When action potential is generated in muscle fibre, it causes the release of Ca++ from sarcoplasmic reticulum. Calcium binds with subunit of troponin (troponin - C) on actin filaments and activates it. Activated actin binds to the myosin head to form a cross bridge. This bridge pull the actin filaments causing them to slide over the myosin filaments and there by causing contraction.