Axon Axopatch 200B, Теория работы

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PRINCIPLES OF OPERATION • 87
Chapter 8
Moreover, the membrane potential, V
m
, is given by V
m
= V
cp
/(1+s
τ
s
) = V
c
/(1+s
τ
srp
), or
V
m
= V
c
(1 - e
-t/
τ
srp
). Therefore, advancing the PREDICTION potentiometer setting to 80%
gives R
srp
of 2 M
Ω
and
τ
srp
of 100 µs. That is, the speed with which the membrane potential
responds to a voltage command is improved 5 fold over that which is achieved with 0%
PREDICTION. PREDICTION of 98% gives R
srp
of 200 k
Ω
and
τ
srp
of 10 µs. The
membrane potential will now respond to a step voltage command with a 10-90% risetime of
about 22 µs and will settle to within 1% of its final value in less than 50 µs.
Saturation Effects
Note that the equation presented above for V
cp
( i.e. , the command potential plus
PREDICTION signal) can be used to define the maximum allowable
%PREDICTION for a given size step voltage command (this limit should not be
confused with limitations imposed by the stability of the PREDICTION circuit
itself). The command plus PREDICTION signal is attenuated at the headstage by a
10:1 voltage divider. Since the circuitry in the Axopatch 200B mainframe will
saturate at about ±11-12 V, V
cp
is limited in absolute value to about 1.1 to 1.2 V.
To be conservative, we will use 1.1 V in the following calculations:
The peak amplitude of V
cp
for a step voltage command, V
c
, is given by V
c
(R
s
/R
srp
)
which can be rewritten as V
c
/(1 - %PREDICTION/100). So we may state the
limitation on V
c
as a function of % PREDICTION as:
V
c
≤
1.1(1 - %PREDICTION/100)
or the limitation on % PREDICTION as a function of V
c
as:
% PREDICTION
≤
100(1 - V
c
/1.1)
Thus, for example, if it is known that the maximum command step to be used in a
particular experiment is 100 mV, PREDICTION may be set at 91% without fear of
saturation of V
cp
; this is true regardless of the value of R
s
or C
m
. In fact, this is a
rather conservative estimate since it is derived on the assumption that the signal V
cp
will instantly jump to its maximum value following a step voltage command. In
fact, due to limitations in the speed of the PREDICTION circuitry, this over-
estimates the maximum value of V
cp
, particularly when % PREDICTION is large.
In actual practice, PREDICTION can typically be set to about 94% for a 100 mV