Input of anterior and posterior semicircular canal interneurons encoding head-velocity to the dorsal Y group of the vestibular nuclei

Neurons in the Y group of the vestibular nuclei are activated disynapticall y from the ipsilateral VIIIth nerve and polysynaptically from the contralat eral nerve. The ipsilateral anterior and posterior semicircular canals proj ect to the Y group via interneurons in the Vestibular nuclei. Candidate int erneurons located in the rostrolateral corner of the superior (SVN) and in the caudal medial (MVN) vestibular nuclei were retrogradely labeled by the iontophoretic injection of biocytin into the Y group. The physiology of the se interneurons named Y-group projecting neurons (YPNs) was studied in the SVN. SVN-YPNs were activated antidromically by electric pulse stimulation i n the Y group. The properties of SVN-YPNs are distinct from those of SVN fl occulus projecting neurons (FPNs). Namely, YPNs have a lower resting rate t han FPNs, have more irregular interspike intervals, show a different phase and gain during the vestibuloocular reflex, and are located differentially within the SVN. After the injection of biocytin into the Y group, the locat ions of Purkinje cells that project to the Y group were confined to the ver tical zones of the flocculus and ventral paraflocculus. However, mossy fibe rs originating in the Y group terminate in both the vertical and horizontal zones of the flocculus and ventral paraflocculus as well as in the ipsilat eral nodulus.