The rostral projections of the primate vestibular nuclei: An experimental study in macaque, baboon and chimpanzee

The ascending projections of the vestibular nuclei were studied with the selective silver methods of Nauta‐Gygax (Nauta,'57), Voneida ('67) and Fink et al. ('67) in macaque, baboon and chimpanzee. Degenerated axons pass from lesions in the vestibular nuclei into the medial medullary and pontine tegmentum bilaterally, the abducens nuclei, the contralateral vestibular complex and in the medial longitudinal fasciculus to the nuclei of the III and IV nerves. A few fibers pass rostral to the oculomotor complex into the interstitial nuclei of Cajal, the nuclei of Darkshewitsch and the nuclei of the posterior commissure. No degenerated fibers originating in vestibular nuclear lesions were observed to ascend rostral to the level of the posterior commissure. Degenerated fibers rostral to this level could be traced, in sagittal, horizontal and frontal sections, to incidental damage to the superior cerebellar peduncle and the posterior column nuclei. Damage to the acoustic striae produced ascending degeneration in the auditory system, but this did not pass rostral to the inferior colliculus. The incidentally damaged tracts could be seen, in sections in three planes, to be separate in their course and terminations from the fibers ascending from the vestibular nuclei in the medial longitudinal fasciculus. The experimental evidence lends no support to the supposition that direct secondary vestibular connections to the diencephalon exist. Previous anatomical descriptions of direct vestibulothalamic connections appear to result from failure to discriminate between fibers originating in the vestibular nuclei and fibers arising in adjacent regions. Vestibular impulses reaching the diencephalon must pass over third or higher order neurons after synapse at lower levels.