Axon MultiClamp 700A, Теория и эксплуатация

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Reference Section
MultiClamp 700A Theory and Operation, Copyright 2000, 2001 Axon Instruments, Inc.
The ‘Prediction’ Control
After switching on R
s
Compensation in the MultiClamp Commander, the Prediction
control adds a transient signal to the command potential, speeding the rate at which the
true membrane potential will change in response to a step voltage command. It is
similar to the idea of “Supercharging” introduced by Armstrong and Chow (1987).
The signal added to the command is derived from the command input and from the
setting of the Whole Cell compensation parameters. It enables the actual membrane
potential to be a faithful replica of the command potential;
i.e. the effects of series
resistance in distorting the command potential at the cell membrane are removed up to
the percentage setting of the control (
e.g. a 98% setting means that, in effect, only 2%
of the original series resistance remains in terms of command potential). The signal
added by Prediction is injected through the C2 capacitor used by whole cell
capacitance compensation (See Figure 4.7). The magnitude and time constant of this
signal are determined by the pF and M
Ω settings under Whole Cell and by the
Prediction setting.
For example, consider a whole-cell voltage clamp situation where R
s
= 10 M Ω and
C
m
= 50 pF and the resting membrane resistance R
m
is very large with respect to R
s
.
Assume that Whole Cell pF and M Ω are set at 10 M Ω and 50 pF, respectively, so that
the whole-cell capacity transient is perfectly canceled. If the Prediction control is 0%,
the signal applied to the headstage capacitor C2 (5 pF for 500M range and 53 pF for
50M range) in response to a step voltage command will have a time constant of 500
µ s
and an amplitude that is appropriate to cancel a whole-cell capacitance transient arising
from these parameters (about 10 V
c
). With 0% Prediction nothing is added to the
command potential waveform. In response to a step voltage command the cell
membrane potential will change to its new value with a time constant of 500
µ s (R
s
C
m
).
If the % Prediction control is advanced to 50%, a transient will be added to the
command potential step, V
c
, with a time constant of 250 µ s and an amplitude equal to
that of the command step itself. This will have the effect of changing the cell
membrane potential in response to a step command with a time constant given by
R
s
C
m
(1 - % Prediction /100); here this is 250 µ s. More formally, the command
potential with the Prediction signal included, V
cp
, can be expressed in terms of the
command input, V
c
, by:
V
cp
= V
c
(1+s
τ
s
)/(1+s
τ
srp
)