MICROCAPILLARY ELECTRODES FOR INTRACELLULAR RECORDING 



shank of the electrodes. The majority of the figures quoted, however 

 have shown a much sharper taper at the tip, and it seems that electrodes 

 with gradients not greater than 1/10 are usable for most purposes {Figure 

 35.5): thus when such electrodes are introduced a distance of 10 // the 



Figure 35.5 Illustration of tapers J 1 10 and lj5 



actual puncture will be 1 to 1-5 ^ diameter. This does not seem to 

 inconvenience some relatively small cells such as motor neurones which 

 apparently tolerate the inscition of electrodes with tips of 1 /^ (Eccles^). 

 Electrodes with gradients of 1/10 are more readily produced than those 

 with lesser tapers, and the typical electrode shown by Alexander and 

 Nastuk^^ produced by their instrument is of this kind. The shorter tipped 

 electrodes will have a lower electrical resistance, which will minimize errors 

 in measuring both voltage levels and rapid signals. 



The strength of the electrode may be an important factor in some kinds 

 of experiment, such as the study of cells within ganglia (Eccles^^). It is 

 possible that the strength may be improved by the use of thick-walled 

 tubing. When studying cells within the depths of a tissue it may be necessary 

 to grade the shape of electrodes in several steps as, for example, by successive 

 drawing under microscopic control (Brock, Coombs and Eccles^^). For 

 the purpose of electrical measurements errors will be minimized if the 

 electrodes have as low a resistance as possible. 



Methods of measuring the resistance of microelectrodes 



The resistance of an electrode immersed in a conducting medium is 

 readily determined by measuring the effect of a known resistance introduced 

 into the circuit. Three such methods are shown in Figure 35.6. The experi- 

 mental arrangement illustrated in (a) is suitable when isolated tissues are 

 studied in a fluid bath which is insulated from earth. The deflection is 

 noted when a voltage (=^10 mV) from a source of low output impedance 

 is applied between the bath electrode and earth, and determined again 

 with a known resistance R connected between the input lead and earth. 



If Vq is the initial deflection and V^ the deflection with R in the circuit, 

 the electrode resistance is given by 



The approximate value of the electrode resistance can be determined by 

 connecting a bank of resistances in series with a switch and finding that 

 value which reduces the initial deflection to half. The resistances used can 

 be 1,2, 5, 10,20,50,70, 100 MQ. 



In the method illustrated in Figure 35.6b a voltage is applied between 

 earth and the input lead and the effect of placing a known resistance R in 

 series with the input determined. In this case 



> R ^ R — 



543 



