Lecture 18 – Muscle Physiology VI

Cause a whole sheet of muscle to contraction no

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Cause a whole sheet of muscle to contraction No concept of a motor unit You can only get a graded response by controlling the force of contraction This is done by regulating the amout of Ca  2+  in the cytosol of the muscle cell phasically active cells can be autorhythmic Cross Bridge activation Activation of myosin molecule by a rise of cytosolic Ca 2+ If you get an influx of Ca 2+  from the ECF or the SR, the calcium binds to calmodulin It is inactive until Ca  2+  is bound Then calmodulin complex activates MLCK MLCK is then activated Dephosphorylates an ATP and adds that phosphate to one of the light chains Myosin 2 in smooth muscle allows P to bind to a light chain Cause the myosin to stand up and comes close enough to the actin thin filaments so  that it binds to the active sites of the actin filaments
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CB forms Will continue until the phosphate group is taken of the myosin If you pump Ca  2+  out of the cell, it changes the dynamic of the MLC-P and it will  dephosphorylate the myosin Relaxation Must dephosphorylate the myosin MLC-P is always active and catalyzing the removal of phosphate group Forward rxn of MLCK is favored by high cytosolic calcium, back reaction in the opposite High Ca  2+  concentrations make the rate of myosin phosphorylation higher than that of  the MLC-P reaction Latch State Adaptation to reduce ATP use when sustained contraction is necessary Sphincters ATP hydrolysis is taking place in the cytosol (?) and in CB cycling
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