Reflex effects of muscle afferents on antagonist studied on single firing motor units in man.

The effects of electrical stimulation of mixed nerves and of the evoked tendon reflex on single firing MUs of antagonists were investigated. The following muscles were studied: m. biceps fem., m. tibialis ant. and m. triceps surae. Under a weak voluntary contraction MU potentials were picked up. Post-stimulus histograms of MU potentials were plotted and the duration of interspike intervals in the post-stimulus period was compared with that in the prestimulus period. Under electrical stimulation of the nerve (Ia afferents) a distinct reciprocal inhibition was observed on MUs of all the 3 investigated muscles. In some cases a weak excitatory drive could be revealed along with the reciprocal inhibition. The tendon (knee) reflex was accompanied by a distinct excitatory effect on MUs of the antagonist. The dependence between the efficiency of the inhibitory volley and the moment within an interspike interval at which the volley arrived was found. The lengthening of an interval was observed only when an inhibitory drive occurred close to its end. The peculiarities of reciprocal inhibition in the firing motoneurone, as well as the possible mechanism of the excitatory effect on the antagonist accompanying the tendon reflex, are discussed.