What forms the cross‑bridge by binding to actin during contraction?

• A. Myosin
• B. Actin
• C. Tropomyosin
• D. Troponin

Answer: (A)

Cross-bridge formation is driven by myosin heads attaching to actin filaments. When muscles contract, the myosin head binds to specific sites on actin, creating a cross-bridge. This connection allows the myosin head to pivot (power stroke), pulling the actin filament toward the center of the sarcomere and shortening the muscle. Actin provides the binding sites for this interaction, but it does not itself form the cross-bridge; that role belongs to the myosin motor head. Tropomyosin and troponin regulate whether the binding sites on actin are exposed: calcium binds troponin, causing tropomyosin to move away from the binding sites, enabling myosin to attach. Thus, the cross-bridge is a myosin–actin interaction powered by ATP, with regulatory proteins controlling access to the binding sites.