A quantitative study of the vestibular commissures in the gerbil

Direct commissural connections between the bilateral vestibular nuclear complexes (VNC) were investigated in the gerbil using ionophoretic injections of horseradish peroxidase into individual vestibular nuclei. Labelled commissural neurons were counted, the cell counts adjusted by the relative nuclear volume, and the results treated quantitatively. The medial nucleus (MVN) contained the greatest number of commissural neurons. The MVN projected to each of the contralateral vestibular nuclei, but most strongly to the contralateral MVN and superior (SVN) nucleus. The SVN projected modestly to the contralateral VNC. Commissural connections of the descending nucleus were weak. Commissural afferents to the MVN were topographically organized. The crossed fastigiovestibular projection was also investigated.