Secondary vestibular projection to the reticular formation of rabbit lower brainstem with reference to reciprocity

The secondary vestibular projection to the RF of lower brainstem was studied in the rabbit by retrograde transport of HRP and WGA-HRP. The projection is rather strong and originates bilaterally in all the main vestibular nuclei. The majority of afferents to the RF arise in the ventrolateral and ventromedial regions of MV and IV, respectively, whereas projections from SV and the ventral region of LV are weaker. Although no clear-cut topographical relationship was noted between location of neurons in the VNC and projection sites in the RF, some degree of preferential connections was found. All four vestibular nuclei project to RGc, though SV and LV contribute less than the others. Rpc alpha is supplied by fibers from MV and IV. RPc and Rpc receive projections from MV and/or IV. A minute connection was traced from the interstitial nucleus of the VIII nerve to RGc. The RF nuclei relaying vestibular information may plan an important role in the control of eye and head movements as well as in the postural adjustments required during animal displacement. The present results are discussed in comparison with those of earlier studies and with reference to reciprocity.