Contraction - bound to the thin filament preventing...

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Contraction Myosin is an ATPase . It hydrolyzes an ATP molecule but remains bound to the ADP and phosphate which activates the head and "cocks" it in an extended position (towards the actin filament). In this extended position, it can bind to an active site on actin (which is revealed when troponin and tropomyosin bind to calcium and change shape). When bound to actin, it pulls the thin actin filament as it releases the ADP and phosphate and "lays" back down. With the binding of a second molecule of ATP, the myosin head releases the actin filament, "cocks" back up, and attaches to a new active site further down the thin filament. This repeated cock-bind-pull-release action results in the sliding of the thick and filaments along their length, pulling the Z discs closer together and shortening the sarcomere. At any given moment, half of the myosin heads are
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Unformatted text preview: bound to the thin filament, preventing slippage of the filaments during contraction. Note that the thin and thick filaments do not become shorter, but slide past each other and the amount of overlap of the filaments changes as the sarcomeres shorten -- a process known as the sliding filament theory of muscle contraction. Muscle cells shorten because their individual sarcomeres shorten, pulling the Z discs within the cell closer together. The myofilaments are attached to the plasma membrane via a protein known as dystrophin ; thus when they shorten due to the action of actin and myosin, they pull on the dystrophin and the plasma membrane of the muscle causing the contraction of the whole muscle cell....
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