MICROCAPILLARY ELECTRODES FOR EXTRACELLULAR RECORDING 



high frequency response when compared with electrolyte-filled microelectrodes. 

 Their properties at zero frequency however are not as stable as the electrode 

 surface is actually in contact with the tissues. 



The resistance of the metal conductor of such electrodes is small, but that 

 of the electrolyte surrounding the tip is appreciable in comparison. The 

 resistance of a 2 /z diameter wire of the commonly used metals such as silver, 

 copper, tungsten, platinum, iron, tin and lead lies approximately between 

 5 and 70 D. per mm. The resistivity of Ringer's fluids is of the order 10^ Q. cm. 



/?. 



Figure 35.14 Circuit of a metal-filled microelectrode with tip immersed in a 



saline medium 



/?j electrode resistance 



J?yj resistance of Ringer's fluid at tip 



Rl amplifier input resistance 



C, capacitance of electrode surface 



Cg capacitance to earth of electrode insulation 



C,- amplifier input capacitance 



but the dimension of the conducting path at the electrode approaches the 

 size of the tip itself, and may be greater than 10^ Q. for a tip 2 [x diameter 

 (Svaetichin"*"). The impedance of the electrode surface itself is frequency 

 dependent, decreasing as the frequency increases. Electrodes of this kind 

 generally behave as if they possessed a capacitance in series with the input 

 {Figure 35.14). 



Polarization — Metal-filled electrodes show polarization in varying degree 

 when current is passed through them (Chapter 34). A polarizable electrode 

 changes its potential with respect to the fluid medium in a manner not 

 accounted for by Ohm's law when current passes, while a non-polarizable 

 one does not do so. The polarization can be determined by measuring the 

 potential difference between the electrode in question and a third indifferent 

 electrode (Rothchild^^). 



Polarizable electrodes behave as if they had a capacitance in series with 

 the input, and this capacitance may seem to be large. The tip of a 30 ^a 

 platinized platinum wire may appear to have a capacitance of 1 /iF when a 

 direct current of 10~^ A is suddenly applied through a high resistance. This 

 is due to the formation of an electrical double-layer at the surface and to 

 electrochemical mechanisms which take some time to reach a steady state. 

 The double-layer can have a capacitance of approximately 20-40 /^F/cm^ 



559 



