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Sodium concentration during action potential generation

  1. Nov 25, 2016 #1
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    At the peak of the graph, Is the concentration of Na outside the cell is more than inside? because that must be to overcome the electrical force in the opposite direction but my book says that that inside the cell there are more Na than outside

    When is the concentration of Na+ greatest inside the cell? At the peak assuming the Na+ channel close immediately at it then it is also greatest at 3. If there is still passage of Na+ then 3 is the maximum ( Is that right?)

    One last question, is the inside of membrane actually negative, or only negative relative to the outside? In a galvanic cell for example, a given rod isn't actually negative, but it is negative relative to another.

    Then why do they say that there are some negative proteins inside the cell that makes the negative charge?
  2. jcsd
  3. Nov 26, 2016 #2


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    The Na+ inflow is driven by concentration. Normally, it remains higher outside. Your book may have a typo or be talking about some unhealthy situation.
    The difference in the concentration of Na+ ions across the membrane drives the flow of the ions across the membrane. The flow of ions (charges) through a type pf channel will drive the change in membrane potential.

    Point 3 on the diagram is poorly drawn. Earlier points showed the Na+ ions flowing up through their channels. The K+ flow should be in the opposite direction which would be down in a better drawn diagram. The pump directions make sense.
  4. Nov 26, 2016 #3


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    The measurements (between two electrodes) are made with a bath ground and an electrode inserted into a cell. The bath ground is electrically continuous with the outside of the cell in most cases. The other electrode is on the other side of the membrane. Its that measurement, relative across the membrane. That's also what the voltage sensitive ions channels sense, the potential across the membrane.
    Multiple electrodes in a dish would be compared to a common ground, in most cases.
  5. Nov 26, 2016 #4
    The diagram is actually from hyperphysics not my book.

    The problem is in our exams they say that at the highest voltage the Na+ concentration is higher inside the cell which I know that is wrong and you confirmed it. Not sure what I should do now because they will probably give me a wrong answer if I type that it is higher outside

    Might not have understood what I meant. I was talking about why there is a negative charges inside. Some say that it is because the negative proteins inside the cell. Do they actually outnumber the positive charges inside? Or do they say that it is negative relative to the outside of the cell and the proteins just contribute to the voltage?
  6. Nov 26, 2016 #5


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    Discuss with your teacher.

    I would guess the proteins only contribute to the potential. There are other things besides negatively charged proteins that can influence cell potential, like non-neutral ion pumps, leak currents (constant current flows across the membrane), and other ion distributions such as Cl-.

    There are also organelles like mitochondria and chloroplasts that maintain their own potential offset from that of the cell in which they reside, which in turn is offset from the outside of the cell. The mitochondria potential is maintained by pumping of H+ ions.
  7. Nov 26, 2016 #6


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    The anions in the cell are molecules (generally proteins) that have extra electrons on them. The total cation charge is approximately equal to the total anion charge both inside and outside the neuron when at rest. The -60 resting potential is due to the diffusive "force" on the ions, not a charge difference.
  8. Nov 26, 2016 #7


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    At time point 3, the sodium concentration is highest in the cell. That is correct, if we are comparing the sodium concentration in the cell at time point 3 to the sodium concentration in the cell at other times.

    On the other hand, the sodium concentration outside the cell is always higher than the sodium concentration inside the cell, even at time point 3.

    When the cell is at rest, ie. the potential inside the cell is -70 mV relative to the outside of the cell, then it is true that the inside of the cell has more negative charge than positive charge. The outside of the cell is essentially Na+Cl-, the inside essentially K+A-, where A- are negatively charged species such as proteins. At rest, the membrane is mainly permeable to K+, so some K+ inside the cell leaves to the outside due to K+ concentration being lower on the outside, until the equilibrium membrane potential difference for K+ is reached. Since the K+ leaves the cell, but A- does not leave the cell, there is unbalanced negative A- in the cell.
    Last edited: Nov 26, 2016
  9. Nov 27, 2016 #8
    Thank you so much for the clarification. Really appreciate it.

    And thank you Pythagorean and BillTre.
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