Tropomyosin
This is the protein of the thin filament of the muscle fiber.
If calcium ions are ejected to a high degree from the t-tubules (in case of stimulus via nerve), they bind to the tropomyosin. Thus the binding to the actin thread is released and the myosin heads of the myosin filaments can hook into the actin.
When the energy carrier ATP from the mitochondria is consumed to ADP, the myosin heads bend off and pull the actin filaments in. This is the basic mechanism of muscle contraction.