Tutorials • 25
Chapter 3
6. Press the Auto button opposite Cp Fast. This will cancel the fast component of the
transient.
The residual step, due to current flow through the “input resistance” of the model
cell, can be canceled using the Leak Subtraction feature of the MultiClamp 700A.
This subtracts from Scaled Output a current that is scaled linearly from the voltage
command. (See Leak Subtraction, Reference section.) Check the Leak
Subtraction checkbox and press the button (or use the glider to obtain a flat trace).
Figure 2.19
The optimum value is about 500 MΩ, the “input resistance” of the model cell.
Manual adjustments of Whole Cell and Cp Fast are necessary to perfectly
compensate the response.
Directly to the left of the Leak Subtraction button is the Output Zero button, which
provides a slightly different way of removing offsets in the Scaled Output trace.
Output Zero acts like a high-pass filter, subtracting a constant DC offset without
regard to the voltage command. To illustrate its use, switch off Leak Subtraction,
and check and press Output Zero (with Holding set to a large negative value, as
described in step 3 of this Tutorial). The Scaled Output trace is baselined but
unlike with Leak Subtraction, the step due to Seal Test is not subtracted.
7. The series resistance (Rs), which typically originates near the tip of the recording
electrode, can be thought of as an unwanted resistance that is interposed between
the headstage circuitry and the membrane of the cell. Since Rs can cause serious
errors in voltage clamp mode, it needs to be reduced as much as possible. This can
be done both mechanically (e.g. by using lower-resistance electrodes) and
electronically. Full details are given in Chapter 5, but the following exercise gives
a foretaste of electronic Rs compensation.
Ensure that Seal Test is running (10 mV, 100 Hz) and both Cp Fast and Whole
Cell compensation have been adjusted as at the end of step 6 above. Switch off
Output Zero and Leak Subtraction and increase Seal Test amplitude to 50 mV.
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