CHAPTEE XCI 



RECIPROCAL INNERVATION 



Reciprocal Inhibition. — It might appear that to bend a joint or to 

 move the eyeball the only muscular action required would be contrac- 

 tion of the muscles which flex the joint or rotate the eyeball, and that 

 the antagonistic muscles would merely become passively elongated. 

 When Ave remember, however, that all the muscles of the body are or- 

 dinarily in a condition of slight contraction, or tone, and that this tends 

 to become increased when the muscles are passively stretched, then we 

 see that for efficient movement there must be inhibition of the tone of 

 the muscles which oppose those that are contracting. This reciprocal 

 inhibition, as it is called, is a very widespread function throughout the 

 animal body. Sometimes it is purely peripheral in origin, as in the claw 

 of the crayfish, where stimulation of the nerve causes an opening of the 

 claw due to the contraction of one set of muscles and the simultaneous 

 inhibition of their antagonists. Instances of peripheral reciprocal in- 

 hibition in the higher animals are not so common, but are illustrated in 

 the case of the myenteric reflex, where it will be remembered a contraction of 

 the intestine over a bolns of food is accompanied by inhibition in front of 

 the bolns. The reciprocal action in this case is probably dependent on 

 the myenteric plexus. 



On the other hand, reciprocal inhibition of central origin is very com- 

 mon in the higher mammalia. Thus, in the case of the lateral movement 

 of the eyes, if we cut the third and fourth nerves to one eye, say, the 

 left, the external rectus of that eye will alone be under the control 

 of the nervous system, through the sixth nerve; nevertheless, if we after- 

 ward cause the animal to look toward the right, as by holding some ob- 

 ject in that direction, it will be found that the left eye as well as the 

 right follows the object. Obviously there must be an inhibition of the 

 external rectus muscle of the left eye, an inhibition which is pronounced 

 enough to bring about a movement of the eyeball, and which exactly cor- 

 responds in point of time with the contraction of the external rectus of 

 the right eye. This movement, due to the atonicity of the external rec- 

 tus, does not however succeed in causing the eye to rotate beyond the 

 midline of the field of vision. This is an instance of n willed reciprocal 

 inhibition ; i. e., a reciprocal inhibition brought about by stimuli coming 



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