What event initiates cross-bridge formation during muscle contraction?

• A. Acetylcholine release directly triggers cross-bridge formation.
• B. Calcium binds the troponin complex, exposing myosin-binding sites on actin.
• C. Tropomyosin prevents cross-bridge formation by blocking the myosin-binding sites.
• D. Myosin heads attach to actin only after ATP hydrolysis.

Answer: (B)

Cross-bridge formation begins when calcium ions bind to the troponin complex on the thin filament. This Ca2+-troponin interaction causes tropomyosin to move away from the myosin-binding sites on actin, exposing them so a myosin head can attach and form a cross-bridge. The role of acetylcholine is to start the neural signal that leads to calcium release, not to directly trigger the cross-bridge; tropomyosin normally blocks the binding sites until calcium removes that block; and ATP hydrolysis energizes the myosin head but cross-bridge formation happens when the actin sites are exposed, not strictly after ATP has been hydrolyzed.