[Neuroscience] Series Resistance compensation, what the hell?

Bill via neur-sci%40net.bio.net (by connelly.bill from gmail.com)
Tue Apr 3 20:30:30 EST 2007

I though I understood Rs compensation, evidendtly I have no idea.
I thought you figured out Rs, say 10MOhm, the command voltage is 100mV
the amplifier wants to pass say 1nA, it knows it would generate 10mV
across Rs, and hence when it thought the cell was at 100mV, it was
actually at 90mV. Hence compensation multiplies passed current by a
proportional to Rs, bringing the voltage behind the seal to a value
above the command voltage, but getting the cell to the correct value

But I know realize that that logic is flawed, as the voltage drop
across Rs, is proportional to Rs/Rs + Rin, i.e. a voltage divider.

So how does the amp know how much voltage is lost across Rs, when that
is proportional to Rin, a value which changes all the time (synaptic
currents, voltage gated currents etc)?

Furthermore, reading the axon manual, I do not understand the
"The amount of compensation achievable is limited by two
considerations. First, as the compensation level (a) approaches 100%,
the increase in the command potential hyperbolically approaches
infinity. For example, at 90% compensation, the command potential is
transiently increased by a factor of ten (Vcmd/(1 - a)). Thus at large
compensation levels the electronic circuits approach saturation"

Transiently? I thought the current was always increased by a factor
proportional to the compensation %? And why would 100% need infinite
current to pass, how would this overcome the finite Rs?

Thanks for your time, and any information.

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