Cellular Level
Skeletal muscle is composed of cells collectively referred to as muscle fibers. Each muscle fiber is multinucleated with its nuclei located along the periphery of the fiber. Each muscle fiber further subdivides into myofibrils, which are the basic units of the muscle fiber. These myofibrils are surrounded by the muscle cell membrane (sarcolemma), which form deep invaginations called transverse tubules (T-tubules) within the myofibril. Each myofibril contains contractile proteins, described as thick and thin filaments, which are arranged longitudinally into units called sarcomeres.
The main unit of the thick filament is the large protein myosin, which is formed by two pairs of light chains, and one pair of heavy chains. The two heavy chains of myosin twist around each other to make the helical tail of the myosin, whereas the light chains interact with the heavy chains to form the two heads of the myosin at the other end. Upon the heads lies an important binding site which facilitates the interaction of myosin with actin, a protein belonging to the thin filament.[1]
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The other contractile filament in myofibrils is the thin filament, mainly composed of three proteins: actin, tropomyosin, and troponin. Actin’s monomeric, globular form called G-actin, is polymerized into two strands that coil and intertwine around each other to give rise to filamentous actin, referred to as F-actin. Down the length of the F-actin are myosin-binding sites that are obscured by the filamentous protein tropomyosin. The function of tropomyosin is to prevent actin and myosin from interacting when the muscle is at rest, consequently preventing muscle contraction. Troponin is a three-protein complex located along the tropomyosin filaments. The first protein, Troponin T, facilitates the binding of troponin to tropomyosin. Troponin I serves the same purpose as tropomyosin in stopping the actin-myosin interaction by blocking the myosin-binding sites. Lastly, troponin C binds calcium to initiate muscle contraction.[2]
As mentioned previously, the thick and thin filaments of myofibrils are arranged in units called sarcomeres. The sarcomere is the fundamental contractile unit of the myofibril. Z lines separate each sarcomere. The A bands, located at the center of each sarcomere, contain the thick filaments, which may overlap with thin filaments. The A band further divides into the H zone, which contains no thin filaments. The prominent M line bisects the H zone and serves to connect the middle portions of the thick filaments. Located on both sides of the A band are the I bands, which contain both the thin filaments and the Z line that runs down the middle of each I band.
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