# 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.

> IB DP Biology Higher Level (2023 syllabus) — B3.3 Muscle and motility (HL only) · Explain · 4 marks

## Mark scheme (4 marks)

1. A motor neuron axon branches terminally, with each branch forming a neuromuscular junction (NMJ) on a separate muscle fibre.
2. At each NMJ, the action potential triggers vesicle fusion and release of acetylcholine (ACh) into the synaptic cleft.
3. ACh binds to receptors on the motor end plate, generating a depolarisation (end-plate potential) that propagates as an action potential along the sarcolemma.
4. 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.

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