# Explain how the structure of the neuromuscular junction ensures that a signal is transmitted in one direction only, from motor neuron to muscle fibre.

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

## Mark scheme (4 marks)

1. Synaptic vesicles containing acetylcholine (ACh) are located exclusively in the presynaptic (motor neuron) terminal / knob
2. Acetylcholine receptors (nicotinic receptors / ligand-gated ion channels) are present only on the postsynaptic membrane (motor end plate / sarcolemma)
3. Acetylcholinesterase in the synaptic cleft rapidly breaks down ACh (into acetate and choline), preventing prolonged or reverse stimulation
4. The muscle fibre (postsynaptic cell) lacks the machinery to release neurotransmitter / cannot depolarise the motor neuron terminal, so signal flow is inherently unidirectional

## Key terms

- [neuromuscular junction](https://www.gradenine.co.uk/glossary/neuromuscular-junction)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-the-c72f678d) · Published by Druglandscape Ltd.