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Guide to Electrophysiological Recording
MultiClamp 700A Theory and Operation, Copyright 2000, 2001 Axon Instruments, Inc.
Microelectrode Capacitance
The settling time of a microelectrode depends not only on R
e
but also on the
transmural capacitance (C
t
) from the inside of the microelectrode to the
external solution. For fastest settling, C
t
must be as small as possible. C
t
is
usually 1-2 pF per mm of immersion. In order to reduce the effect of C
t
, two
approaches may be taken. One is to electronically compensate C
t
using the
Pipette Capacitance Neutralization control in the MultiClamp Commander.
This is discussed below, in the section on “Impaling Cells”. The other
approach is to minimize the problem by careful experimental design, as
follows.
In an isolated preparation, lowering the surface of the solution as far as
possible can reduce Ct. For a long slender microelectrode, 200
µ
m or less is
regarded as a low solution level; 500
µ
m is tolerable. Deep is regarded as 1
mm or more. For a microelectrode that tapers steeply (
i.e.
a stubby
microelectrode) deeper solutions can be used with less loss of performance.
When working with very low solution levels there is a risk of evaporation
exposing the cells to the air unless a continuous flow of solution is provided
across or through the preparation. If evaporation is a problem, try floating a
layer of mineral oil on the surface of the solution. If used, this layer of oil will
have the additional advantage of automatically coating the microelectrode as it
is lowered into the solution.
Precautions must be taken to prevent surface tension effects from drawing a
thin layer of solution up the outer wall of the microelectrode. If this film of
saline is allowed to develop, C
t
will increase substantially. Because the film of
saline has axial resistance the contribution to C
t
will be very nonlinear, and the
voltage decay after a current pulse will either be biphasic or slow, even when
capacitance neutralization is used. To prevent the saline film from developing,
the microelectrode should be coated with a hydrophobic material. This can be
done just before use by dipping the
filled
microelectrode into a fluid such as
silicone oil or mineral oil. Another method is to coat the microelectrode with
Sylgard #184 or Q-dope (model airplane glue). The selected material should
be painted onto the electrode to within 100
µ
m of the tip.
