Ascending connections of a midbrain auditory area in a crocodile, Caiman crocodilus

Abstract

AbstractThe torus semicircularis is a tonotopically organized auditory region in the midbrain of the crocodile, Caiman crocodilus (Manley, '71). Two distinct regions of the torus can be distinguished in Caiman: an external nucleus, which is continuous with the deep layers of the optic tectum, and a central nucleus. Ascending connections of the central nucleus were studied with the light microscope by means of the Fink‐Heimer procedure after unilateral, anodal stereotaxic lesions. Efferent axons leave the central nucleus in the lateral lemniscus to enter the tecto‐reuniens tract where they course as far rostrally as nucleus Z. At this level, this bundle bifurcates. The majority of these fibers turn medially and then pass through and probably synapse on interposed neurons of nucleus Z, prior to their termination in nucleus reuniens. The remaining axons continue ventrally past nucleus Z to enter the ventral supraoptic decussation where they travel anteriorly to a level just posterior to the optic chiasm. Here the fascicles cross the midline and swing caudally, still in the ventral supraoptic decussation, until they reach nucleus Z of the contralateral side. At this level, these fibers enter the tecto‐reuniens tract and turn medially, to pass through and perhaps end on the intercalated neurons of nucleus Z, prior to their termination in nucleus reuniens. The neuronal architecture of nucleus Z and that of the nucleus reuniens complex are described. The latter, a midline nuclear group, consists of two subdivisions: a pars centralis and a pars diffusa. Each of these subdivisions is segregated into two neuronal aggregates at the midline. Efferents of the central nucleus terminate massively in the pars centralis and, to a considerably lesser extent, in the pars diffusa of nucleus reuniens.The results of this study are compared with similar ones in pigeons and mammals. Parallels in the fiber connections of midbrain auditory and visual areas and the segregation of these modalities in the mesencephalon and diencephalon are discussed.