Which statement describes a critical event that occurs at the neuromuscular junction?

The statement that acetylcholine is released by axon terminals of the motor neuron accurately describes a critical event at the neuromuscular junction, which is the site where the motor neuron communicates with the muscle fiber to initiate contraction.

When a nerve impulse arrives at the axon terminal, it triggers the opening of voltage-gated calcium channels, allowing calcium ions to enter the terminal. This influx of calcium causes synaptic vesicles containing acetylcholine (ACh) to fuse with the presynaptic membrane, leading to the release of ACh into the synaptic cleft. Once in the cleft, ACh binds to receptors on the muscle fiber's membrane, leading to depolarization and, ultimately, muscle contraction. This process is fundamental to the functioning of skeletal muscles, making it crucial for movement.

The other options do not accurately describe events that occur at the neuromuscular junction. Sodium ions enter the muscle fiber after acetylcholine binds to its receptors, but they are not released by the fiber itself. Neurotransmitter synthesis occurs within the neuron and not in the muscle fiber, and protons are not exchanged at the axon terminals during this process, as the events involve neurotransmitter release and receptor binding rather than proton exchange