The mechanism of cooperativity in thin filament activation studied in demembranated fibers of slow skeletal muscle

Regulation of contraction in striated muscle is due to mechanisms on both thin and thick filaments. Thin filament is rapidly activated by Ca2+ binding to troponin C leading to a structural change in tropomyosin that makes the actin sites available for the interaction with the myosin motors. Biochemical and structural evidence suggests that the actin site availability is modulated by motor attachment to actin, which could also be responsible for the spread of activation along the thin filament, under a cooperative mechanism, still poorly defined, that determines the steepness of the sigmoidal isometric force-pCa relation (estimated by the Hill coefficient nH).