Organization of efferent projections from the spinal cervical enlargement to the medullary subnucleus reticularis dorsalis and the adjacent cuneate nucleus: a PHA-L study in the rat.

The distribution and organization of projections from the spinal cervical enlargement to subnucleus reticularis dorsalis (SRD) and the neighbouring Cuneate nucleus (Cu) area was studied in the rat by using microinjections of Phaseolus vulgaris leucoagglutinin (PHA-L) into different laminae around the C7 level. The Cu received very dense projections from the dorsal horn, with the highest density being observed following injections into the medial part of laminae III-IV. The SRD received dense projections from laminae V-VII of the cervical enlargement, particularly 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 terminals and boutons were observed mainly in the SRD areas subjacent to the 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 that there are two parallel systems in neighbouring structures of the caudal medulla, viz. the Cu and the SRD, which, respectively, relay lemniscal and nociceptive information from the spinal cord to the thalamus.