234 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



excite 



fiber. 



VI-10 



If the curve is graphically extrapolated until it becomes asymptotic to 

 a line paralleling the time axis for a value of current equal to B, then B 



is called the rheobase. 



The chronaxie is the time re- 

 quired for a strength 2B to 

 activity in the nerve 

 It is labeled T in Fig. 

 Thus, if it has taken 

 an infinite time to initiate an 

 impulse with 15 volts in frog 

 nerve, it would take 1.75 milli- 

 seconds, i.e., chronaxie time, 

 T, to initiate the impulse with 

 30 volts. 



Analytically an expression for 

 the chronaxie may be obtained 

 from the equation 



1 2 3 



Time in milliseconds (1/1000 sec) 



Fig. VI-10. A typical strength duration 

 curve. T is the chronaxie, the shortest 

 time that an intensity equal to IB will ini- 

 tiate the nerve excitation. 



t 



k 



log 



V - B 



When, as in Fig. VI-10, V = 2B, the chronaxie for direct current is 



m 1 , 0.693 



r = * log « 2 = — 



Thus the chronaxie decreases with increase in k, the speed of subsi- 

 dence of the excitatory process per unit state of excitation. 



Experimentally an increase in temperature (Blair [1935]) is accom- 

 panied by an increase in the magnitude of the rheobase B together with 

 an increase in k. If an increase in temperature implies an increase in 

 excitability, an increase in k and its expected theoretical decrease in 

 chronaxie must reflect an increase in the excitability of nerve fiber. 



A tissue with a small coefficient of subsidence (k) must, therefore, in 

 general have a slow rate of formation of the excitatory state or require a 

 relatively large amount of excitatory process to initiate an impulse, or 

 both. Tissue possessing small chronaxie can therefore be more easily 

 excited. 



In practice, in order to determine the chronaxie, all that is necessary 

 is to find the threshold stimulus or rheobase (B), to double this value, 

 and then, with a proper timing device, to find the shortest time of appli- 

 cation of the stimulus 2B which will cause the least observable response 

 of the tissue. The practical value of the chronaxie lies in the fact that 

 it has been used as a definite measure of the excitability of a tissue. 



