Guide to Electrophysiological Recording • 47
Chapter 4
keep it from getting into the holder. Holders that become contaminated with
solution should be disassembled and sonicated in ethanol or pure deionized
water, and allowed to dry thoroughly before being used again. It is also a good
idea to periodically clean the holders by sonication even if no fluid has been
observed in them.
Seal
The seal will usually be the dominant noise source if it is only a few gigaohms.
Seal resistances in excess of 20 G must be obtained if exceptionally low
noise single-channel recordings are to be routinely achieved. The noise
depends also on the depth of the pipette tip below the surface of the bathing
solution since the effective pipette capacitance increases as the depth of
immersion increases. The voltage noise of the headstage interacts with the
pipette capacitance to produce a noise source that rises with frequency. In
order to minimize noise when recording from excised membrane patches, the
electrode tip should be lifted until it is just under the surface of the bathing
solution.
Signal Generator
One last potential noise source to consider is the noise in the signal generator
that provides the command. In the MultiClamp 700A we have succeeded in
minimizing this noise by heavily attenuating the external command. However,
it is possible for this noise source to be significant, particularly if the command
signal comes from a D/A converter.
Sharp Microelectrode Recording
The CV-7A headstage of the MultiClamp 700A contains both an Axopatch-like
current-to-voltage converter and an Axoclamp-like voltage follower circuit. The
former is activated when VC (V-Clamp) mode is selected in the MultiClamp
Commander, the latter when I=0 or IC (I-Clamp) mode is selected. Although the
I-Clamp circuit is designed to be used with high-resistance sharp microelectrodes, it
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