Final answer:
When a sarcomere contracts, myosin heads attach to actin forming cross-bridges and pull the actin filaments inward. This shortens the sarcomere, resulting in muscle contraction. This process is driven by ATP and is described by the sliding filament model.
Step-by-step explanation:
When a sarcomere contracts, it results in the shortening of the muscle fiber. This process is described by the sliding filament model of muscle contraction. The active sites on actin filaments are exposed as calcium ions bind to troponin, which allows myosin heads to attach to these active sites forming cross-bridges. Once attached, the myosin heads undergo a power stroke that pulls the actin filaments toward the center of the sarcomere, thereby shortening it. This action is powered by ATP, which when hydrolyzed to ADP and phosphate, provides the energy for the myosin heads to return to their 'cocked' position and prepare for another cycle of binding and pulling.
The overall effect of this cycle of cross-bridge attachment, pivoting of the myosin heads, and detachment is the sliding of the thin filaments over the thick filaments, leading to a shortened sarcomere, narrower I band, closer Z lines, and a contracted muscle. It's important to note, the A band remains the same width throughout this process as it is defined by the length of the myosin filaments, which do not change. As contraction continues, the zone of overlap between actin and myosin filaments increases, maximizing the force production of the muscle.