On the projections from the vestibular and perihypoglossal nuclei to the spinal trigeminal and lateral reticular nuclei in the cat.

The projection from the vestibular and perihypoglossal nuclei to the spinal trigeminal and lateral reticular nuclei has been studied in cats where the wheat germ agglutinin-horseradish peroxidase complex has been used as a retrograde tracer. All injections were made at the level of the caudal pole of the inferior olive. The medial and descending vestibular, and the perihypoglossal nuclei were found to project to the spinal trigeminal nucleus. The projection to the lateral reticular nucleus reaches its medial-most part only, and originates in the lateral vestibular nucleus. The lateral part of the reticular formation also appears to be the target for some vestibular efferent fibres, mainly from the descending vestibular nucleus. The retrogradely labelled cells within the medial and descending vestibular nuclei are of all sizes and distributed throughout their entire territory. Certain observations furthermore indicate that the fibres reaching the lateral reticular nucleus are collaterals only from the vestibulospinal tract. The projections are bilateral. The observations confirm and extend previous observations on the afferent projections to the spinal trigeminal and lateral reticular nuclei.