OTHER ELECTRODES 



An amalgamated zinc rod is placed in one arm of a £/ tube full of saturated 

 zinc sulphate, the other arm being blocked by a ball of china clay soaked 

 in saline. From the clay ball a cotton saline-soaked wick or a saline bridge 

 with a wick makes contact with the tissues. The zinc ions are very mobile 

 and will diffuse into the wick after about 24 hours so that new electrodes 

 should be made up daily, otherwise tissue damage will occur. 



SIMPLE METAL ELECTRODES 



Although theoretically non-polarizable electrodes should always be used, 

 in practice a great deal of work is carried out with simple wire electrodes. 

 In the case of platinum, owing to its very low gas over-voltage, the plain 

 metal is not polarized by very small short currents. Silver wire, as soon as 

 it comes into contact with chloride-containing biological fluids, rapidly 

 acquires a chloride layer and therefore is not easily polarized. Stainless 

 steel and tungsten are chiefly used as micro- and semi-microelectrodes for 

 recording action potentials from deep structures which can only be reached 

 by electrodes with good mechanical properties. These two metals can be 

 reduced electrolytically to very fine points comparable in size to glass 

 pipette microelectrodes. Pure gold is difficult to handle owing to its softness 

 but it may be used where an extremely inert metal electrode is necessary 

 to check the performance of electrodes made of more reactive metals. It is 

 particularly valuable in the measurement of oxidation-reduction potentials 

 and oxygen tension in tissues. 



The simplest forms of stimulating and recording electrodes consist of a 

 suitable metal wire of appropriate thickness, insulated to within a short 

 distance of the tip and fixed to some form of handle. Stimulating electrodes 

 may be made in the form of a straight wire, a wire circle round the structure 

 to be stimulated or, where large areas of stimulation are required, as a 

 plate. A convenient form of electrode for stimulating or recording from 

 whole nerve preparations can be made from a piece of Tufnol tubing fitted 

 at each end with a plug of Perspex or sealing wax through which run the 

 electrode wires. 



It is frequently convenient to use unipolar electrodes as these can be 

 made very small and there is only one possible stimulation point on the 

 relevant structure. This system of course requires that there shall be some- 

 where in the preparation an indifferent electrode which is usually of a 

 relatively large size giving a low current density. For acute experiments 

 on whole animals a metal cylinder inserted into the animal's rectum makes 

 a very efficient indifferent electrode, or a metal plate, usually of silver, can 

 be placed subcutaneously. This latter method is also useful for chronic 

 work. For measurement of oxidation-reduction potentials, pH and oxygen 

 tension reference electrodes of simple silver wires plated with silver chloride 

 and placed subcutaneously are quite satisfactory. 



IMPLANTED ELECTRODES 



Stimulation of, or recording from, deeply placed structures, in particular 

 brain tissue, requires some modification of the simple metal electrode. 

 The requirements for implanted electrodes are as follows : (a) they must be 



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