery
What can you explain now?
Which ion enters the presynaptic terminal to trigger acetylcholine release?
Compare with the explanation
Calcium ions
Calcium entry triggers vesicle fusion.
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Make it stick
How do calcium ions connect an impulse to transmitter release?
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