Hello Geng Pan ,

The first problem is to assume that a cell is a homogeneous electrical component. This is not the case and it is not a component at all.

Secondly, the cell is rather composed of an electrolytic water medium and therefore electrochemistry applies in the first place.

It is possible to measure some sort of equivalent resistance only at constant current and only when that current is stabilised. It must be remembered that this current in the electrolyte consists of a positive and a negative current/ionic fluxes and that the passage of the (external) current causes the ionic change of the medium by redox reactions.

As a result, the value of this resistance fluctuates with the duration of the observation/measurement.

For more information, see the excellent book:

https://www.amazon.fr/1-Modern-Electrochemistry-John-Bockris/dp/1461574692/

Best regards,

Dear Bernard:

If one would like to measure the series resistance of a neuron (i.e., input resistance), it is correct to measure at I=0. However, the current amplitude should not be so large as to affect any type of ionic currents.

SN

Dear Sheng-Nan Wu ,

Since U=RI if I=0 then U=0 and under these conditions it becomes difficult to measure a resistance without current or voltage.

Moreover, this does not change the electrochemical problem at the interfaces (electrodes). As soon as there is an applied current, you change the medium by injecting OH- ions into the medium and you produce hydrogen and chlorine.

Does science have to change the medium under study? No!

The major problem with electrophysiology is that it ignores interfaces and their consequences.

The current is not absolute in value and is relative.

Assuming that voltage difference is 1 mV and current different is 1 pA, then the resistance would be (delta V)/(delta I)= 1 gigaOhm.

Well Sheng-Nan Wu ,

It does not matter whether the current is absolute or relative, it is still a current that flows through the measuring device and/or is injected into the living circuit.

The only important thing that does not appear in your calculation is the way the current is applied (dt).

The current has to be applied for a minimum of time in order to have a steady state where it is possible to obtain this famous equivalent resistance (but this is only an analogy with electricity).

This resistance is variable with di/dt or dv/dt.

I usually use the term "series resistance" to refer to the resistance of the recording pipette that is in series with the cell. The resistance of the cell itself would be called "input resistance" or "membrane resistance".

It is relatively simple to evaluate the input resistance. In voltage clamp, you would pass a small voltage (e.g. -5 mV) for 5 or 10 ms and measure the resulting current, then work out the resistance (R = V / I). You don't need to do a full I-V curve. The input resistance that you want to aim for depends on the cell. A big, leaky neuron might have an input resistance of 50 or 200 M-Ohm. A tight, small neuron might have an input resistance of 1 G-Ohm. If the number gets lower than normal, it means you are losing your patch, and the clamp quality is not reliable.

To evaluate input resistance in current clamp, you would pass a small current and allow the resulting voltage change to reach steady-state. For example, a 40 M-Ohm cell given a 100 pA current would depolarize by 4 mV, which is probably little enough to minimize effects on voltage-dependent conductances.

The series resistance is measured in current clamp by adjusting the bridge balance, and reading the value off the knob. In voltage clamp, you perform series resistance compensation, and read the value off the series resistance knob (on an Axopatch 200B) or hit the Auto button (on a Multiclamp 700B). The method is explained in the Axon Guide. The issue is, if the series resistance is high, then the voltage difference between the pipette and the cell is large, and clamp is poor. In my lab, we deal with currents on the order of 10 nA, so a 40 M-Ohm series resistance would give us a 400 mV error, which would be intolerable. We try to keep our series resistance as low as possible (< 10 M-Ohm), and use series resistance compensation to take care of the rest of the error. There are electrophysiologists who consider voltage clamp too unreliable because of series resistance and space clamp issues. They have a point, but, hey, you do what you can.

-Matthew

Dear Matthew Xu-Friedman ,

Your answer is well documented and would be perfect if you were dealing with conductors and components that exchange electrons. In an electrolyte and under the effect of a current/voltage, you exchange electrons through chemical reactions (redox).

This is the basic principle of electrolysis.

The negative electrode supplies electrons (source) and the positive one receives them (sink). Between the two there is an ionic movement governed by the shape and intensity of the applied current. There are two streams of charged particles and therefore two opposite currents.

You only measure the integration (in the mathematical sense) of these flows and you cannot describe at an instant t and a location x the ionic content of the electrolyte which is in the living frame, polyionic and thus includes several flows of different speeds.

The electrophysiological vision is naive, reductionist but not scientifically well founded.

Rseries is mainly due to the resistance of the tip of your patch electrode when you are in whole-cell, voltage-clamp configuration. This resistance (called also “access resistance”) affects significantly the recording conditions and can cause even current distortion.

You can get an estimation of Rseries by applying a small voltage step, say 10 mV, from a holding potential of -80 mV. The current record is composed of a fast transient (capacitative current) and of a sustained component (resistive current). Measure the peak of the capacitative transient (in pA). Then divide the voltage step (10 mV) by the peak of capacitative transient and you obtain the R series. The higher the capacitaive current peak, the lower Rseries. Of course, you need to eliminate the capacitive transient due to the patch pipette glass before getting whole-cell recording, otherwise you’ll get a wrong estimate of R series.

Albertino

Dear Albertino Bigiani ,

As mentioned above, imposing a current or a voltage automatically leads to electrolysis: you change the environment.

Moreover, the transient voltage injected (10 mV) is the assurance of injecting an ionic charge into the medium: It will disperse in the medium but gives little/no information on a hypothetical resistance.

