The arrangement of globular actin molecules in an actin filament. The molecules are packed into a tight helix with about two actin monomers per turn. Tropomyosin is a long molecule made up of two helical chains. A muscle thin filament showing the position of tropomyosin and troponin along the actin filament. Each tropomyosin has seven evenly spaced regions of homologous sequence, each of which is thought to bind to an actin filament.
Calcium acts as a trigger to initiate the contraction process.
Myosin head groups are known to link the thick filaments to the thin filaments. X-ray diffraction shows that these heads can swivel back and forth like oars, first attaching to then detaching from adjacent actin filaments. This uncovers the actin binding sites for the myosin head groups which then bind, swivel and slide the actin filaments farther into the A band. The head detaches only after a new ATP molecule is bound, and the head subsequently binds to the next actin site. If no ATP is available, the head remains stably attached to the actin.