In skeletal muscle, what event directly initiates an action potential in the muscle fiber (sarcolemma)?

• A. Increased binding of troponin to actin, which opens voltage-gated Na${+}^{}$ channels in the sarcolemma
• B. ATP binding to myosin heads, causing a conformational change that triggers a sarcolemmal action potential
• C. Binding of acetylcholine to nicotinic receptors at the motor end plate, opening ligand-gated cation channels and depolarizing the sarcolemma to threshold
• D. Release of Ca$2^{+}$ from the sarcoplasmic reticulum through ryanodine receptors, which directly generates the sarcolemmal action potential

Answer: (C)