Muscle fibres are lined by the plasma membrane (sarcolemma) enclosing sarcoplasm. Muscle fibres contains multiple nucleus (syncytium) and the sarcoplasm contains relatively higher amount of glycogen granules, myoglobin. Moreover high amount of calcium stored in endoplasmic reticulum. Parallel arrangement of myofilaments in sarcoplasm is one of the characteristic nature of the muscle fibre. Two types of myofilaments, known as myofibril present in the muscle fibres (Figure 16.2). Based on the thickness, myofibril termed as thin and thick myofilament which is made up of actin and myosin filaments respectively. Actin filament otherwise termed as ‘I’ band or isoelectric band whereas myosin filament termed as ‘A’ band or anisotrophic band. Actin and myosin fibrils are arranged alternatively and run across the muscle fibres longitudinally. Z – line and M – line, which are fibres with elastic in nature present in between I band and A band respectively. The portion in between to A band is termed as sarcomere which is the functional unit of the muscles which is responsible for contraction

Figure 16.2:  A thin myofilament

Actin: Actin is one of the major muscle proteins, which is a polymeric in nature. G-actin is the monomer unit of the actin filament and it is globular in nature. Its molecular weight is around 43 kda. Number of actin monomer units polymerize and form multimeric fibrous F-actin. Two F-actins coil around each other to form α-helice (Figure. 16.3). Thickness of F-actin filament is around 6-7 nm and it consists of 14 G-actin molecules per turn. Association with tropomyosin and troponin, F-actin forms I band of the myofibril. Troponin masks the active binding sites for myosin on actin filaments.