The vestibular nuclei in the domestic hen (Gallus domesticus). IV. The projection to the spinal cord.

Horseradish peroxidase was injected at various levels of the spinal cord of the hen and the cells in the brain stem, labeled due to retrograde transport of the tracer enzyme, were mapped with particular reference to the vestibular nuclear complex. The Deiters' nuclei project ipsilaterally to the spinal cord. The projection from the nucleus Deiters ventralis reaches down to the lumbosacral spinal cord. The spinal projection from the nucleus Deiters dorsalis is somatotopically organized. Cells localized rostrally within the nucleus project to upper parts of the spinal cord cells localized caudally project to the lumbosacral spinal cord. The medial and the descending nucleus contain labeled cells in two compartments of the nuclei. In each of them a rostral one projects to the upper cervical cord, a caudal one projects to the thoracical spinal cord. In addition, labeled cells are observed in the reticular formation, mainly in the pons and the medulla oblongata, in the red nucleus, in the raphe nuclei and in the periaqueductal grey. Very few labeled cells are observed in the locus ceruleus, the nucleus intercalatus and in the nucleus of the solitary tract. In some cases, the number of labeled cells in the various nuclei in the brain stem projecting to the spinal cord was counted to get an impression of the quantitative importance of the vestibulospinal projection relative to afferents to the spinal cord from the other nuclei. The findings are discussed in the light of what is known of the vestibulospinal projection in mammals.