The somatotopic organization of the nucleus raphe magnus and surrounding brain stem structures as revealed by HRP slow-release gels.

Horseradish peroxidase (HRP) in slow-release gel was unilaterally implanted in the dorsal lateral funiculus (DLF) of either cervical, midthoracic, lumbar or sacral spinal cord levels of adult male rats. Complete mappings and cell counts were made of HRP-filled cells from the level of the rostral trapezoid body caudal to the inferior olive. Following cervical or midthoracic placements, labelled cells often formed a continuous band surrounding the medial, dorsal and lateral aspects of the ipsilateral pyramid and extending into the ventral region medial to the facial nucleus and lateral to the pyramid. It appears that this population of HRP-filled cells in the nucleus raphe magnus (NRM) and reticularis magnocellularis (Rmc) corresponds to the serotonergic cell group B3 of Dahlström and Fuxe. In lumbar and sacral cases, there was a noticeable difference both in the pattern and number of labelled cells, with fewer labelled cells and greater tendency for a 'gap' to occur in the population of cells midway over the pyramid. Based on the patterns of labelling and a review of the literature, its appears that the actual functional unit involved in the descending control of nociception may be the combined population of Rmc and NRM.