increase in electrode resistance

Doktor DynaSoar targeting at
Wed Mar 24 18:03:23 EST 2004

On 24 Mar 2004 02:56:32 -0800, ethomas at (Elizabeth Thomas)

} I am currently doing sharp electrode current clamp recordings on
} thalamic slices.  I have an intermittent problem (but it occurs with
} high frequency) with the recordings that has been bothering me.  When
} I start out with the electrodes in the bath, I can start at very
} reasonable resistances like 90-120 Mohms.  But once I penetrate a
} neuron and stablize it with hyperpolarizing currents - the resistance
} climbs up very high to more than 300 MOhms.  At this point it is no
} longer possible for me to continue the experiment.
} This problem only occurs after I have penetrated a neuron.  The
} resistance is stable even when I leave the electrode in the bath for a
} long time.  I also have the impression that the problem is very
} reduced during lunch hour.
} Has anyone had this problem before?  Any suggestions?
} Sincerely
} Elizabeth

I'm not familiar with your technique, but am with biosignal analysis
in general.

Is it possible you're encountering cells coupled by either electric
field (Sperelakis N, McConnell K..Electric field interactions between
closely abutting excitable cells. IEEE Eng Med Biol Mag. 2002
Jan-Feb;21(1):77-89. Review. ) or gap junction (Van Rijen HV, Wilders
R, Van Ginneken AC, Jongsma HJ. Quantitative analysis of dual
whole-cell voltage-clamp determination of gap junctional conductance.
Pflugers Arch. 1998 Jun;436(1):141-51. )? Capacitance difference may
result in apparent impedance differences. This may be possible to
determine with some comparative signal induction (Donnelly DF. 
A novel method for rapid measurement of membrane resistance,
capacitance, and access resistance. Biophys J. 1994 Mar;66(3 Pt
1):873-7. ).

That being said, is a 3x increase in impedance really so much?
Differences in electrode impedance are really only significant if
they're outside the range of the amplifier impedance. Impedance itself
is usually not really an issue; impedance matching is. Mismatch, of
course, causes frequency dependent drop off. Even so, it may be
possible to correct for the induced error (Rohlicek V, Rech F.
Improvement of the accuracy by the measurement of the electrical cell
membrane parameters. Physiol Res. 2002;51(2):169-77.)

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