ABSTRACT
Attachment of the myosin head to the actin filament results in the release of ADP and Pi from the myosin head, and the myosin head then moves towards the center of the sarcomere (the power stroke). The crossbridge is then said to be in the 'rigor' state. If ATP is available, this will then bind to the myosin ATPase, briefly detaching the myosin head from the actin filament before the ATP is split again and the myosin head is 'charged' again with ADP and Pi. If calcium ion concentrations remain high, further interactions between myosin and actin will again take place, and a further power stroke will result. Hundreds such interac tions between myosin and actin take place along the length of the sarcomere, meaning that shortening of the sarcomere occurs even though some myosin heads are detached from actin filaments at all points during contraction. There is enough redundancy in the system for shortening of the sarcomere to take place in a ratchet-like fashion; when some myosin heads are detached from actin, others will be in the 'rigor' state and will be sufficient to hold the sarcomere at a given length. Only when the muscle fiber action potential is over and the Ttubule has become repolarized will calcium ion concentrations around the myosin heads begin to fall. This prevents any interaction between myosin heads and actin filaments as the troponin-tropomyosin regulatory complexes cover over the myosin-binding sites on actin. Thus, muscle contraction requires both
ADP. Pi
