CHRONAXIE 233 



Rheobase 



The quantity B = kb/K is technically known as the rheobase (rheo = 

 to flow). If the equation for ^ is rewritten 



KV 

 V = ^ (1 ~ e~ kt >) 



the excitatory process p increases exponentially with time to attain a 

 maximum limiting value, when t = <x> , which is KV/k. 



If B is the least voltage which will stimulate when the excitatory proc- 

 ess has stopped growing, then 



Pmax — — 



k 



Thus 6 is the magnitude of the local excitatory process necessary for the 

 initiation of an impulse. Under these conditions, the so-called rheobase 

 (Fig. VI-10) is 



kb 

 B ~K 



that is, the rheobase is k times the amount of local excitatory process 

 necessary for adequacy, divided by the growth per second of the excita- 

 tory process per unit stimulus (Blair [1935]). Since this method necessi- 

 tates waiting a very long time, the chronaxie (2B) is used instead of B 

 to test for the threshold to initiate a nerve impulse. 



Chronaxie 



It has been found experimentally that, if the time of application of a 

 stimulus is increased, the strength of the stimulus must be decreased in 

 order to obtain the threshold quantity to initiate the nerve impulse. 



Lapicque's [1926] data seemed to verify the assumption that the time 

 of application must be inversely proportional to the strength of the stimu- 

 lus in order to initiate the nerve impulse. Under these circumstances 

 the strength-duration curves would be equilateral hyperbolas of the 

 form I X t = constant when a current of intensity / is used. In terms 

 of Blair's theory this statement describes the conditions only very 

 approximately, since the expected relation is an exponential one. 



In Fig. VI-10 are depicted more accurate data which show successive 

 smaller stimuli of direct current that must be applied to initiate the 

 nerve impulse during successively longer periods of time. This strength- 

 duration relation is an exponential curve, as predicted by the theoretical 

 analysis. 



