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junction potential calculation

Mark Farrant m.farrant at ucl.ac.uk
Tue Jan 30 12:02:49 EST 1996

Alexander Reiprich wrote:
> Does anybody know how to calculate junction potentials between complex intra-
> and extracellular solutions in whole cell recordings ( containing
> TRIS-phosphate for example]? Is there free software available?
> How can I calculate the series resistance error in wcr?

The calculation for liquid junction potentials is perfectly 
straightforward as long as you have access to values of ionic mobility.
The equation required is the Generalized Henderson Liquid Junction 
equation and the calculation is easy to implement either in Mathcad or 
Excel, for example.  Essential reading (and sources of mobilility data) 
are Barry & Lynch (1991; J.Memb.Biol. 121, 101-117), Ng & Barry (1995; 
J.Neurosci.Meth. 56, 37-41), Neher (1995; Chapter 6 in Single-Channel 
Recording, eds Sakmann and Neher, Plenum Press) and references therein.

I can't remember if either of the Barry papers give data on 
Tris-phosphate.  Things start getting difficult if you try to include 
every constituent (eg ATP or EGTA, for which I can find no data).

There is a commercial package sold by Peter Barry called JPCalc.

As to series resistance errors in wcr, this is an entirely different 
question. I suggest you read selected chapters in the aforementioned 
Single-Channel Recording, in Microelectrode Techniques (The Plymouth 
Workshop Handbook, Ed D. Ogden 2nd edition 1994) or the quite useful Axon 
Guide from Axon Instruments.

Mark Farrant
Dept. Pharmacology

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