Intersegmental neuronal pathways in sacrococcygeal spinal cord (S3-Co3) activated by electrical stimulation of tail muscle nerves with low threshold in low spinal cats

Abstract

The aim of this study was to elucidate the neuronal pathways mediating propriospinal reflexes of the tail with special reference to the intersegmental connections between group I muscle afferents and motoneurons. Experiments were performed on 25 unanesthetized, spinalized (L1) cats of both sexes. The effects of electrical stimulation of the afferents embedded in different levels of the following muscles: m. extensor caudae medialis (ECM), m. extensor caudae lateralis (ECL), m. flexor caudae longus (FCL), and m. flexor caudae brevis (FCB), were observed in tail motoneurons in the first or second coccygeal (Co1 or Co2) spinal segment. Stimulation of group I muscle afferents from the cranial parts of the tail muscles produced predominantly excitatory postsynaptic potentials. In ECM and FCB, some motoneurons had significant excitatory effects exerted by group I afferent inputs from homonymous and/or synergistic muscles during vertical tail movement, and some motoneurons had predominately excitatory effects from antagonistic muscles during vertical tail movement. Excitatory effects in ECL and FCL motoneurons occurred more often and were larger in size after stimulating the ipsilateral tail muscles than after stimulating the contralateral tail muscles. The stimulation of group I muscle afferents from the caudal parts of the tail muscles produced predominantly inhibitory effects. The shortest neuronal pathways for excitatory postsynaptic potentials and inhibitory postsynaptic potentials were mono- and disynaptic neuronal pathways, respectively. The results of the present experiments suggest that continuous tail movements along the tail are performed by group I reflex pathways.