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Muscle - M uscle Both cardiac and skeletal m uscle are...

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Muscle Both cardiac and skeletal muscle are striated . Smooth muscle is not. Myoblasts do not produce muscle cells after childhood. Satellite cells , located adjacent o muscle fibers undergo differentiation to form new muscle cells. Muscle is composed of muscle fibers (muscle cells), that are composed of myofibrils (proteins), which are composed of many sacromeres . Each sacromere is composed of thick filaments of myosin and thin filaments of actin. o There is a regular arrangemet: each think filament is surrounded by a hexagonal array of six thin filaments. Each thin filament is surrounded by a triangular arrangement of three thick filaments. Muscle Contraction: Contraction does not mean shortening; one can shorten or elongate a muscle via this mechanism. In a shortening mechanism, cross bridges forces the thin filaments to move toward the center of the sarcomere. o A common pattern of muscle shortening involves on end of the muscle remaining at a fixed position while the other end shortens toward it. Two major components in contraction: o Actin: polypetide with abinding site for myosin. o Myosin: two large chains and four smaller light chains. These form two globular heads that have a binding site for actin and one for ADP + P i . Pages 254-266,268-269; 270-278; 180-185
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Muscle Acts as the enzyme ATPase that hydrolyzes the bound ATP. o The myosin molecules in the two ends of each thick filament are oriented in opposite directions, so that their tail ends are directed toward the center of the filament. At any instant during contraction, only a portion of the cross- bridges are attached to the thin filaments, producing tension, while others are in a detached state. In a resting mscle fiber, the myosin cross-bridges cannot bind to actin. The cross-bridges are energized with ATP, or therefore holding potential energy. The binding of actin to myosin triggers the power stroke . After a power stroke has occurred, an ATP molecule comes in and binds to myosin. This acts as a way to weaken interaction b/ w myosin and actin so that the whole process can occur again. o This ATP is hydrolyzed to ADP + P i so that the whole cycle is returned. Therefore: o ATP hydrolysis : provides the energy for cross bridge movement o ATP binding : breaks the link formed b/w actin and myosin during the cycle. Rigor mortis is the stiffening of skeletal muscles after death, due to ATP deficiency. Roles of Troponin, Tropomyosin, and Calcium in Contraction: Tropomyosin: a rod shaped molecule approximately 7 actin molecules long. o Partially cover the myosin-binding site on each actin molecule, preventing corss bridges from forming. Pages 254-266,268-269; 270-278; 180-185
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Muscle Troponin: composed of three subunits, and binds to each tropomyosin and regulates the access to myosin-binding sites on actin.
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