Final answer:
During skeletal muscle contraction, adjacent Z lines are pulled closer together as the sarcomere between them shortens, causing the muscle to contract. This is due to the sliding of thin filaments towards the center of the sarcomere, powered by ATP. The sliding filament theory provides the basis for understanding this process.
Step-by-step explanation:
During skeletal muscle contraction, adjacent Z lines (or Z discs) are pulled closer together as the sarcomere between them shortens. A sarcomere is the basic unit of a muscle's cross-striated myofibril and is defined as the segment between two neighboring Z lines. When a muscle fiber receives a signal from a nerve, the myosin heads within the sarcomere latch onto the actin filaments and pull, which results in the sliding of thin filaments towards the center of the sarcomere. This action shortens the sarcomere, thereby shortening the muscle fiber and ultimately the entire muscle.
As the sliding filament theory explains, the contraction occurs without the thick and thin filaments themselves shortening; instead, they slide past one another. The Z lines, which border each sarcomere, move closer, and the distance between them decreases. This process is powered by ATP, which is hydrolyzed by the myosin heads to provide the necessary energy for the conformational changes that drive this sliding mechanism.
The regions within the sarcomere affected during contraction include the H zone, which contains only thick filaments and shortens, and the I band, which contains only thin filaments and also shortens. The A band, which contains both types of filaments, does not shorten but instead moves closer together in adjacent sarcomeres. At full contraction, the thin and thick filaments overlap completely, making the sarcomere and, therefore, the muscle itself shorter.