Rhythmic Activity of the Nervous System. 



279 



III. Rhythmic Responses to Simple Stimuli. 



The simple stimulus usually evokes a muscular response characterised by 

 contraction of one muscle and relaxation of its antagonist. 



Occasionally in the ipsilateral flexion-reflex the flexor contraction dies 

 down during the process of the stimulus and at the same time an extensor 

 contraction appears. At the cessation of stimulation there may he a flexor 

 rebound. In a similar manner the contralateral extension-reflex occasionally 

 exhibits contraction of both the antagonists. Tracings demonstrating these 

 points have already been published.* 



A third type of reaction is seen in the rhythmic responses which are 

 occasionally evoked. 



In the low spinal preparation these are rare but may be evoked after a 

 mechanical stimulation of the upper part of the lumbar portion of the 

 divided spinal cord. This often evokes a state of maintained extension, 

 and a flexion-reflex then super-added may produce a rhythmic reaction. 

 This resembles the " balanced " phase of the progression reaction obtained on 

 rapid division of the spinal cord.f 



So too may a reflex stimulus applied shortly after such a rapid division — 

 when the phenomenon is in the third phase of maintained extension. 



In the decerebrate preparation there is a much greater tendency to the 

 appearance of rhythmic reactions in response to simple stimuli. Tracings 

 in which this is evident have already been published.* These may appear 

 when, as was described above in the case of the low spinal preparation, there 

 is no such artificial background of maintained extension. But it must be 

 remembered that in the decerebrate preparation there is always the back- 

 ground of decerebrate rigidity — that is, a maintained extension, and there 

 is usually evident contraction of both antagonists. These rhythmic reactions 

 resemble the movements of progression and are reciprocal in the two muscles. 

 It happens very rarely that they resemble the movements of the scratch- 

 reflex. Both muscles contract together, and the " beats " seem to occur as 

 relaxations of these contractions. They occur alternately in the two muscles, 

 the point of maximum relaxation in one corresponding to the point of 

 maximum contraction in the other. 



Another rhythmic response in the simple reflex is the "rhythmic 

 rebound " already described by the author.* This occurs especially in the 

 decerebrate preparation and after cessation of a stimulus. The movements 

 undoubtedly resemble those of progression. 



* -Quart. Journ. of Exp. Physiol.,' 1911, vol. 4, p. 331. 

 t ' Eoy. Soc. Proc.,' 1911, B, vol. 84, p. 308. 



