Final answer:
The chemically-gated ion channel at the neuromuscular junction is the ligand-gated channel, which opens when a specific neurotransmitter binds to it.
Step-by-step explanation:
The structure of the neuromuscular junction that is a chemically-gated ion channel is the ligand-gated channel, which is another name for an ionotropic receptor for which a neurotransmitter is the ligand. When a specific neurotransmitter binds to these channels, they open and allow ions such as sodium, calcium, and potassium to pass through.