132 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



far as they go, would indicate that in the cerebrum and the cord 

 the nerve cells react with a certain rhythm. 



The Refractory Period of the Nerve Cell. The peculiar 

 rhythm of the active nerve cell just referred to hi the para- 

 graph above is explained most satisfactorily by an assumption 

 first used in connection with the rhythmical beat of the heart. As 

 will be explained more fully in the section on the physiology of 

 the heart, it has been found that after the contraction of the 

 heart begins it is unirritable to artificial stimuli, and that its 

 irritability is recovered during the period of rest, the diastole. 

 The heart has, therefore, alternate periods of irritability and 

 unirritability. The latter phase, the condition in which the heart 

 muscle will not respond to stimulation, is known as the refractory 

 period, or refractory phase. Inasmuch as it appears immediately 

 after the contraction, it is usually explained as being due to some 

 product of the chemical reaction causing the contraction, in fact, 

 a state of temporary fatigue. A similar conception has been applied 

 to the nerve cell. The experiments cited in the preceding para- 

 graph would indicate that, after the discharge of an impulse, the 

 cell falls into a refractory phase for a period of time lasting about 

 0.1 sec. The idea is a convenient one, although we have no explana- 

 tion of what is the immediate cause of this temporary loss of irrita- 

 bility. Reasoning from analogy with the muscle, we might suppose 

 that in this case also it is due to some product of the chemical reaction 

 that is assumed to underlie nervous activity. Using this terminol- 

 ogy, it is probable that the cells in different parts of the nervous 

 system may have different refractory periods. In the case of the 

 normal nerve fiber (see p. 112) it will be recalled that the refractory 

 period is very brief, say, 0.006 sec., but varies with the condi- 

 tion of the fiber, since in the narcotized fiber it may be as much 

 as 0.1 sec. 



