1909.] 



Innervation of Antagonistic Muscles. 



265 



in the antagonistic muscles obvious contraction concurrently in both. 

 But though both the muscles are contracting the contraction of each is 

 seen to be less than it would be were there not then in action the reflex 

 which produces contraction of its antagonist. And this is so because 

 some reflex inhibition of the motoneurones of each accompanies the reflex 

 ■excitation of the motoneurones of the other. Thus the contraction of 

 semitendinosus is less than it would be were not the contralateral afferent 

 being concurrently stimulated, and the contraction of vastocrureus is less 

 than if the ipselateral afferent were not concurrently stimulated. As to 

 which of the two antagonists exhibits the greater contraction, that feature 

 is determined by the relative intensity of the stimulation of the two 

 .antagonistic afferents. To obtain the type 3 of result the effects of the two 

 ■opposed afferents have, of course, to be sufficiently nearly balanced, and the 

 intensity of the one must not wholly overbalance that of the other. Within 

 these limits, however, all degrees of grading of combination of the two seem 

 ■obtainable by simple adjustment of the relative intensity of the stimulation 

 of the opposed nerves^ In short, a delicate grading of the intensity of 

 -contraction is effected by bringing into play a double reciprocal innervation. 

 The result of the double reciprocal action is that each of the two muscles 

 •displays simultaneously with its antagonist an algebraic summation of reflex 

 inhibition and reflex excitation. In other words, antagonistic reflexes add 

 themselves algebraically with unlike signs. 



To obtain this result with spinal reflexes, e.g. in the decapitated mammal, 

 •or with reflexes produced by stimulation of afferent nerves in the decerebrate 

 .animal, it is necessary to stimulate synchronously two or more afferents of 

 opposed effect as regards the antagonist muscles selected. Stimulation of 

 ■ either of such opposed afferents taken alone never suffices, in my experience, 

 i to produce simultaneous contraction, e.g. of the knee flexor and extensor. In 

 short, concurrent contraction of the antagonists denotes that two antago- 

 nistic reflexes are synchronously at work and are operating with intensities 

 not far removed from balancing one another. In this respect there 

 is a significant resemblance between spinal reflex co-ordination and that 

 exercised by the motor cortex cerebri. Electric stimulation at one cortical 

 .point giving contraction of, e.g. flexor of elbow with relaxation of the 

 . antagonist extensor, and stimulation at another cortical point giving contrac- 

 tion of the antagonist extensor with relaxation of the flexor, synchronous 

 .stimulation of the two cortical points together does often, I find, give, 

 if suitably graded, a concurrent contraction of both flexor and antagonist 

 .extensor together. 



V. — Apart from the intensity of the stimuli applied to the opposed afferent 

 vol. lxxxi. — b. u 



