MICROCAPILLARY ELECTRODES FOR INTRACELLULAR RECORDING 



by placing the metal electrode away from the tissue, where the chemical 

 environment is not subject to variation. 



The glass microcapillary electrode serves as an electrolyte bridge between 

 the tissues and the electrode surface ; a fluid junction is established at the 

 tip within the tissues. When two different electrolyte solutions form a 

 junction a P.D. is established across it, due to the different mobilities and 

 concentration of ions present, so that the choice of the medium is important. 

 Thus at a sodium chloride junction the smaller Cl~ ion will tend to diffuse 

 faster than the Na+ ion, causing a P.D. to be established. A P.D. of over 

 3 mV is found between two solutions of sodium chloride with concentration 

 ratio 1 : 10. However with potassium chloride solutions, in which the K+ 

 and Cl~ ions have similar mobilities, the same concentration ratio gives a 

 P.D. of only 0-4 mV. Concentrated solutions of potassium chloride — 

 usually 3 M — are used to establish fluid junctions in order to reduce the 

 diffusion potential due to the other ions present. As a result of their high 

 concentration the junction currents are largely carried by K+ and Cl~ ions, 

 thus reducing the potential. When 3 M potassium chloride solutions are 

 employed the ionic content of small cells may be altered significantly by 

 the diffusion from the tip. It was calculated by Nastuk and Hodgkin that 

 6 X 10^^* mole per sec would diffuse from the tip of a sample electrode, 

 of 0-4 [JL outside diameter. This quantity would cause negligible change 

 in the constitution of a muscle fibre, but could appreciably alter the content 

 of a neurone (Eccles^). 



Capillary electrodes of such small dimensions possess a high resistance. 

 Nastuk and Hodgkin' found that the resistance of their electrodes filled 

 with isotonic potassium chloride was 5 to 7 times greater than when filled 

 with 3 M solution. It is advantageous therefore to use such concentrated 

 solutions to reduce the resistance to a minimum. Nevertheless, the intra- 

 cellular electrodes generally used have resistances not less than 5 MQ when 

 measured in Ringer solution, and range up to about 100 MQ. When 

 measuring potentials with such electrodes the input resistance of the 

 amplifier must be many times greater than that of the electrode. If i?i 

 represents the resistance of the electrode and tissue, R^, the input resistance 

 of the amplifier, and E the voltage applied, the potential recorded will be 

 E . RJiRi + ^2)- Thus even when the ratio Rj^ : R^ = I : 10 the recorded 

 potential will be £ X 0-91. It is therefore necessary that the input resistance 

 of the amplifier should be not less than about 10^° MD, in order to measure 

 the applied voltage. 



Construction of glass microcapillary electrodes 



Electrodes for intracellular studies by membrane puncture are usually 

 required to have tips of almost submicroscopic dimension, often less than 

 0-5 fi external diameter. As a result their true proportions can be decided 

 only with the electron microscope. Nevertheless the tips have to be suffici- 

 ently strong, in some cases, to be introduced through layers of tissue without 

 breaking. The most commonly used glass for such electrodes is a hard 

 borosilicate, usually either Pyrex or Phoenix, which has a high content of 

 silica with BgOg added to reduce the softening temperature, and the propor- 

 tion of alkali oxides is small. In addition to its strength borosilicate glass 



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