Is there a three neuron arc in the cat utriculo-trochlear pathway?

The utriculo-ocular pathway was examined in decerebrated and anesthetized cats, in which all the vestibular afferents in the labyrinth, except for those innervating the utricular (UT) macula, had been transected. The UT nerve was stimulated with tungsten electrodes which were insulated except for 200 microns at the tips. Stimulation of the UT nerve evoked a small negative (N1) potential in the vestibular nuclei, with a threshold (N1T) less than 25 microA. The stimulus evoked disynaptic EPSPs in ipsilateral abducens (AB) motoneurons. The threshold and latency of the excitatory postsynaptic potentials (EPSPs) was 1.3 x N1T and 1.2 ms, respectively, in accordance with the data of Schwindt et al. (1973). On the other hand, EPSPs with a clear rising phase and short latency, suggesting the existence of a disynaptic pathway, were never observed in any contralateral trochlear (TR) motoneurons, even when triple shocks at intensities of up to 4 x N1T were applied. This stimulus strength was strong enough to activate the UT nerve. Thus it seems very likely that a disynaptic pathway from the UT nerve to contralateral TR motoneurons, is absent or very poorly organized.