Afferent and efferent connections of the primary octaval nuclei in the clearnose skate, Raja eglanteria.
Horseradish peroxidase techniques were employed to trace the central projections of afferents from the individual endorgans of the membranous labyrinth and to delineate the efferent projections from the primary octaval nuclei to the spinal cord and midbrain octavolateralis area in the clearnose skate, Raja eglanteria. First-order octaval afferents project ipsilaterally to five primary octaval nuclei, namely: magnocellular, descending, posterior, anterior, and periventricular. Octaval afferents also terminate in the reticular formation, nucleus intermedius (primary mechanoreceptive lateral-line nucleus), and vestibulolateral lobe of the cerebellum. Each primary octaval nucleus receives afferent input from each labyrinthine endorgan, with the possible exception of macula neglecta input to the magnacellular nucleus. Within the anterior, descending, and to a lesser extent posterior and magnocellular nuclei, this input is largely nonoverlapping. Semicircular canal cristae afferents terminate ventrally, saccular and lagenar afferents dorsally, utricular afferents laterally, and macular neglecta afferents course ventrally but terminate largely dorsally within these nuclei. In the vestibulolateral lobe of the cerebellum, cristae afferents project primarily to the pars medialis, whereas macular endorgan afferents terminate in the pars lateralis. Primary afferent input to the reticular formation is predominantly from the horizontal canal crista. The densest projections to nucleus intermedius are from the utriculus and sacculus. Vestibulospinal projections originate primarily from the magnocellular and descending nuclei. Second-order auditory neurons are most likely located in dorsomedial parts of the descending and anterior nuclei. Cells in these nuclei project directly to the auditory area of the midbrain octavolateralis complex, but projections to this area originate predominantly from nuclei C1 and C2, which are possible superior olivary homologues.