Long ascending propriospinal projections from lumbosacral to upper cervical spinal cord in the rat

The retrograde tracer cholera toxin beta-subunit (CTB) was used to trace long ascending propriospinal projections from neurons in the lumbosacral spinal cord to the upper cervical (C3) gray matter in adult male Sprague–Dawley rats. Following large 0.5 μl CTB injections restricted mainly to the upper cervical ventral horn (n = 5), there were many lumbosacral CTB-positive neurons (14–17/section) in the intermediate gray and ventral horn (dorsal lamina VIII, medial VII extending into X) contralaterally, with fewer at corresponding ipsilateral locations. Labeled cells (4–8/section) were also observed in contralateral laminae IV–VI and the lateral spinal nucleus, with fewer ipsilaterally. Few labeled cells (< 2/section) were observed in superficial laminae I–II. Smaller (0.15 μl) microinjections of CTB restricted to the upper cervical ventral gray matter labeled cells in contralateral laminae VII–VIII (~ 6–9/section) with fewer ipsilaterally. There were relatively fewer (< 2/section) in the intermediate dorsal horn and very few (< 1/section) in lamina I. Larger (0.5 μl) CTB injections encompassing the C3 dorsal and ventral gray matter on one side labeled significantly more CTB-positive neurons (> 6/section) in contralateral lamina I compared to ventral horn injections. These results suggest direct projections from ventromedially located neurons of lumbar and sacral segments to the contralateral ventral gray matter of upper cervical segments, as well as from neurons in the intermediate but not superficial dorsal horn. They further suggest that some lumbosacral superficial dorsal horn neurons project to the upper cervical dorsal horn. These propriospinal projections may be involved in coordinating head and neck movements during locomotion or stimulus-evoked motor responses.