Calcium, Troponin, and Tropomyosin
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State
At rest, tropomyosin blocks actin's myosin-binding sites; released calcium binds troponin, shifting tropomyosin aside and exposing those sites so the sliding cycle, per Bio 3115, can begin.
Show
Calcium removal from the cytoplasm lets tropomyosin snap back over actin's sites, ending contraction.
Watch for
Calcium enables contraction by exposing binding sites — it does not itself generate the pulling force.