472 



ANNALS NEW YORK ACADEMY OF SCIENCES 



also reveals the significant fact that there is, apparently, no minimal 

 current strength that must be exceeded before the rhythmic discharge 

 from the chemically sensitized nerve is modified. Any change in the 

 direct current, no matter how small, flowing across the membrane of 

 these chemically modified nerves, alters the rhythmic activity of the 

 nerve and is reflected in the altered frequency of the propagated im- 

 pulses. This is in contrast to the limiting threshold of current 

 strength necessary for the excitation of a conducted impulse in a nerve 

 with normal calcium content. 



We have already said that potassium chloride causes a decrease in the 

 frequency of the impulses developed in a calcium-deficient portion of 



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8 



Ul 

 (0 



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 bJ 

 Q. 



m 



bJ 

 O) 



-I 



2 



40 60 



CONCENTRATION 



Figure 14. Frequency of response due to action of isotonic solutions containing various propor- 

 tions of sodium chloride and sodium citrate. Concentration in terms of per cent of isotonic solu- 

 tion of sodium citrate. 



a nerve. This is the effect of potassium chloride when its action has 

 reached a steady state, but it is preceded by a transient increase in the 

 number of impulses discharged per second. This stimulating action of 

 potassium chloride also occurs in nerves with normal calcium content, 

 but there it is of even shorter duration. Both the maximum frequency 

 and the duration of the impulse discharge, caused by an increase of 

 potassium chloride, are greater, the lower the calcium content of the 

 fluid bathing the nerve. -° Thus, the removal of calcium from a nerve 

 makes it more sensitive to the transient stimulating action of a moder- 

 ate increase in the concentration of potassium chloride. 



This increased sensitivity of calcium-deficient nerve to other chem- 

 ical agents is further revealed in the experiments shown in figure 14. 

 There, the nerve was made active by bathing a portion of it in isotonic 



