THE ROLE OF REFLEX INHIBITION 593 



3. "Identical Innervation" and "Reciprocal Innervation" 



A second office which reflex inhibition performs in the 

 interest of co-ordination regards antagonistic muscles. In 

 the anatomical arrangement of the musculature individual 

 muscles are frequently so placed as to exert their pull in 

 directly opposed directions. A striking example of this occurs 

 at hinge-joints such as the elbow and the knee, where extensor 

 and flexor muscles act on the same bony lever in exactly 

 opposite senses. Experiment shows that in simpler reflex 

 actions which produce flexion of the joint the stimulation of 

 the afferent nerve evokes at one and the same time a reflex 

 contraction of the flexor muscle and a reflex relaxation of the 

 extensor (fig. 3). Conversely a simple reflex contraction of 

 the extensor is accompanied by a reflex relaxation of the 

 flexor. In this case, as in all others we are discussing here, 

 the inhibition is not peripheral but central, that is, it has its 

 seat not in the muscle but in the nervous centre about the 

 starting-point of the final common path. The muscle relaxes 

 because the motor discharge from that centre is abated. 



The utility of this arrangement seems obvious. The same 

 cause which increases the contraction of the protagonist muscle 

 simultaneously diminishes the contraction of its antagonist. 

 And the two are dealt with in proportionate degree ; the 

 intense stimulus which evokes a strong contraction of pro- 

 tagonist produces likewise a full relaxation of its antagonist ; 

 the weak stimulus evokes weak contraction and correspondingly 

 weak inhibition. Expenditure of energy to merely overcome 

 the contraction of one muscle by the contraction of another, 

 or the motor activity of one nerve-centre by the motor activity 

 of another is thus avoided as waste. This " reciprocal innerva- 

 tion " of the antagonistic centres and through them of their 

 muscles ensures co-ordination by harmony. 



Instances of reciprocal innervation of antagonistic muscles 

 are easily obtained for study in the laboratory. For example, 

 in an appropriate reflex preparation it is easily seen during 

 reflex flexion at the elbow that while the flexor muscle is 

 thrown into contraction the extensor muscle becomes relaxed 

 and if before the stimulus any contraction were present in the 

 latter, that contraction disappears as the reflex takes place. 

 But such an instance does not fully exemplify reciprocal 



39 



