Final answer:
The main transmitter at mammalian neuromuscular junctions is acetylcholine, which is crucial for muscle activation and part of the cholinergic system.
Step-by-step explanation:
The main transmitter used at the mammalian neuromuscular junctions (NMJ) is acetylcholine. At the NMJ, motor neurons release acetylcholine to activate muscles, which then respond by contracting. Acetylcholine is part of the cholinergic system and operates differently from other neurotransmitters, such as GABA (gamma-aminobutyric acid), which is the primary inhibitory neurotransmitter in the central nervous system.
The other options listed, dopamine, serotonin, and GABA, serve different functions in various areas of the nervous system, but they are not the principal neurotransmitters used to stimulate muscle action at the NMJ.