ORGANIZATION OF EFFERENT PROJECTIONS FROM THE SPINAL CERVICAL ENLARGEMENT TO THE MEDULLARY SUBNUCLEUS RETICULARIS DORSALIS AND THE ADJACENTCUNEATE NUCLEUS - A PHA-L STUDY IN THE RAT

The distribution and organization of projections from the spinal cervi cal enlargement to subnucleus reticularis dorsalis (SRD) and the neigh bouring Cuneate nucleus (Cu) area was studied in the rat by using micr oinjections of Phaseolus vulgaris leucoagglutinin (PHA-L) into differe nt laminae around the C7 level. The Cu received very dense projections from the dorsal horn, with the highest density being observed followi ng injections into the medial part of laminae III-IV. The SRD received dense projections from laminae V-VII of the cervical enlargement, par ticularly from the reticular and medial aspects of lamina V, lamina VI , and the dorsal part of lamina VII. By contrast, the superficial part of the dorsal horn (laminae I to IV) and the dorsal part of lamina X provided only sparse projections to the SRD. Clusters of labelled term inals and boutons were observed mainly in the SRD areas subjacent to t he Cu. In the caudorostral axis, labelled terminals were spread along the whole SRD from the cervicomedullary junction up to the caudal-most part of the area postrema. Contralateral projections to the SRD were scarce and were observed mainly after injections into the medial part of laminae VI-VII. These data give further support to the proposal tha t there are two parallel systems in neighbouring structures of the cau dal medulla, viz. the Cu and the SRD, which, respectively, relay lemni scal and nociceptive information from the spinal cord to the thalamus. (C) 1996 Wiley-Liss, Inc.