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● note: at the start of contraction ca+ binds to troponin which cause tropomyosin to move. This this binding sites on action are accessible. ➔ When a bound ATP on the myosin head is hydrolyzed to ADP and inorganic phosphate, the myosin head extends to attach to binding site on actin forming cross bridge ➔ Triggers power stroke ➔ Actin pulled towards M line ➔ Atp and pi released during power stroke ➔ Myosin remains attached to actin until... ➔ NEW atp attaches to myosin head ➔ This frees the head to either carry on with more contraction or to allow muscle to relax Therefore actin and myosin filaments slide over each other. They don’t actually become shorter them selves.