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INSTRUMENT OPERATION
Axopatch 200B, Copyright 1997-1999, Axon Instruments, Inc.
COMMAND sensitivity changes to 2 ÷ β nA/V while the HOLDING COMMAND
sensitivity changes to 200 ÷ β pA/ turn (for toggle set to x1; the range is five-fold greater
with the toggle set to x5).
There are at least two possible ways to achieve a well-behaved transition between voltage-
clamp and current-clamp modes:
1) Dedicate one external source to voltage clamp and the other to current clamp. Connect
each of them to one of the EXT. COMMAND BNCs on the rear panel. While in the
I = 0 mode, turn off the EXT. COMMAND corresponding to voltage clamp and turn on
the current clamp EXT. COMMAND. Also, while in the I = 0 mode, readjust (or turn
OFF) to HOLDING COMMAND control for use in I-CLAMP.
2) Design your setup to be sensitive to the MODE TELEGRAPH BNC and have your
program to automatically scale a single external command accordingly so that a smooth
transition between modes is obtained (see Telegraph Output section for values).
Whole-Cell Parameters
Although WHOLE-CELL CAP. is disabled in current-clamp mode, SERIES RESISTANCE
compensation is active, and may be used to correct for pipette resistance. Thus, one may
use the Axopatch 200B to accurately record synaptic, action and field potentials from cells
and tissues as well as to record currents from patches and cells. Please note, however, the
risetime limitations listed in the
SPECIFICATIONS
chapter for current clamp. Due to the
nature of patch-clamp amplifiers, current clamp mode in a patch-clamp amplifier will not
clamp cell current with the fidelity that is possible with a conventional microelectrode
amplifier (see below).
Electrode Capacitance Compensation
The speed of the pipette time constant depends on the pipette resistance (R
p
) and the pipette
capacitance (C
p
). As a first-order approximation, the pipette time constant depends on the
product R
p
C
p
. The FAST and SLOW ELECTRODE CAPACITANCE COMPENSATION
controls can be used to electronically reduce the effective value of C
p
, thus reducing the
pipette time constant. This is achieved by injecting a transient current into the headstage
input to charge and discharge C
p
during signal changes.
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