Final answer:
When a sarcomere contracts, actin filaments are pulled towards the M line, sliding past the myosin filaments, which leads to muscle contraction. During this process, the Z lines come closer and the I band shortens while the A band remains unchanged.
Step-by-step explanation:
When a sarcomere contracts, the actin filaments are pulled towards the M line, sliding past or over the myosin filaments. This process, described by the sliding filament model of muscle contraction, involves myosin heads binding to actin to form cross-bridges. As the heads pivot towards the M line, they pull the actin along with them in a movement called the power stroke, resulting in a shortened sarcomere and the subsequent contraction of the muscle.
During this contraction, the Z lines move closer together, the I band becomes smaller, and the A band stays the same width. At full contraction, the thin and thick filaments overlap completely. The central H zone, which contains only thick filaments, shortens as the I band shortens, while the A band's length remains unchanged.
The comsumption of ATP is vital for muscle contraction, providing the necessary energy for the myosin heads to change their conformation and perform the power stroke, as well as for the detachment of myosin from actin, allowing the cycle to repeat for continuing muscle contraction.