Reference Section • 119
Chapter 5
However, it should be remembered that dynamic voltage errors and bandwidth errors
can still occur in the above example, because these depend on R
s
and C
m
and not on the
size of the membrane current. Even if you are measuring only small membrane
currents in a whole-cell recording, application of R
s
compensation can greatly improve
the fidelity of the voltage clamp.
As a general rule, it is best to try R
s
compensation to see if it makes a difference. This
is certainly advisable in all whole-cell recordings. Compensation is rarely useful with
isolated membrane patches, which typically have small capacitance and membrane
currents. Indeed, the Whole Cell controls (which must be set before using R
s
compensation) are disabled with the 5 and 50 G feedback resistors typically used for
isolated patch recordings. An exception is macropatches or nucleated outside-out
patches, in which the currents can be quite large and for which R
s
compensation may
be necessary.
If R
s
compensation is found not to be necessary, it is best to turn it off. This is because
R
s
compensation increases noise.
Adjusting R
s
Compensation
It is recommended that you practice adjusting R
s
compensation with the PATCH-1U
model cell before using compensation in a real experiment. (See Chapter 5,
MODEL
CELL
.) Connect the CELL connector to the CV-7A headstage. Set the feedback
resistor to 500 MΩ (for Voltage Clamp) and Seal Test to 100 mV at 50 Hz.
When adjusting R
s
compensation, the Filter on Scaled Output should always be set at
10 kHz or above. Check the Seal Test checkbox and observe Membrane Current at a
fast sweep speed on an oscilloscope, triggering the oscilloscope so you can clearly see
the rising edge of the signal (Figure 4.15).
To Next Page
Loading...