Final answer:
The neurotransmitter released at the neuromuscular junction is acetylcholine (ACh).
Step-by-step explanation:
The neurotransmitter released at the neuromuscular junction after the action potential arrives is acetylcholine (ACh).
Neurotransmitter release occurs when an action potential travels down the motor neuron's axon, resulting in altered permeability of the synaptic terminal membrane and an influx of calcium. The Ca²+ ions allow synaptic vesicles to move to and bind with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft. ACh then diffuses across the synaptic cleft and binds with ACh receptors on the motor end plate, causing depolarization and initiating an action potential in the muscle cell.