Final answer:
Acetylcholine (ACh) is the neurotransmitter that triggers muscle contraction when it binds to receptors on the muscle fiber.
Step-by-step explanation:
The neurotransmitter that triggers muscle contraction when it binds to receptors on the muscle is acetylcholine (ACh). When a nerve impulse reaches the motor neuron's axon terminal, it causes the release of ACh from synaptic vesicles. The ACh then travels across the synaptic cleft and binds to ACh receptors on the muscle fiber, initiating a muscle contraction.