slidingfilamentmodel

The sliding filament model describes how contraction of striated muscle occurs through the relative movement of thin actin filaments past thick myosin filaments within the sarcomeres of muscle fibers. The lengths of the thick and thin filaments remain essentially constant, while their overlap increases as the muscle shortens.

Contraction is driven by cross-bridge cycling. When a motor neuron initiates an action potential and calcium ions are released, calcium binds to troponin, shifting tropomyosin away from myosin-binding sites on actin. Energized myosin heads attach to actin, perform a power stroke that pulls the actin filaments toward the center of the sarcomere, and then detach after ATP binds. The cycle repeats rapidly, causing filaments to slide and the sarcomere to shorten.

In the sarcomere, actin filaments are anchored at the Z-discs and partially overlap with myosin in the central A-band. Contraction reduces the I-band and the H-zone while the A-band remains constant in length. The synchronized shortening of many sarcomeres produces overall muscle contraction. Force and speed depend on calcium availability, ATP supply, the degree of filament overlap, and the regulatory proteins.

Historically, the model provided a unifying explanation for how muscles generate force and shorten, aligning structural observations with biochemical and physiological data. It remains foundational to our understanding of skeletal and cardiac muscle contraction and informs research into muscle mechanics, energetics, and motor control.