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Patch clamp electrophysiology experiments typically use an intracellular solution that mimics the ionic concentrations of neuronal cytosol. At the same time, the extracellular solution is meant to mimic cerebrospinal fluid. If these two sets of ion concentrations are similar to their in vivo physiological values, then one might expect a patch-clamped neuron to also have physiological ionic reversal potentials. If, on the other hand, they are dissimilar, then we might expect the behavior of the patched neuron (for example its intrinsic excitability or synaptic currents) to be different from the behavior of the same neuron in vivo. In principle, one would like to use solutions that are as close as possible to physiological for this type of experiment.

My question is: how do we know what these physiological values (concentrations or reversal potentials) really are? It is easy to find these values in a textbook, but I would like to know what original research was performed to determine these values in the first place.

In many patch clamp experiments, the solutions are designed to set the reversal potentials to non-physiological values, or are otherwise constrained by the practicalities of the experiment. Let's consider these issues to be outside the scope of this question.

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There are various methods to determine the intracellular ionic concentrations. One method is to register action potentials by the fowling method: inject a Voltage Active dye VAD (something like a -2di Anepp or s similar dye) into the cell or tissue and using an Optic Mapping System register the Action potentials and then determine the intracellular ionic concentrations by Nernst equations.

We could also register Action potentials by "ion-sensitive glass electrodes method" by introducing ion-sensitive glass electrodes directly into the interior of the cells. The problem with these electrodes is that they cannot register very low ionic concentrations especially of Calcium. And so the above method is superior using VAD.

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  • $\begingroup$ Given that multiple ionic currents set the behavior of the cell during an action potential, it seems infeasible to determine ion concentrations from only a voltage recording. Can you point to an example of this method being used to determine ion concentrations? $\endgroup$
    – Luke
    Mar 9, 2016 at 5:55
  • $\begingroup$ Sorry for the delayed response! Busy with routine.I recommend to read the information posted on utdallas.edu/~tres/microelectrode/microelectrodes_ch11.pdf for understanding how ionic concentration is measured inside the cell. $\endgroup$
    – user118008
    Mar 10, 2016 at 21:10
  • $\begingroup$ Thanks for the review! That is an excellent seed for digging through the ion-selective electrode literature. So far I have been able to find measurements in oocytes, epithelial cells, photoreceptors, neuroblastoma cells, purkinje fibers, muscle, etc. No measurements, however, from neurons in anything resembling a physiological preparation. From your reference, it seems like it would be very difficult to make such measurements in cells as small as most neurons, as this would lead to excessive cell damage. $\endgroup$
    – Luke
    Mar 11, 2016 at 5:33

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