Projection sites of superficial or deep dorsal horn in the dorsal reticular nucleus.

The termination sites in the medullary dorsal reticular nucleus (DRt) of axons arising from superficial or deep dorsal horn cells were analysed by anterograde tracing with biotinylated dextran (BD). Following BD injections in spinal laminae I-III or in these laminae plus the dorsal funiculus, labelled fibres and boutons were located in the dorsal most ipsilateral portion of the DRt. Injections covering laminae III-V or laminae I-V resulted in bilateral labelling of the ventral DRt and ipsilateral labelling of the dorsal DRt. These findings, together with previous data concerning the origin of DRt projections to the superficial or deep dorsal horn, suggest the occurrence of reciprocal anatomical loops connecting the dorsal or the ventral areas of both regions.