Segmental Analysis of the Vestibular Nerve and the Efferents of the Vestibular Complex.

Use of a segmental approach in the study of vestibular centers in the hindbrain improves morphological and functional understanding of this region controlled by Hox genes, among other molecular determinants. Here, we review accrued data about segmental organization of vestibular afferents and efferents. Inner ear-originated vestibular fibers enter the hindbrain, together with auditory ones, through the alar plate of rhombomere 4, then branch into descending and ascending branches to reach appropriate vestibular nuclei along the vestibular column. Classical vestibular nuclei (superior, lateral, medial, and inferior) originate in eight successive rhombomeric segments, which suggests internal subdivisions correlated with distinct connections and functions. The vestibular projection neurons identified for various targets aggregate in discrete groups, which correlate topographically either with rhombomeric units, or with internal subdivisions within them. Each vestibular projection system (e.g., vestibulo-spinal, vestibulo-ocular, vestibulocerebellar) has a characteristic ipsilateral/contralateral organization. Comparing them as a connective mosaic in different species shows that various aspects of this segmental connective organization are conserved throughout evolution in vertebrates. Furthermore, certain genes that control the development of the rhombomeric units in the hindbrain may determine, among other aspects, the specific properties of the different neuronal subpopulations related to their axonal navigation and synaptogenesis. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc.