Vestibular nucleus neurons relaying excitation from the anterior canal to the oculomotor nucleus.

A morphological approach was undertaken to determine which vestibular nucleus neurons relay excitation from the anterior canal to the IIIrd nucleus. In anesthetized rabbits HRP was iontophoresed into the IIIrd nucleus and cells filled with HRP reaction product (positive cells) searched for within the vestibular nuclear complex. By lesioning the MLF or brachium conjunctivum immediately after iontophoresis it was demonstrated that positive cells in the dorsum of the superior vestibular nucleus are backfilled via their axons which ascend in the brachium conjunctivum. By contrast positive cells in the center of the superior nucleus are backfilled via their axons in the MLF. In electrophysiological experiments in the presence of a severed MLF the anterior canal was selectively stimulated for orthodromic, and the 3rd nucleus stimulated for antidromic, activation of vestibular nucleus neurons. Recording extracellularly with glass microelectrodes filled with fast green FCF the only cells both ortho- and antidromically activated were localized to the dorsum of the superior vestibular nucleus. It is concluded that cells dorsally located in the superior nucleus relay the disynaptic excitatory vestibulo-ocular reflex from the anterior canal to the contralateral 3rd nucleus via their axons which ascend in the brachium conjunctivum.