Link the neuromuscular junction to calcium release
Trace how a motor-neurone impulse, neuromuscular junction, T-tubules and sarcoplasmic reticulum stimulate contraction.
Section 15.1 · Control and coordination in mammals
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
Where is calcium stored before it is released for muscle contraction?
What you will learn
- Describe the role of the neuromuscular junction in stimulating a muscle action potential
- Explain how T-tubules and sarcoplasmic reticulum lead to calcium release
From motor neurone to muscle calcium release
| Feature | Meaning |
|---|---|
| Neuromuscular junction | Motor neurone releases acetylcholine onto the muscle membrane |
| T-tubules | Carry the action potential into the muscle fibre |
| Sarcoplasmic reticulum | Releases calcium ions into the sarcoplasm |
| Next step | Calcium binds troponin, permitting actin–myosin interaction |
Build the mechanism
At a neuromuscular junction, an action potential in a motor neurone causes acetylcholine release. The transmitter binds receptors on the muscle-fibre membrane and initiates a muscle action potential. The action potential spreads along the sarcolemma and down T-tubules, triggering the sarcoplasmic reticulum to release calcium ions into the sarcoplasm. Calcium binds to troponin and enables actin–myosin interaction.
Keep the sequence precise
The T-tubule carries the electrical change into the fibre; the sarcoplasmic reticulum stores and releases calcium. Do not say the T-tubule itself releases calcium or that calcium travels down the motor neurone.
Pause and explain
A diagram labels the sarcoplasmic reticulum as the site where acetylcholine binds. Correct the two locations.
Worked example
Which structure carries the muscle action potential into the fibre interior?
Show the worked solution
- The sarcolemma conducts across the surface.
- T-tubules carry depolarisation inward.
Answer T-tubules carry the muscle action potential from the sarcolemma into the fibre.
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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.
Trace the events from motor-neurone impulse to calcium in the sarcoplasm.
Give me a hint
Name the first event, then follow each link in order.
Compare my reasoning
- Acetylcholine triggers a muscle action potential.
- T-tubules carry depolarisation inward; the sarcoplasmic reticulum releases calcium.
A motor-neurone impulse causes acetylcholine release at the neuromuscular junction. This starts a muscle action potential that travels along the sarcolemma and T-tubules, triggering calcium release from the sarcoplasmic reticulum.
Look for these in your work
- Mechanism stated
- Steps linked in order
A blocker prevents T-tubule depolarisation from reaching the fibre interior. Predict the effect on contraction.
Give me a hint
Use the terms in the question and state what changes.
Compare my reasoning
- The signal does not efficiently reach internal membrane regions.
- Calcium release from the sarcoplasmic reticulum is reduced.
The action potential may not trigger normal calcium release from the sarcoplasmic reticulum, so fewer actin–myosin interactions can occur and contraction is reduced.
Look for these in your work
- Evidence interpreted
- Biological direction explained
A diagram labels the sarcoplasmic reticulum as the site where acetylcholine binds. Correct the two locations.
Give me a hint
Apply the mechanism to the new context without adding unsupported claims.
Compare my reasoning
- Acetylcholine receptors are at the muscle membrane at the neuromuscular junction.
- The sarcoplasmic reticulum releases stored calcium after depolarisation.
Acetylcholine binds receptors on the muscle-fibre membrane at the neuromuscular junction. The sarcoplasmic reticulum is an internal calcium store that releases calcium after the muscle action potential spreads through T-tubules.
Look for these in your work
- Mechanism applied
- Limits preserved
Common mistakes
- Swapping the T-tubule and sarcoplasmic-reticulum roles
- Saying acetylcholine directly shortens actin filaments
What can you explain now?
Where is calcium stored before it is released for muscle contraction?
Compare with the explanation
In the sarcoplasmic reticulum
T-tubules carry the action potential inward; the sarcoplasmic reticulum releases calcium.
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Make it stick
How do the neuromuscular junction, T-tubules and sarcoplasmic reticulum work in sequence?
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