What directly triggers contraction after an action potential in skeletal muscle?

• A. Rise in intracellular Ca2+ from the sarcoplasmic reticulum
• B. Increase in intracellular Na+ concentration
• C. Depletion of ATP
• D. Phosphorylation of troponin

Answer: (A)

When a skeletal muscle fiber fires an action potential, the voltage signal causes calcium to be released from the sarcoplasmic reticulum into the cytosol. That rise in intracellular Ca2+ directly triggers contraction because Ca2+ binds to troponin C on the thin filament, causing tropomyosin to move away from the myosin-binding sites on actin. With those sites uncovered, myosin heads can attach and pull, producing cross-bridge cycling and contraction. The other options don’t directly start this process: a rise in intracellular Na+ is part of the action potential itself, ATP depletion would stop contraction by removing energy for cycling, and phosphorylation of troponin modulates sensitivity but doesn’t initiate the immediate trigger for contraction.