GLASS MICROCAPILLARY ELECTRODES 



The earliest method of filHng microelectrodes was to boil them for 30 

 minutes or more in 3 M potassium chloride ; however, this may cause 

 damage to the fine tips. A more efficient and less destructive process Is to 

 boil under the reduced pressure of a water pump. The solution is heated 

 and the flask evacuated for 5-10 minutes. The bubbles are fine and the 

 process takes less time than boiling at atmospheric pressure. Heating can 

 be avoided simply by evacuating the flask containing the filling solution 

 and electrodes, clamping the inlet and leaving for some hours. 



Other slightly more elaborate methods have also been described, and 

 may have to be used in some cases. In one of these (Tasaki, Policy and 

 Orego^^) the capillaries are first gently boiled in alcohol at reduced pressure. 

 They are then placed in distilled water for a few minutes and finally trans- 

 ferred to 3 M potassium chloride which may also be evacuated. The method 

 described by Caldwell and Downing^^ is probably the most gentle of all. 

 The electrodes are allowed to fifl with distilled water by capillarity and 

 the large air bubbles in the stem eventually dislodged with a probe. When 

 filled they are transferred to, and left in, 3 M potassium chloride. 



A method of producing pre-filled microelectrodes has been described by 

 Kao^^. Before drawing the electrodes the tubes are fiUed with 3 M potassium 

 chloride and the ends left immersed in this solution during the drawing 

 process : successfully filled electrodes result. A variation of this method is 

 described by Crain^*. 



Once filled with electrolyte media, either physiological fluid or 3 M 

 potassium chloride, the useful life of stored electrodes is about one week. 

 Deposits and growths appear, and often the tips are found broken. The 

 best way of preserving is possibly in boiled distilled water or alcohol. Among 

 the methods which have been employed in attempts to prolong the useful 

 life of microelectrodes are storage in the dark in a refrigerator, and an 

 addition of small quantities of bacteriostatics or antiseptic such as methylene 

 blue, which dissolves very slightly in 3 M potassium chloride. Their 

 efficacy is doubtful. It is advisable to reduce the number of examinations 

 and the transfer of electrodes to a minimum, since mechanical agitation, 

 surface tension forces, and drying can cause damage. 



Dimensions of electrodes required for intracellular observations 



The size and shape of the microelectrodes used for intracellular work 

 vary and a compromise is often reached between three factors ; the nature 

 of the tissue, the physical properties of the electrodes, and those electrodes 

 which can be made. 



The size of cells varies considerably with the corresponding variation 

 in the size of acceptable electrodes. Crab muscle fibres continue to function 

 with electrodes over 50 [jl in diameter inserted, but at the other end of the 

 scale small neurones 10 to 20 /* in diameter, such as are found in the lateral 

 geniculate body, the spinal cord, and peripheral nerve, do not react well 

 to penetration. It is possible that the leakage around an electrode is 

 insignificant for large cells but may cause small cells to depolarize and lose 

 much of their contents (Nastuk and Hodgkin'^; Tasaki, Policy and Orego^^). 



To minimize damage, electrodes of less than 0-5 /^ with a taper of 1/60 

 were used by Ling and Gerard^. This taper probably refers to the 



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