Anatomical study of vestibulospinal neurons in lampreys

The present study was carried out to characterize anatomically the vestibul ospinal (VS) system of lampreys. Cobalt-lysine or Texas Red dextran amines were applied in vitro to the rostral spinal cord. Two distinct populations of VS neurons were labeled in the ventral nucleus of the area octavolateral is. The rostral group, comprising the intermediate octavomotor nucleus (ION ), contained between 100 and 150 neurons, having somata of variable size an d morphology. Intracellular injections of Lucifer Yellow in single neurons revealed ION VS neurons with dendrites extending in the ventrolateral alar plate as well as medially in the basal plate. The caudal group, comprising the posterior octavomotor nucleus (PON), contained approximately 65 neurons , most of which were unipolar with round or oval somata. To study the proje ctions of VS axons, cobalt-lysine was injected into the ION or PON regions in the brainstem. Axons from the ION projected to the ipsilateral spinal co rd, whereas PON axons decussated within the basal plate giving out descendi ng and ascending branches. The descending branch projected to the contralat eral spinal cord. Injections of two fluorescent dextran-amines, each restri cted to one side of the spinal cord, did not double-label VS cells in eithe r octavomotor nuclei, indicating that the projections of each nucleus are r estricted to one side. Injections of horseradish peroxidase further caudall y in the spinal cord revealed that VS axons from the ION reached past the g ill region. Our results indicate that the organization of the VS system of lampreys is similar to that observed in other vertebrates. J. Comp. Neurol. 407:512-526, 1999. (C) 1999 Wiley-Liss, Inc.