Brainstem afferents to the lateral mesencephalic tegmental region of the cat.

The lateral mesencephalic tegmental region (LTR) is a part of the midbrain reticular formation characterized by the presence of neurons exhibiting head movement-related discharge modulation. In addition, the LTR contains directionally selective visual units. Possible sources for these vestibular and visual signals were studied by retrograde axonal transport of horseradish peroxidase and three different fluorescent tracers (rhodamine, fast blue, and fluorogold) injected into various parts of the LTR. All injections into the LTR traced afferents from the vestibular nuclei and from the nucleus prepositus hypoglossi. Predominant projections were derived from the ipsilateral nucleus prepositus hypoglossi and the ipsilateral medial vestibular nucleus, whereas the observed inputs from the inferior, lateral, and superior vestibular nuclei were much weaker. Further inputs to the LTR originated in the deep and intermediate layers of the ipsilateral superior colliculus and the ipsilateral periaqueductal gray, the contralateral LTR, and the contralateral marginal nucleus of the brachium conjunctivum. Tracer deposits in medial parts of the tegmentum neighboring the LTR never produced the pattern of afferents observed after injections into the LTR. Our results suggest that afferents from the deeper layers of the superior colliculus are probably the source of visual signals in the LTR and that head movement-related responses are likely to be derived from the nucleus prepositus hypoglossi and the medial vestibular nucleus.