What occurs during the power stroke in the cross-bridge cycle?

• A. Myosin head pivots and pulls the thin filament toward the M line
• B. Myosin head detaches from actin
• C. Calcium is pumped back into the SR
• D. Tropomyosin slides to cover active sites

Answer: (A)

The power stroke is the force-generating step of the cross-bridge cycle: the myosin head pivots like a lever and pulls the actin (thin) filament toward the middle of the sarcomere, the M line. This action uses the energy previously stored when the myosin head was cocked after ATP hydrolysis. As the head pivots, the actin slides past the myosin, shortening the sarcomere and producing contraction. After this stroke, ADP is released and a new ATP must bind for the myosin head to detach and reset for another cycle. The other events happen at different times: detachment occurs when a new ATP binds; calcium reuptake into the sarcoplasmic reticulum ends activation; and tropomyosin moves back to cover actin’s active sites when calcium levels fall.