ssrpk
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Low Ca++ enhances Na+ permeablity in newironms and therefor make it more excityable!
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| mdwannabe
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Ca is necessary for activation of Ca pump that removes Ca from cytoplasm to SR. Once concentration falls, the same amount of Ca is released at stimulus, but more Ca diffuses out due to Ca concentration difference, therefore, less Ca is avalable for stimulation of the pump and removal of Ca, hence, sarcomere remains in contracted state.
As far as extblty goes, Ca stabilazes membrane's charge, once less Ca is avalible, membranes threshhold drops, less stimuls then is needed to discharge a cell. This especially worrisome in a heart where cells are electrically coupled.
Edited by mdwannabe on 06/03/05 - 08:42 AM
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| KrazyK
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Thanks again mdwannabe for the muscle explanation (is that the phospholambin pump)?
I don't quite understand why Ca2+ stabilizes the membrane's potential. I don't think it directly affects the RMP since the cell is impermeable to Ca2+. The only thing I can come up with is that a decrease in free calcium ion means the ICF is less negative than it was before relative to the ECF since less of a + ion had made the ECF less positive . However, I though other changes in ECF environment (increased binding of organic anions, etc) might minimize the electrical change in the ICF due to the hypercalcemia.
So is the mechanism of increased excitability in nerve as simple as I made it out to be above, or is more complicated.
sspk, if you could explain how Ca2+ increases Na2+ permeability, I would appreciate it. Thanks.
Thanks again.
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| ssrpk
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it's a speculation tht Ca++ binds to outer surface of sodium channels and prevent it's opening ;therefore in case Ca++ goes down Na+ channels will get more excitable of neurons in particular!
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| mdwannabe
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thats true ssrk.. however, based on that speculation we always give Ca in cases of hypoMg and nypoK exactly for that reason.
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| KrazyK
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Thanks.
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| dr tsuka
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Hello,
This is my first post by the way. Concerning hypocalcemia and tetany, I believe hypocalcemia affects the MOTOR NEURONS, NOT THE MUSCLE, as previously stated that muscle do not have membrane calcium channels. Lowered plasma calcium, increases excitability of these Motor neurons. Clinically, this can be tested by Trousseau sign (carpal spasm) occluding blood flow to the forearm, or tapping facial nerve.
Hope this helps, bye
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| SDK
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as ssrpk said...inc: ca.....dec: Na+ influx in the neurons...& dec: ca .....inc: inc: Na+ flux..
This b/c of Ca+ compete for Na+ in the Na+ channels in the neuronal membrane....
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| SmokyWaters
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I wud go with ssrpk theory ....n thats what we were taught 
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