MICROCAPILLARY ELECTRODES FOR EXTRACELLULAR RECORDING 



The actual values will be determined by the proportion of the external 

 resistance to the total resistance of the circuit, so that small masses of 

 tissue tend to show greater interstitial voltages than larger ones. Values of 

 potential difference so measured within the central nervous system are 

 generally less than 1-2 mV, though larger potentials have been recorded. 



The most common arrangement used for recording is to measure the P.D. 

 between an electrode in the region of activity and an indifferent electrode 

 placed on inactive tissue, which can also be earthed: at the inactive point 

 the tissues may be merely quiescent or deliberately destroyed. The difference 

 in electric potential between the electrodes at any instant is that due to the 

 flow of current through the medium between them : its value is given by the 

 integral of the P.D. along a line between the two points. It can be regarded 

 as the sum of all the products IR taken over small finite distances along that 

 line. The studies of Lorente de No^^ have shown that the potential in a 

 volume of conducting fluid approximates to the second derivative of the 

 tissue membrane potential and is due to the transverse membrane current. 



The smallest kind of microelectrode is not often used for extraceflular 

 studies. The potential recorded from such electrodes tends to be dominated 

 by the activity of a single cell nearby, probably because it can approach 

 closely without distortion of tissues. However electrodes of 1-3 fx can be 

 used to examine the external field of individual cells. 



Resistance of electrodes 



When recording extracellularly the electrical noise of the electrode may be 

 comparable in voltage to that of the signal, giving a relatively low signal-to- 

 noise ratio. The noise due to the resistance R of an electrode is given 

 by 1-8 X 10~^ {R)^ volts R.M.S. for the audible range of frequencies. In 

 order to reduce the noise level the resistance should be kept at a minimum. 

 When electrodes of about 2 fi diameter are employed they can be filled with 

 concentrated sodium chloride solution (5-6 M). This is used in preference 

 to potassium chloride as the effect of any leakage from the tip will be less 

 harmful. Electrodes of this kind have resistances of the order of 1 MQ with 

 a R.M.S. noise level of approximately 20 fi\. 



When electrodes larger than approximately 3 microns tip diameter are 

 employed they are usually filled with isotonic solutions, either Ringer's or 

 sodium chloride, so that leakage of contents will not be deleterious. The 

 diameter of the tip is made as large as can be tolerated in order to reduce the 

 resistance. In addition a silver wire can be introduced as far down as possible 

 into the shank. Some further details of this are given later. 



Reduction of the electrode resistance will not effect 'noise' generated by 

 tissues. Such noise is recorded in a mass of tissue, as the central nervous 

 system, particularly when the electrode is close to a cell. Under some 

 conditions the tissues themselves may have a high resistance which may 

 generate noise in the circuit. 



Some methods of making microelectrodes for extracellular use 



Glass microcapillary electrodes for extraceflular use can be made by any 

 of the methods described for the smaller type. Mechanical methods however 

 are much more satisfactory than hand drawing. 



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