What happens at the neuromuscular junction during muscle contraction?

• A. Muscle fiber releases acetylcholine to the nerve causing repolarization
• B. Calcium is released from the motor neuron into the synapse to trigger contraction
• C. The nerve impulse bypasses chemical signaling and directly triggers actin-myosin binding
• D. Motor neuron releases acetylcholine, which binds receptors on the muscle fiber, causing depolarization and calcium release that triggers cross-bridge cycling

Answer: (D)

At the neuromuscular junction, signaling starts when a motor neuron releases acetylcholine into the synaptic cleft. This acetylcholine binds to nicotinic receptors on the muscle fiber’s membrane, producing an depolarizing signal (end-plate potential). That depolarization propagates into the muscle fiber, triggering a muscle action potential. The action potential then travels along the sarcolemma and into the T-tubules, causing the calcium release channels in the sarcoplasmic reticulum to open. Calcium floods the cytosol and binds to troponin, which moves tropomyosin away from actin’s binding sites, allowing cross-bridge cycling with myosin heads and driving contraction. Acetylcholine is quickly degraded by acetylcholinesterase to terminate the signal, ensuring the contraction is precisely controlled.