BRINK AND OTHERS: CHEMICAL EXCITATION OF NERVE 471 



If the current is in the opposite direction, so that it flows out of the 

 fiber in the calcium-deficient region, the sequence of frequency changes 

 is reversed. Then, as the current starts, there is a transient increase 

 of frequency, followed by a temporary depression when the current is 

 interrupted. 



Still obscure are the cellular mechanisms which account for an in- 

 crease in the frequency of impulses from calcium-deficient nerve, when 

 positively charged ions move outward across the fiber interface, or for 



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Figure 13. The maximum change in frequency (impulses per second) in a chemically excited 

 frog a fiber is proportional to the magnitude of polarizing current (in microamperes). Nerve ex- 

 cited as described in figure 12. 



Positive current : anode in the treated region ; negative current : cathode in treated region. 



a decrease of frequency, when the ionic movement is reversed. The 

 effects are, however, consistent with the long-established fact that a 

 cathodally polarized region of nerve is more irritable, while anodally 

 polarized nerve is less irritable. Accordingly, the effects are also in 

 agreement with the view that an agent which reduces the stability of 

 the nerve structure increases the frequency of chemically induced 

 activity. 



The magnitude of the transient increase or decrease in the frequency 

 of impulses, caused by the passage of an electric current through a 

 chemically activated nerve, depends upon the strength of current. Fig- 

 ure 13 shows that there is, indeed, a linear relationship between the 

 current strength and the maximal increase or decrease of impulse fre- 

 quency caused by the current flow, within certain limits. This figure 



