UTRICULOOCULAR REFLEX ARC OF THE CAT

1. Intracellular recordings of synaptic potentials in extraocular moto neurons were studied to determine the connectivities between the utric ular nerve and the extraocular motoneurons in cats. 2. Stimulating ele ctrodes were placed within the left utricular nerve, while other branc hes 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 plate au (submaximal stimulation). Stimulus spread to the other nerve branch es appeared as an additional increase in N-1 amplitude after the plate au discontinued (supramaximal stimulation). 3. Intracellular recording s were made from 200 identified motoneurons in the bilateral III, IV, and VI cranial nuclei. 4. Stimulation of the utricular nerve at submax imal intensity evoked a longer latency depolarizing and hyperpolarizin g potentials in contra- and ipsilateral medial rectus motoneurons, res pectively. Complex potentials with longer latencies also were recorded in ipsilateral inferior oblique and contralateral trochlear motoneuro ns after stimulation of the utricular nerve at a submaximal intensity. Monosynaptic and disynaptic connections between the utricular nerve a nd 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 develope d, whereas polysynaptic circuits from the utricular nerve to inferior oblique and trochlear motoneurons may play a role in eye rotation duri ng head tilt.