Ascending and descending projections of the nucleus reticularis gigantocellularis in the cat demonstrated by the anterograde neural tracer, Phaseolus vulgaris leucoagglutinin (PHA-L)

Ascending and descending projections of the nucleus reticularis gigantocellularis (NRGc) were studied in the cat by the anterograde Tracer, Phaseolus vulgaris luecoagglutinin. Ascending fibers from the left or the right NRGc coursed through the bilateral medial reticular formation and some of them reached the diencephalon. In the brainstem, PHA-L-labeled fibers and their terminals were observed in the medial reticular formation, the cranial motor nuclei (III, IV, V, VI, VII, XII), the vestibular complex, the LC complex, the raphe nuclei, the periaqueductal gray, the red nucleus, the Edinger-Westphal nucleus and the interstitial nucleus of Cajal. In the diencephalon, they were observed in the dorsal thalamus and the hypothalamic regions. In the caudal medulla, labeled fibers and their terminals were observed in the nucleus prepositus hypoglossi, the nucleus intercalatus and the inferior olive. Descending axons from the NRGc coursed bilaterally through the ventral and ventrolateral funiculi as far caudal as the upper thoracic cord. Single axon collaterals arising from the descending axons gave off terminal fibers to be left or the right gray matter. Their terminals were located in laminae V–X, mainly in laminae VII and VIII. In lamina IX, they were distributed mainly in the medial portion. A few fibers originating from the descending axons ipsilateral to the PHA-L injection side coursed through the anterior or posterior commissure, and ended in laminae VI, VII and VIII. The functional implications of these findings are discussed in relation to the behavioral state control and the generalized motor inhibition.