MORPHOLOGY OF INTERNEURONS IN PATHWAYS FROM GROUP-II MUSCLE AFFERENTSIN SACRAL SEGMENTS OF THE CAT SPINAL-CORD

The morphology of 12 sacral interneurones with peripheral input from g roup II muscle afferents was analyzed after intracellular injection of horseradish peroxidase (HRP). The neurones were located in Rexed's la minae III-V overlying the pudendal (Onuf's) motor nucleus. The interne urones had medium sized elongated somata and dendrites projecting radi ally. All Of the interneurones were funicular neurones and fell into t wo categories depending on whether their axons ran within the dorsal p art of the lateral funiculus (DLF; n = 7) or within the ventral funicu lus, or the ventral Part of the lateral funiculus (VF or VLF; n = 4). The latter were located more rostrally. Within the DLF similar proport ions of stem axons and secondary axonal branches descended and ascende d. Within the VF and VLF all of the axons ascended. Collaterals of axo ns running in the DLF arborized primarily within the dorsal horn and t he intermediate zone; none were found to approach the motor nuclei. In contrast, collaterals of axons running in the VF/VLF arborized in bot h the intermediate zone and the ventral horn and passed close to the m otor nuclei. We conclude that sacral interneurones with group II input are morphologically nonhomogenous and that only those located most ro strally might have direct actions upon motoneurones. Both the axonal p rojections and the input (from group II but not from group I muscle af ferents and from skin afferents) of sacral interneurones indicate that they are homologous to dorsal horn group II interneurones in the midl umbar segments. They appear, however, to form part of more local neuro nal networks than their midlumbar counterparts. (C) 1993 Wiley-Liss, I nc.