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USE OF THE PATCH CLAMP – A TUTORIAL
Axopatch 200B, Copyright 1997-1999, Axon Instruments, Inc.
Current Clamp (Model Cell)
The usual mode of operation of a patch clamp is voltage clamp. In V-CLAMP mode the
membrane potential is controlled and the current needed to maintain that potential is
recorded. It is, however, often useful to allow the membrane potential to change while
keeping the current constant. This can be done in I-CLAMP mode in which the membrane
current is controlled (often at zero) while the membrane potential is recorded.
Current-clamp mode can be used in either the WHOLE CELL or PATCH configurations.
However, it is most often used in the WHOLE CELL configuration. All time varying
current command signals must be generated externally and brought in through the EXT.
COMMAND BNCs. A DC holding current can be generated in I-CLAMP using the
HOLDING COMMAND control; in current clamp, SEAL TEST generates a 50 pA (β = 1)
or 500 pA (β = 0.1) current steps.
To obtain the most accurate cell parameter measurements while in I-CLAMP, it is
important to have the PIPETTE CAPACITANCE COMPENSATION set correctly
before
entering I-CLAMP mode. This is because in I-CLAMP (for a first approximation) the
pipette capacitance (C
p
) appears to be in parallel with the membrane capacitance (C
m
). If
the PIPETTE CAPACITANCE COMPENSATION is set too low, the measurement of C
m
will be too high and vise versa.
Typically, you will change to I-CLAMP mode after establishing whole-cell recording in
V-CLAMP mode. Assuming you have just completed going through the whole-cell voltage
clamp tutorial, you have already set the PIPETTE CAPACITANCE COMPENSATION
controls to minimize fast transients. You may turn off CORRECTION, PREDICTION and
WHOLE CELL CAP. if you wish; they will all be disabled automatically when you switch
from V-CLAMP to TRACK (I=0) or I-CLAMP. Do not change the PIPETTE OFFSET
control as that will lead to erroneous V
m
measurements.
Switch the MODE to TRACK (I=0). You are now in a slow current clamp, but all
commands are being ignored and the current is being clamped at zero. Now is the time to
set the external command signal and holding current command desired when you go into I-
CLAMP mode.
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