Abstract

Ascending projections from the several nuclei of the medullary reticular formation were examined using the autoradiographic method. The majority of fibers labeled after injections of [ 3Hleucine into nucleus gigantocellularis ascended within Forel's tractus fasciculorum tegmenti which is located ventrolateral to the medial longitudinal fasciculus. Nucleus gigantocellularis injections produced heavy labeling in the pontomesencephalic reticular formation, the intermediate layers of the superior colliculus, the pontine and midbrain central gray, the anterior pretectal nucleus, the ventral midbrain tegmentum including the retrorubral area, the centromedian-parafascicular complex, the fields of Forel/zona incerta, the rostral intralaminar nuclei and the lateral hypothalamic area. Nucleus gigantocellularis projections to the rostral forebrain were sparse. Labeled fibers from nucleus reticularis ventralis, like those from nucleus gigantocellularis, ascended largely in the tracts of Forel and distributed to the pontomedullary reticular core, the facial and trigeminal motor nuclei, the pontine nuclei and the dorsolateral pontine tegmentum including the locus coerulus and the parabrachial complex. Although projections from nucleus reticularis ventralis diminished significantly rostral to the pons, labeling was still demonstrable in several mesodiencephalic nuclei including the cuneiform-pedunculopontine area, the mesencephalic gray, the superior colliculus, the anterior pretectal nucleus, the zona incerta and the paraventricular and intralaminar thalamic nuclei. The main bundle of fibers labeled by nucleus gigantocellularis-pars alpha injections ascended ventromedially through the brainstem, just dorsal to the pyramidal tracts, and joined Forel's tegmental tract in the midbrain. With the brainstem, labeled fibers distributed to the pontomedullary reticular formation, the locus coerulus, the raphe pontis, the pontine nuclei, and the dorsolateral tegmental nucleus and adjacent regions of the pontine gray. At mesodiencephalic levels, labeling was present in the rostral raphe nuclei (dorsal, median and linearis), the mesencephalic gray, the deep and intermediate layers of the superior colliculus, the medial and anterior pretectal nuclei, the ventral tegmental area, zona incerta as well as the mediodorsal and reticular nuclei of the thalamus. Injections of the parvocellular reticular nucleus labeled axons which coursed through the lateral medullary tegmentum to heavily innervate lateral regions of the medullary and caudal pontine reticular formation, cranial motor nuclei (hypoglossal, facial and trigeminal) and the parabrachial complex. Parvocellular projections to the medial pontomedullary reticular formation were light and few ascended rostrally beyond the mid-pons. Labeled fibers from reticular formation injections that included the A1 noradrenergic area coursed laterally through the brainstem and distributed significantly to the locus coeruleus, the parabrachial complex, the dorsolateral tegmental nucleus and mid-lateral regions of the pontine gray. Labeling was light throughout the core of the pontomedullary reticular formation. Labeling in the diencephalon was pronounced. Several cell groups of the hypothalamus were heavily labeled including the median eminence, the arcuate nucleus, the periventricular nucleus, the supraoptic nucleus, the paraventricular nucleus and the lateral hypothalamic area. In the thalamus, light labeling was observed in the rostral intralaminar nuclei and midline nuclei (reuniens and rhomboid) moderate labeling was present in the paraventricular thalamic nucleus. In contrast to the other cases, significant numbers of labeled fibers reached the rostral forebrain and distributed to parts of the amygdaloid complex and the ventral striatum as well as to the medial septum and the diagonal band nucleus. The present results indicate that the ascending projections from the bulbar reticular formation are connectionally heterogeneous. We predict that this heterogeneity will be manifested by differences in function.

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