Using the Patch Clamp EPC9 Manual 79
provide the necessary rapid compensation; however, the slower settings will provide
compensation that is less prone to high-frequency oscillations from misadjustment of
the controls. How much compensation you can apply is also determined by tu. If τ
u
is larger than about 100 µs, you can use any degree up to the maximum of 90%
compensation without serious overshoot or ringing in the voltage-clamp response.
For smaller values of τ
u
, the %-comp setting should be kept below the point where
ringing appears in the current trace.
As in the case of patch recording, there is rarely need to use the full bandwidth of the
Filter 1 in whole-cell recording. This is because typical membrane charging time
constants (even after R
s
-compensation) are considerably longer than 16 µs, which is
the time constant corresponding to a 10 kHz bandwidth. Thus, the current monitor
signal is expected to contain no useful information beyond this bandwidth.
In whole-cell recording, the voltage and current monitor signals follow the usual
convention, with outward currents being positive. This is because the pipette has
electrical access to the cell interior.
Current-Clamp Recording
To switch from voltage-clamp to current-clamp recording, just select the Current
Clamp mode. This provides a recording of the cell membrane potential, which can be
monitored at the V-mon output of the EPC9.
When you switch to the Current Clamp mode, the following things happen inside the
EPC9 main unit: The current monitor circuitry is forced into the intermediate Gain
Range (if it was not before), I-membrane is set to a value that will keep the membrane
voltage constant, internal stimulation is stopped, the 10 kHz Bessel characteristic for
Filter 1 is selected, C-slow Ranges is set to Off, and the value of C-fast is reduced by 0.5
pF to avoid oscillations. R
s
-compensation will now act as a “bridge balance”. For the
user, these changes are of little consequence and are mainly designed to make
current-clamp recording simple and reliable. The holding current will be displayed
in the original V-membrane display (now converted to I-membrane) and the membrane
potential is shown in the V-mon display of E9Screen. In Pulse, the I-mon display is
converted to V-mon.
Once you have switched to Current Clamp, you can use I-membrane to set a holding
current, and you can apply stimulus pulses via Stim. In. The rule for the scaling of
current stimuli is easy to remember: Any combination of applied stimulus and Stim.
Scale settings that would provide 1 mV of stimulus in the Voltage Clamp mode
provides 1 pA of stimulus in the Current Clamp mode.
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