GLASS MICROCAPILLARY ELECTRODES 



either in the cathode circuit of an input cathode follower or between the 

 output of the latter and the input to the next stage. These methods are 

 described in an earlier chapter. 



Error in measurements of potential levels with microcapillary electrodes — It 

 has been found that submicroscopic capillary electrodes may behave as if 

 they were semi-permeable to certain ions; thus such electrodes sometimes 

 give a potential which is a function both of the electrical potential present 

 and of the concentration and type of ions at the tip. Observations of Nastuk^^, 

 del Castillo and Katz^'* and Adrian^^ have shown that such electrodes filled 

 with 3 M potassium chloride can commonly show potentials of 30 mV or 

 more when immersed in Ringer's fluid, the inside being usually negative to 

 the outside. Adrian^^ calculated that under these conditions the liquid 

 junction potential at the tip between 3 M potassium chloride and Ringer's 

 fluid should be only about 2 mV. He found that the potential depended on 

 the concentration and type of ions in the external medium. The observations 

 of del Castillo and Katz^* and Adrian^^ have demonstrated that the potential 

 at the tip is reduced or abolished when the resistance falls, consequent on 

 breaking the tip, and, conversely, rises when there is an increase of resistance. 

 The potential disappears when the composition of the external and internal 

 medium is the same. This behaviour of electrodes has been ascribed by these 

 authors to a blockage in the tip, possibly by protein, causing a reduction of 

 the pore size. Electrodes in this condition may allow some ions to pass more 

 readily than others, depending on the charge on the surface of the glass, and 

 on the size and charge of the ions. Another mechanism which can be sugges- 

 ted is that the glass wall of the electrode behaves as an ion electrode for 

 alkali ions similar to a pH electrode for hydrogen ions. Such an electrode 

 will have a high resistance, which may be short-circuited by the electrolyte 

 of the pore. When the resistance of the pore is increased the short-circuiting 

 is less effective and the measured potential may increase. Electrodes which 

 show a potential at the tip generally have a high resistance. 



The result of this type of behaviour is that measurements of potential 

 difference may show considerable error if the composition of the medium 

 around the tip varies. The magnitude of the variation will depend on the 

 actual electrode used, but with microelectrodes of resistance greater than 

 10 MQ the resting membrane potential of muscle cells can be reduced by 

 about 30 mV (Adrian^^) or occasionally increased. Sometimes electrodes 

 may show such properties only intermittently, and del Castillo and Katz^^ 

 describe changes in potential and resistance while actually inside a muscle 

 fibre. They also found that electrodes with such potentials behaved as 

 rectifiers, allowing current to pass more readily inwards than outwards. 



To avoid errors due to such properties of the electrode tip it is necessary to 

 measure the electrode resistance and potential regularly. Adrian used only 

 electrodes with a potential of less than 5 mV but with resistance greater than 

 5 MlQ. These values can be determined during an experiment, when an 

 increase in resistance may be indicated by an increased noise level and greater 

 susceptibility to electrical interference. 



The method employed for filhng and storing electrodes may be a factor 

 inducing these properties. Nastuk^^ found that boiling in 3 M potassium 

 chloride caused electrodes to develop such potentials, this may even happen 



550 



