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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)
Thu Apr 2 23:04:39 EST 2009

```Thanks,

> Icap = C dV/dt = 0 because during voltage clamp, the voltage is
> ordinarily constant.  Of course, there is an enormous transient
> capacitative current at the time of the voltage step.

Aah, that is exactly where I am having the issue. What happens during
the voltage step. Now I appreciate at the point of the step, dV/dt =
infinity (or close enough), so Icap is infinity, and the capcitor
fills instantly. However, because of the series resistance of the
electrode, peak delta I = delta V * Rs. But this is where I fall over.
Because what happens during Non-step voltages (ramps etc).

I also get that in a parallel RC circuit

Itot = V/Rm + C *  dV/dt

But that leaves me in the same position, with dV/dt = infinity. So it
needs some kind of modifier, but I can't figure it out.
```