An animal performs most of its behavior through the contraction and relaxation of muscles. Skeletal muscles have an organization in which bundles of protein filaments make up muscle cells, and bundles of muscle cells make up muscles. Muscle cells are the products of the fusion of many different cells, so each muscle cell contains many nuclei. The subcellular organization of a muscle cell also includes the previously mentioned bundles of protein filaments, called myofibrils. Lying end-to-end within these myofibrils are the actual contractile units, the sarcomeres. When the sarcomeres within a muscle cell contract, the muscle itself contracts and shortens. In the accompanying animation, we examine the mechanism by which a sarcomere shortens during contraction.
A muscle contraction can be explained by the cycle of molecular events that take place between actin and myosin filaments. In a single cycle, a myosin head binds to an actin filament, performs a power stroke, and then releases. Note that for the two filaments to disconnect, the myosin head must bind to a fresh molecule of ATP. After myosin releases actin, it hydrolyzes its ATP and initiates another cycle of actin/myosin interactions.
Although we focused on a single myosin head, in fact a myosin filament has many myosin heads. Each myosin filament is also surrounded by six actin filaments to which the different myosin heads can bind. Therefore, when a myosin head breaks its contact with actin, other myosin heads still connect to actin filaments and thus prevent the sarcomere from sliding back to its relaxed position.
The relaxation of the sarcomere occurs after calcium returns to the sarcoplasmic reticulum. Whereas the release of calcium from the sarcoplasmic reticulum is by a passive event in which calcium moves through ion channels, the return of calcium is an active event that requires energy. The control of muscle contraction happens at the level of free calcium in the cytoplasm—all other components involved in muscle contraction are always present and essentially await calcium ions to begin the action.
Textbook Reference: Concept 33.1 Muscle Cells Develop Forces by Means of Cycles of Protein–Protein Interaction