Relationship between vestibular primary afferents and vestibulospinal neurons in lampreys

The distribution of vestibular primary afferents as well as their relationship with vestibulospinal and other brainstem neurons were studied in lampreys using anatomical tracers. Afferents from the anterior (aVIIIn) and the posterior (pVIIIn) branches of the vestibular nerve were located mainly in the ventral nucleus of the octavolateral area. The relationship between afferents and vestibulospinal neurons was studied by applying one fluorescent tracer to the whole vestibular nerve or one of its branches and applying another tracer to the spinal cord. Some afferents showed large, bulb-like enlargements (bulbs) and about 20 of these were found in the anterior and the intermediate octavomotor nucleus, whereas about 40 were found in the posterior octavomotor nucleus. Some of the bulbs made apparent contact with vestibulospinal neurons in the intermediate octavomotor nucleus and originated mostly from the aVIIIn, whereas bulbs in the posterior octavomotor nucleus originated from the pVIIIn. Applications of biocytin to hemisegments of rostral spinal cord labeled vestibulospinal neurons located in the ipsilateral intermediate octavomotor nucleus and the contralateral posterior octavomotor nucleus. In addition, vestibular primary afferents with bulbs in apparent contact with vestibulospinal neurons were transneuronally labeled by biocytin. They were observed in the ipsilateral aVIIIn and the contralateral pVIIIn and could be followed in the labyrinths, where they innervated the vertical and horizontal arms of the semicircular canal crests. Taken together, these results indicate that vestibular primary afferents from the aVIIIn innervate predominantly vestibulospinal neurons of the intermediate octavomotor nucleus, whereas afferents from the pVIIIn innervate vestibulospinal neurons in the posterior octavomotor nucleus. This anatomical organization suggests that afferents carrying bulbs convey dynamic information to vestibulospinal neurons, which, in turn, project to the spinal cord networks.