A MORPHOPHYSIOLOGICAL STUDY OF THE SUPERIOR VESTIBULAR NUCLEUS IN THE CAT
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The present study is an attempt to increase the understanding of the role of the superior vestibular nucleus (SVN) in the vestibulooclar reflex (VOR) and, in general, in the control of gaze by brain stem neurons. The prime objective is to investigate the morphophysiology of the nucleus, that is, the electrophysiological responses of the SVN neurons in connection with their morphological characteristics.;Intracellular recording from SVN neurons, following electrical stimulation of the ipsilateral and contralateral vestibular nerves and of the oculomotor complex, is combined with intracellular horseradish peroxidase staining and serial reconstruction of these neurons.;Six types of SVN neurons are described in this study according to their projections to the oculomotor complex, the cerebellum, the reticular formation, the contralateral vestibular nuclei and the motor trigeminal nucleus. Vestibuloocular neurons are located throughout central and caudal SVN, while the other cell types are preferentially located in the periphery of the nucleus. Most of the somadendritic features of the different SVN cell types are similar. An isodendritic branching pattern of the vestibuloocular neurons and a branching pattern of the other neurons exhibiting allodendritic characteristics are described. SVN neurons receive an excitatory monosynaptic input from the ipsilateral vestibular nerve and are located throughout the nucleus. In more that one third of the SVN neurons an inhibitory postsynaptic potential is evoked by stimulation of the contralateral vestibular nerve. SVN neurons display only a few axon collaterals while Golgi type II interneurons are not encountered in the SVN. These findings support the implication that each SVN neuron is conveying information to only one site outside the nucleus and that there is no communication between SVN neurons on the same side of the brain.