A myofilament is a chain of protein molecules found in the myofibrils of a striated muscle. There are two types of myofilament, thin and thick, with the thin filaments being made primarily of the protein actin and the thick ones primarily comprised of the protein myosin. In striated muscle tissue, the filaments are arranged within the myofibrils in repeating polypeptide complexes called sarcomeres. The protein molecules play an important role in muscle contraction and relaxation.
Striated muscle is found throughout the body in the form of skeletal muscle, which is a type of muscle whose contractions and movements can be consciously controlled. Cardiac muscle is also striated, and is found only within the walls of the heart. Within the cells of striated muscles are tubular organelles called myofibrils. The myofibrils are made up of bundles of protein in the form of thick and thin myofilaments.
In striated muscle tissue, a myofilament has a set length that doesn't change, even when the muscle shortens as it contracts or elongates as it relaxes again. A thin myofilament is primarily made of a protein called actin, which assembles itself into a ladder-like scaffold during muscle contraction that the myosin filaments can then use to generate force. With the thin structures, actin protein molecules are bound to two other types of protein called nebulin and troponin. A thick myofilament is primarily made of the motor protein myosin, which is attached to the actin network of proteins with another protein called titin.
These structures play a role in muscle contraction, using a model called the sliding filament model. Action potential impulses from the central nervous system trigger the release of calcium ions from within skeletal muscle tissue. Calcium ions bind with the protein tropomyosin, which obscures myosin binding sites on the actin myofilaments. The calcium bond changes the structure of the tropomyosin molecules, allowing the myosin filaments to form chemical bonds with the actin filaments at the myosin binding sites.
Once myosin and actin filaments are bound together, the process of contraction really begins. Muscles shorten, or contract, as myosin myofilaments slide over the passive network of actin proteins. Once the muscle is done contracting, a nucleotide called adenosine triphosphate (ATP) binds with the myosin filaments, weakening their bond with the actin filaments. The myosin filaments use ATP to gain energy so they can detach from the actin filaments and move back to their original position. This causes the muscle to lengthen and return to a relaxed position.