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PRINCIPLES OF OPERATION
Axopatch 200B, Copyright 1997-1999, Axon Instruments, Inc.
command step. Figure 4 shows maximum % PREDICTION as a function of
voltage step.
As the PREDICTION potentiometer is advanced the signal applied to the 5 pF capacitor
(C2) in the headstage is modified appropriately so that it will continue to cancel the whole-
cell capacity transient despite the fact that the speed of this transient has increased. This is
simply accomplished by reducing the time constant of this signal as % PREDICTION is
increased. If the circuitry worked perfectly, and if the whole-cell capacity transient had
been perfectly canceled with 0% PREDICTION, no transient would appear as %
PREDICTION is increased up to the maximum allowable values. However, due to the
complexity of this circuitry and a variety of non-ideal characteristics, cancellation of
whole-cell capacity transients does not remain perfect as % PREDICTION is increased.
The small residual transient that emerges can, however, be completely removed by small
readjustments of the setting of the WHOLE CELL CAP., SERIES RESISTANCE, FAST
MAG and FAST τ controls. A detailed description of the required procedure is provided in
the Series Resistance
section of the
TUTORIAL
.
It should be noted that PREDICTION will work for any command waveform, not just steps.
This may be useful for capacitance measurements using phase sensitive techniques or lock-
in amplifiers.
Although PREDICTION can greatly speed the response time of the true membrane
potential with respect to the command potential and, thus, overcome one important effect
of series resistance, it does not correct for the effects of series resistance associated with
the flow of membrane ionic current (i.e., IR drops and filtering effects described above).
This is the role of the CORRECTION potentiometer. CORRECTION feeds back a portion
of the measured membrane current; this signal is added to the command potential. The
percentage set by the CORRECTION potentiometer refers to the setting of the SERIES
RESISTANCE control of WHOLE-CELL PARAMETERS. For example, if the SERIES
RESISTANCE control is set at 10 MΩ, a 90% setting of the CORRECTION control means
that 9 MΩ of series resistance is compensated; the residual (uncompensated) series
resistance in terms of CORRECTION, R
src
, is 1 MΩ.
The LAG potentiometer is used to determine the time constant of a one-pole RC filter
through which the CORRECTION signal is passed prior to being summed with V
c
. The
-3 dB bandwidth of this filter is given by 1/2πτ
LAG
, where τ
LAG
is the setting (in seconds) of
the LAG control. For example, a LAG of 5 µs corresponds to filtering the CORRECTION
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