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Unformatted text preview: MOTOR SYSTEMS LIFE 47 47.1 How do muscles contract? Skeletal muscle is responsible for all voluntary movements, such as running or playing a piano. It also generates the unconscious movements of breathing. Cardiac muscle is responsible for the beating action of the heart. Smooth muscle creates the movement in many hollow internal organs , such as the gut , bladder, and blood vessels, and it is under the control of the autonomic (involuntary) nervous system. 47.1.1 Sliding filaments cause skeletal muscle to contract- Skeletal muscle cells, called muscle fibers , are large and have many nuclei-A specific muscle such as your biceps (which bends your arm) is composed of hundreds or thousands of muscle fibers bundled together by connective tissue-Muscle contraction is due to the interaction between the contractile proteins actin and myosin . Actin filaments ( thin filaments ), and myosin filaments ( thick filaments ) lie parallel to each other. When muscle contraction is triggered, the actin and myosin filaments slide past each other in a telescoping fashion-The myofibril consists of repeating units called sarcomeres . Each sarcomere is made of overlapping filaments of actin and myosin , which create a distinct banding pattern. The bundles of myosin filaments are held in a centered position within the sarcomere by a protein called titin . Titin is the largest protein in the body; it runs the full length of the sarcomere from Z line to Z line. Each titin molecule runs right through a myosin bundle. Between the ends of the myosin bundles and the Z lines, titin molecules are very stretchable, like bungee cords. In a relaxed skeletal muscle , resistance to stretch is mostly due to the elasticity of the titin molecules .-Each sarcomere is bounded by Z lines , which anchor the thin actin filaments. Centered in the sarcomere is the A band , which contains all the myosin filaments. The H zone and the I band , which appear light, are regions where actin and myosin filaments do not overlap in the relaxed muscle. The dark stripe within the H zone is called the M band ; it contains proteins that help hold the myosin filaments in their regular arrangement- sliding filament theory : as the muscle contracts, the sarcomeres shorten, and the band pattern changes. The H zone and the I band become much narrower, and the Z lines move toward the A band as if the actin filaments were sliding into the region occupied by the myosin filaments 47.1.2 Actin-myosin interactions cause filaments to slide-A myosin molecule consists of 2 long polypeptide chains coiled together, each ending in a large globular head. A myosin filament is made up of many myosin molecules arranged in parallel, with their heads projecting laterally from one or the other end of the filament-An actin filament consists of two chains of actin monomers twisted together like two strands of pearls in a helix. Twisting around the actin chains is another protein , tropomyosin , and attached to the tropomyosin...
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This note was uploaded on 09/02/2009 for the course BIO 315H taught by Professor Payne during the Spring '08 term at University of Texas at Austin.
- Spring '08