GROUPS OF GLASS MICROELECTRODES 



then with platinum. The properties of these electrodes are those of a 

 platinum surface electrode. 



GROUPS OF GLASS MICROELECTRODES 



The grouping of microelectrodes may be conveniently considered to be 

 of two types ; two or more electrodes at a single site or a cluster of individual 

 electrodes at a fixed distance apart. The first of these really consists of a 

 single structure with a number of channels. Two channel electrodes were 

 described by Renshaw, Forbes and Morrison*^ who employed them to 

 study potential gradients in the brain. Structures such as this have been 

 used to inject substances in the central nervous system (Kennard^°), to 

 inject and record (Coombs, Eccles and Fatt^^) and for surface application 

 to muscle cells (del Castillo and Katz^^). 



Construction of a two-channel capillary 



Two glass tubes 5-10 mm outside diameter and 1 ft. long are bound 

 together tightly with wire, rubber bands, or a cement (Picene). At one end 

 both tubes are then fused and closed. Heat is applied a few in. from the 



Figure 35.15 Stages in the lateral fusion of two glass tubes 



end and when soft both tubes are blown so that they stick together. By 

 repeated heating and blowing the tubes at this point can be fused together 

 by varying amounts. The least degree of fusion is that when the tubes 

 simply form a figure 8, while complete fusion gives a structure with oval 

 cross-section containing two D shaped compartments back to back {Figure 

 35.15). As the heating proceeds the glass tends to flow from the dividing 

 wall to the exterior, and if excessive may lead to a very thin and fragile 

 partition. When the required degree of fusion is achieved the glass is drawn 

 down to a diameter of 1-2 mm, while it is gently blown by mouth. Hence- 

 forth the tube can be treated as a single channel microcapillary and drawn 

 down further manually or by machine. Often however, as a result of 

 unequal stresses, the tube tends to flatten and occlude, and this can be 

 prevented by applying a slight air pressure (4 in. water) during the draw. 

 Another method of making a two-channel tube is to fuse a glass partition 

 (e.g. microscope slide) across the middle of a wide-bore tube. 



Electrical leakage — To prevent electrical leakage between the two channels 

 at the stem an angled tube can be cemented into each channel by means 

 of a low melting point wax such as Picene or dental wax. Alternatively 

 small portions of the unfused parent tubes can be left and slightly separated^^. 



Leakage between the two compartments in the shank or near the tip 

 may also cause difficulty. Leaks can be detected sometimes by comparing 



561 



