MEASUREMENTS WITH MICROCAPILLARY ELECTRODES 



as a result of storage. In concentrated potassium solution some of the ions 

 may perhaps enter the structure of the glass and it is possible that this may 

 be linked with the actual mechanism of the potential. The electrode proper- 

 ties of glass alter with constitution and increase to some degree with respect 

 to the ions contained. Entry of ions also makes glass more brittle and this 

 may be one cause of the short life of microelectrodes. Nastuk filled electrodes 

 by means of an alcohol replacement method, and long-term storage in 

 distilled water or alcohol may be advantageous. 



c; 50 



2 



(U 



o 



I 30 



<D 

 Q. 



E 



10 - 



J_ 



0-5 



2 U 



Frequency 



10 

 kc 



Figure 35.9 Plot of impedance of a microcapillary electrode against frequency: 

 electrode filled with 3M KCl. Drawn from data given by Tasaki^'' 



Measurements of rapid signals — The impedance of microcapillary electrodes 

 is frequency sensitive. Figure 35.9 shows the type of curve obtained when 

 the impedance is measured with the electrode tip immersed in Ringer's fluid 

 (Tasaki^^). As the frequency is increased to 5 kc impedance falls to approxi- 

 mately one tenth or less of that at d.c. The cause of this is that at higher 

 frequencies the microcapillary no longer behaves as a simple pore electrode. 

 The impedance of the glass wall falls and becomes comparable to that of the 

 electrolyte filled lumen. The wall of an electrode suitable for intracellular 

 puncture may be only 50 i^fx thick at the tip. It behaves as the dielectric of a 

 capacitor with concentric plates. A capacitance of this order however 

 extends up the shank of the electrode, because as the effective separation of 

 the plates increases with greater wall thickness the total area per unit length 

 also increases with increasing diameter. These properties of microelectrodes 

 were described and examined by Nastuk and Hodgkin'^. 



When a microelectrode is inserted into a cell within a bath of fluid or a 

 mass of tissue, the source of voltage — at the membrane — is connected between 

 the fluid at earth potential and the tip of the electrode. The resistance of the 

 electrolyte in the capillary, R, is connected in series with the source, which it 

 links to the input of the amplifier, while the capacitance of the wall of the 

 electrode, C, is placed in parallel with the source, between the lumen and the 

 earthed fluid {Figure 35.10a). This can be represented by the arrangement in b. 



If a voltage step, K^- {Figure 35.10b), is applied between the input and earth, 

 the output voltage, v^, across the capacitor will not attain a steady value 



551 



