low resistance grounding bridges in patch clamp

Chris Elliott cje2 at york.ac.uk
Fri May 5 07:23:49 EST 1995

In a microelectode lab, we had to put a pole in the ground and run the 
earth to this; the earth on the mains was faulty


Brad Keele 
(nkeele at beach.utmb.edu) 
wrote: : 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
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