SCI-VIS IFC-CNR / SCIENCE PHOTO LIBRARY SCI-VIS IFC-CNR / SCIENCE PHOTO LIBRARY
In the presence of calcium ions (white dots), chemical bonds (cross bridges) are formed between 'heads' on the myosin filaments (darker stalks in foreground) and myosin binding sites on the actin filaments. The myosin heads pull the actin filaments along the myosin filaments resulting in contraction. When the calcium has been removed (absence of white dots) the myosin heads detach, the actin filament slides back, the sarcomere extends and the cardiac muscle relaxes.
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