MICROCAPILLARY ELECTRODES FOR INTRACELLULAR RECORDING 



electrode itself is inserted into a narrow rubber tube at the end of the 

 Ringer agar bridge {Plate 35.3), which is filled with the Ringer's solution 

 just before use. In the arrangement employed by Nastuk and Hodgkin' 

 the wire was held within the agar by means of a wax seal, which thus also 

 reduced electrical leakage and prevented concentrated salt solution from 

 escaping. 



A more rigid form of holder made of plastic has been used, and consists 

 of a split Perspex tube widening at its lower end, with a sliding ring to 

 tighten the tube around the electrode. 



The electrode should be rinsed with distilled water before use in order 

 to remove external potassium chloride and increase the surface resistance. 



Screening of electrodes 



It has been found necessary to mount high resistance microelectrodes 

 close to the amplifier input in order to improve the high-frequency response 

 of the recording system. For this purpose, following the practice of Nastuk 

 and Hodgkin', electrodes are usually supported by the structure actually 

 holding the input valve, thus reducing the distance between them to a 

 matter of inches. The stray capacitance to earth of the wire connecting 

 an electrode to the amplifier will tend to act as a high-frequency shunt. 

 To reduce the stray capacitance still further the intervening link is shielded 

 with a screen connected to the cathode of a 'cathode follower' input 

 (Chapter 18). By this method the capacitance is effectively decreased, as 

 only a small fraction of the input voltage appears between the grid and 

 cathode, so reducing the flow of charge. 



A cathodal screen will also serve to shield the input from interference. 

 Electrodes of high resistance readily 'pick up' signals by capacitive 

 coupling and a screen may be essential. Thus when stimulating and recording 

 from the same muscle fibre Fatt and Katz^" had to interpose a cathodal 

 screen to shield the electrode almost to the level of the fluid contained in 

 the tissue bath in order to reduce the recorded artefact. 



A simple form of screen can be provided by a cathodally connected 

 metal tube around the electrode. A convenient form of this is shown in 

 Figure 35.7a. The electrode is attached to a glass tube containing Ringer- 

 agar or a silver wire, while the tube is tightly held by a polythene support 

 within a metal tube linked to the cathode. The cathodal screen is extended 

 by an inner sliding tube which while allowing access for connection to be 

 made can be brought down to shield the electrode. 



Screening can also be provided by a metal film deposited on to the glass 

 of the electrode surface (Marmont^^) and a form of such an electrode was 

 used by Frank and Fuortes^^. Pt.Cl. or silver can be deposited on to 

 electrodes before they are filled. Platinum was used by Marmont and a 

 platinum resinate has been found satisfactory (Liquid Platinum, Johnson 

 and Matthey F 104). When heated to 500°C a conducting film of platinum 

 is deposited. The fluid is applied to the surface of clean and dry electrodes 

 with a small brush or bristle, leaving a free margin of at least J in. at the 

 upper end of the stem and about 1 mm away from the tip. When the screen 

 is to be cathodal a layer of insulating varnish must be applied to that part 

 of the surface which will come into contact with tissue or bath fluid. Two 



545 



