Morphological types of spinomesencephalic neurons in the marginal zone (lamina I) of the rat spinal cord, as shown after retrograde labelling with cholera toxin subunit B.

Retrogradely labelled lamina I neurons were studied after intramesencephalic injections of subunit B of cholera toxin. The tracer was visualized with a mixture of two monoclonal antibodies followed by the peroxidase-antiperoxidase technique that produced Golgi-like staining of the labelled cells. A morphological and morphometric analysis in the three anatomical viewing planes disclosed two structural neuronal types which were recognized as the fusiform and pyramidal cells of our Golgi-based classification of rat marginal cells. Fusiform cells had a bipolar longitudinally elongated dendritic arbor, were located in the lateral third of lamina I, and appeared to project mainly to the contralateral parabrachial nuclei. It is asserted that these cells may convey the spinal input which elicits visceral responses generated in that area. Pyramidal cells were longitudinally oriented pyramids with the triangular base straddling the dorsal horn/white matter border and a dendritic arbor extending lateromedially and mainly rostrocaudally throughout superficial lamina I and the dorsal funiculus. These cells occurred along the entire mediolateral extent of lamina I and seemed to have a prevalent projection to the contralateral caudal ventrolateral periaqueductal gray. They may represent the ascending branch of the spinal-midbrain loop centered in that zone which controls nociceptive transmission postsynaptically in the dorsal horn.