Talk:Voltage clamp

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Untitled

I've moved this page from Electrophysiology, which I thought was too long. I've been working to clean it up, but it still needs work. Here's some material that I took off the page because I thought it was too informal and maybe not necessary. delldot | talk 04:08, 25 November 2005 (UTC)

Say you are standing at the bottom of a hill holding a soccer ball. You have a friend standing up on the hill who wants the ball. The friend can move up and down the hill at will. Your friend, or more accurately, the altitude of your friend, is the command potential. The altitude of the soccer ball is the cell’s membrane potential. The difference between the altitude of your friend and the altitude of the ball is the error signal. You are the electrode.

In discontinuous single-electrode ball movement, you move the ball up the hill with your feet. You cannot judge the distance to your friend and see the ball at the same time (say you have to look down to see the ball, and up to see your friend). So you judge the distance to your friend (measure the voltage difference) then kick the ball toward your friend (an epoch of current passing). The distance is too great to make it to your friend in one kick, so you run up the hill after the ball, (entering the next duty cycle) judge the distance again and kick the ball up the hill again. You repeat this until the ball reaches your friend (and thus, the error signal is zero). Now, if the speed at which you can run up the hill is too slow compared to the ball’s speed at rolling back down the hill, you won’t make any progress. But if you are significantly faster than the ball, you can run up the hill, and kick it again before it’s had much of a chance to roll back down the hill. This is basically what is happening during SEVC-d. The electrode is kicking the cells membrane potential toward a goal, pausing to measure the voltage, and then kicking it again before it can decay significantly. Of course, such analogies can go only so far in explaining. Here for instance, it doesn’t really need to be your friend on the hill. It could just as well be your enemy (although why you’d want to give your enemy a soccer ball is beyond me). But your enemy might want to torment you by constantly changing the command potential, so maybe its better if you stick with a friend.

Just to carry this analogy to its logical extreme, two electrode voltage clamping would be the same, except you could just carry the ball up the hill without ever taking your eyes off of your friend. SEVC-c would be like giving the ball to a little kid who you ‘’think’’ is reliable, but you’re not completely sure, and asking the kid to carry the soccer ball to your friend. You lost your glasses, so you can only kind of see the kid, so you think he’s doing the job, but you are not really sure.

Voltage gated channels and voltage clamp

The article states with a reference that voltage gated channels operate normally. Is this correct? Clamping the membrane voltage interferes with the voltage gated channels. I'm revising to reflect this Kghose 01:36, 7 January 2007 (UTC)

Broken link - Axon Guide?

Can this be found anywhere else? The links http://www.moleculardevices.com/pages/instruments/axon_guide.html and http://www.moleculardevices.com/pdfs/Axon_Guide.pdf both seem to be broken. 130.243.156.168 (talk) 20:47, 7 May 2012 (UTC)

First sentence - incorrect terminology/confusing phrasing?

I found the first sentence of this article:

"The voltage clamp is used by electrophysiologists to measure the ion currents across the membrane of excitable cells, such as neurons, while holding the membrane voltage at a set level."

To be a bit confusing. Initially I thought "one doesn't measure a current across something, you measure the current through something (in normal circuit analysis you do this by breaking the circuit and adding a current measuring device such as an ammeter to close the circuit again). Current is literally the measure of electrons flowing through a given point. You do however measure the voltage (literally the difference in electrical potential-a.k.a electro-motive force) between 2 points, or across something.

However several following statements in the article seem to suggest that the concept of current being referred to cannot be accurately applied to the way that I understand it (viewing It from the perspective of a person who studies electrical engineering).

It makes sense to me that a voltage clamp is a way of taking variations in voltage across the cell's membrane out of the equation, and that by keeping them constant, eliminates one variable so that the V in Ohm's law becomes a constant and the I and R can then be measured, and their implications as they pertain to ion channels and ion flux throughout a cell be deduced successfully.

The first sentence just struck me as perhaps wrong, or possibly just rather unclear and hard to parse. Of course there's the very real possibility that I'm wrong, because as I alluded to I'm not an electrophysiologist. — Preceding unsigned comment added by 71.204.252.142 (talk) 05:33, 9 January 2013 (UTC)

Hi, I'm an electrophysiologist by background, but I also believe strongly that Wikipedia needs to be written for non-specialists. I've just spent a long time staring at that sentence in response to your comments, and I ended up changing "across" to "through", while also feeling a little ambivalent about that edit. Your understanding is entirely correct, but the paradox of terminology arises, I suspect, from the biochemical structure of biological membranes. The currents in question do indeed travel through the central pores of individual ion channel proteins. But there are many such proteins, spread out side-by-side in the lipid bilayer of the cell membrane, so it's more like a very large number of wires in parallel. Electrophysiologists who study single ion channels via patch clamp often say that current goes "through" the channel proteins, but electrophysiologists who do voltage clamp at the whole-cell level are more likely to say "across", probably because it's not quite logical to think of current going through the whole membrane, most of which is the non-conducting lipid bilayer. Ultimately, I think "through" and "across" are almost interchangeable in meaning here, so I guess it's probably better to say "through" in this sentence. --Tryptofish (talk) 00:53, 10 January 2013 (UTC)

current during AP in voltage clamp

Can someone add an example what happens with the current (that can apparently also be recorded) during an action potential? I would add in the abstract: For exmaple, during an action potential of an excitatory cell (positive voltage) the recorded current would be negative. Would that be correct, and possibly help further readers? If yes, please add, or confirm and I'll add. The reason to add is the confusion between flow of ions (as already stated in abstract) and recorded current (EPSC?). Source Greetasdf (talk) 11:25, 15 March 2021 (UTC).