7.11—Sliding filament theory

An action potential releases calcium from the sarcoplasmic reticulum. Calcium binds troponin, moves tropomyosin away from actin sites, and allows myosin heads to form cross-bridges and pull actin towards the sarcomere centre.

ATP lets a myosin head detach and its hydrolysis resets the head for another power stroke. Repeated cycles bring Z discs closer; when calcium is pumped back, binding sites are blocked and relaxation follows.

The filaments remain the same length while the sarcomere shortens. Without ATP, myosin cannot detach, explaining rigor mortis after death.

ATP is needed for detachment as well as the recovery stroke, and calcium exposes binding sites rather than directly pulling actin.