Cytology and intrinsic organization of the perihypoglossal nuclei in the cat.

The morphology of the neurons in the perihypoglossal nuclei (nucleus prepositus, nucleus intercalatus, and nucleus of Roller) of the cat was studied in normal Nissl material, and by intracellular injection of horseradish peroxidase. The neurons in the nucleus prepositus were morphologically heterogeneous. Many of the cells in the ventromedial part of the caudal prepositus had relatively large somata, and complex dendritic trees which arose from numerous proximal dendrites and ramified extensively in the ventromedial aspect of the prepositus. These neurons had thick axons which typically did not give rise to local collaterals. The cells in the dorsolateral part of the caudal prepositus tended to have small somata, and dendritic trees which arborized in that region of the nucleus. The axons of these small cells frequently gave rise to local collaterals which terminated in the prepositus. Most of the cells in the prepositus had medium-sized somata and relatively few dendrites which branched in an isodendritic manner and extended for long distances, frequently leaving the nucleus. These "principal" prepositus neurons had axons which arborized unilaterally, and often gave rise to collaterals which terminated in either the ipsilateral or contralateral prepositus. The neurons in the nucleus of Roller and nucleus intercalatus which were intracellularly injected with horseradish peroxidase resembled the multidendritic and small prepositus cells, respectively. The intrinsic connectivity of the perihypoglossal nuclei was also studied by injecting horseradish peroxidase or 3H-leucine into the prepositus nucleus. The results of these experiments suggest that the perihypoglossal nuclei are highly interconnected bilaterally, although the large cells in the ventromedial prepositus and the nucleus of Roller contribute little to these intrinsic connections, and are not major recipients of intrinsic inputs. On the other hand, the magnitude of the reciprocal connections between the prepositus and the nucleus intercalatus suggests that they are functionally related.