Talk about muscle contraction.

When a muscle is stimulated, acetylcholine is released and depolarizes the sarcolemma, electrical impulse is conducted through the T tubules to the sarcoplasmic reticulum, which then releases Ca++ ions and then binds to the troponin causing conformational change in the complex, and the tropomyosin then is displaced to the interior of the F actin helix groove. Myosin heads will then attach to the exposed actin filaments binding sites, bridges are formed and sliding process begins, Myosin heads attach and detach several times to produce tension and pull the actin filaments and the Z line toward the center of the sarcomere, towards the H zone, the length of the I band is shortened leading to a decrease in the size of the sarcomere while A band is the same size. So contraction occurs simultaneously in all sarcomeres of all myofibril, muscle cells shorten and so does the muscle.