What substance acts on the neuromuscular junction to excite muscle fibers?

Acetylcholine plays a critical role at the neuromuscular junction, where it is released from the motor neuron. It binds to receptors on the muscle fiber's membrane, specifically the sarcolemma, leading to depolarization of the muscle membrane and ultimately triggering muscle contraction. This process is essential for voluntary muscle movements.

Dopamine, norepinephrine, and serotonin, while important neurotransmitters in the nervous system, do not primarily function at the neuromuscular junction. Dopamine is primarily involved in reward and motor control, norepinephrine functions mainly in the autonomic nervous system and attention, while serotonin is associated with mood regulation, sleep, and other psychological functions. Therefore, acetylcholine is uniquely designated as the substance that directly influences excitation at the neuromuscular junction.