[Subcortical pathways related to optokinetic nystagmus in the cat]

In recent experiments, it has been disclosed that the nucleus of the optic tract (NOT) is the visut-motor relay between the retina and preoculomotor structures in the pathway mediating optokinetic nystagmus (OKN). In the present study, how OKN signals are transmitted from the NOT in the brainstem was investigated using biocytin and triticum vulgaris (Wheat Germ) peroxidase labeled (WGA-HRP) as anterograde and retrograde tracers, respectively. Following biocytin injections into the NOT, labeled fibers were observed in each of the following efferent pathways: 1) those that project to the contralateral NOT via the posterior commissure; 2) those that course through the nucleus pontis orklis to terminate in the Edinger-Westphal complex and nucleus reticularis tegmenti pontis; and 3) those that descend via the medical lemniscus to the level of the medulla to terminate in the dorsal cap of the inferior olive, during which their axons branch to the dorsolateral pontine nucleus, nucleus prepositus hypoglossi, and the nucleus pontis caudalis, superior and lateral. Furthermore, differences in the distribution of labeled cells in the NOT were observed following WGA-HRP injections into the nucleus prepositus hypoglossi and medial vestibular nucleus. The retrograde labeled cells in the NOT were distributed to the medial area at the rostral level following the tracer injections into the medial vestibular nucleus. On the other hand, labeled cells were recognized in the part of the caudal NOT following the tracer injections into the nucleus prepositus hypoglossi. A recent neurophysiological study demonstrated that areas adjacent to the medial vestibular nucleus apparently to participate in the production of OKN, because both the slow component and after-nystagmus similar to OKN and optokinetic after nystagmus were elicited by stimulation of the vestibular nuclei. The present study shows that this direct projection from the NOT to the vestibular nuclei may serve to drive velocity storage in the vestibular nuclei.