ActinThin filament

Actin forms the thin filaments of the sarcomere. Two strands twist around each other, wrapped by the regulatory proteins tropomyosin and troponin, which physically block the sites where myosin would otherwise bind.

It matters because that blocking is the switch that controls contraction. When a nerve impulse releases calcium inside the muscle fibre, calcium binds troponin, tropomyosin shifts, the binding sites are exposed and myosin can pull. Remove the calcium and the muscle relaxes.

In practice this is why calcium handling matters to muscle function and why fatigue is partly a failure of that handling rather than simply running out of fuel.