230 



Prof. C. S. Sherrington. Reciprocal [Nov. 13, 



It seems, therefore, that in the combination of two reciprocal reflexes of 

 opposite effect on the symmetrical muscles of the limbs there is, besides 

 simple algebraic summation of the respective excitation and inhibition of the 

 components, a further factor sometimes present. An actual reversal of the 

 weaker element of one of the components appears to take place. Thus, the 

 excitatory effect on the contralateral extensor tends occasionally to be 

 reversed to inhibitory effect, and the inhibitory effect on contralateral flexor 

 tends to be reversed to excitatory effect. 



There remains the question whether identical concurrent contraction of 

 the symmetrical extensor pair can be obtained as a direct reflex. With 

 concurrent stimulation of both nerves, right and left, it can occasionally, 

 when the stimulation is weak, especially if one or both nerves be then giving 

 ipsilateral extensor contraction. Experiment shows that it can also be 

 obtained by a procedure qxvite other than that followed for producing the 

 identical innervation which has symmetrical relaxation for its result, namely 

 by weak stimulation of the afferent nerve of one side alone, right or left. 

 The range of intensity of stimulus over which reciprocal innervation of the 

 muscle pair results from excitation of a limb afferent is wide and ranges 

 from weak intensities without break right up to the very strongest. But 

 with stimuli of little above threshold value the result changes in the 

 decerebrate preparation. With these stimuli although the response in the 

 contralateral muscle remains reflex contraction, in the ipsilateral muscle the 

 response becomes reflex contraction instead of inhibitory relaxation. This 

 alteration is that noted previously by Miss Sowton and myself.* That it 

 is accompanied by concomitant contraction of the symmetrical extensor of 

 the fellow knee shows that with change merely of intensity of the stimulus 

 the reflex innervation of the fellow muscles alters from reciprocal to identical. 

 The reflex contraction so obtained is weak but quite indubitable and clearly 

 concomitant (fig. 10). 



Over a range of stimulus-intensities running from threshold value up to 

 weak or moderate, the exact limit upward varying with the condition of the 

 preparation, the reflex result in the decerebrate animal passes from identical 

 innervation (fig. 10, A) to an admixture of identical and reciprocal innervation 

 (fig. 11), until with stronger stimuli reciprocal innervation is fully established 

 (fig. 10, B). In the admixed form of result the reflex opens with bilateral 

 contraction, i.e. contraction of both right and left extensor muscles, and then 

 passes over into ipsilateral inhibitory relaxation, with contralateral pure 

 contraction (fig. 11). This transition is speedier the less weak the stimula- 

 tion. It is brought about by a change occurring, as noted by Miss Sowton 

 * 'Boy. Soc. Proc.,' 1911, B, vol. 83, p. 435. 



