294 



Dr. A. Forbes. Reflex Rhythm [Apr. 11, 



(b) Oscillations Resulting from a Single Stimulus. 



In several cases I have observed oscillations during the application of an 

 inhibitory (ipsilateral) stimulus alone. The significance of these I take to be 

 dependent on the fact reported by Sherrington and Sowton,* that by 

 modifying the character of electrical stimulus applied to an ipsilateral 

 afferent nerve, the usual inhibitory reflex effect in the extensor muscle may 

 be replaced by excitation. They find that weak or inabrupt stimuli tend to 

 produce the excitatory effect, while stronger and more abrupt stimuli 

 inhibit. Evidently, the stimulus applied to the ipsilateral nerve exerts 

 both an inhibitory and an excitatory influence on the extensor moto- 

 neurones, and it depends on the character of the stimulus, perhaps also 

 on the condition of the preparation, which influence dominates. 



The authors propose an explanation of their findings, on the assumption 

 that there are two sets of afferent nerve fibres responding in different 

 degrees to different kinds of stimuli. I have already proposed an alternative 

 explanation,! namely, that there is only one set of afferent fibres, but that 

 they branch on reaching the cord, and are connected with two centres of 

 opposite influences. In support of this is the well-known fact that most 

 afferent fibres do branch on reaching the cord, also the fact that there are 

 known to be two centres of opposite influence upon extensor muscles. 

 There is a local centre in the spinal cord through which an afferent nerve 

 may inhibit the extensors, and there is a centre in the region between the 

 hind edge of the pons and the corpora quadrigernina which is essential to 

 the maintenance of decerebrate rigidity, and is thus shown to exert an 

 excitatory influence upon the extensor. 



Altogether, four possible explanations may be framed for the presence of 

 the opposed influences resulting from the single stimulus, as follows : — 



A. A single set of fibres acts upon a single centre, but exerts a different 

 influence upon that centre under one kind of stimulus from what it does 

 under another kind. 



B. A single set of fibres branches, and acts on two reflex centres of 

 opposite influences. 



C. Two sets of afferent fibres from qualitatively different receptors, and 

 themselves differently sensitive, act with opposite effects upon a single 

 centre. 



D. Two sets of fibres, of qualitatively different sensitivity, act on two 

 centres of opposite influence, and each set of fibres passes to its proper 

 centre. 



* ' Koy. Soc. Proc.,' B, vol. 83, p. 435. 



+ Loc. cit. 



