Current clamp
Current clamping is used mainly with whole-cell recording. In current clamp mode, the
VOLTAGE HOLD, JUNCTION ZERO, AUTO ZERO, and CAP COMP controls are inactive. CURRENT HOLD
becomes active, as well as the V
m
x10 output.
In current clamp mode, an error voltage drop I
m
RP is generated by current through the
electrode. Series R comp (10 m/turn) cancels the error voltage by feeding back a portion of the
output current.
Switching into current clamp
1) Begin by switching the MODE SELECTOR toggle (in the OUTPUT section) to I
o
(zero current).
2) Turn off all applied commands and test signals to prevent inadvertently applying
unwanted signals that could damage the cell preparation when you switch to current
clamp or voltage clamp.
Notes: Although command signals are disengaged in zero current mode, they become
active immediately on switching out of zero current mode.
Do not press zap. It remains active in zero current mode and can break the patch.
Do not change the cap comp or leak subtraction setting from those made while in voltage
clamp mode. They are still valid when switching back to voltage clamp mode. Changes
made now can't be monitored, can result in over-compensation, feedback oscillation, and
can immediately destroy the patch, or at least may cause errors in series resistance
compensation.
3) Pre-set the holding current. Unfortunately, this is not as straightforward as one would
hope. Briefly:
• Select V
c
+H IN on the meter. The meter will display units of mV. Disregard the units.
• Set the polarity of the HOLDING CURRENT toggle switch (eg, + or –).
• Adjust the CURRENT HOLD knob until the meter reads the desired current.
IMPORTANT NOTE ! The actual current at the pipette is 10x that of the meter display.
For example, when the meter reads 20 mV, then the actual current is 200 pA.
• Then switch the HOLDING CURRENT toggle switch off until you are ready to use it.
4) Finally, switch the MODE SELECTOR toggle from I
o
(zero current) to I
c
(current clamp).
The amplifier is now ready to apply a current clamp when desired.
Activate the clamp mode by turning the HOLDING CURRENT toggle switch to on (eg, + or -).
Recording in current clamp
Commands can now be applied with the pre-set holding current, an externally generated
test protocol applied to the COMMAND IN BNC, or manually with the HOLDING CURRENT control
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