Final answer:
Acetylcholine (ACh) is the substance that acts at the neuromuscular junction to excite the muscle fibers of a motor unit. It binds to receptors in the motor end plate and triggers depolarization, leading to muscle contraction.
Step-by-step explanation:
The substance that acts at the neuromuscular junction to excite the muscle fibers of a motor unit is Acetylcholine (ACh). ACh is a neurotransmitter released by motor neurons that binds to receptors in the motor end plate. When ACh binds, ion channels open, allowing Na ions to cross the membrane into the muscle cell, triggering depolarization and leading to muscle contraction.