Explanation

Each actin (thin) filament is made up of two ‘F’ (filamentous) actins which are helically wound to each other. Each F-actin is a polymer of monomeric G (globular) actins. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length. A, complex protein troponin is distributed at regular intervals on the tropomyosin. In the resting state, a sub-unit of troponin masks the active binding sites for myosin on the actin filaments