Final answer:
1) Acetylcholine (ACh) is the neurotransmitter that acts at the neuromuscular junction to excite muscle fibers, triggering an action potential that leads to muscle contraction.
Step-by-step explanation:
The substance that acts at the neuromuscular junction (NMJ) to excite the muscle fibers of a motor unit is 1) acetylcholine (ACh).
ACh is a neurotransmitter that is released into the synaptic cleft when a nerve impulse, or action potential, reaches the axon terminal of a motor neuron.
The ACh molecules then bind to ACh receptors on the motor end plate of the muscle fiber, causing sodium ions (Na+) to flood into the muscle cell.
This influx of ions initiates depolarization, changing the voltage of the cell membrane and leading to the generation of an action potential.
The action potential triggers a cascade of events that lead to muscle contraction.
While ATP is crucial to power contraction and release of the cross-bridges in muscle fibers, it does not excite muscle fibers at the NMJ.