MEASUREMENTS WITH MICROCAPILLARY ELECTRODES 



Thus when a brief signal is appHed to the tip of an electrode the full 

 voltage may not be recorded at the output unless the capacitance has had 

 time to be charged. The time constant is a measure of the speed of the 

 response, and can be used to construct the frequency response curve to 

 steady-state signals. At any frequency the fraction of the voltage measured 

 will be determined by the proportion of the reactance of the capacitor to the 

 total impedance (Figure 3.24), 



i.e. v^=Vi. (— ;•) — 



where Xc is capacitive reactance and Z is total impedance. 



As described in Chapter 3, the frequency at which the response is reduced 

 by 3 dB's or to 71 per cent is obtained from 



/ • 



liT.C.R. 



where /is the required frequency in c/s. 



It can be seen by substitution that when the time constant is 50 /^sec the 

 response will be reduced to 71 per cent at 3-2 kc, and with 100 /isec at 1-6 kc. 

 In order to reduce the shunt capacitance of the recording system the method 

 of cathodal rather than earthed screening can be used. 



To examine the properties of an electrode under conditions similar to those 

 when recording from within a cell, Nastuk and Hodgkin' devised an arrange- 

 ment which simulates this closely. A potential is applied to the tip of an 

 electrode while a length of shank nearby is surrounded by fluid at earth 

 potential. An electrode is passed through a globule of saline supported on 

 a wire ring connected to earth. The tip just touches the surface of the fluid 

 in a bath beneath, and the voltage source is connected between earth and 

 the bath. The capacitance of the electrode wall in the fluid was about 1 pF 

 per mm of length. The capacitance of the shank in air is a small fraction of 

 the value in saline, as effectively the wall capacitance is in series with the air 

 capacitance to earth. However with cathodal screening the total capacitance 

 usually ranges between 3-10 pF. Of this total 1-3 pF may be due to the 

 cathode-follower input stage while the rest is principally that of the electrode. 



Microelectrodes used in a bath of tissue fluid with cathodal screening 

 generally have a time constant of 30-100 //sec. The degree of distortion 

 introduced depends on the activity studied and will vary with temperature 

 and other factors. For example, published data for the action potential of 

 frogs muscle show that the height may be reduced approximately 1-8 per 

 cent under these conditions. At low temperatures greater time constants 

 can be used. The greatest effects of time constant distortion are on the rate 

 of rise of a potential. Generally a time constant of 30 /zsec will introduce 

 little distortion in recording frog muscle potentials. It has been estimated 

 that more rapid events, such as the action potential of some mammalian 

 neurones — 0-5 //sec duration, will require a time constant of 15 //sec to avoid 

 appreciable distortion (Woodbury^^) 



The time constant can be reduced by decrease in both the input resistance 

 and capacitance. It is difficult to decrease the resistance without increasing 

 the dimensions of the electrode. Very small tips may be required for work 



553 



