Talk:I–V curve

I think we should talk about this before inserting this edit..

"This convention stems from the experimental approach used to obtain an I/V curve. During a voltage clamp experiment, when the neuron is experimentally clamped at a specific membrane potential, ionic currents will attempt bring back the membrane potential to the equilibrium potential of that ion. To prevent this, the voltage-clamp amplifier will inject a current of the exact magnitude, but of opposite polarity to clamp the membrane potential. Thus, a positive current where positive ions enter the cell will be countered by the amplifier with a negative current of the same magnitude. This will show up as a negative-going current in the I/V curve."

Much of the basic information is correct, for instance, a voltage clamp does in fact inject opposite polarity current, but this does not have anything to do with the convention. In fact, most voltage clamp invert the signal on their output to give it the same polarity as the cell signal. But the "inward/outward" convention is just that, a convention. It's arbitrary. The British used to use the opposite convention. If it comes from anywhere, it comes from the intuition that "inward" is more like 'dropping down into' than "rising up out of".

Synaptidude, I beg to differ, the I/V curve is really a representation of the voltage-clamp amplifier output. Thus, when you look at a current trace you are really looking at the current output of the amplifier, which mirrors the cell's current. The I/V curve is read directly from this current output. Yes, at some point negative and positive are arbitrary conventions, but I think my explanation, that you chose to remove, makes far more intuitive sense than saying inward is like dropping into. As far as the British and their backward currents go, well... they also liketo drink warm beer! Nrets 00:20, 3 August 2005 (UTC)[reply]

Nrets, I respect your opinion, but I have to differ right back. But at the same time you hit it on the head. At some point, positive and negative are arbitrary conventions. So are positive and negative when applied to current polarity. The "at some point" is at their core. The voltage clamp is merely an instrument for reading the current, it itself does not determine what you call the polarity. Whether or not your particular voltage clamp inverts the current polarity on the output (most do; by output, I mean the place where you attach the cable to read the output,not the Op-amp output) is really a side issue, but illustrative. When you plot an I/V curve you are plotting the cell's current (which you read off the v-clamp). But even if you inverted the output on the clamp (and you knew it), you would still plot an inward current as negative, even if the output of the clamp was positive. Inward/outward, postivie/negative are just conventions, pure and simple.

The I/V curve is not something the cell does, it is an arftificial construct used to understand underlying cell function. As with any graph, what you are looking at is directly the result of the scientific instrument used to produce it. Thus, even though what we read off of the voltage-clamp amplifier is what we call by convention the cell's current, it really is the amplifier current (and thus subject to a bunch of errors, but that is another story). Therefore the inward current is negative: the amplifier injects negative current to keep it from depolarizing, its that simple, whether or not the signal is later inverted is, as you say, a side issue. I say put the edit back, see if anyone else complains. Nrets 15:13, 3 August 2005 (UTC)[reply]

One more thing, the parallels to Ohm's law are basically correct, but it is a very unintuitive way to introduce the concept of an IV curve, especially since this is something so dependent on equilibrium potentials, which become harder to model by a simple voltage across a resistor. Nrets 02:02, 3 August 2005 (UTC)[reply]

Here I really need to differ. I/v curves are all about Ohm's law. It may be unintuitive, but it is absolutely correct. Even the name I/v is two of three terms of Ohm's law. As for equilibrium potentials, they exist only as a voltage across a resistor! What else could they be? They are merely an offset that moves the origin off of zero in the I/V plot. The offset is caused by the introduction of a battery, the potential caused by the K concentration gradient across the resistance of the membrane. I'm curious though, how else would you describe it. I'd be in favor of adding a more intuitive explanation as long as it did not supplant the physically correct explanation offered by Ohm's law. Figure 2, which shows the equilibrium potential for K, would be a good place.
- It's not that your description is technically incorrect, but in reality (as far as I know) the only context where an I/V curve is used is in electrophysiology, and since the circuit analogy is a bit unintuitive, it makes sense to give that aspect less precedence. Whenever I teach these concepts I really find that introducing them first as circuits and then as membranes just confuses everyone. I usually start with a more intuitive explanation and then show how this can be modelled by a simple circuit. In other words, figure 1 does not really help in grasping figure 2. What if we switch them around? Or not, I guess I see arguments for leaving it as it is, just to me is not very helpful. And yes, the Erev is really just a battery. If I think of a better way to introduce the topic I'll add it in. Nrets 15:13, 3 August 2005 (UTC)[reply]
  - OK, as I said, if you think of a more intuitive way to put it, then by all means, add it. But one thing, I/v curves do have general applicability outside of biology, and in that case, fig 1 is the more useful general model. Synaptidude 22:02, 3 August 2005 (UTC)[reply]