470 



ANNALS NEW YORK ACADEMY OF SCIENCES 



charged from that region, despite the fact that the impulses originate 

 without the mediation of such potential gradients. The influence of 

 these gradients on the excitability of calcium-deficient nerve, as meas- 

 ured by changes in the frequency of impulse discharge, is in agreement 

 with the usual effects of current flow from an external source: depres- 

 sion at the anode and excitation at the cathode. 



Although the chemical excitation of nerve does not depend upon the 

 development of a steady current flow, the discharge of chemically 

 initiated impulses can be modified by an externally imposed potential 



§250 



o 

 u 



UJ 

 (O 



cm 

 h!200 



UJ 



-J 



a. 

 ZI50 



100 



i 



POLARIZE 



RECORD 



10 



20 



30 



40 



50 



SECONDS 



Figure 12. The frequency of response in a single fiber stimulated by topical application of 

 isotonic sodium citrate is reduced wlien the treated region is anodally polarized (first arrow). 

 When circuit is opened (second arrow), there is a transient increase in response. The current 

 passed into nerve in the treated region as shown in the diagram. 



gradient. This was first reported by Fessard,^^ who observed such an 

 effect in crab nerves which had been excited to activity by the applica- 

 tion of alcohol or sodium thiocyanate. To study this problem further, 

 we have passed a polarizing current through a calcium-depleted region 

 of frog nerve, during the period of constant frequency of discharge. 

 When the direction of current flow is such that it enters the nerve 

 fiber in the chemically activated region, the average frequency of im- 

 pulses is reduced for a brief time. As shown in figure 12, only a slight 

 depression continues after a few seconds. When the polarizing current 

 is terminated, there is a temporary increase in the frequency of im- 

 pulses from the chemically activated region, followed by a return to 

 the frequency that preceded the beginning of the current flow. 



