INSTRUMENT OPERATION
•
51
Chapter 6
Seal Test
The SEAL TEST command generator provides a convenient source of test pulses to be
used during seal formation. The step is gated by the internal line frequency oscillator; its
duty cycle is between 40% and 60%. In voltage clamp mode, the step size is 5 mV; in
current clamp, it is 50 pA (β = 1) or 500 pA (β = 0.1).
Current Clamp
The Axopatch 200B can be used in current-clamp mode similarly to a conventional
microelectrode amplifier. Series Resistance compensation is functional in this mode.
There are three current-clamp modes. I=0 for voltage recording without an applied current
command, I-CLAMP NORMAL for use with pipette resistances greater than 1 MΩ and
I-CLAMP FAST for use with pipette resistances greater than 10 MΩ. The accuracy of the
response can be improved by using the FAST and SLOW ELECTRODE CAPACITANCE
COMPENSATION as well as the SERIES RESISTANCE COMPENSATION controls.
Membrane Potential
In voltage-clamp mode, V
m
is simply equal to the command potential. In current-clamp
mode, V
m
is not clamped and automatically becomes the voltage needed to allow the
commanded current to flow.
Note:
To avoid introducing errors, you
should not
change the PIPETTE OFFSET control
after you switch from V-CLAMP to I-CLAMP.
Whole-Cell Current Clamp
Current-clamp can be used from the very beginning of the experiment, or you can switch
into current-clamp mode at any time (see Current Clamp section of the
TUTORIAL
).
However, if whole-cell recording has been established, special care must be taken when
switching between voltage-clamp and current-clamp modes because in voltage-clamp mode
the EXT. COMMAND sensitivity is 20 mV/V and the HOLDING COMMAND sensitivity
is 20 mV/turn (for x1; 100 mV/turn with the toggle set to x5). In current-clamp the EXT.
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