1. Intracellular recordings of synaptic potentials in extraocular motoneurons were studied to determine the connectivities between the utricular nerve and the extraocular motoneurons in cats.
2. Stimulating electrodes were placed within the left utricular nerve, while other branches of the vestibular nerve were removed. Subsequently, the N-1 field potentials evoked by utricular nerve stimulation were recorded in the vestibular nuclei. The potential typically grew until reaching a plateau (submaximal stimulation). Stimulus spread to the other nerve branches appeared as an additional increase in N-1 amplitude after the plateau discontinued (supramaximal stimulation).
3. Intracellular recordings were made from 200 identified motoneurons in the bilateral III, IV, and VI cranial nuclei.
4. Stimulation of the utricular nerve at submaximal intensity evoked a longer latency depolarizing and hyperpolarizing potentials in contra- and ipsilateral medial rectus motoneurons, respectively. Complex potentials with longer latencies also were recorded in ipsilateral inferior oblique and contralateral trochlear motoneurons after stimulation of the utricular nerve at a submaximal intensity. Monosynaptic and disynaptic connections between the utricular nerve and ipsilateral abducens motoneurons and interneurons were recorded as described previously.
5. The results of the present study confirm our initial findings that a disynaptic pathway from the utricular nerve to contralateral trochlear motoneurons is absent or very poorly developed, whereas polysynaptic circuits from the utricular nerve to inferior oblique and trochlear motoneurons may play a role in eye rotation during head tilt.