Mesencephalic projections from superficial and deep laminae of the medullary dorsal horn.

The mesencephalic projection pattern of axons arising from the medullary dorsal horn (MDH) was studied on the basis of axonal transport of Phaseolus vulgaris Leucoagglutinin (PHA-L). After large injections of PHA-L into both the superficial and deep laminae of the MDH, labeled fibers were observed in the anterior pretectal area (ATP), internal gray matter of the superior colliculus (InG), and rostral linear raphe nucleus (RLi) in the contralateral mesencephalon, and also in the ipsilateral parabrachial nucleus (PBA). Restriction of PHA-L to only the superficial laminae resulted in heavy axon labeling and varicosity in the APT and little labeling in the lateral part of the InG of the contralateral mesencephalic nuclei and the dorsal part of the ipsilateral PBA. On the other hand, after injections into the deep laminae, labeled axons were distributed mainly in the contralateral InG and RLi. Therefore, it is concluded that there are two different major pain pathways from the superficial and deep laminae of the MDH to the mesencephalic nuclei, processing nociceptive information in the trigeminal system.