Which neurotransmitter causes muscles to contract?

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

Answer: (D)

The signal that starts muscle contraction at the neuromuscular junction is acetylcholine. When a motor neuron fires, it releases acetylcholine into the synaptic cleft, where it binds to nicotinic receptors on the muscle fiber. This binding opens ion channels, letting sodium enter and depolarize the muscle membrane. The depolarization travels along the membrane and into the muscle via T-tubules, triggering calcium release from the sarcoplasmic reticulum. Calcium enables actin-myosin interactions, allowing cross-bridge cycling and contraction. The signal ends as acetylcholine is broken down by acetylcholinesterase, returning the muscle to its relaxed state. Other neurotransmitters like dopamine, GABA, and serotonin play roles in the brain and CNS functions, not directly in causing muscle contraction.