The efferent vestibular neurons in the toad ( Bufo bufo L.): their location and morphology. A horseradish peroxidase study

Abstract

Retrogradely labeled neurons are observed in the central nervous system of the toad after peripheral application of peroxidase to the anterior and posterior stumps of the VIIIth nerve. These efferent vestibular neurons are localized in the brainstem only ipsilaterally to the treated nerve; they are restricted within a region close to the motor nucleus of the VIIth nerve, outside the vestibular nuclear complex, and are predominantly localized to the borders between the gray and white matter in an arrangement that seems to surround the motor nucleus of the VIIth nerve. No evidence was found for the existence of labeled Purkinje cells in the cerebellar cortex and therefore of a cerebellolabyrinthine pathway. The efferent vestibular neurons are medium-sized cells with two prominent dendrites, often oriented in a mediolateral direction. A comparative analysis between these neurons and the contiguous motoneurons of the VIIth nerve permitted differentiation of the two groups of neurons as to: (a) distribution in a mediolateral direction, (b) distribution in depth from the brainstem surface, (c) longitudinal extension in the caudorostral direction and (d) morphologic characteristics and dendritic arrangement of the neurons.