Final answer:
Acetylcholine (ACh) released by motor neurons binds to receptors in the motor end plate, leading to muscle contractions.
Step-by-step explanation:
Acetylcholine (ACh) is a neurotransmitter released by motor neurons that binds to receptors in the motor end plate. When an action potential travels down the motor neuron's axon, it causes the release of ACh, which then diffuses across the synaptic cleft to bind with ACh receptors on the motor end plate. This binding results in the opening of ion channels and the depolarization of the muscle fiber, leading to muscle contraction.