Which neurotransmitter is released at the neuromuscular junction to stimulate muscles?

• A. GABA
• B. Serotonin
• C. Dopamine
• D. Acetylcholine

Answer: (D)

Acetylcholine is the signal that directly triggers muscle contraction at the neuromuscular junction. When motor neurons fire, they release acetylcholine into the synaptic cleft, where it binds to nicotinic acetylcholine receptors on the muscle’s motor endplate. This binding opens ligand-gated sodium channels, depolarizing the membrane and creating an end-plate potential that reaches threshold to generate an action potential along the muscle fiber. The action potential then causes calcium release from the sarcoplasmic reticulum, enabling the interaction of actin and myosin for contraction. Acetylcholine is rapidly broken down by acetylcholinesterase to stop the signal and allow the muscle to relax.

The other neurotransmitters listed (GABA, serotonin, dopamine) do not directly stimulate skeletal muscle at the neuromuscular junction; they have other roles in the nervous system and different synapses.