Differential Attenutation of EPSPs in Voltage Clamp vs Current Clamp

[Bill.Connelly]

I was reading a paper by Stephen Williams that showed that you get
more distance dependent attenuation of EPSPs when you record in
voltage clamp than in current clamp. I modelled it using NEURON and I
got the exact same result. Why is this?

I would have thought the (albiet partial) voltage control along the
cable would mean that voltage clamp would reduce capacitive currents
(i.e. less C*dv/dt), and therefore would retain more longitudial
current and you would get less attenuation.
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[r norman-4]

On Fri, 7 Aug 2009 02:15:31 -0700 (PDT), Bill
<connelly.bill@...> wrote:

>I was reading a paper by Stephen Williams that showed that you get
>more distance dependent attenuation of EPSPs when you record in
>voltage clamp than in current clamp. I modelled it using NEURON and I
>got the exact same result. Why is this?
>
>I would have thought the (albiet partial) voltage control along the
>cable would mean that voltage clamp would reduce capacitive currents
>(i.e. less C*dv/dt), and therefore would retain more longitudial
>current and you would get less attenuation.

It is easier to see this doing the rather complex math, but here is an
attempt at an explanation.

With current clamp, there is essentially no "loading" or alteration of
the conditions for voltage and current spread down a cable and the
voltage decays with distance in a particular fashion.

With voltage clamp, there is essentially a "short circuit" across the
cable at the point of the clamp.  This causes a rather drastic
alteration in the pattern of current flow pattern when the clamp is
close to the synapse.  However when the clamp is farther down the
cable, the current flow near the synapse is virtually unchanged.  As a
result, when the clamp is near the synapse it "steals" pretty much all
the current, far more than would pass that way unclamped.  When the
clamp is far from the synapse, it only gets what current would
normally end up passing that far anyway.  Hence an apparent "faster
decay with distance" in voltage clamp than unclamped.

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[Bill.Connelly]

Oh, so when the current is generated far away, the voltage clamp kind
of acts like a lot of open channels, i.e that end of the cable appears
to have low resistance?

On Aug 8, 1:33 am, r norman <r_s_nor...@...> wrote:

> With voltage clamp, there is essentially a "short circuit" across the
> cable at the point of the clamp.  This causes a rather drastic
> alteration in the pattern of current flow pattern when the clamp is
> close to the synapse.  However when the clamp is farther down the
> cable, the current flow near the synapse is virtually unchanged.  As a
> result, when the clamp is near the synapse it "steals" pretty much all
> the current, far more than would pass that way unclamped.  When the
> clamp is far from the synapse, it only gets what current would
> normally end up passing that far anyway.  Hence an apparent "faster
> decay with distance" in voltage clamp than unclamped.

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[r norman-4]

A voltage clamp always acts like a low resistance at that point on the
cell.  It is just that when the synapse is far away, it doesn't "see"
the clamp very much so the longitudinal flow of currents down the
dendrite are not much affected.  However when the synapse is very
close, then a major portion of synaptic current is shunted through the
clamp and the pattern of longitudinal current is seriously altered.

On Fri, 7 Aug 2009 16:22:30 -0700 (PDT), "Bill.Connelly"
<connelly.bill@...> wrote:

>Oh, so when the current is generated far away, the voltage clamp kind
>of acts like a lot of open channels, i.e that end of the cable appears
>to have low resistance?
>
>On Aug 8, 1:33 am, r norman <r_s_nor...@...> wrote:
>
>> With voltage clamp, there is essentially a "short circuit" across the
>> cable at the point of the clamp.  This causes a rather drastic
>> alteration in the pattern of current flow pattern when the clamp is
>> close to the synapse.  However when the clamp is farther down the
>> cable, the current flow near the synapse is virtually unchanged.  As a
>> result, when the clamp is near the synapse it "steals" pretty much all
>> the current, far more than would pass that way unclamped.  When the
>> clamp is far from the synapse, it only gets what current would
>> normally end up passing that far anyway.  Hence an apparent "faster
>> decay with distance" in voltage clamp than unclamped.

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