why is the resting potential negative?

Discussion in 'MCAT Study Question Q&A' started by unsung, May 27, 2008.

  1. unsung

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    If the resting membrane potential is defined as when the rate of passive diffusion of ions equals the rate of the action of Na+/K+ pump, why is the resting potential negative?

    I.e. the Na+/K+ pump actively transports 3 Na+ out for every 2 K+ in, but 3 Na+ are passively diffusing back in, and 2 K+ are passively diffusing back out.

    ???
     
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  3. koopa_troopa

    koopa_troopa Junior Member

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    Na+ is not diffusing back in.
     
  4. unsung

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    Yes, it is. Per EK: "As the electrochemical gradient of Na+ becomes greater, the force pushing the Na+ back into the cell also increases. The rate at which Na+ passively diffuses back into the cell increases until it equals the rate at which it is being pumped out of the cell. The same thing happens for potassium. When all rates reach equilibrium, the inside of the membrane has a negative potential difference (voltage) compared to the outside. This potential difference is called the resting potential."
     
  5. rocuronium

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    The rates may be the same, but that doesn't mean that the concentration of ions on either side of the membrane is the same.

    Think of a chemical reaction where the equation is very right shifted. At equilibrium, the rates will be equal, but the concentrations of products and reactants are not the same. It is similar for the membrane potential.

    Does that help?
     
  6. supertrooper66

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    i don't get the question...it's negative, then Na+ flows in for AP and it gets positive shooting up from -70 to +whatever. it starts out negative because of the low ion concentration inside compared to that of outside the cell. it's a -70 differential...difference between the inside and outside. the inside is more negative because that leaky K+ channel is causing K+ to leak out during "rest" and thus making the outside more positive than inside. so, yes, there are "positive" ions inside but a greater concentration of them outside. so, comparing the inside to outside, the there is an overall negative charge differential. remember...the voltage differential is measured across the membrane. it's not just measured from the inside only and voila we have negative. it's negative compared to the outside.

    think i answered your question...i know i skimped on the technical details but whatever.
     
  7. phospho

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    i wasn't aware Na could do that, I thought neuronal leakage only applied to K+ (passive diffusion).

    If what you're saying is true, then I would assume that the rates need to reach equilibrium AFTER the membrane potential has been reached. I could definitely be wrong and would like to see what others say.
     
  8. unsung

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    Yep, I wasn't aware Na could do that, either. We only learned about K+ leak channels in class. But there you have it. So, let me see if I get this. The Na+/K+ pump works to set up a negative potential inside the cell, and it keeps working until the potential decreases to ~-70mV. If we think of that as the forward reaction, and passive diffusion as the backwards reaction, then the two reactions only achieve equilibrium at ~-70mV of potential for the cell, meaning the forward reaction is more likely to occur than the backwards reaction. I guess that makes sense. Correct me if I'm wrong.
     
  9. G1SG2

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    There's relatively more sodium ions outside the membrane. Also, there are negatively charged proteins inside the membrane, as well as negatively charged chloride ions that help establish the negative resting potential.
     
  10. supertrooper66

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    yeah, but i remember reading somewhere that the negatively charged proteins don't add much to the negative resting potential. it's mainly due to the K+ leaky channel. also, there isn't that much Cl- inside, either, because the Cl- generally follows the Na+. the Cl- is going to be mainly with the Na+ outside the neuron during rest.

    the Na+/K+ pump also runs during rest to try to counter the leaky K+ channel, but i'm pretty sure the leaky K+ is still the primary reason it's negative.
     
  11. phospho

    phospho SDN Lifetime Donor
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    I agree 100% with everything you just said.

    Hopefully we're not both wrong:D

    :luck:
     
  12. koopa_troopa

    koopa_troopa Junior Member

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    While it is true that Na+ does leak back, it is relatively insignificant. I wouldn't try to think about the rates of forward and backwards rates, they do not explain why the RMP is negative. What is important is the Nernst Equilibrium for each ion (K+, Na+, Cl-, and Ca2+) and the conductance of each ion. The Nernest equilibrium of K+ is negative and Na+ is positive. The conductance of most ions is low except for K+ (due to the leak channels). As a result resting membrane potential is closer to the equilibrium potential of K+, which is negative. If the conductance of Na+ was most significant, then RMP would be positive.
     
  13. supertrooper66

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    okay, i just reviewed this topic. so now i have better answer...

    the Na/K pump maintains the potential across the membrane. it maintains it by balancing out the Na and K after an AP.

    once at rest, the inside is more negative than the outside because
    1) K+ moves out down its concentration gradient passively via the leaky K+ channel
    2) negatively charged proteins inside
    3) the membrane is barely permeable to Na+ when at rest, so you primarily only get K+ leaving. the Na+ that manages to get through the membrane into the cell is pumped back out by the Na+/K+ pump. so, you're also getting 2 K+ pumped back in but 3 Na+ out, thus you still overall have more positive ions leaving than entering through the pump

    that's it.
     

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