 Here students, in this topic we shall discuss the substructure or ultrastructure of myofilaments. A myofilament is composed of thick and thin filaments. The thick filaments extend the entire length of a band. These filaments are about 16 nanometer in diameter and are composed of about 300 molecules of myosin protein. A myosin molecule is a protein which consists of two identical heavy chains. A protein has two heavy chains and these two heavy chains are coiled together and form a long tail. Apart from this tail, there are two globular heads. These heads are also formed of two heavy chains along with three or four calcium binding light chains. In addition to two heavy chains, there are three or four light chains and these are calcium binding chains. These are the heads which form cross bridges between the thick and thin myofilaments during contraction. During contraction, the cross bridges are formed because of these heads. These students, the thin filaments are about 7 to 8 nanometer thick and they extend across the eye band. Apart from the eye band, they overlap in the dark regions of the eye band with the myosin filaments. Apart from these three filaments, they also overlap in the eye band with the myosin filaments. However, these three filaments are not present in the eye band but in the dark regions of the eye band. These thin filaments are composed of actin molecules which is a protein. Dear students, in the thin filaments, actin molecules are arranged in two chains which twist around each other. In the thin filaments of actin, two actin molecules are formed in two chains which twist around each other. Around this actin, there are two more proteins called tropomyosin which twist around each other. In a relaxed muscle fiber, tropomyosin prevents the actin and myosin from becoming cross bridges. These thin filaments also have a complex troponin. This troponin is made up of three polypeptides. Troponin is not continuous but it is present at intervals of about 14 nanometers along the length of thin filaments. Dear students, the troponin polypeptide which is a three polypeptide complex has three different types of polypeptides. These polypeptides include the TNI which is inhibitory and binds to the actin. The second is the troponin which is called TNT that binds with the tropomyosin and helps to position it on the actin. The tropomyosin which is twisted around the actin is used to place TNT or polypeptide on the actin. The third type of troponin polypeptide is called TNC that binds with the calcium ions. The troponin and tropomyosin attached to the actin help to control the actin-myosin interactions which are involved in muscle contraction. Muscle contraction is a process which is produced on the requirement. Sometimes it is muscle-contacted and sometimes it is relaxed. This is because of the interaction between the actin and the myosin. When these interactions are done, the cross-bridges are formed and the muscle is contacted. Otherwise, when the muscle is not contacted, it is also necessary to keep the cross-bridges locked. In this, the troponin plays an important role. These are the students. The myofilament has an edge zone in the central portion of the A-band. This edge zone appears lighter in colour than the other regions of the A-band. This edge zone is the region which contains only the thick filaments. This is the region where thick and thin filaments do not overlap. As I told you, the A-band has thick and thin filaments that overlap. There is no overlap between the actin and myosin in this region. If we look at the edge zone from the center, there is an M-line. This is the region which contains an enzyme which is involved in energy metabolism.