Utriculoocular reflex arc of the cat

Abstract

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 N1 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 N1 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.

Acoustic Clicks Activate both the Canal and Otolith Vestibulo-Ocular Reflex Pathways in Behaving Monkeys

“…In this study, the minimum latency of crossed, excitatory response was 1.02 ms, which was plausible for a response mediated by a disynaptic circuit given the small difference in the vestibular afferent minimum latency to electric pulses and clicks (0.33 vs. 0.4 ms). Third, as shown by several earlier studies (Uchino et al 1994(Uchino et al , 1996(Uchino et al , 1997Isu et al 2000), the latency analysis involves not only the minimum latency but also the range and distribution of the latency. According to Broussard et al (1995), the median latency of crossed, excitatory response was 1.5 ms.…”

“…To elucidate the neural pathways that mediate the two excitatory responses, we measured abducens neurons' latencies to ipsilateral and contralateral clicks and compared them to the published latencies of vestibular afferents and abducens neurons to clicks as well as to brief electrical pulses. As shown in Figure 5, it has been well established that the contralateral canal-abducens connection is excitatory and disynaptic (Scudder and Fuchs 1992) and the ipsilateral utricular-abducens connection is excitatory and monosynaptic (Uchino et al 1994(Uchino et al , 1996. The latency analysis, in combination with the well-established VOR circuits, provided important insights into understanding how clicks activate specific VOR pathways.…”

“…The otolith-ocular pathways, however, have been shown to have a very different circuitry. Specifically, the utricule projections make monosynaptic and disynaptic excitatory connections to the ipsilateral abducens neurons and trisynaptic inhibitory connections to the contralateral abducens neurons (Imagawa et al 1995;Schwindt et al 1973;Uchino et al 1994Uchino et al , 1996Uchino et al , 1997. The saccular projections make no connections to the horizontal extraocular motoneurons.…”

“…For the horizontal AVOR, extensive studies in monkeys and cats have established that the direct AVOR pathways are mainly organized in a disynaptic threeneuron-arc pattern, i.e., from the lateral canal afferents to the interneurons in the vestibular nuclei, then to the abducens motoneurons that innervate the lateral rectus muscle of the contralateral eye (Lorente De No, 1933;Scudder and Fuchs 1992). For the horizontal LVOR, Uchino et al (1994Uchino et al ( , 1996 showed that utricule afferents make monosynaptic excitatory connections to the abducens motoneurons that innervate the lateral rectus muscle of the ipsilateral eye in cats. Thus, by analyzing the sound-evoked responses in abducens neurons on both sides, we were able to determine how clicks activate the canal and otolith VOR pathways in behaving monkeys.…”

Differential Sensorimotor Processing of Vestibulo-Ocular Signals during Rotation and Translation

“…Finally, there is increasing evidence that the synaptic organization of the otolith-ocular and canal-ocular pathways is different. Whereas the shortest latency RVOR pathways are mediated dominantly by excitatory projections to the contralateral abducens, the shortest latency TrVOR pathways are instead excitatory to the ipsilateral abducens (Schwindt et al, 1973;Uchino et al, 1994Uchino et al, , 1996Uchino et al, , 1997Imagawa et al, 1995). These differences at both sensory and motor levels, as well as the existence of unique neuroanatomical connections, suggest that the sensorimotor processing of canal and otolith signals in the RVOR and TrVOR is at least partially distinct.…”