Connections of the lobus inferior hypothalami of the clearnose skate Raja eglanteria (Chondrichthyes).

The afferent and efferent connections of the lobus inferior hypothalami of the clearnose skate were demonstrated by the anterograde and retrograde transport of horseradish peroxidase. The main source of afferents is from the midbrain tegmentum and telencephalon. The major midbrain input is from cells of the ipsilateral nucleus tegmentalis lateralis and the caudal tegmental area. Another prominent, mostly ipsilateral, projection arises from nucleus F of the isthmic region. A few labeled cells also occur in the nucleus interpeduncularis, nucleus raphes superior, and lateral reticular formation. Afferents from the telencephalon arise from cells of the area preoptica, area superficialis basalis, striatum, nucleus septalis lateralis, and area subpallialis 1. Of the pallial structures, the pallium mediale and anterior as well as posterior subdivisions of the pallium dorsale pars centralis appear to have strong projections to the inferior lobe. Efferent connections of the inferior lobe consist of ascending and descending pathways. Fibers of the main ascending efferent pathway course within the basal forebrain bundle and distribute to subpallial areas. The descending efferent pathways course within the tractus lobobulbaris and tractus lobocerebellaris. Of these, the former is traceable to the level of the facial motor nucleus, issuing fibers enroute to the midbrain tegmentum and to the lateral reticular formation. The lobocerebellar tract courses dorsolateral to the lobobulbar tract, and its fibers terminate within the ipsilateral granular ridge of the rostral pole of the cerebellar corpus. There appears to be a topological organization of the inferior lobe connections. In general, pallial areas project mainly to the lateral subdivision of the inferior lobe nucleus at midlobic levels, whereas connections with the brainstem arise from or terminate within the dorsal and intermediate subdivisions at midlobic as well as caudal levels. The widespread ascending and descending connections indicate that the hypothalamic inferior lobe of the clearnose skate is a major relay center between the telencephalon and brainstem.