Muscle Structure and Function PDF

## 17.2.1 Structure of Contractile Proteins Each actin (thin) filament is made of two F (filamentous) actins helically wound to each other. Each F actin is a polymer of monomeric G (Globular) actins. Two filaments of another protein, tropomyosin also run close to the F actins throughout its length...

## 17.2.1 Structure of Contractile Proteins Each actin (thin) filament is made of two F (filamentous) actins helically wound to each other. Each F actin is a polymer of monomeric G (Globular) actins. Two filaments of another protein, tropomyosin also run close to the F actins throughout its length. A complex protein Troponin is distributed at regular intervals on the tropomyosin. In the resting state, a subunit of troponin masks the active binding sites for myosin on the actin filaments. - Each myosin (thick) filament is also a polymerized protein. Many monomeric proteins called Meromyosins constitute one thick filament. Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called the heavy meromyosin (HMM) and the latter, the light meromyosin (LMM). The HMM component, i.e., the head and short arm projects outwards at regular distance and angle from each other from the surface of a polymerized myosin filament and is known as cross arm. The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin. ## 17.2 Muscle You have studied in Chapter 8 that the cilia and flagella are the outgrowths of the cell membrane. Flagellar movement helps in the swimming of spermatozoa, maintenance of water current in the canal system of sponges and in locomotion of Protozoans like Euglena. Muscle is a specialized tissue of mesodermal origin. About 40-50 per cent of the body weight of a human adult is contributed by muscles. They have special properties like excitability, contractility, extensibility and Elasticity. Muscles have been classified using different criteria, namely location, appearance and nature of regulation of their activities. Based on their location, three types of muscles are identified: - Skeletal - Visceral - Cardiac. ### Skeletal Muscles Skeletal muscles are closely associated with the skeletal components of the body. They have a striped appearance under the microscope and hence are called striated muscles. As their activities are under the voluntary control of the nervous system, they are known as voluntary muscles too. They are primarily involved in locomotory actions and changes of body postures. ### Visceral Muscles Visceral muscles are located in the inner walls of hollow visceral organs of the body like the alimentary canal, reproductive tract, etc. They do not exhibit any striation and are smooth in appearance. Hence, they are called smooth muscles (nonstriated muscle). Their activities are not under the voluntary control of the nervous system and are therefore known as involuntary muscles. They assist, for example, in the transportation of food through the digestive tract and gametes through the genital tract. ### Cardiac Muscles As the name suggests, Cardiac muscles are the muscles of heart. Many cardiac muscle cells assemble in a branching pattern to form a cardiac muscle. Based on appearance, cardiac muscles are striated. They are involuntary in nature as the nervous system does not control their activities directly. The image shows a diagrammatic cross-sectional view of a skeletal muscle, showing muscle bundles and muscle fibers. It is labeled with the following: - Muscle fibre (muscle cell) - Sarcolemma - Blood capillary - Fascicle (muscle bundle) The description states that a muscle fiber is lined by the plasma membrane called sarcolemma, enclosing the sarcoplasm. Muscle fibre is a syncitium as the sarcoplasm contains many nuclei. The endoplasmic reticulum, i.e., sarcoplasmic reticulum of the muscle fibers is the store house of calcium ions. A characteristic feature of the muscle fibre is the presence of a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils. Each myofibril has alternate dark and light bands on it. A detailed study of the myofibril has established that the striated appearance is due to the distribution pattern of two important proteins - Actin and Myosin. The light bands contain actin and is called I-band or Isotropic band, whereas the dark band called 'A' or Anisotropic band contains myosin. ## Myosin and Actin Filaments - Both the proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils. - Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively. - In the centre of each I band is an elastic fibre called Z line which bisects it. The thin filaments are firmly attached to the Z line. - The thick filaments in the A band are also held together in the middle of this band by a thin fibrous membrane called M line. - The A and I bands are arranged alternately throughout the length of the myofibrils. - The portion of the myofibril between two successive Z lines is considered as the functional unit of contraction and is called a sarcomere. - In a resting state, the edges oft hin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments. - This central part of thick filament, not overlapped by thin filaments is called the H zone.