How does a muscle contract according to the sliding filament theory?

The sliding filament theory describes how muscle contraction occurs at the microscopic level, particularly in skeletal muscle fibers. According to this theory, the contraction process is primarily due to the interaction between two types of protein filaments: thick filaments, which are primarily made of myosin, and thin filaments, which are composed mainly of actin.

When a muscle fiber receives a signal to contract, the thick filaments generate force by pulling on the thin filaments. This pulling action causes the thin filaments to slide toward the center of the sarcomere, the functional unit of muscle contraction, effectively decreasing the distance between the Z lines of the sarcomere. As a result, the entire muscle fiber shortens, leading to muscle contraction.

Therefore, the concept of thin filaments sliding along thick filaments is central to understanding how muscles contract. This mechanism is facilitated by cross-bridges that form between the myosin heads and actin binding sites, driven by the hydrolysis of ATP. The sliding filament mechanism explains not only how muscle shortening takes place but also how force is generated during muscle contraction.