What is a critical event that occurs at the neuromuscular junction?

Neurotransmitter release

At the neuromuscular junction, the critical event is the release of neurotransmitters (acetylcholine) from the presynaptic neuron into the synaptic cleft. This release of neurotransmitters is triggered by an action potential reaching the presynaptic terminal. The action potential causes voltage-gated calcium channels in the presynaptic membrane to open, allowing calcium ions to influx into the neuron. The increase in intracellular calcium concentration triggers the release of acetylcholine-containing vesicles by a process called exocytosis.