Explain the mechanics of contraction; how does a sarcomere contract its length?
Contraction begins when the myosin is stimulated by a nerve impulse; when stimulated, the globular head of a myosin protein binds with the actin to form actomyosin. Once bound, the myosin head is able to break ATP into ADP and Pi releasing energy which causes the myosin head to pull the actin toward the center of the sarcomere. The myosin head then releases from the actin and repeats the binding and pulling action when stimulated by subsequent nerve impulses causing the two sets of filaments to slide past each other. As a result, the actin filaments, anchored at their midpoints to the Z lines, exert a pulling force against the Z lines. The Z lines are pulled closer together shortening the length of each sarcomere. As each sarcomere is shortened, the length of each myofibril and fiber is shortened and, thus, the length of the muscle itself is shortened as well.
