Troponin (tro-po'-nin) is a complex of proteins that is integral to muscle contraction. Troponin is attached to tropomyosin (another type of protein) and lies within the groove between actin (muscle)filaments. In a relaxed muscle, tropomyosin blocks the attachment of cross bridges to actin, thus preventing contraction. When the muscle cell is stimulated to contract, mechanisms cause the concentration of calcium in the sarcoplasm to rise. Some of this calcium attaches to troponin, causing a conformational change that moves troponin and tropomyosin out of the way so that the cross bridges can attach to actin and produce muscle contraction. Without troponin, there can be no muscular contraction.