Vestibulo-thalamic neurons give off descending axons to the spinal cord

Differences between otolith- and semicircular canal-activated neural circuitry in the vestibular system

In the last two decades, we have focused on establishing a reliable technique for focal stimulation of vestibular receptors to evaluate neural connectivity. Here, we summarize the vestibular-related neuronal circuits for the vestibulo-ocular reflex, vestibulocollic reflex, and vestibulospinal reflex arcs. The focal stimulating technique also uncovered some hidden neural mechanisms. In the otolith system, we identified two hidden neural mechanisms that enhance otolith receptor sensitivity. The first is commissural inhibition, which boosts sensitivity by incorporating inputs from bilateral otolith receptors, the existence of which was in contradiction to the classical understanding of the otolith system but was observed in the utricular system. The second mechanism, cross-striolar inhibition, intensifies the sensitivity of inputs from both sides of receptive cells across the striola in a single otolith sensor. This was an entirely novel finding and is typically observed in the saccular system. We discuss the possible functional meaning of commissural and cross-striolar inhibition. Finally, our focal stimulating technique was applied to elucidate the different constructions of axonal projections from each vestibular receptor to the spinal cord. We also discuss the possible function of the unique neural connectivity observed in each vestibular receptor system.

Reorganization of the vestibulothalamic projections in lesions to the interpositus nucleus of the cerebellum and the vestibular nucleus of Deiters

The potential for plastic reorganization in the vestibulothalamic system was studied in adult cats. Preliminary (three months) lesioning of the contralateral nucleus interpositus of the cerebellum or the lateral vestibular nucleus of Deiters led to reorganization of vestibulothalamic projections with formation of ipsilateral projections to the ventrolateral nucleus of the thalamus from the nuclei of the vestibular complex, along with changes in the normal representation of the contralateral projections of the vestibular complex to this thalamic nucleus. The distribution and morphological composition of cells in the vestibular nuclei forming the new projections to the ventrolateral nucleus of the thalamus were studied.

Posterior canal-activated vestibulocortical pathways in cats

This study was undertaken to investigate vestibulothalamocortical pathways in anesthetized cats. Synaptic connections of posterior canal-activated excitatory vestibuloocular relay (PC) neurons to thalamic neurons were examined by a spike-triggered averaging technique. The averaged potentials evoked in the ventrobasal complex of the thalamus revealed a negative wave with latencies from 0.8 to 1.5 ms. Thirty-six thalamic neurons, which were activated by nose-up head rotation and by contralateral labyrinth stimulation, were mainly located in the ventrobasal complex. Thirteen of these neurons were antidromically activated from the anterior suprasylvian sulcus or postcruciate dimple of the cortex. These results suggest that the PC neurons participate, at least in part, in the vestibulocortical pathways contributing to spatial orientation.

Posterior canal-activated excitatory vestibuloocular relay neurons participate in the vestibulocortical pathways in cats

We have previously reported that axon collaterals of posterior canal-activated excitatory vestibular (PC) neurons project to the contralateral oculomotor nucleus, and rostrally to the thalamus. To elucidate the vestibulothalamocortical pathways we investigated the synaptic connections of the PC neurons with the thalamic neurons by post-spike averaging of compound potentials triggered by spikes of the PC neuron in anesthetized cats. The averaged field potential evoked in the ventrobasal complex (VBC) revealed a spike followed by a negative wave. Latencies of the wave ranged from 0.8 to 1.5 ms. Next, we examined the location and axonal projection of 36 thalamic neurons which were activated by nose-up head rotation and by contralateral labyrinth stimulation. Most of them were located in the VBC and some in the medial geniculate body. Thirteen of the 36 neurons were antidromically activated from the anterior suprasylvian sulcus or postcruciate dimple of the cortex. These results suggest that the PC neurons participate, at least in part, in the vestibulocortical pathways contributing to spatial orientation.

Ascending projections of posterior canal-activated excitatory and inhibitory secondary vestibular neurons to the mesodiencephalon in cats

The axonal projections of 62 posterior canal (PC)-activated excitatory and inhibitory secondary vestibular neurons were studied electrophysiologically in cats. PC-related neurons were identified by monosynaptic activation elicited by electrical stimulation of the vestibular nerve and activation following nose-up rotation of the animal's head. Single excitatory and inhibitory neurons were identified by antidromic activation following electrical stimulation of the contralateral and ipsilateral medial longitudinal fasciculus, respectively. The oculomotor projections of identified neurons were confirmed with a spike-triggered averaging technique. The axonal projections of the identified neurons were then studied by systematic, antidromic stimulation of the mesodiencephalon. Excitatory neurons showed two main types of axonal projections. In one type, axonal branches were issued to the interstitial nucleus of Cajal, central gray, and thalamus including the ventral posterolateral, ventral posteromedial, ventral lateral, ventral medial, centromedian, central lateral, lateral posterior, and ventral lateral geniculate nuclei. The other type was more frequently observed, giving off axon collaterals to the above-mentioned regions and to Forel's field H as well. Inhibitory neurons issued axonal branches to limited areas which included the central gray, interstitial nucleus of Cajal, its adjacent reticular formation and caudalmost part of Forel's field H, but not the rostral part of the Forel's field H and the thalamus. These results suggest that PC-related excitatory neurons participate in the genesis of vertical eye movements and in the perception of the vestibular sensation, and that PC-related inhibitory neurons seem to take part only in the genesis of vertical eye movements.