MONOSYNAPTIC AND DISYNAPTIC CONNECTIONS IN THE UTRICULO-OCULAR REFLEX ARC OF THE CAT

1. Connections from the utricular (UT) nerve to motoneurons and interneurons in the ipsilateral abducens (AB) nucleus were studied in anesthetized and decerebrated cats. Bipolar electrodes were fixed on the left UT nerve under visual observation. The other branches of the vestibular nerve and the facial nerve were transected in the left inner ear. 2. Stimulation of the UT nerve evoked a small positive-negative (P/N) deflection and a negative (N-1) potential in the vestibular nuclei, with mean latencies of 0.56 and 0.84 ms, respectively. In the AB nucleus a small P/N deflection with a mean latency of 0.72 ms was recorded, which was considered as a incoming volley of the UT nerve. 3. Excitatory postsynaptic potentials (EPSPs) were recorded from AB motoneurons with short latencies after UT nerve stimulation. They were classified into two types, M and D. M-type EPSPs, which followed repetitive high-frequency stimuli and were recorded from the majority of AB motoneurons, had latencies ranging from 0.9 to 1.2 ms. Double shocks to the UT nerve evoked EPSPs that had the same latency. It was suggested that the AB motoneurons had monosynaptic connections with the UT nerve. D-type EPSPs, which were recorded from most of the AB motoneurons, had slightly longer latencies ranging from 1.2 to 1.8 ms. They showed temporal facilitation when double shocks were provided to the UT nerve. They did not follow repetitive high-frequency stimuli (less than or equal to 2.5-ms interval). It was suggested that D-type EPSPs were disynaptically evoked via secondary vestibular neurons. Interneurons in the AB nucleus had the same characteristics as AB motoneurons. 4. P/N action potentials were recorded from the UT nerve in the inner ear after stimulation of the AB nucleus, suggesting that the primary afferents of the UT nerve project to the AB nucleus. The latencies of the positive and negative peaks were 0.5 and 0.8 ms, respectively. 5. Secondary vestibular neurons that were activated both from the UT nerve orthodromically and from the AB nucleus antidromically with low thresholds were located in the ventral part of the lateral nucleus and the lateral part of the descending nucleus.