Final answer:
Acetylcholine is the neurotransmitter facilitating communication between a motor neuron and a skeletal muscle, triggering muscle contraction. It is broken down after its action to allow the muscle to relax.
Step-by-step explanation:
The neurotransmitter that crosses the synapse between a motor neuron and a skeletal muscle is acetylcholine. When acetylcholine is released from the neuron, it diffuses across the synaptic cleft and binds to receptors on the surface of the muscle fiber, at the neuromuscular junction (NMJ). This binding leads to the opening of ligand-gated ion channels, allowing an influx of sodium ions, which results in the depolarization of the muscle fiber's membrane. Consequently, this depolarization initiates a sequence of events that leads to muscle fiber contraction.
After its action, acetylcholine is rapidly degraded by the enzyme acetylcholinesterase, preventing continuous muscle contraction and allowing the muscle fiber to relax. Thus, acetylcholine is essential in converting the electrical signal from the motor neuron into a mechanical response, which is the contraction of the skeletal muscle fiber.