Myofilaments

Composed of myofilaments (contractile protein filaments) whose highly organized arrangement results in the striations observed in the single muscle fiber.

a) Thick filaments - The thick filament is composed of myosin. The projections from the myosin filament are called crossbridges. The crossbridge is believed to be a molecular motor that converts chemical energy into mechanical work.

b) Thin filaments - The thin filament is composed of actin, troponin, and tropomyosin. The cyclic attachment and detachment of myosin crossbridges to actin produces muscle active force and/or shortening. The troponin and tropomyosin comprise the calcium sensitive regulatory proteins. In the absence of intracellular calcium, the troponin/tropomyosin prevent the interaction of actin and myosin.