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# [Neuroscience] Re: Equation that explains the behaviour of a circuit in voltage clamp

Bill.Connelly via neur-sci%40net.bio.net (by connelly.bill from gmail.com)
Tue Apr 7 20:50:31 EST 2009

```On Apr 6, 6:13 pm, r norman <r_s_nor... from comcast.net> wrote:

> Are you actually attempting to do voltage clamp experiments or are you
> trying to solve homework problems?

More general curiosity than anything. What I was REALLY trying to do
was understand the behaviour of the op amp circuitry of your basic
headstage, i.e. the current to voltage converter and then the
differential amp that subtracts Vcmd. I thought I needed to know the
equation for the feedback current, but I didn't. I was trying to prove
to myself that even with a significant series resistance, the op amp
circuitry still behaved properly (even if it doesn't clamp the cell
properly).

I've gotten quite obsessive about trying to understand every detail
about e-phys recently. I was at a neuroscience course, where a lot of
the other students (some of whom were post docs and had far 'fancier'
papers than I do, doing whole-cell voltage clamp) really had no idea
about what they were actually doing during voltage clamp. They just
kinda assumed that the entire cell was clamped exactly where they set
the dial on the preamp, and the reported current was exactly equal to
the transmembrane current.

It just kinda reminded me of how anyone can be taught to use a
calculator, and 9 times out of 10, they'll get the right answer, but
if they don't understand math, that 10th time, when they times 2 by 2,
and they get 46873, they wont think it is odd.

It was really quite interesting looking back on Hodgkin and Huxley's
papers, when they 'invented' series resistance compensation... and the
resistance they were compensating for was only a couple of kiloOhms
(though the whole fact that they use opposite directions for current
and voltage makes it a bit harder to read than it should be).

But I just really like seeing those equations. I think dV/dt = I/C is
really quite nice, and while not quite as succinct Vm = Vcmd - (Vm/Rm
+ C dVm/dt)  * Rs is pretty cool too.

```