SECTION VIII 

 POLARISATION PHENOMENA IN NERVE 



ELECTROTONIC CURRENT. If a constant current be passed through 

 a nerve fibre through the electrodes x and y, x being the anode and y the 

 cathode and the extrapolar portions of the nerve ab, cd be connected with 

 galvanometers, the needles of both are deflected, ^nd the direction of the 

 deflection shows the existence of a current in the extrapolar portions of 

 the nerve a to 6, and from c to d. 



We thus see that a passage of a current through a part of a nerve gives 



G' 62 



FIG. 124. Diagram showing electrotonic currents, p, polarising circuit ; 

 G 1 , G 2 , galvanometers. 



The galvanometers will indicate, before the passage of the polarising current, 

 the ordinary demarcation current of the nerve resulting from the cross-section at 

 the upper end. This current flows, in the outer circuit, from equator to cut end, 

 and therefore in the nerve fibre from a to 6, and from d to c. The effect of closing 

 the polarising current will be to increase the current of rest between a and 6, 

 and to diminish that between c and d. 



rise to a current flowing through a considerable portion of the nerve fibre 

 on each side of the polarising current and in the same direction. This 

 current is called the electrotonic current. It must not be confounded with 

 the current of action, which originates at one of the poles, only at make or 

 break of the current, and is transmitted thence in the form of a wave with 

 a measurable velocity (in the frog) of about 30 metres per second The 

 electrotonic current is developed instantaneously, and lasts the whole time 

 that the current is flowing through the nerve. Its production is dependent 



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