A two dimensional view of a hypothetical sarcomere is shown here, consisting of three sets of thin filaments and two thick filaments. During contraction, the myosin heads on the thick filaments maintain a smooth and continuous pull on the thin filaments.
Contraction starts when some of the myosin heads attach to the G-actins and begin to pull the thin filaments towards the center of the sarcomeres. In this simulation, only a few myosin heads appear to move and attach to the thin filaments. There are many more thick filaments in actual sarcomeres, and each is surrounded by six thin filaments. About half of the myosin heads attach to the thin filaments at any given time. Just when ATP breaks the first set of bonds between actin and myosin, a second wave of myosin heads attach to the thin filaments and continue to pull them towards the center of the sarcomere. This replacement cycle continues until nerve cell stimulation stops, and the muscle fiber relaxes.
