[Neuroscience] Re: electrode filling solution

r norman via neur-sci%40net.bio.net (by r_s_norman from _comcast.net)
Mon Mar 12 16:44:01 EST 2007

On Mon, 12 Mar 2007 12:47:40 -0500, "Khaled Moussawi"
<khaledm8 from gmail.com> wrote:

>hey guys,
>does anybody know how  the electrode filling solution (mainly for
>extracellular recording) would affect the signal recorded. For example,
>saline vs 2M NaCl, or 0.5M Na acetate or .....

The filling solution should not affect the signal itself except
indirectly.  The higher the concentration, the lower the resistance
which might make some difference if your electrode tips is very small
and the resistance very high.  High resistance, combined with stray
uncompensated capacitance, reduces high frequency response.

More important, how much material leaks out the tip and what effect
will that have on the system you are recording from?  If saline leaks
out, no harm done.  If you fill the electrode with 3M KCl (as was long
traditional for intracellular recording) and some leaks out, it might
significantly alter the local [K+] and affect the system.   That
doesn't usually have much influence intracellularly except that Cl-
leakage can indeed alter the reversal potential of inhibitory
synapses. The larger the tip, the more leakage you will have.  

Another factor is how you plan to connect your electrode to your
electronics.  At some point you have to have a metal-solution
interface.  If that occurs with a chlorided silver wire, you must dunk
it in a solution containing a reasonable [Cl-] in order to get a
reasonable redox reaction occurring at the interface.  If your
solution is connected through a liquid junction to a solution of KCl
or NaCl where the silver wire is, then no problem.  If you don't have
a reversible redox reaction occurring at the metal-liquid interface,
you will be using capacitative coupling between the metal and the
liquid and you will low frequency components in the signal and you
will not be able to record steady (DC) potentials.  That is usually
not a problem with extracellular recording where DC and very low
frequencies are not ordinarily of interest.

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