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

r norman via neur-sci%40net.bio.net (by r_s_norman from comcast.net)
Mon Apr 6 01:13:23 EST 2009

```On Sun, 5 Apr 2009 14:52:11 -0700 (PDT), "Bill.Connelly"
<connelly.bill from gmail.com> wrote:

>On Apr 6, 3:16 am, r norman <r_s_nor... from comcast.net> wrote:
>
>> Vcmd = I Rs + Vm  -- you have this correct.
>>
>> Now just write    Vm = Vcmd - I Rs.
>
>And as I = Vm/Rm + C dVm/dt, then
>Vm = Vcmd - (Vm/Rm + C dVm/dt)  * Rs.

That gives you a differential equation you can solve for Vm knowing
Vcmd.

BUT.....

The real question is just why are you doing all this algebra (and
calculus)?

Are you really interested in voltage clamp on real cells?  What type
of cell, what type of activity?  A real cell is NOT a passive RC
circuit.  Any interesting cell has active spots, whether synaptic or
electrically excitable in which case Rm varies outside your control.

The usual purpose of voltage clamping is so you can study just how Rm
(more usually written 1/Rm = gm) varies with Vm and with time.  When
Rm changes, you can NOT solve the equation you just wrote down.

So just what are you after with all these voltage clamp questions?

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