Answer:
Acetycholine (Ach)
Step-by-step explanation:
Acetylcholine is an excitatory neurotransmitter that can act on both neuronal synapses and motor plates that send signals to the muscles.
A neuromuscular junction is the junction between the terminal part of a motor axon (neuron) with a motor plate (or neuromuscular synapse), which is the region of the membrane plasmtic of a muscle fiber, where the meeting occurs. The nerve and muscle allowing to trigger muscle contraction. For this neuromuscular junction happens, it's necessary the acetycholine.
In motor plates (neuron - muscle), receptors in muscle fibers recognize acetylcholine as a sign of contraction.