The binding of the myosin heads to the muscle actin is a highly regulated process. The thin filament is made of actin, tropomyosin, and troponin. The contraction of skeletal muscle is triggered by nerve impulses that in turn stimulate the release of Ca2+. The release of Ca2+ from the sarcoplasmic reticulum causes an increase in the concentration of Ca2+ in the cytosol. Calcium ions then bind to troponin, which is associated with tropomyosin. Binding causes changes in the shape of troponin and subsequently causes the tropomyosin isoform to shift its position on the actin filament. This shifting in position exposes the myosin-binding sites on the actin filament, allowing the myosin heads of the thick filament to bind to the thin filament.