The Contraction Cycle begins…..
The SR also contains Ca++ Active Transport Pumps that use ATP to move Ca++ constantly from the cytosol into the SR. As muscle action potentials continue to propagate through the T-tubules the Ca++ release channels are open. Ca++ flows freely into the cytosol more rapidly than they are transported back by the pumps. After the last action potential, the Ca++ release channels close. As the pumps move Ca++ back into the SR, the concentration of Ca++ ions in the cytosol decreases. As this level drops, tropomyosin covers the myosin-binding sites, and the muscle fibre relaxes.
Sliding Filament Mechanism
Muscle contraction occurs because myosin heads attach to and “walk” along the thin filaments at both ends of the sarcomere, progressively pulling the thin filaments toward the M-line. The result, the thin filaments slide inward and meet at the centre of the sarcomere, even possible overlap. As the thin filaments slide inward, the Z-discs come closer together, and the sarcomere shortens. The lengths of the thick and thin filaments does NOT change. Shortening of the sarcomere causes shortening of the whole muscle fibre leading to shortening of the entire muscle.
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