Lumbosacral predominance of vestibulospinal fibre projection in the rat.

Abstract

The somatotopic organization of the vestibulospinal tract (VST) and the percentages of cells of the lateral vestibular nucleus (LVN) contributing fibres to different regions of the spinal cord of the rat have not been studied before. These problems are investigated using the technique of retrograde cellular degeneration. Varying degrees of spinal cordotomies were performed at upper cervical, thoracic, and lumbar segments in 75 adult rats of Wistar strain. The animals were killed after varying survival periods and the brain stems were processed in the usual way. Chromatolytic neurons were observed in the ipsilateral lateral vestibular nucleus, and no such neurons were seen contralaterally. Following lumbar lesions, reacting neurons were condensed in the caudal two thirds, while with cervical and thoracic lesions, such neurons occupied more cranial levels in the nucleus. It is concluded that vestibulospinal fibres projecting to lumbosacral segments arise from the caudal two thirds of the nucleus, while those projecting to cervical and thoracic segments come from the cranial two thirds, those to the cervicals arise from the most cranial levels. The dense lumbosacral projection was attributed to the role played by the vestibulospinal fibres in maintaining extensor tonus of antigravity musculature, especially in the rat, which can effectively stand on its hindlimbs and use its free forelimbs for handling and examining objects.