You can read the following paper written by Erwin Neher, Nobel Prize for inventing the patch-clamp technique:

https://pubmed.ncbi.nlm.nih.gov/3357753/

Good luck

I'm not sure that reading his paper changes any of the theories of chemistry or changes the redox equations.

I understand that you don't want to talk about this anymore.

It would be, in the spirit of openness, to read a good electrochemistry textbook;

https://www.amazon.fr/dp/B00FAZ4ZK6/

Actually, I suggested the paper by Neher to Geng Pan, who started this chat.

Sorry for the misunderstanding.

Albertino

Albertino Bigiani

What kind of misunderstanding?

The argument from authority is not a scientific argument.

Should we rather believe the chemical industry, which manufactures millions of tons of chemicals thanks to electrochemistry and its theories? Products that you use every day in your laboratories.

Should we reject the principles of Ame Tisselius who uses these same principles in electrophoresis?

Is Rudolph A. Marcus wrong when he talks about charge transfer?

Or should we reject all these evidences and retain only one theory which disregards redox and assumes that an electrolyte can be replaced, at will, by an electrical component?

I would tend to trust the scientific evidence, wouldn't you?

Good luck.

Bernard

I believe electrochemistry even rules this discussion. The electrochemical potential of a logical scientific argument about currents at the tip of the pipette, some millimeters away from any electrode, created BS- ions in the social media channels. (For some reason, known only to social science, BS ions always act negative.) We now observe a mixed current of knowledge and hysteria that derails a perfectly sober discussion through aggressive trolling. The impartial observer from the electrochemical industry who stumbles across this thread will find it hard to discern at which time t and with which answer bs the mix‘s redox potential turned sour.

In summary: there is absolutely nothing wrong with Matthew Xu-Friedman ‘s answer. Unless your electrode sits in the tip of the patch pipette, which it never does. The expert’s answers regarding redox potentials can be safely moved to a discussion of scanning ion conductance microscopy with activated tips. There they belong and they might be valuable. In any classical cellular ephys the electrochemical potential gradient at the mouth of glass pipette is by far dominated by the voltage and not local activities or redox potentials. If you really need to look into modifications of simple Ohm‘s law, look into

Dear Andreas Neef

I sense a hint of tension in your response but it is true that there is a certain resistance to accepting the inevitable.

How does being a few millimetres away from the electrode change the facts?

Does it change the movement of the ions induced by the voltage/current you apply between these two electrodes? No.

Is there no current flowing through the electrical device you are connecting to the system under observation? No

Doesn't the applied voltage cause redox? No.

Conclusion: you are changing the medium AND you cannot apply ohm's law.

ps: a reference or link is missing from your kind reply.

Just trolling a troll. No reference is legitimizing that.

My dear Andreas Neef

If describing the facts as they are observed in electrochemistry is trolling then I bow out.

If, on the other hand, you refuse any scientific discussion and are not willing to accept the facts.

Be curious as my master Albert used to say.

Try to answer the simple questions that the neophyte asks you instead of closing your mind to discovery and knowledge.

So you are not trolling, but looking for a scientific exchange? Let’s try: calculate the amount of Cl coming off or on the electrode for a 200 ms long 50 pA current injection. The result should be 1E-10 Coulomb divided by Faraday‘s constant: roughly 1E-15 mol or one femtomole. Now let’s calculate the amount of chloride in the pipette. Often the concentration is around 150 mmol/ L. The volume can be approximated by a cylinder 20 mm long, 1 mm in diameter. That gives 1.5E-1 mol/L * 2E-2 m * Pi * 1E-6 m^2 * 1000 L/m^3. That’s roughly 10E-6 mol or 10 micromol.

Such a current injection changes the chloride concentration by 1 part in one hundred million.

So, if you listen to scientific arguments, think about the quantities involved. Nobody said there was no release of ions. It’s just not relevant in this specific case. What’s more, we can calculate when it becomes relevant, we can relate that to the ion exchange flux of neuronal chloride pumps and do science.

If you are a troll, you will of cause repeat in the same cloudy, non-quantifiable terms that electrochemistry rules the measurement of electrical resistance at the tip of a pipette. You will repeat that everyone but you is blind to the truth, that you alone - out of all the people thinking about this - you alone clearly see the mistakes. To which I will not answer any more. Precisely because I do value science.

You are looking in the wrong place my dear Andreas Neef .

You think you are injecting a constant current of 200ms duration but the slew rate has a slope of 2000mA per µs (manufacturer's data) and you are only interested in the plateau whereas the energy is in the rise time. This is the famous dV/dt or dI/dt that you superbly ignore.

Physiologists usually say that it is not important, because it does not last long: your current is established in less than a nanosecond: who cares. But 2A/µs is also the equivalent of 2,000,000 A per second! Do you think the neuron will like it?

And you persist in ignoring the fact that electrons do not pass through your cell environment: the carriers are ions and it is proven. This is the subject of my intervention.

As explained above, you can read the series resistance value from the amplifier after compensation of series resistance. However, you might sometimes want to have a continuous monitoring of series resistance, e.g. during a LTP experiment where a decrease in EPSC or EPSP amplitudes often results from increases of series resistance during the long-lasting experiment). We often don't compensate for series resistance in these experiments. You can continuously monitor series resistance and input resistance by measuring peak and steady-state current amplitudes, respectively, in response to small hyperpolarizing pulses. Input resistance is calculated as explained in the first response. Series resistance can be estimated as R=V/Ipeak. Please let me know if you are interested in the theory why that works. Best, Jakob