Which chemical transmitter stimulates skeletal muscle by binding to receptors on the muscle cell?

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

Answer: (A)

Skeletal muscle contraction at the neuromuscular junction is triggered when a chemical messenger from the motor neuron binds to receptors on the muscle cell. The transmitter in this case is acetylcholine. It binds to nicotinic acetylcholine receptors on the motor end plate, which are ligand-gated ion channels. When acetylcholine binds, these channels open and allow sodium ions to rush into the muscle cell (with some potassium leaving), depolarizing the membrane and generating an action potential that travels into the muscle. This electrical signal then prompts the release of calcium from the sarcoplasmic reticulum, leading to contraction. Acetylcholine is quickly broken down by acetylcholinesterase, which ends the signal.

Norepinephrine, dopamine, and serotonin don’t directly bind to receptors on skeletal muscle to trigger contraction. They act on different receptors in the nervous system or other tissues, so they don’t produce the direct muscle depolarization needed at the neuromuscular junction.