According to the University of New Mexico, regulatory proteins known as troponin and tropomyosin form a complex and block the active sites on actin molecules. The troponin tropomyosin complex prevents the actin myosin crossbridge from forming and allows the muscle tissues to stay relaxed.
The troponin tropomyosin complex consists of three different components, states the University of New Mexico. TnT binds to tropomyosin, TnI binds to actin and inhibits it from moving, and TnC binds calcium ions in the muscle. When excess calcium flows into the muscle tissues, the calcium ions bind to the TnC and cause the troponin tropomyosin complex to move. This allows the active site on actin to be exposed and for the crossbridge to form and muscles to contract.