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

RSNorman via neur-sci%40net.bio.net (by r_s_norman from comcast.net)
Thu Oct 20 15:25:23 EST 2016

On Thu, 20 Oct 2016 11:52:16 -0700 (PDT), mathewjones from wisc.edu wrote:

>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.

It has been a number of decades, now, since I did microelectrode
recording of epsp's.  Perhaps what you refer to is now rather standard
terminology and technique but I don't see exactly what type of
recording system you might refer to.  Your picture is not all that
clear and I don't understand what you mean by "wire".

Definitely the shape of the epsp is controlled by the cable properties
of the neuron including the RC properties in the immediate vicinity of
the synapse and the propagation down the dentrite which changes both
the amplitude and shape of the signal.  But you seem to be concerned
more with resistances and even capacitances associated with the
recording apparatus.

Any system for measuring voltage must have a very high input
resistance, so high that negligible current, what you call Ix, is
drawn from the system.  In that case any series resistance can be
ignored because, with essentially no current flowing through it, there
will be essentially no voltage drop across it.  Classic glass
microelectrodes might have resistance of 10 to 100 Mohm but the
electronics should draw less than 1 pA of current.  And people using
electrodes at the higher end of the series resistance scale use
electronics with smaller current draw.  I use glass microelectrodes
because you refer to the "pipette" capacitance.  There are "negative
capacitance" compensation circuits which must definitely should be
used and adjusted correctly so that the effective rise time of the
recording system is well under 1 ms.  This eliminates that possible
source of signal corruption.

If you are using glass pipettes to do some form of clamping, then
other considerations apply.  That depends entirely on the effective
circuit of your recording situation and the properties of the
equipment (and electrodes) you are using.  I don't see enough detail
in your question to help you here.  However the questions you raise
about recording system resistance and capacitance are dealt with
routinely by people accustomed to working in this area.  Are you in a
lab with experienced experimenters who can far better help you?

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