I want to perform an elecrophysiological recording and staining of the neurons from visual cortex but facing problems due to D.C. shift during the experiment.
I m using sharp pipette with resistance of 70 M ohms to 90 M ohms for recording. I cover the open area partially with agar4% in D. water and then low melting point wax within the chamber. For the recording, I use axoclamp 2A and use narishige hydraulic micro-manipulator.
Thanks for help in advance!
Hi Mohit,
One important aspect is that you should NOT ONLY LOOK at your amplifier (or the Op-amp) BUT PAY SERIOUS ATTENTION to the input impedance of your amplifier circuit, or the op-amp, as the case may be. Since the pipettes you are using are also having very high resistance >70M Ohms, certainly it is needed that you have an amplifier (or op-amp) with higher (>100Mohm) input impedance. For such a high resistance, the input bias currents of nA range would certainly produce a voltage drop at the input of the amplifier and you have your gain to multiply this voltage to appear at the output.
Second reason might be eventually the potential arising due to electrochemical effects between the electrodes and biological fluids.
Hope this answer is helpful to you in sorting out the problem.
Wish you success in your experiment!
Is the problem that you can't zero the voltage trace?
Animal is grounded?
Thanks for helping me!
Yes sir, that is the problem. When i set it to zero after a minute it again get positive or negative by more than 10 mV. Thus cannot determine resting membrane potential of the neuron penetrated by pipette or whether penetrated the neuron or not until spikes are visible.
Make sure that the silver wires (both the pipette and the reference ones) are properly covered with chloride. Then check that you have no drifts if you put your pipette in a dish of saline.
I agree with Michael Okun; chloride your electrodes, set up your recording situation without the animal prep and determine whether you still get drift. If you do, then use the model cell circuit that comes with the Axoclamp amplifier to test for the same thing. If you get drift with the model cell, I'd suspect leakage current in the amplifier is causing the problem. You should be able to zero this out.
I thank for the suggestions, I am already using electrode covered by chloride (for pipette) , but just silver wire as reference. It work perfectly good as I always use it to set pipette resistance. I observer no drift with model cell at all and when I do rodent experiment , there is no drift.
Dear Mohit,
If the model cell simulates a patch clamp experiment (low access R) you may indeed not see a leak current (with standard glass pipette a dV/dt of 10mV /60s my reflect a leak of a few 10s nA. Checking your leak current with a capacitor and a voltmeter is easy and zeroing it cannot hurt. Cheers
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