Final answer:
The movement of a Myosin-II head on an actin filament involves attachment, power stroke, and detachment, known as the cross-bridge cycle, which is essential for muscle contraction.
Step-by-step explanation:
A cross-bridge forms when the myosin head attaches to actin, and inorganic phosphate (Pi) is released, leading to a conformational change that produces a power stroke. This stroke pulls the actin filament approximately 10 nm toward the M line. The sarcomere shortens as a result, causing the muscle to contract. After the power stroke, ATP binds to the myosin head, causing it to detach from actin. Hydrolysis of ATP to ADP and Pi energizes the myosin head, positioning it for a new cycle of interaction with actin. This process, known as the cross-bridge cycle, is crucial for the sliding filament model of muscle contraction, which allows thin filaments to slide past thick filaments as the myosin heads repetitively cycle through attachment, power stroke, and detachment.