A reinvestigation of the spinovestibular projection in the cat using axonal transport techniques

There are numerous discrepancies within the literature concerning the sources of spinovestibular fibers and their distribution in the vestibular complex. Sources of afferents from all spinal levels were sought using the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. Following injections of this tracer in all portions of the vestibular complex retrograde labelling was densest at upper cervical levels, especially within the contralateral central cervical nucleus. Labelling was also observed in laminae VI (ipsilaterally), IV, V, VII, and VIII (bilaterally). At progressively more caudal levels, numbers of labelled cells decreased but were similarly distributed in these laminae. The terminal distribution of spinal efferent fibers within the vestibular complex was revealed by injecting wheat germ agglutinin conjugated to horseradish peroxidase or tritiated amino acids into various levels of the spinal cord. These studies showed that all spinal levels project to the descending vestibular nucleus and group x. The f-tail of the descending vestibular complex receives projections from upper cervical and thoracic levels. Terminations within the medial vestibular nucleus arise from both upper cervical and lumbar levels. No conclusive evidence was found supporting the presence of substantial direct spinal projections to the lateral vestibular nucleus, superior vestibular nucleus, or group z. Possible functional roles for the spinovestibular projection in posture and gaze are discussed.