What is described as cross-bridge formation in the cycle?

• A. High-energy myosin head attaches to actin of the thin filament
• B. Myosin head pivots and pulls the thin filament
• C. ATP binds to the myosin head causing detachment
• D. Tropomyosin moves to block actin binding sites

Answer: (A)

Cross-bridge formation is the moment the energized myosin head attaches to an actin site on the thin filament, creating a link between the thick and thin filaments. This attachment happens after the myosin head is energized by ATP hydrolysis and the actin binding sites are exposed, allowing the myosin to grab onto actin. This bridge is what enables the myosin to perform the power stroke, pulling the actin filament toward the center of the sarcomere. Detachment occurs later when a new ATP binds to myosin, resetting the cycle. Tropomyosin movement to expose binding sites is the regulatory step that sets the stage for this attachment, but the actual cross-bridge is the myosin–actin link.