When an action potential reaches the axon terminal of a motor neuron:

• Voltage-gated channels in the presynaptic terminal open causing enters the neuron.

• This triggers exocytosis of vesicles containing acetylcholine (ACh) into the synaptic cleft at the neuromuscular junction.

• ACh binds to receptors on the muscle fibre membrane causing depolarisation of muscle fibre leading to contraction.

• Later, acetylcholinesterase breaks down ACh to terminate the signal.

• So the immediate event at the axon terminal is: ACh release into the neuromuscular junction.