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ANNALS NEW YORK ACADEMY OF SCIENCES 



fiber. Obviously, these properties will be modified by the chemical 

 excitant, qualitatively by the nature of the chemical change, and to a 

 degree that is determined by the amount of substance added to, or re- 

 moved from, the nerve structure. Therefore, it is not surprising to 

 find that the number of impulses discharged per second from the treated 

 region depends upon the concentration of the calcium ion, as shown in 

 FIGURE 4. 



Figure 4. The average frequency of impulse;^ recorded from a single fiber depends UDon the 

 concentration of calcium ions. Upper record, concentration of Ca* is 0.3 mM ; middle, 0.1 mM ; 

 lower, no calcium. In this experiment, the 0.1 mM calcium was applied first, then tiie 0.3 mM, 

 and finally the calcium-free solution. Time in 1/5 seconds. 



The frequency of impulses initiated by a given reduction of calcium 

 ion concentration, or by other chemically stimulating media, also de- 

 pends upon the previous duration of the chemical action. The electric 

 threshold begins to fall almost at once after the application of the 

 solution (figure 5). Further time is required for changes in the intra- 

 cellular processes which must precede the development of conducted 

 impulses. Indeed, the first impulse may not develop for some minutes, 

 and then at a time when the threshold has fallen to zero. This same 

 gradual loss of stability, continuing further, is manifest in the pro- 

 gressive increase in the average frequency of impulses. 



When calcium is removed from a nerve, by diff'usion into a solution 

 containing less than the normal amount of calcium chloride, the dis- 



