Lecture 17 – Muscle Physiology V

Sphincters are normally totally contracted until you

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Sphincters are normally totally contracted until you want them not contracted at all Stomach and intestines have cycles of contraction Peristalsis Normally medially contracted – blood vessels Have continuous muscle tone Bladder, esophagus – normally totally relaxed until they contract completely Cross Bridge Activation in Smooth Muscle Rise in cytosolic calcium concentrations Main regulator of contraction Ca binds to a calcium binding protein called calmodulin Changes into an active form Binds to another cytosolic protein, MLCK MLCK uses ATP to phosphorylate the light chains on the globular head of myosin
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If you phosphorylate the light chain on myosin, you change the conformation of the  myosin so that it is forced into an upward position and a cross bridge can form May be the case that it is not changing length, and just changing tension Isotonic contraction MLC-phosphatase Takes the phosphate off in response to a decreased amount of calcium Always active In [Ca] is high enough that it is higher than the rate that the dephosphorylation is  happening, then cross bridge will happen Relaxation Myosin needs to be dephosphorylated and then it cannot bind to actin MLC-P is always active Latch States Some smooth muscles have this but not all When stimulation is persistent at a high level, Ca levels remain high but the rate of ATP  hydrolysis decreases even though isometric tension remains constantly high This means myosin is still bound to actin Happens when a cross bridge is formed and becomes dephosphorylated when it is still  detached Tension is maintained without movement, the myosin will not detach When ATP binds, the cross bridge will eventually detach This means you can stay maximally contracted with only a little ATP consumption This is VITAL for SM that needs to be contracted most of the time E.g. sphincters Doesn’t happen in phasic contraction or arterioles/bronchioles
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Blood flow and airflow would stop Smooth v skeletal
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05:57
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05:57
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