840 



CENTRAL NERVOUS SYSTEM 



appropriate groups of muscles. Such continuous activity on the parl 

 of nerve cells might be due to either (1) the passage of a series of dis- 

 tinct impulses along the nerve fiber, or (2) to a continuously main- 

 tained activity. Experiment has shown that the first conception is th< 

 correct one. Two impulses set up in rapid succession in the nerv( 

 fibers of a nerve muscle preparation from the frog may cause a greater 

 contraction in the muscle than a single impulse. If the interval betweei 

 these stimuli is decreased to less than 0.0025 of a second the effect due 

 to the second stimulus disappears and it has evidently failed to initi- 

 ate a second nerve impulse. This result occurs whether the two stimuli 

 are applied to the same, or to different parts of the nerve fiber, showing 

 that it is not due to the impossibility of setting up the process of ex- 



100 



50 



01 -02 



Time since previous stimulus (seconds) 



03 



Fig. 210. The recovery of excitability in the nerve fiber after the passage of a nerve impulse. 

 After a brief time in which no second excitation is effective, the excitability gradually returns to 

 normal, and then becomes temporarily greater than normal. The changes in conductivity following 

 the passage of a nerve impulse over the nerve follow a similar course. (From Adrain and Lucas.) 



citation in one place twice in such rapid succession, but that the nerve 

 fiber is incapable of conducting a second nerve impulse until a sufficient 

 period of recovery has intervened. Moreover by cooling the nerve be- 

 tween the point of stimulation and the muscle, without affecting the 

 temperature in the place at which the process of excitation occurs, the 

 period of recovery may be increased three fold, because at a low tem- 

 perature it takes longer for the nerve fiber to regain its power to con- 

 duct. This period of recovery during which a second impulse cannot 

 pass along a nerve fiber is called the refractory period of conduction. It 

 is quite analogous to the refractory period which occurs in the heart 

 (page 179) and like it imparts certain characteristics to the tissue in 

 which it occurs. 



