Afferent and efferent connections of the dorsal column nuclear complex and adjacent regions in the turtle.

Anterograde and retrograde tracer substances were injected into the dorsal spinomedullary region (DSM), which in the turtle comprises especially the dorsal column nuclear complex (DC) and the solitary nucleus (Sol). The fibers arising from the DSM ascended ipsi- and contralaterally via the lateral and the ventral fiber tracts respectively. These tracts could not be assigned clearly to particular target areas except that the dorsal thalamic regions presumably receive their afferents exclusively from the lateral tract. The efferent projections to rhombencephalic and mesencephalic regions were restricted essentially to the nucleus visceralis secundarius, the nucleus profundus mesencephalicus and the nucleus interstitialis commissuralis posterior. Diencephalic target areas were found in the hypothalamus, the nucleus suprapeduncularis, the nucleus reuniens, the rostral perirotundal region and the area triangularis. Most impressive were the telencephalic projections to the areas c, d and h. Unlike the thalamic projections which were predominantly labeled ipsilaterally, the telencephalic target regions were more heavily involved contra- than ipsilaterally. Area d, in addition, gave rise to a predominantly ipsilateral projection to DSM. Other areas reciprocally connected with DC and/or Sol were the periventricular and intermediate regions of the hypothalamus, the nucleus interstitialis commissuralis posterior, the nucleus visceralis secundarius and the nucleus profundus mesencephali. A major descending projection originated from the griseum centrale (including the nucleus laminaris of the torus semicircularis), while minor areas of origin, apart from isolated reticular cells, were the nucleus and the interstitial nucleus of the fasciculus longitudinalis medialis, the red nucleus, the locus coeruleus and the raphe nuclei. The demonstrated projections were discussed in regard to their relations to DC and/or Sol and compared with corresponding connections in birds and mammals.