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Recent anatomical studies have identified a network of central neural circuits that appear to integrate vestibular and autonomic information. Like vestibulo-ocular and vestibulospinal circuits, these pathways appear to be under inhibitory modulation by distinct regions in the medial aspect of the cerebellar cortex. These central circuits have the potential to explain the known influence of vestibular stimulation on autonomic motor responses through descending effects on brain stem autonomic regions. In a more global context, the extensive convergence of vestibular and autonomic information in both vestibular and autonomic brain regions is consistent with the concept that vestibular and visceral information (for example, blood pooling and visceral proprioception) are used to form a central representation of gravitoinertial parameters during movements. This representation can influence neural circuitry involved in postural control, cardiovascular control, perception of the spatial vertical and emotional or affective responses.

Clinical observations have long indicated a vestibular influence on autonomic function. Neuroanatomical studies in the rabbit and in the cat have identified descending vestibulo-autonomic pathways from the caudal portion of the medial vestibular nucleus and the inferior vestibular nucleus to the dorsal motor nucleus of the vagus nerve, the nucleus of the solitary tract, and some brain stem medullary sympathetic regions. This study describes vestibulo-autonomic pathways in rats. One group of Long-Evans rats received injections of tetramethylrhodamine dextran into the caudal aspect of the vestibular nuclear complex. Anterogradely labeled descending fibers were traced bilaterally to lateral, ventrolateral, and intermediate subnuclei of the nucleus of the solitary tract and the dorsal motor nucleus of the vagus nerve. A small number of axons also projected bilaterally to the nucleus ambiguus, the ventrolateral medulla, and the nucleus raphe magnus. Finally, anterogradely labeled ascending fibers were traced from the caudal medial vestibular nucleus and the inferior vestibular nucleus to the medial, lateral, ventrolateral, and Kolliker-Fuse regions of parabrachial nucleus. A second group of rats received iontophoretic injections of Flu oro-gold into the nucleus of the solitary tract to identify the cells of origin of the vestibulo-solitary projection. Similar to findings in the rabbit (Balaban and Beryozkin, 1994), retrogradely labeled cells were observed in the caudal medial vestibular nucleus and the inferior vestibular nucleus. These findings are consistent with the hypothesis that a common pattern of vestibular nuclear projections to autonomic regions is shared by rabbits, cats, and rats.

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