The time course of changes in monosynaptic reflex amplitude, after conditioning from both ipsi- and contralateral sural nerves at different stimulus strengths, was studied on two antagonistic motoneuronal pools acting on ankle muscles in spinal cats. Attention was focused on late effects, namely those appearing after a dely of more than 30 ms from the cutaneous stimulus. With low-threshold afferent activation, at conditioning-test intervals to 30 ms, the ipsilateral extensor monosynaptic reflex, recorded from the proximal stump of L7 to S1 ventral roots, showed marked inhibition; at longer intervals, a late facilitation period (LFP) lasting to 100 ms was observed. Increasing stimulus strength did not modify the time course of reflex excitability, but might enhance the amount of the facilitatory effecct. On the flexor monosynaptic reflex, sural conditioning induced, after the expected early facilitation, a second facilitatory period, starting at about 30 ms and recovering at about 130 ms. The excitability of antagonistic contralateral motoneuronal pools was also influenced, showing again a LFP with the same time course. The LFP was present after stimulation of the sural and saphenous nerves and was absent after stimulation of a muscle nerve. These late, long-lasting and nonreciprocal facilitatory effects on flexor and extensor ipsilateral motoneurons were quite distinct from the early reciprocal responses, and were evoked by large cutaneous fibers. An interpretation is put forward in light of the primary afferent hyperpolarization of Ia afferent terminals. A correlation is tentatively proposed with the mechanism subserving the stumbling corrective reaction.
ASJC Scopus subject areas