Axon Axopatch 200B, Теория работы

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USE OF THE PATCH CLAMP – A TUTORIAL • 13
Chapter 3
in the model cell by the 5 mV SEAL TEST command. The total height of the pulse is
about 5 volts. (Note: With a 10 M
Ω
resistance a 5 mV step will generate 0.5 V; the x10
gain provides a 10-fold amplification resulting in a 5 V output). While the SEAL TEST
circuit puts out only a +5 mV signal, the current signal you see will be 2.5 volts above zero
and 2.5 volts below. This is because the TRACK circuit keeps the total current at zero. It
achieves this by supplying the appropriate command that, when summed with the 5 mV
command, keeps the integral of the current at zero. What you see on the oscilloscope is
comparable to what you would see immediately following the immersion of the pipette tip
in the bathing solution surrounding your cells.
Now turn off the SEAL TEST and look at the value on the meter. Adjust the PIPETTE
OFFSET control until the TRACK voltage is zeroed. At this point, the tracking circuit
does not have to put out a voltage in addition to the pipette offset voltage to achieve zero
current. This specifies the proper setting of the PIPETTE OFFSET control to zero junction
potentials and electrode asymmetries.
Using the V-CLAMP Mode
Some investigators prefer not to use a tracking circuit but do their offset adjustments and
sealing in V-CLAMP. The choice is a matter of personal preference. Both methods enable
you to follow drifts in the electrode. In TRACK mode, drifts are observed as changing
voltages on the panel meter when V
TRACK
is selected. In V-CLAMP mode, drifts are
observed as changing currents. To test this approach, change the OUTPUT GAIN to 5,
switch to V-CLAMP mode, turn the meter switch to I, and again turn on the SEAL TEST.
Now you will see a 2.5 V, about 8 ms (10 ms for 50 Hz line frequency) pulse that goes in
the positive direction. Change the position of the PIPETTE OFFSET control and notice
that the DC position of the rectangular pulse is altered as you do so. You could achieve
rough zeroing of pipette offsets by simply adjusting the control until the rectangular pulse
starts from zero on the oscilloscope screen. For more accurate adjustment, turn off the
SEAL TEST and adjust the PIPETTE OFFSET control until the meter reads zero.