Which sequence describes the cross-bridge cycle in skeletal muscle?

• A. Myosin head detaches from actin, then re-cocks, then attaches, then power stroke.
• B. Myosin head attaches to actin, performs a power stroke, detaches when ATP binds, then re-cocks via ATP hydrolysis.
• C. Myosin head binds ATP, releases ADP, and immediately detaches without a power stroke.
• D. Myosin head attaches to actin, detaches, binds ADP, rebinds to actin.

Answer: (B)

The sequence describes how the myosin head uses energy from ATP to generate force in skeletal muscle. First, the myosin head binds to actin to form a cross-bridge. Then a power stroke occurs, during which the head pivots and pulls the actin filament, shortening the sarcomere. After the stroke, a new ATP molecule binds to myosin, causing detachment from actin. The ATP is then hydrolyzed to re-energize and re-cock the myosin head, preparing it to bind again and repeat the cycle. This core flow—attach to actin, perform the power stroke, detach with ATP binding, and re-cock via ATP hydrolysis—matches the described sequence.