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

Synaptic vesicles containing acetylcholine (ACh) are confined to the presynaptic terminal of the motor neuron, so ACh is released only into the synaptic cleft.

The postsynaptic membrane of the muscle fibre (the motor end‑plate) carries nicotinic acetylcholine receptors, which are ligand‑gated ion channels. These receptors are absent from the presynaptic terminal, so the neuron cannot respond to ACh released from the muscle.

ACh that enters the cleft is immediately hydrolysed by acetylcholinesterase into acetate and choline, terminating the signal and preventing ACh from diffusing back to the neuron.

Because the muscle fibre lacks the machinery to synthesise or release ACh and cannot depolarise the motor neuron terminal, the signal can only travel from neuron to muscle.