Tropomyosin and Troponin
Actin looks like a double stranded string of pearls. Tropomyosin wraps along this string and effectivley covers the actin binding sites. Troponin sits along this line with binding sits for Ca+. When Ca+ is bond, it flexes, moving the Tropomyosin off the binding sites, so that the myosin can bind to the actin.