Compensation Procedures EPC9 Manual 59
7. Compensation Procedures
Series Resistance Compensation
In whole-cell voltage-clamp recording, the membrane potential of the cell is
controlled by the potential applied to the pipette electrode. This control of potential
is not complete, but depends on the size of the access resistance between the pipette
and the cell interior, and on the size of the currents that must flow through this
resistance. This access resistance is called the series resistance (R
s
) because it
constitutes a resistance in series with the pipette electrode. Part of the series
resistance arises from the pipette itself, but normally the major part arises from the
residual resistance of the broken patch membrane, which provides the electrical
access to the cell interior. In practice, we find that the series resistance usually cannot
be reduced below a value about two times the resistance of the pipette alone.
Series resistance has two detrimental effects in practical recording situations. First, it
slows the charging of the cell membrane capacitance because it impedes the flow of
the capacitive charging currents when a voltage step is applied to the pipette
electrode. The time constant of charging is given by τ
u
= R
s
x C
m
, where C
m
is the
membrane capacitance. For typical values of R
s
= 5 MΩ and C
m
= 20 pF, the time
constant is 100 µs. This time constant is excessively long for studying rapid, voltage-
activated currents such as Na
+
currents in neurons, especially since several time
constants are required for the membrane potential to settle at its new value after a
step change. The second detrimental effect of series resistance is that it yields errors
in membrane potential when large membrane currents flow. In the case of R
s
= 5
MΩ, a current of 2 nA will give rise to a voltage error of 10 mV, which is a fairly
large error; for studying voltage-activated currents, errors need to be kept to ~2 mV
at most.
Electronic compensation for series resistance in voltage-clamp systems has been in
common use since the days of Hodgkin and Huxley. The principle of the
compensation in the case of a patch clamp is that a fraction of the current monitor
signal is scaled and added to the command potential (correction pathway, see figure
below). When a large current flows in the pipette, the pipette potential is altered in a
way that compensates for the potential drop in the series resistance. This
arrangement constitutes positive feedback, and can become unstable when
overcompensation occurs.
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