MEASUREMENTS WITH MICROCAPILLARY ELECTRODES 



the response to a square pulse is improved. Effective time constants under 10 

 jbisec can be obtained. 



The most common method of minimizing distortion is to use electrodes of 

 the lowest resistance possible with the provision of cathodal screening. 



Graphical and mechanical methods of correcting for time constant distor- 

 tion — In order to examine and correct for distortion introduced by the 

 response time constant of the input the method described by Burch^^ and 

 by Lucas^* or the mechanism given by Rushton^^ can be used. The method 

 is based on the fact that the subtangent of an exponential curve is constant 



(a) (b) 



Figure 35.12 Graphical correction for time constant distortion 



at all parts of the curve: if the value of the subtangent be found for an 

 experimental arrangement, it can then be used to correct for distortion in a 

 recording. 



In Figure 35.12a the response to a voltage step is given by the exponential 

 curve BCD, and the level which it will finally attain is AD. At any point C, 

 the value CF is less than the full response FE by an amount CE. If the 

 tangent at C be drawn to cut AD, then EG is the subtangent. The value of 

 EG (in sec) is constant at all points of the curve. When EG is known, CE 

 can be determined. 



A correction can now be applied to an experimental curve (as in Figure 

 35.12b) obtained under the same recording conditions of resistance and 

 capacitance. At any point Q, a vertical line and a tangent are drawn. The 

 point E^ is marked on the vertical where the horizontal line E^G is equal to 

 the subtangent EG. The point E^ represents the true voltage at this time on 

 the response curve. The correction is performed at successive points on the 

 curve. 



Recording within a mass of active tissue — Records may be required from a 

 structure within a tissue mass, when activity is widespread and varies from 

 point to point. The electrode may have to pass through one active region in 

 order to study another beyond it. It is possible that activity may be recorded 

 by capacitive coupling through the walls of the shank of the electrode 

 (Hodgkin^). The degree of interference will depend on the relative voltages 

 of the source and interfering region, and on the dimensions of the latter. 



A simple experiment to illustrate this phenomenon can be performed by 

 applying a 10 kc sine wave to a bath of fluid while an electrode is introduced: 



555 



