Long spinal and pyramidal actions on hindlimb motoneurons of the marsupial brush-tailed possum, Trichosurus vulpecula

Abstract

The existence of descending propriospinal reflex linkages between forelimbs and hindlimbs has been established in the brush-tailed possum (Trichosurus vulpecula). In animals under chloralose anesthesia and with intact brain stem, forelimb volleys evoked facilitation of flexor and extensor monosynaptic reflexes of both hindlimbs, more pronounced on the ipsilateral side. Powerful inhibition of briefer latency and restricted to ipsilateral flexor digitorum longus (FDL) motoneurons was also brought about by forelimb volleys; at latencies exceeding 20-30 ms, FDL inhibition was usually replaced by facilitation. Distinctness of the two long spinal actions was shown by differences in forelimb receptive fields and in threshold of the executant afferent fibers. The field for reflex inhibition was located distally in the forepaw region, that for facilitation being wider, including deep as well as superficial tissues. Threshold of afferent fibers evoking inhibition was lower than that for facilitation. The descending long spinal actions were compared with those set up by repetitive stimulation of the motor cortex contralateral to the test hindlimb reflexes. In agreement with previous work, strong facilitation of most flexor or extensor motoneurons was produced, including those of quadriceps and ankle flexors, as well as gastrocnemius and hamstring motor nuclei; inhibition consistently appeared only in the FDL motoneuron pool. Weak and inconstant inhibitory action was occasionally observed in other motor pools. Pyramidal tract section abolished the cortical inhibition of FDL, but had little effect on facilitation; both long spinal actions were unchanged. Pyramid-sparing brain stem section greatly reduced both cortical and long spinal facilitatory action, but had little or no effect on FDL inhibition from either source. Interaction experiments demonstrated facilitation of weak inhibitory actions on FDL motoneurons of forelimb and cortical stimulation when elicited together, suggesting a sharing by the two inputs of common interneuronal elements. The observation is consistent with the notion that the long propriospinal system responsible for FDL inhibition from the forepaw might provide the pathway for pyramidal inhibition of the same group of motoneurons.