Relationship between vestibular primary afferents and vestibulospinal neurons in lampreys

The distribution of vestibular primary afferents as well as their relations hip with vestibulospinal and other brainstem neurons were studied in lampre ys using anatomical tracers. Afferents from the anterior (aVIIIn) and the p osterior (pVIIIn) branches of the vestibular nerve were located mainly in t he ventral nucleus of the octavolateral area. The relationship between affe rents and vestibulospinal neurons was studied by applying one fluorescent t racer to the whole vestibular nerve or one of its branches and applying ano ther tracer to the spinal cord. Some afferents showed large, bulb-like enla rgements (bulbs) and about 20 of these were found in the anterior and the i ntermediate octavomotor nucleus, whereas about 40 were found in the posteri or octavomotor nucleus. Some of the bulbs made apparent contact with vestib ulospinal neurons in the intermediate octavomotor nucleus and originated mo stly from the aVIIIn, whereas bulbs in the posterior octavomotor nucleus or iginated from the pVIIIn. Applications of biocytin to hemi-segments of rost ral spinal cord labeled vestibulospinal neurons located in the ipsilateral intermediate octavomotor nucleus and the contralateral posterior octavomoto r nucleus. In addition, vestibular primary afferents with bulbs in apparent contact with vestibulospinal neurons were transneuronally labeled by biocy tin. They were observed in the ipsilateral aVIIIn and the contralateral pVI IIn and could be followed in the labyrinths, where they innervated the vert ical and horizontal arms of the semicircular canal crests. Taken together, these results indicate that vestibular primary afferents from the aVIIIn in nervate predominantly vestibulospinal neurons of the intermediate octavomot or nucleus, whereas afferents from the pVIIIn innervate vestibulospinal neu rons in the posterior octavomotor nucleus. This anatomical organization sug gests that afferents carrying bulbs convey dynamic information to vestibulo spinal neurons, which, in turn, project to the spinal cord networks. (C) 20 00 Wiley-Liss, Inc.