Which event initiates cross-bridge cycling in skeletal muscle contraction?

• A. Calcium binds to troponin C on actin, initiating cross-bridge formation.
• B. ATP binds to myosin heads to start contraction.
• C. Potassium influx triggers cross-bridge formation.
• D. Cross-bridges form spontaneously without regulatory proteins.

Answer: (A)

Calcium binding to troponin C on actin triggers cross-bridge cycling because it moves the troponin-tropomyosin complex away from the myosin-binding sites on actin, exposing those sites for attachment. When Ca2+ is released from the sarcoplasmic reticulum, it binds to troponin C, causing a shift that unblocks actin. Energized myosin heads can then attach to actin and start the cycle, leading to the power stroke and continued movement as long as Ca2+ remains elevated and ATP is available. The other events don’t initiate this process: ATP binding to myosin is needed to detach and re-energize the head, not to start attachment; potassium influx is part of electrical signaling, not the mechanical activation of the contractile apparatus; and cross-bridges won’t form spontaneously without the regulatory proteins enabling the binding sites to be exposed.