Myosin has another binding site for ATP at which enzymatic activity hydrolyzes ATP to ADP, releasing an inorganic phosphate molecule and energy.ĪTP binding causes myosin to release actin, allowing actin and myosin to detach from each other. Myosin binds to actin at a binding site on the globular actin protein. This action requires energy, which is provided by ATP. The motion of muscle shortening occurs as myosin heads bind to actin and pull the actin inwards. One subunit binds to tropomyosin, one subunit binds to actin, and one subunit binds Ca 2+ ions. Troponin consists of three globular subunits. Strands of tropomyosin block the binding sites and prevent actin–myosin interactions when the muscles are at rest. Actin has binding sites for myosin attachment. Two other components of the thin filament are tropomyosin and troponin. The primary component of thin filaments is the actin protein. The tail of a myosin molecule connects with other myosin molecules to form the central region of a thick filament near the M line, whereas the heads align on either side of the thick filament where the thin filaments overlap. Thick filaments are composed of the protein myosin. Thick and thin filaments are themselves composed of proteins. Myofibrils are connected to each other by intermediate, or desmin, filaments that attach to the Z disc. Both the Z disc and the M line hold myofilaments in place to maintain the structural arrangement and layering of the myofibril. The middle of the H zone has a vertical line called the M line, at which accessory proteins hold together thick filaments. This central region of the A band looks slightly lighter than the rest of the A band and is called the H zone. Thin filaments do not extend all the way into the A bands, leaving a central region of the A band that only contains thick filaments. The region at which thick and thin filaments overlap has a dense appearance, as there is little space between the filaments. Thin filaments attach to a protein in the Z disc called alpha-actinin and occur across the entire length of the I band and partway into the A band. Thick filaments occur only in the A band of a myofibril. There are two main types of filaments: thick filaments and thin filaments each has different compositions and locations. Myofibrils are composed of smaller structures called myofilaments. Many sarcomeres are present in a myofibril, resulting in the striation pattern characteristic of skeletal muscle. Figure 19.35.Ī sarcomere is the region from one Z line to the next Z line. A myofibril is composed of many sarcomeres running along its length, and as the sarcomeres individually contract, the myofibrils and muscle cells shorten (Figure 19.35). One sarcomere is the space between two consecutive Z discs and contains one entire A band and two halves of an I band, one on either side of the A band. The Z discs mark the border of units called sarcomeres, which are the functional units of skeletal muscle. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes the entire cell to appear striated or banded.Įach I band has a dense line running vertically through the middle called a Z disc or Z line. The striated appearance of skeletal muscle tissue is a result of repeating bands of the proteins actin and myosin that are present along the length of myofibrils. It has one nucleus per cell, is branched, and is distinguished by the presence of intercalated disks. Like skeletal muscle, cardiac muscle is striated, but unlike skeletal muscle, cardiac muscle cannot be consciously controlled and is called involuntary muscle. Smooth muscle has no striations, is not under voluntary control, has only one nucleus per cell, is tapered at both ends, and is called involuntary muscle.Ĭardiac muscle tissue is only found in the heart, and cardiac contractions pump blood throughout the body and maintain blood pressure. Smooth muscle tissue occurs in the walls of hollow organs such as the intestines, stomach, and urinary bladder, and around passages such as the respiratory tract and blood vessels. Skeletal muscle also has multiple nuclei present in a single cell. Actin is a globular contractile protein that interacts with myosin for muscle contraction. The striations are caused by the regular arrangement of contractile proteins (actin and myosin). Skeletal muscles are long and cylindrical in appearance when viewed under a microscope, skeletal muscle tissue has a striped or striated appearance. Because it can be controlled by thought, skeletal muscle is also called voluntary muscle. Skeletal muscle tissue forms skeletal muscles, which attach to bones or skin and control locomotion and any movement that can be consciously controlled.
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