Axon Axopatch 200B, Теория работы

Страница: 102 / 153

86 • PRINCIPLES OF OPERATION
Axopatch 200B, Copyright 1997-1999, Axon Instruments, Inc.
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). PREDICTION works with the setting of the SERIES RESISTANCE
potentiometer and the WHOLE CELL CAP. potentiometer that, in conjunction with the
percentage set on the PREDICTION control, determine the magnitude and time constant of
the compensating signal added to the command input. Simultaneously, the signal applied
to the C2 capacitor used to cancel whole-cell capacity transients is appropriately modified
as the percent PREDICTION is increased. (The value of C2 is 5 pF with
β
= 1 and 50 pF
with
β
= 0.1).
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 the SERIES RESISTANCE control is set at 10 M
Ω
and the WHOLE CELL CAP.
control is set at 50 pF so that the whole-cell capacity transient is perfectly canceled. If the
PREDICTION control is OFF (0%), the signal applied to the headstage 5 pF capacitor
(50 pF for
β
= 0.1) in response to a step voltage command will have a time constant of
500 µs 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 µs (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 µs 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 µs. 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
)
where
τ
s
= R
s
C
m
,
τ
srp
= R
srp
C
m
, where R
srp
is the residual (uncompensated) series resistance in
terms of PREDICTION given by R
srp
= R
s
(1 - %PREDICTION/100), and, in the frequency
domain s = j w ( w is the natural frequency, w = 2
π
f), or in the time domain s is the operator
d/dt. Thus, V
cp
= V
c
(1 + (R
s
/R
srp
-1)e
-t/
τ
srp
).