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- Discussion:
    - muscle contraction is initiated w/ release of ionic calcium, from terminal cisternae into the sarcoplasm;
    - complexing of calcium by troponin shifts the protein conformations in thin filament & allows formation of crossbridges between actin & 
         myosin, which then moves the actin molecule up long axis of the myosin filament;
    - thick & thin filaments comes from cross-bridges undergo a conformational change which end up pulling thin filaments past the thick 
         filaments;
    - as long as the calcium concentration in the cell is maintained in range of 10-6 M, the myosin ATPase remains active permiting the fibril to
         stay in its contracted form;
    - when energizing impulse terminates, Ca is rapidly pumped out of sarcoplasm;
    - as ionic concentration drops, calcium is withdrawn from troponin, and when sufficiently low, the ATPase on myosin is inactivated;
         - the troponin-actin-tropomyosin B complex again impedes the crossbridging between actin and myosin