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enter image description here

At the peak of the graph, is the concentration of Na+ outside the cell more than inside? That must be to overcome the electrical force in the opposite direction.

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 this true or false?

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.

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Below is a schoolbook example of the membrane potential combined with the individual conductivities of Na+ and K+ during an action potential (AP) (Fig. 1);

enter image description here Fig. 1. Membrane potential and conductivities of the two most important ions in AP generation. source: Physiology Web

And in Fig. 2 is an example of the currents of Na+ and K+ during an action potential (AP) (Fig. 1). Note that K+ stays high due to amanipulation of ion concetrations.

enter image description here Fig. 2. Sodium and potassium currents during AP generation.

However, you ask about the concentrations of Na+ in the cell. As far as I know, this has not been investigated directly. Intracellular Ca2+ concentrations, for example, can be imaged with various imaging techniques, but absolute Na+ and K+ concentrations have to be measured with intracellular electrodes. So while the conductivities and the currents are depicted quite clearly in the above figures, they do not show the absolute concentrations of Na+ or K+ and I doubt it can be found anywhere in the literature. It can of course be mathematically deduced, by calculating the number of ions entering the cell using the Nernst equation and correcting that for the background effect of the sodium-potassium pump and other factors that alter Na+ concentration in the cell (protein transporters and what not). It will at least give an approximation of [Na+]i.

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    $\begingroup$ Thank for the information about the current and how absolute concentration weren't measured. Can you just solve this question for me? 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 Because our book shows that at the peak the concentration of Na outside is lower than inside which is not reasonable to me. $\endgroup$ Nov 25 '16 at 11:57
  • $\begingroup$ @user3733086 the idea I have is that, even though the peak Na+ concentration inside is still greater than outside, the amount of Na+ that exits the cell is sufficient to revert the the electric potential. Remember that there are other ions intervening, and that the potential differences are measured in mV $\endgroup$ Mar 25 '17 at 11:53
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    $\begingroup$ See my answer to another question: the emphasis there is on resting potential but it is relevant to this one too. tl;dr: Very few ions actually move across the membrane (relative to the total number of ions present) during an action potential. The intracellular concentration of sodium does not change much during a spike, far less than 1%. $\endgroup$
    – Bryan Krause
    Mar 25 '17 at 18:45

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