This a tutorial for setting up RTXI with the Axoclamp 2 electrophysiology amplifier by Molecular Devices. We will follow their tutorials for practicing bridge balancing and capacitance neutralization with the CLAMP-1U model cell. Here, the computer has a National Instruments 6052 E-series DAQ card installed and is connected to an Axoclamp 2B through a National Instruments BNC-2110 breakout box. The ME1 PROBE has a headstage with gain 0.1 (HS-2A-x0.1LU). The toggle switch of the model cell is set to “Cell.” We will use the Istep module to apply step currents to the model cell and help us balance the bridge. This settings file will recreate the tutorial.
Connections
Here are the physical connections between the DAQ card and the Axoclamp:
- Analog Input 0: 10Vm Output (membrane potential recorded by ME1 with a gain of 10)
- Analog Input 1: Im Output (membrane current for ME1)
- Analog Output 0: EXT. ME1 COMMAND (voltage converted into ME1 current command)
We expect that the current measured by ME1 and acquired on analog input channel 1 matches the signal defined by the Istep module. Plot Analog Input 0 and 1 on the oscilloscope so you can verify your signals. With no applied current, you should read zero for both channels. If this is not the case, try calibrating your DAQ card. Some COMEDI devices have the ability to measure internal stable voltage references and control gains and offsets using programmable DACs, such as the NI E-series cards:
$ sudo comedi_calibrate --reset --dump --calibrate --results --verbose /dev/comedi0
For an NI M-series card, the configuration is done in software:
$ comedi_soft_calibrate /dev/comedi0
System Control settings
For this tutorial, it’s fine to leave the real-time period as 1kHz. However, you must set the correct gain on each of the analog channels listed above. From the Control menu on the RTXI toolbar, choose the System Control panel. The default device should be your DAQ card listed as /dev/comedi0. For Analog Input 0, the amplifier specifies that the signal has a gain of 10 applied. Invert this and enter a gain of 0.1 V/V. Click the “Active” toggle button and click “Apply.” For Analog Input 1, the amplifer specifies that there is a gain of 10 ÷ H mV/nA. Since the headstage gain on ME1 is 0.1, the conversion is 100 mV/nA or 0.1 V/nA. Invert this to get 10 nA/V. For Analog Output 0, the EXT. ME1 COMMAND specifies a gain of 10 x H nA/V which comes to 1 nA/V. Inverting this we get 1 gigaV/A. Make sure that all these channels are set to “Active” and that you have used the “Apply” button to activate your changes. For your own reference, you can set the “Scale” dropdown box to either “Volts” or “Amps.” This does not affect the computation.
Istep module
The Istep module can be used to deliver a sequence of step current commands. Here, we’ve set it to the recommended amplitude, 5 nA, and set it to repeat for 20 cycles. Initially, use a period of 1 second and a duty cycle of 50%. Connect the output of the module the the output channel of the DAQ card. Toggle the “Pause” button to start and stop the signal. Double check that the amplitude of the current pulses are correct since these values are determined by the gain you set earlier. When you have finished bridge balancing, you can change the duty cycle to 4% to give a 40 ms pulse as suggested.
Oscilloscope
Here is what you should see if you did everything correctly! You can download this settings file to explore the System Control panel and recreate this screenshot. If you are using different headstages, you will need to change the gains. Make sure you have downloaded and installed the Istep module first.
