GLASS MICROCAPILLARY ELECTRODES 



the voltage recorded at 5 mm depth may be double that at the surface. 

 However in order to examine the behaviour of an electrode under conditions 

 resembling those when recording, a method similar to that of Nastuk and 

 Hodgkin'^ can be used. This was done by passing an electrode through a 

 piece of agar-Ringer 3 mm thick, placed on a silver plate with a hole in the 

 centre. The tip of the electrode was immersed in Ringer's fluid below the 

 silver plate (Figure 35.13a). The potentials were compared when the tip 

 was given a voltage step with the agar earthed, and when the agar received 

 the pulse with the tip earthed. (The responses were as sketched in Figure 



Electrode 



Ringer-agar 



A- 



Response when voltage 

 ■applied to Ringer's fluid 



Response when voltage 

 -applied to Ringer-agar 



Silver plate 



Ringer 



(a) (b) 



Figure 35.13 Text for the recording of a signal tliroi'gh the wall capacitance 



35.13b; of exponential form and of similar time constant. The ratio of their 

 heights was approximately 20 : 1. The input capacitance of the cathode 

 follower was 3 pF under these conditions, but if this were smaller, a greater 

 potential would be recorded through the wall of the electrode. It is not 

 likely that this kind of interference would be significant when recording 

 large intracellular potentials, but if small extracellular voltages are examined 

 and a large region along the shank were to become active it is possible 

 for some significant interference to occur. This type of interaction may be 

 more serious with other types of electrode, such as varnished metal needles, 

 which may possess thinner insulation and greater wall capacitance. 



When it is necessary to reduce interference of such kind electrode screening 

 can be used. A metal such as platinum or silver can be deposited on to the 

 electrode down to a millimetre or less from the tip, with an additional layer 

 of insulating varnish : the method has been described earlier. It will usually 

 be necessary to use cathodal rather than earthed screening to minimize 

 capacitive distortion. 



MICROCAPILLARY ELECTRODES FOR EXTRACELLULAR 



RECORDING 



Saline conducting medium 



The voltages recorded in the conducting medium surrounding active tis- 

 sues are but a fraction of those generated across the tissue cell walls themselves. 



556 



