The rubrospinal tract. II. Facilitation of interneuronal transmission in reflex paths to motoneurones.

Abstract

Summary. 1. The effect of stimulation of the red nucleus on transmission of synaptie actions from different systems of primary afferents to alpha motoneurones has been invest/gated in cats, mainly with intracellular recording from motoneurones. 2. The dominating effect is facilitation, presumably caused by excitatory action exerted from the rubrospinal tract on interneurones of reflex arcs. The time course of facilitation suggests that the minimal linkage from the rubrospinal tract to these interneurones is monosynaptic. 3. Interneuronal transmission in reflex pathways from the following afferent systems is facilitated : a) Ia inhibitory between flexors and extensors, l~ubrospina] facilitation did not reveal Ia inhibitory pathways between adductors and abductors at the hip. b) Ib excitatory and inhibitory. There is marked facilitation of the reciprocal effects evoked by Ib afferents from extensors but also of other Ib pathways, for example inhibitory from extensors to flexor nuclei and from flexors to extensor nuclei and excitatory from flexors to extensor nuclei, e) Low threshold joint, inhibitory and excitatory, presumably from afferents with Ruffini endings. d) Low threshold cutaneous, excitatory and inhibitory. Since transmission from these afferents could be facilitated under conditions when there was no effect on transmission from high threshold muscle afferents it is postulated that the effect is exerted on pathways which are not part of the common pathways from the flexor reflex afferents, e) Flexor reflex afferents, excitatory and inhibitory. Facilitation of these pathways is not found regularly, in some eases there was no effect and in others inhibition. 4. The effeets are discussed in relation to the complex effects evoked from the rnbro spinal tract in motoneurones and to supraspinal regulation of proprioceptive reflexes. 5. It is postulated that in complex movements alternative Ib patterns may be mobilized, whereas flexion-extension movements are subserved by the Ib pattern found in the spinal eat. 6. Facilitation of the Ia inhibitory pathway is taken to indicate "a-y-linkage" in reciprocal inhibition. It is pointed out that convergence from Ia and descending impulses on a common inhibitory interneurone may play an important role in the regulation of a-y-linked flexion-extension movements.