MICROCAPILLARY ELECTRODES FOR INTRACELLULAR RECORDING 



4 [JL, and further it appears from electromicrographs of Nastuk and Hodgkin'' 

 that this was also true at the very tip (Hodgkin^). 



Mechanically produced microelectrodes — Several methods varying in 

 degree of mechanization have been employed to make the production of 

 electrodes less subject to variation in timing, temperature, strength and 

 direction of pull. All these are difficult to maintain constant when drawing 

 manually. 



The simplest mechanical aid employs gravitational force for extension, 

 and the electrodes are drawn in one or more stages. In the single-stage 

 method the glass tube, 1-2 mm external diameter, is clamped in a vertical 

 position between two rubber cushions and the coil, when used, lifted on a 

 vertical slide to surround the tube. A weight is attached either by means of 

 a clamp with rubber jaws (Templeton^") or more reliably to a hook formed 

 from the end of the tube. Weights applied range up to 500 g, depending 

 on the thickness of the glass. Heat is applied either with a gas flame or 

 an electrically heated coil or wire. The gas jet can be made from a No. 20 

 Record needle or better from a glass tube with narrow tip, brought up by 

 hand. The electrically heated coil is preferable as it can more readily be 

 controlled. Coils of two or three turns can be used, between 3-5 mm 

 in diameter and 3 mm long. Weale^^ used oxynickelnichrome wire of 

 2 n/in., with power from a 6 V battery in series with a rheostat; however 

 wire between 0-3-0-5 mm diameter and a heavy duty variable transformer 

 can also be used. When the tube has extended some 5 to 7 mm heat is 

 turned off". 



With this method capillary tubes between 3-30 [x can readily be made 

 but greater difficulty is encountered in producing electrodes of less than 1 fi. 

 The one advantage of this method as against hand drawing is that the 

 electrode shank can be made collinear with the parent stem. 



Electrodes drawn in several stages — Gravitational pull can be used to 

 draw microelectrodes in several stages under controlled conditions. A 

 method employing the de Fonbrume Microforge has been described by 

 Brock, Coombs and Eccles^^ in which considerable care is taken in the 

 manufacture of each electrode. This method is capable of yielding a much 

 higher proportion of successful attempts than the single-stage draw. Pyrex 

 tubing of 4-5 mm outside diameter is pulled by hand down to 0-15 mm 

 with a length of at least 1 cm. A bead is formed at the tip and the tube 

 mounted vertically with the tip down. The succeeding stages are observed 

 with a microscope. The tip is coated with wax (Chatterton's compound 

 or Picene) and a 300 mg weight attached by means of a thread and small 

 hook. Using the electrically heated loop of wire the tube is drawn down 

 to 50 ji diameter, when the weight is reduced to 30 mg and the final draw 

 accomplished. The following modification can be used: a tube is first 

 drawn by hand down to about 0-2 mm and as before a bead formed and 

 thread with hook attached. A weight of 0-5-1 g is suspended and by means 

 of a heated wire loop the tube is drawn under a dissecting microscope. 

 The heating coil is mounted on a glass slide coated on one side with oil or 

 Vaseline and held against a block of Perspex between thumb and fingers. 

 Movements in the vertical plane are performed by the thumb holding the 

 glass slide while those in the horizontal plane are carried out by sliding 



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