How Do Muscles Contract?
The contraction of muscles can be explained by the sliding filament theory. The filaments in this theory are the thin and thick filaments that form the myofibrils.
Thin filaments are constituted by two strains of actin, a protein that is twisted in a double helix (like DNA). Thin filaments have troponin and tropomyosin at binding locations along the double helix af actin. Thick filaments contain groups of myosin, which is also a protein. Myosin strains are rounded on one side, but are twisted around each other, so that it seems that both ends of the thick filaments have thick myosin ends.
Myofibrils have filaments of actin and myosin that lie next to each other. The actin filaments are attached to a Z-disk, while the myosin filaments are lying in between the actin filaments and are not attached to the Z-disk. The area between one Z-disk and the next is called a sarcomere, and this is the unit of the muscle that contracts.
For the contraction of the muscles, ATP is required. When there is no ATP available (for example, when oxygen is not available), the metabolism switches and uses lactic acid instead of ATP. Lactic acid is produced by anaerobic respiration, but can not be directly used by the muscle tissue. A muscle fiber contains enough ATP to contract for about a second. In phosphocreatine molecules (ATP + creatine) energy is stored. Creatine is a product that is often used to gain muscle mass fast. Phosphocreatine is easily broken down, releasing more ATP when the relatively small supply of ATP in the muscle cell is depleted.
ATP binds to the thick end of a myosin filament and then splices into ADP (two phosphates in contrast to ATP which has three) and a molecule of anorganic phosphate. The ADP and the phosphate remain attached to the myosin.
Furthermore, a calcium ion is required for the contraction of muscles. This binds to the troponin of the thin filament, which results in a movement of the tropomyosin, which, in turn, opens up binding locations on the actine strains.
After these locations become available, the myosin binds to the actin. Cross-bridges will be formed between the actin and the myosin, which causes the myosin to release the ADP and the phosphate.
When the ADP and the phosphate are released and bind to the actin, the shape of the thick myosin end changes. During this change, the actin filament slides back to the middle of the sarcomere, while the z-disks on the ends of the sarcomere are pulled closer to each other. The muscle gets shorter: it contracts.
The cross-bridges between actin and myosin are unhooked when the next ATP molecule attaches itself to one of the thick ends of the myosin.