On reciprocal innervation of antagonistic muscles.―Seventh note

Abstract

In a previous note on this subject, instances were pointed out in which the passive posture already obtaining in a limb influences the character of the spinal reflex elicitable from the limb. It was there shown how passive flexion, for instance, favours reflex extension. On resorting to postures assumed and maintained not passively but actively a like influence is evident. A reflex described in the previous note gives good illustration of this. Light pressure applied to the planta of the spinal dog evokes a brisk extension of the limb at hip, knee, and ankle. The pressure applied is such as resembles that which the weight of the dog would, in its own step, apply to the planta on contact with the ground. This reflex―which may be termed the “extensor thrust”―employs the antagonist muscles to those employed by the well-known flexion reflex provocable by noxious stimuli applied to the planta , e. g. , by a prick. This latter reflex, the flexion reflex, although it throws into action a group of muscles different from that thrown into action by the extensor thrust, yet exerts a marked influence over that reaction. If the extensor thrust be compared as elicited before and after a prolonged flexion reflex, the “thrust” reflex is found more facile and vigorous just after the flexion than it was before it.