Each Tm has 7 actin binding sites. The subunit Tn-C controls Tm's position on thin filament. Stringlike titin molecules are thought to act like springs to keep the myosin thick filaments centered in the sarcomere. Myofibrils are bound to one another side by side by a system of desmin intermediate filaments, and the entire array is then anchored to the plasma membrane of the muscle cell by various proteins, including a flexible, elongated actin-binding protein called dystrophin. This protein, which is either absent or defective in individuals with muscular dystrophy, has a close structural resemblance to spectrin, and it may link specific muscle membrane proteins to actin filaments in the myofibril.
Mechanical force is developed as myosin and actin filaments slide past each other.
X-ray data show that internal packing of actin, myosin in thin and thick filaments don't change during contraction. Sarcomere shortening is caused by the myosin filaments sliding past the actin filaments with no change in the length of either type of filament. The ultrastructural basis for force-generating interactions between thick and thin filaments is visible at very high magnification in electron micrographs. Myosin filaments are seen to possess numerous side projections, or cross-bridges, that extend about 13 nm to make contact with adjacent actin filaments. Such complexes are seen under the EM to have the appearance of arrowheads along the actin filament.