mash
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its B
The Na+-K+ pump restores ionic concentrations by exchanging Na+ for K+ in a ratio of 3:2.
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| Bela
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No guys, it's A. Mash, my choice was b, too. Now I am confused b/c apparently I don't know what's going on in phase 4.
I thought that there is an ATPase pump that expels 3 Na+ from the cell for an exchange of 2K+ coming in.
Now, in First, Aid, page 321 (of this year's edition), where they talk about antiarrhythmics, the graph shows what happens at phase 4. Anddddddd, it shows Na+ and Ca+2 going into the cell and K going out of cell, which kinda contradicts my first idea about ATPase.
Now, in the explanation of this question, they were saying that the key is "what moves a + charge into the cell's cytosol." So, they said 1Ca+2 was exchanged for 2K+ going out.
Hmm, can someone straighten this out?
Thnx!
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| mash
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bela ,check this page n it says dat Na - K pump is responsible fr RMP
http://www.mfi.ku.dk/ppaulev/chapter1/Chapter%201...
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| Bela
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Mash, I didn't look at the site yet but u're right. The ATPas pump is what is responsible for RMP. What happens is that 3Na+ go out and 2K+ come in. So, we know that the positive charge is mainly in Na+ and K+ has the "-"charge. Remember, when Na+ comes in during 0 phase, it depolarizes the membrane = makes it more positive.
So, if u utilize Na/K pump, it would make the cytosol more negative b/c all the Na is going out.
However, in the Ca/K pump, Ca is coming in and K is going out, thus making the cytosol more positive.
But my problem is in the discrepancy. I am having trouble understanding which of these is correct or whether they all can happen in phase 4???
1) ATPase pump: 3 Na out/2K in (what i thought happens)
2) Ca in/ 2K out (according to Kaplan)
3) 3Na, Ca in/2K out (according to FA, page 321)
Does it maybe matter whether we are in cardiac muscle or elsewhere?
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| mash
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It depends on the type of fibers.
If it is fast response fibers like his purkinje or cardiac ms, then phase 4 is due to Na -K ATPase activity.
but if it is slow response fibers like SA , AV nodes n specialized cells, then there is spontaneous depolarization ka pacemaker potential & it is composite of inward Na n Ca currents and outward K currents.
ques is asking abt resting cardiac ms so, it wud be Na- K ATPase...
when we talk abt effect of antiarrhythmics (class II n IV), they slow phase 4 in pacemaker fibers. so, it wud be composite effect....
i hope dat explains wat we r looking fr.........
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| Bela
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Regardless that they're asking for the resting cardiac muscle. The point they're making though is that with a Na/K pump, u wouldn't bring + charge inside the cell.
Anyway, this is just a bad question, I think but thnx for the explanation 
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| mash
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o yaeh i dint think abt dat......
lets leave it now ...........may be someone else comes up with a better explanation.
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| rida
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Well Guys this is my understanding of the whole action potential business.
Normally, the function of the Na/K ATPase is to pump 3Na out of the cell and 2K into the cell, however, we are talking about the CARDIAC ACTION POTENTIAL, in which, in order to depolarizse the membrane, Na must move INTO the cell to make is more positive and depolarize, whereas to repolarize, K must move out of the cell to Hyperpolarize the membrane. Remember guys, Depolarization goes towrds Na's equilibrium, whereas Hyperpolarization goes towards K's equilibrium, thats why K has to move out in order to repolarize because we are trying ot reach K's equilibrium. Now I am just going to summarize the four phases:
Phase 0 = Upstroke, incrase Na conductance (cells permeability to Na has increased) therefore, Na comes in and, making charge more positve and going towards its equilibrium hence we have depolarization.
Phase 1 = Now that depolarization has taken place, the cell decreases its permeability to Na and increases it to K causing a small initial repolarization.
Phase 2 = Plateau phase is due to cell's increase permeability to both K and Ca causing Ca in while K out, soOUWARD AND INWARD currents are almost equal, and hence the cell is called a stable cell alt this point.
Phase 3 = Now we are trying to get to complete repolarization tehrefore the permeability to ca's will start decreasing and will completely be towarsds K where K will move out of the cell make the cell go back towards K equilibrium = HYPERPOLARIZATION.
PHASE 4 = is the RESTING MEMBRANE POTENTIAL = wherre INWARD AND OUTWARD currents are now equal, and cell is at K equilibrium potential.
Now the question asks which brings positive charge, the ATPases are not bringing in the positve charge as they move Na out not in, therefore, by process of elimination I would pick Na/Ca. Hope this helps guys!!!
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| mash
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thanx a lot ........
i think dat makes some sense.
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| rida
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Actually another thing I should add as I was confused about the Na/K atpase also in the myocardial cell according to first aid, but when i read physio BRS, ti made so much more sense. It says that the permeability of calcium is based on the gradient established by the Na/K pump. The whole point of this ATPase is that so the Na that is coming into the cell doesn't accumulate in the cell so the ATPase balances it out keeping a gradient. Just another point!
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