Abstract
We have suggested that lamina VII interneurons in the lower lumber segments, which receive inhibition from flexor reflex afferents and a group of Ib interneurons, can mediate a generalized motor inhibition that is induced by stimulating the medullary reticular formation (MRF; Takakusaki et al., 2001, 2003). Here we examined whether the interneurons satisfying above criteria could exert inhibitory effects upon hindlimb motoneurons in the lumbosacral segments. Twenty four lamina VII interneurons in the L6 segment were antidromically excited by stimulating the motoneuron pools at L7 and S1 segments. The minimum latency for the antidromic responses was 1.10 + 0.29 ms. Stem axons of 10 and 3 interneurons were located in the ventral and ventrolateral funiculi, respectively. Effects of single interneurons upon hindlimb motoneurons were studied by spike-triggered averaging. Among the combinations of 38 interneurons and 203 motoneurons, monosynaptic inhibitory connections were revealed between 21 interneurons and 37 motoneurons. Ten and eleven interneurons were identified to inhibit extensor and flexor motoneurons, respectively. Although eleven interneurons induced IPSPs in plural motoneurons, each interneuron did not induce IPSPs in motoneurons of antagonistic relationship. These results suggest that a group of lamina VII interneurons in L6 segment descend through ventral or ventrolateral funiculi and inhibit either extensor or flexor hindlimb motoneurons. Signals from the MRF can simultaneously activate these lamina VII interneurons so that generalized motor inhibition (muscular atonia) is induced. [Jpn J Physiol 55 Suppl:S176 (2005)]
