Autoradiographic studies of the projections of the midbrain reticular formation: Descending projections of nucleus cuneiformis

Abstract

The descending projections of nucleus cuneiformis in the cat were traced by autoradiography in the transverse and sagittal planes following stereotaxically placed injections of 3 H‐leucine. Many descending axons are organized into distinct fiber systems, of which the largest and most well‐defined crosses directly in the midbrain and descends through the ventromedial tegmentum of the brain stem. This fiber system first terminates profusely in n. reticularis tegmenti pontis and then proceeds through the rhombencephalic tegmentum emitting transversely oriented branches to n. reticularis pontis caudalis and gigantocellularis, the raphe magnus and the facial nucleus. Its most caudal field of termination is a restricted portion of the medial accessory olive. Because of its size, compactness, and ventromedial course, this fiber system was termed the ventral tegmental bundle. In contrast, fibers descending on the ipsilateral side are more loosely organized and describe many divergent paths. In their initial course they remain in a dorsolateral position and distribute to the n. locus coeruleus, n. raphe dorsalis, the central gray matter, and n. reticularis pontis oralis. At successively more caudal levels they converge ventromedially and descend through the rhombencephalic tegmentum in a position similar to that of the ventral tegmental bundle. At the level of the facial nucleus a group of fibers splits off to form a separate fiber bundle which descends lateral to the inferior olive and distributes to the lateral reticular nucleus. No evidence was found of projections to the spinal cord or cerebellum. A portion of the tegmentum ventromedial to nucleus cuneiformis was found to project bilaterally to the facial nucleus and to the ipsilateral medial accessory olive. The results of this study are discussed in relation to the role of the midbrain reticular formation in the elaboration of certain forms of behavior.