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[Neuroscience] Re: Help with series resistance and EPSP variations

mathewjones from wisc.edu via neur-sci%40net.bio.net (by mathewjones from wisc.edu)
Thu Oct 20 13:52:16 EST 2016


On Friday, November 23, 2012 at 4:36:59 PM UTC-6, Jeffrey Lopez wrote:
> Hi everybody, there is a question I have since some time but still
> dont have a convinvcing answer.
> When your series resistance changes during a patch clamp experiment,
> 1- which influence does it have on my EPSP slope and amplitude?
> 2-why?
> 3-how would it help or not to use a discontinuous (switching) amplifier?
>  If anyone could help me to understand I will appreciate it, please
> try to be explicit in answering. Thanks a lot in advance!

I might be wrong about this, but...

Just off the top of my head, I would expect an increase in Rs to cause a reduction in EPSP amplitude. The EPSP has already been filtered somewhat by the RC properties of the dendrites by the time it gets to the soma. Now the question is: how much more filtering happens between the soma and the electrode wire, due to the increased Rs? I'll try to draw the equivalent circuit using text (might get mangled by google):

                   Rs               /----| Cm |----\
wire ---V^VV^V--Vm <                            >--------------Gnd
                                      \---V^ Rm ^V---/


The Rs and Rm form a "voltage divider" such that the total voltage between your wire and Gnd is Vm plus the voltage drop across the series resistance, which is IxRs.  So if the Vm (eg peak of the EPSP) is, say 5 mV, you wouldn't see the whole 5 mV at the wire, because there is the voltage drop IxRs. 

As for the shape of the EPSP as measured at the wire, I would expect some sort of additional filtering cause by the RC circuit of Rs plus the pipette capacitance (which are in parallel with each other, Cp not show above). But i would expect this to be pretty small because Cp is very small compared to Cm, and Rs is very small (hopefully), compared to Rm.  So it might slow down the rising phase a bit, but maybe not noticeably slow the decay. 

However, one thing that can badly alter the shapes of things is if you are using Rs compensation (or bridge balance) *incorrectly*.  

I don't think a switching clamp would help any of this, because it is typically used in Vclamp (I think) to get more accurate control of the Vm with sharp electrodes. 

I recommend getting ahold of the Axon Guide, which provides pretty good low-level explanations of a lot of these issues.

Cheers,

Matt



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