In this assignment, based on studies, a comparison has been drawn between the contractions of these two types muscles at the cellular and molecular levels. Before going into this discussion, contraction must be defined. Contraction is defined by activation of the muscle fibers with a demonstrable shortening of the muscle fibers2. The physiological events in relation to muscle contraction have been delineated in great detail, and they reveal that basically, with neural stimulation there occurs a series of molecular events initiated by increase in the cytosolic calcium concentration. In this assignment, these events will be looked at in a greater detail to compare these events between the striated and nonstriated muscles.
Each skeletal muscle is made up of a large number of skeletal muscle fibers, which are thin cylindrical multinucleated cells of variable length, which in turn are made up of myofibrils, which are bundles of filamentous structures running along the length of the fiber. While observing skeletal- or cardiac smooth muscle fibers through microscope, a series of light and dark bands perpendicular to the long axis of the fiber are very conspicuous. Actually, each myofibril consists of repeating sarcomeres, and the alignment of these sarcomeres3 between adjacent myofibrils gives rise to these bands in the skeletal muscle. The structural plan of the myofibril is such that the sarcomeres are placed end to end, and this fundamental contractile unit is visible within skeletal and cardiac muscles. On the contrary, smooth muscle cells do not show a banding pattern. Actually, the arrangement of sarcomeres leads to the striated pattern in skeletal and cardiac fibers, and sarcomeres in turn comprise of alternate arrangements of numerous thick and thin filaments in the cytoplasm to give rise to roughly 1-2 micrometer cylindrical bundles.
Smooth muscles are abundant mainly in the visceral organs, such as, gut, blood vessels, bladder, and uterus, and they are to serve a particular function. For example, smooth muscles present in the sphincters are designed to maintain a steady contraction for long periods of time and then rapidly relax or in other muscles, they remain constantly activated. To suit these varied functions, the smooth muscles consists of sheets of many small spindle-shaped cells linked together by junctional contacts. Although these serve as mechanical continuity between adjacent spindles, actually, in places there are gap junctions that are designed to provide electrical continuity between cells. Each cell is uninucleated, shorter than striated muscles, and broader than a myofibril. Unlike striated muscle, no cross striations are visible, although they contain the same contractile protein filaments, actin and myosin. It is important to note that their arrangement is irregular in a loose lattice unlike the regular arrangement of skeletal and smooth muscles. Myosin has been termed as a molecular motor. This consists of two heavy chains and two pairs of light chains. The two terminal ends of the heavy chains create a coil that aggregate in the cell to create the basic unit of a thick filament. The remainder of this molecule juts outward from the thick filament creating the cross-bridge. Thus, these polypeptides combine