Target neurons of floccular caudal zone inhibition in Y-group nucleus of vestibular nuclear complex

Abstract

Extracellular unit spikes were recorded in and around the Y-group nucleus in the anesthetized cat. Target (T) neurons of floccular caudal zone inhibition were identified by observing cessation of their spontaneous discharges following stimulation of the floccular caudal zone. The axonal trajectories of the T neurons to the rostral brain stem were studied by observing the antidromic responses of single neurons during systematic tracking with a stimulating microelectrode in the brain stem. The axons of the T neurons pass through a region closely ventral to the lateral part of the brachium conjunctivum (BC), continue rostrally in a region between the BC and the lateral lemniscus, arch medially around the rostral part of the nucleus reticularis tegmenti pontis, cross the midline, continue to the contralateral side by about 1.5 mm lateral from the midline, arch rostrally, run in the central tegmental field on the contralateral side, arch dorsomedially around the caudal pole of the red nucleus, and enter the contralateral oculomotor nucleus (OMN) from the ventrolateral side. In the caudal half of the contralateral OMN, the axons of the T neurons branch out and terminate. The T neurons were exclusively located in the dorsal subdivision of the Y-group nucleus (DY), whereas some were in the medial part of the subnucleus lateralis parvocellularis (SLP, Ref. 12) of the lateral cerebellar nucleus. T neurons were not found in the ventral subdivision of the Y-group nucleus (VY). Differences in neuronal connections between the DY and VY neurons were investigated by observing responses of single neurons to stimulation of the contralateral OMN, the ipsilateral floccular caudal zone, the ipsilateral eighth nerve (i8N), and the contralateral eighth nerve (c8N). Most neurons in the DY and the adjacent medial part of the SLP, receiving inhibitory inputs from the ipsilateral flocculus (exclusively from the caudal zone), project to the contralateral OMN, and about one-half of these neurons receive polysynaptic inputs from the i8N and the c8N. On the other hand, most neurons in the VY receive monosynaptic inputs from the i8N, and some of these neurons project to the ipsilateral flocculus. The neuronal tract via the ventral part of the pontine tegmentum demonstrated in the present experiments is distinct from the classically established vestibulooculomotor tracts via the BC, the medial longitudinal fasciculus, or the ascending tract of Deiters. We call this tract the 'crossing ventral tegmental tract'. Previously, we reported that electrical stimulation of the caudal zone elicited conjugate downward eye movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Twitch and nontwitch motoneuron subgroups in the oculomotor nucleus of monkeys receive different afferent projections

“…This finding implies that the nontwitch motoneurons are not directly involved in the generation of the vestibuloocular reflexes carried in these pathways. The inclusion of mv p or the Y group into the injections labeled diffuse terminals over the nontwitch motoneurons in the midline S and C groups, in addition to projections to the twitch motoneuron subgroups (Sato and Kawasaki, 1987). This result speaks for a supplementary tonic role of mv p or the Y group in eye muscle control, perhaps in terms of gaze holding or visual following (McFarland and Fuchs, 1992;Partsalis et al, 1995).…”

Upbeat Nystagmus due to a Small Pontine Lesion: Evidence for the Existence of a Crossing Ventral Tegmental Tract

“…The CVTT has been described in the cat [8][9][10] and probably also exists in the monkey [11] . This tract originates in the superior vestibular nucleus, with an initial rostral course passing laterally and ventrally to the BC in the lateral tegmentum of the lower pons, arching a fi rst time at the midpons level, then coursing almost horizontally towards the midline near the limit between the tegmentum and the basis pontis, crossing the midline close to the upper part of the nucleus reticularis tegmenti pontis, slightly above the midpons level ( fi g. 3 ).…”

“…The concurrent marked reduction in the velocity gain of the upward eye movement elicited by the OCR supports the hypothesis of a transmission of a part of the upward vestibular eye velocity signals through the CVTT in humans. In fact, this double role of the CVTT in the transmission of both eye position and vestibular eye velocity signals could be expected from a tract which belongs to the fi nal part of the central vestibulo-oculomotor connections [9] . Overall, in our patient and probably in other cases of UBN due to pontine lesions, the nystagmus could be the result of a simple imbalance in the central vestibular connections between the two vertical systems, as previously hypothesized [12] : the CVTT failure would result in a relative hypoactivity in the fi nal upward vestibular pathways, compared to the undamaged downward system, with con-sequently a continuous downward slow phase interrupted by upward quick phases, the latter being generated by the saccadic system to recenter the eyes.…”

Chronic Changes in Inputs to Dorsal Y Neurons Accompany VOR Motor Learning

“…The only known source of eye velocity to Y neurons arrives from the flocculus (Partsalis et al 1995b;Rambold et al 2002). The head signal, however, arrives to Y neurons via two pathways: the flocculus, which sends an upward head velocity signal (inhibitory synapses convert the downward head velocity of Purkinje cells to upward head velocity) (Langer et al 1985;Partsalis et al 1995b), and interneurons with either up or down head-velocity signal are located in the superior vestibular nucleus (SVN) (Blazquez et al 2000;Sato and Kawasaki 1987). In the normal gain or naïve animal, Y neuron head-velocity sensitivity arrives almost exclusively from the flocculus because inputs from up-head and down-head velocity interneurons in the superior vestibular nucleus cancel each other at the level of the dorsal Y group, as suggested by pharmacological inactivation of the cerebellar flocculus (Partsalis et al 1995b).…”