Special Considerations for Microelectrodes

The input circuit is a combined voltage divider (attenuator for all frequencies) plus a low-pass filter with time constant R,C,. If we neglect C, for the moment, then 

If we consider a typical commercial preamplifier such as is found in a high-gain unit in an oscilloscope, Ri 3 x 10 Q. R. is usually about 10 Q. Thus 

With conventional amplifiers, the signal is attenuated by two orders of magnitude. Hence a signal of 100 mV at the tip of the micropipette would be only 3 mV at the input terminals of the preamplifier. Even when is 

If we now consider the effect of C, we note another problem. Typical values of Cj are from 2 to 5 pF. Thus for R, = 3 x 10 Q, the minimum time constant is 

We now see the reason for the general rule that glass microelectrodes are used for slowly varying processes (intercellular recording) and metal microelectrodes are used for rapid processes (neuron records). 

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