From genes to behavior in the vestibular system

The central nervous system of all vertebrate embryos is derived from a series of conspicuous segments, called neuromeres, that are particularly visible in the midbrain and hindbrain areas, giving rise to the brain stem sensory and motor nuclei. This article focuses on a series of eight embryonic rhombomeric segments whose progeny can be identified in adults by the locations of iteratively homologous reticulospinal neurons and cranial motor nuclei IV through XII. Evidence shows that these rhombomeric units represent domains of gene expression, lineage restriction, and accordingly, individual vestibular neuronal phenotypes with unique oculomotor and spinal projections. Preliminary electrophysiologic and behavioral correlates of a few vestibulo-oculomotor subgroups are used as examples to illustrate the hypothesis that homologous vestibular phenotypes likely exist in all taxa because the genetic prepattern is already well established in primitive vertebrates. Finally,the segmented hindbrain arrangement responsible for the longitudinally arranged column of vestibular subnuclei is placed in perspective with genetic and molecular approaches that will eventually permit a causal reconstruction of the signaling mechanisms responsible for the development of unique vestibular subgroups.