Crossbridgecykler

Crossbridgecykler, or the cross-bridge cycle, describe the sequence of interactions between myosin heads and actin filaments that powers muscle contraction in striated muscle. The cycle converts chemical energy from ATP into mechanical work through repeated attachment, force generation, and detachment of myosin from actin. It is a central component of the sliding filament model of muscle contraction.

In a resting muscle, ATP is hydrolyzed by the myosin head, which becomes energized and adopts a cocked conformation with ADP and Pi bound. When Ca2+ levels rise and the troponin-tropomyosin complex exposes binding sites on actin, a myosin head binds to actin to form a cross-bridge. Release of Pi triggers the power stroke, during which the myosin head pivots and pulls the actin filament toward the sarcomere center, producing force and shortening the muscle. After the stroke, ADP is released and the myosin head remains bound in a low-energy state until a new ATP molecule binds.

Detachment occurs when a new ATP binds to the myosin head, reducing its affinity for actin and freeing it from the filament. Hydrolysis of this ATP re-energizes and re-cocks the head, preparing it for another cycle. The rate and efficiency of crossbridgecykler depend on ATP availability, calcium handling, and the types of myosin present, which vary among muscle fibers and organisms.

Crossbridgecykler underpin the force–velocity relationship of muscle and are essential for understanding muscle physiology, biomechanics, and related clinical conditions. The concept emerged from mid-20th-century work on the molecular basis of contraction and remains a foundational element of muscle science.