If we attempt to decipher the biological meaning of reciprocal innervation its various instances when marshalled together say plainly that one of the functional problems which it meets and solves is mechanical antagonism. Where two muscles have directly opposed effect on the same lever, “reciprocal innervation” is the general rule observed by the nervous system in dealing with them, and this holds whether the reciprocal innervation is peripheral as with the antagonists of the arthropod claw, or is central as with vertebrate skeletal muscles. Also where one and the same muscle is governed by two nerves influencing it oppositely, reciprocal innervation seems again the principle followed in the co-ordination of the two opponent centres, as has been shown by Bayliss in his observations on vasomotor reflexes. But the distribution and occurrence of reciprocal innervation extend beyond cases of mere mechanical antagonism. The reflex influence exerted by the limb-afferents on symmetrical muscle-pairs such as right knee-extensor and left is reciprocal. Thus right peroneal nerve excites the motoneurones of left vastocrureus, and concomitantly inhibits those of the right. The reflex inhibition of the one is concurrent with, increases with increase, and decreases with decrease of, the excitatory effect on the other. Here the muscles are not in any ordinary sense antagonistic; not only do they not operate on the same lever, but they are not even members of the same limb, nor do they belong even to the same half of the body. They are, however, actuated conversely in the most usual modes of progression—the walking and the running step—though not always in galloping.
Reciprocal innervation and symmetrical muscles