MONOSYNAPTIC AND DISYNAPTIC CONNECTIONS IN THE UTRICULO-OCULAR REFLEXARC OF THE CAT

1. Connections from the utricular (UT) nerve to motoneurons and intern eurons in the ipsilateral abducens (AB) nucleus were studied in anesth etized and decerebrated cats. Bipolar electrodes were fixed on the lef t UT nerve under visual observation. The other branches of the vestibu lar 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, wi th 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, w hich was considered as a incoming volley of the UT nerve. 3. Excitator y postsynaptic potentials (EPSPs) were recorded from AB motoneurons wi th short latencies after UT nerve stimulation. They were classified in to two types, M and D. M-type EPSPs, which followed repetitive high-fr equency 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 ner ve evoked EPSPs that had the same latency. It was suggested that the A B motoneurons had monosynaptic connections with the UT nerve. D-type E PSPs, which were recorded from most of the AB motoneurons, had slightl y longer latencies ranging from 1.2 to 1.8 ms. They showed temporal fa cilitation 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 nucle us had the same characteristics as AB motoneurons. 4. P/N action poten tials were recorded from the UT nerve in the inner ear after stimulati on of the AB nucleus, suggesting that the primary afferents of the UT nerve project to the AB nucleus. The latencies of the positive and neg ative peaks were 0.5 and 0.8 ms, respectively. 5. Secondary vestibular neurons that were activated both from the UT nerve orthodromically an d 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.