low resistance grounding bridges in patch clamp

[Brad Keele]

In article <3nltvg$83n at saba.info.ucla.edu>, 
chrisdn at physci.lifesci.ucla.edu 
says...
>
>has anyone had trouble eliminating noise (line frequency) from patch
>clamp rigs attributable to faulty grounding with an agar bridge 
electrode?
>        
>i have had line frequency problems for about 2 months and have tried 
many
>forms of grounding and only one was successful in eliminating line freq. 
>noise: a simple Ag/AgCl pellet (low res.) in the chamber. it seemed all 
>agar bridges were too high resistance and allowed noise to enter the 
set-
>up. i have recently constructed large diameter bridges which are a bit
>unwieldy in a small chamber such as mine but which apparently have low  
>enough resistance to sufficiently ground the preparation and worked well 
>for two experiments so far. has anyone else had such a problem?

-- 
I have two suggestions, but they don't really answer you question, I'm 
afraid :(

1.  Grounding through an agar bridge increases the series resistance of 
your 
recordings.  That is, the ground of the electrode is in series with the 
membrane resistance.  However, if you are using discontinuous SEVC (w/ an 
Axoclamp 2A, for instance) then there is no error associated with 
Rseries.

2.   Grounding your electrode through the bath will introduce a junction 
potential at the start of an experiment (which is cancelled with 
amplifier 
offset) that is not present in patch or whole-cell mode (since internal 
and 
intracellular solutions are identical).  That is, the corrected offset 
becomes the juction potential.
________________________________________________________
N. Bradley Keele
Neuroscience Graduate Program
UTMB - Pharmacology J-31
Galveston, TX  77445-1031
Voice:  (409) 772-9604
FAX:    (409) 772-9642
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