Low-Noise Recording EPC9 Manual 82
the pipette to remove any spilled solution, fingerprints, etc. before inserting it into
the holder. It might also help to treat the inside of the pipette to prevent the
formation of a film, for example, by shooting some dimethyl-dichlorosilane vapor
(caution: nasty stuff!) into the back of the pipette before or after filling it.
Lower noise is obtained by immersing the pipette a shorter distance into the bath:
this reduces the coupling of noise currents arising in the pipette glass. A significant
amount of noise seems to arise in the sealed membrane itself and is probably lower
in higher-resistance seals. This noise is generally more than one would calculate
from the resistance of the gigaseal.
The usual goal of low-noise recording is better time resolution: if the noise level is
lower, you can use a wider filter bandwidth to observe single-channel events of a
given amplitude. Judicious use of filtering can improve the time resolution of your
analysis. For example, if you are using the 50%-threshold-crossing analysis technique
to analyze channel open and closed times, the best filter bandwidth is the one that
makes the rms background noise about 1/10 of the channel amplitude. Since one
rarely wants to go through the process of choosing the optimum bandwidth during
an experiment, the best procedure is to record the data at a wide bandwidth and
perform any necessary filtering (analog or digital) later, during analysis of the data.
In typical voltage-clamp, whole-cell recordings the predominant noise source arises
from the combination of the access resistance R
s
and the cell membrane capacitance
C
m
. Above 1 kHz or so, the current variance from this source increases with this
resistance and capacitance as
2
= R
s
C
m
so that it is clearly desirable to keep R
s
as small as possible, and, even more
important, to select small cells, if one is interested in low noise. See the chapter by
Marty and Neher (1983) for a more complete description of this and other fine points
of whole-cell recording.
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