Action filament :

1. Each actin (thin) filament is made of two “F” (filamentous) actins helically wound to each other. Each ‘F’ actin is a polymer of monomelic ‘G’ (Globular) actins.

2. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length.

3. A complex protein troponin is distributed at regular intervals on the tropomyosin.

Structure of thick and thin filaments