OTHER ELECTRODES 



in animal tissue, since excess of the commonly used catalysts and accelerators 

 may be toxic. Polyesters have the advantage over most other resins in that 

 they can be both transparent and cold setting, and either hard and rigid or 

 soft and pliable when polymerized. 



A difficulty common to all electrodes is that of determining where the 

 tip has been during stimulation or recording. The smaller the electrode 

 the more difficult the task of tracing the track through the tissues during 

 histological reconstruction after an experiment. If the electrode has been 

 used in one place only the tissue around it should be fixed by perfusion 

 while the electrode is in situ as this will leave the electrode track clearly 

 visible under the microscope. This has the added advantage that an estimate 

 of the degree of shrinkage of the tissue during fixation can be made by 

 measuring the length of the electrode track before and after fixation. If 

 many tracks have been made and if it is necessary to follow each one some 

 sort of marker system is essential. In the case of relatively large electrodes 

 there will usually be a small amount of haemorrhage which will render 

 the tracks visible in sections, but in many cases, and particularly where 

 very small electrodes are used, no such convenient marking takes place. 

 Before insertion an electrode may be coated with some form of pigment 

 such as carbon, which will be left adhering to the track walls, but results 

 with this method are very variable and may be misleading as to the position 

 of the tip of the electrode. Small electrolytic lesions can be made around 

 the tip of some electrodes at the end of an experiment by passing a small 

 direct current through the electrode which will give a general indication 

 of the position of the point of the electrode in relation to surrounding 

 nerve tracks and nuclei. If steel wires are being used iron can be deposited 

 electrolytically in the tissue from the point of the electrode and this is 

 detected later by a histochemical Prussian blue reaction (Marshall^, Adrian 

 and Moruzzi^). 



After using microelectrodes it is frequently impossible to identify the 

 track or position of the tip in a histological reconstruction of the tissue. 

 It is possible that if such an electrode were coated before insertion with 

 some form of radioactive dust or colloid, sufficient radioactive material 

 might be transferred to the tissues to be detectable afterwards by auto- 

 radiographs. ^^C might well be a suitable 'marker' substance. 



METAL MICROELECTRODES 



Metal electrodes with very fine tips for recording from very small areas or 

 even from single cells have the advantage over glass pipette microelectrodes 

 of being relatively robust, and can be used for both acute and chronic 

 work. In all other respects however they are generally inferior to glass 

 electrodes for critical work. 



Steel microelectrodes can be made from ordinary darning needles but 

 are better produced from a good stainless steel wire such as Staybrite. 

 They should be used in conjunction with some form of mechanical mani- 

 pulator as the fine tip is very fragile, but it is possible to leave them imbedded 

 in brain tissue for long periods provided they are firmly fixed in position. 

 They can be made relatively easily with tips down to 10 fi diameter and 



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