Lecture 18 – Muscle Physiology VI

Cb cycle will continue as long as phosphate group is

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CB cycle will continue as long as phosphate group is attached Tension can be maintained without movement Dissociate the cros bridge from the thin filaments by binding ATP Because CBs are not moving forward, they do not need more ATP CB is waiting for an ATP, not phosphorylated, no tension is maintained CB in SM is much slower than it is in a skeletal muscles Sources of Ca  2+  in cytosolic Ca  2+ Key regulator Sources Extracellular Ca  2+  that enters the cell through the plasma membrane Ca that eners the cytosol from the SR Relative amount contributed from each source varies across smooth muscle types Extracellular Ca  2+ Voltage gated, ligand gated, and stretch gated Ca  2+  channels in the plasma membrane Different types in different cells Opening Ca  2+  channels in the plasma membrane results in a large influx in Ca  2+  because of concentration differences Because of small cell size Ca  2+  does not have to diffuse very far Ligand gated  Ach or epinephrine or other hormones Depolarization can happen without NS input Can be totally hormonal
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after Ca  2+  enters, the mechanism is the same since it is all the same effect – increase in cytosolic Ca SR in SM Amount of SR in SM is much smaller Poorly developed No cisternae (because no T-tubule) Generally located very close to the plasma membrane so that it acts like skeletal muscle  AP in plasma membrane can result in AP in SR Ligand binding to receptors Ach, epinephrine Can initiate signal transduction pathways via g-proteins Gq pathway
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