Explain how the structure of a motor neuron and the neuromuscular junction allows a single nerve impulse to result in contraction of multiple muscle fibres.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A motor neuron axon branches terminally, each branch forming a neuromuscular junction (NMJ) on a separate muscle fibre.
At each NMJ, the action potential causes vesicle fusion and release of acetylcholine (ACh) into the synaptic cleft.
ACh binds to receptors on the motor end plate, generating a depolarisation (end‑plate potential) that propagates as an action potential along the sarcolemma.
Because all muscle fibres innervated by one motor neuron are stimulated simultaneously, a single nerve impulse causes the contraction of all fibres in that motor unit.
At each NMJ, the action potential causes vesicle fusion and release of acetylcholine (ACh) into the synaptic cleft.
ACh binds to receptors on the motor end plate, generating a depolarisation (end‑plate potential) that propagates as an action potential along the sarcolemma.
Because all muscle fibres innervated by one motor neuron are stimulated simultaneously, a single nerve impulse causes the contraction of all fibres in that motor unit.
Examiner tips
- Use the word ‘branch’ to show axonal branching; mention NMJ and motor end plate. Explain the sequence: action potential → vesicle fusion → ACh release → receptor binding → end‑plate potential → sarcolemma AP. State that all fibres in a motor unit contract together, linking structure to function.
- common_mistakes
- :
- Forgetting to mention axonal branching or NMJs. Confusing ACh release with depolarisation of the motor neuron instead of the muscle fibre. Saying only one fibre contracts instead of the whole motor unit.
Mark scheme (4 marks)
- A motor neuron axon branches terminally, with each branch forming a neuromuscular junction (NMJ) on a separate muscle fibre.
- At each NMJ, the action potential triggers vesicle fusion and release of acetylcholine (ACh) into the synaptic cleft.
- ACh binds to receptors on the motor end plate, generating a depolarisation (end-plate potential) that propagates as an action potential along the sarcolemma.
- Because all muscle fibres innervated by one motor neuron are stimulated simultaneously, a single nerve impulse causes the contraction of all fibres in that motor unit.
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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