Auditory and lateral line inputs to the midbrain of an aquatic anuran; Neuroanatomic studies in Xenopus laevis

Computation of rate in auditory signals is essential to call recognition in anurans. This task is ascribed to a group of central nervous sytem nuclei in the dorsal midbrain or torus semicircularis, homologous to the inferior colliculus of mammals. We have mapped the connections of the subnuclei of t he torus semicircularis in Xenopus laevis to determine which receive audito ry and which receive lateral line information. Relative to terrestrial anur ans, the torus of X. laevis is hypertrophied and occupies the entire caudal , dorsal midbrain. Auditory input to the torus, that arising directly from the dorsal medullary nucleus, is present only in the laminar nucleus. The p rincipal and magnocellular nuclei receive their input from the lateral line nucleus of the medulla. All three nuclei of the torus also have reciprocal connections with the superior olive and the nucleus of the lateral lemnisc us. Ascending efferents from all three nuclei of the torus innervate centra l and lateral thalamic nuclei, and all have a weak reciprocal connection wi th the posterior thalamus. The laminar and magnocellular nuclei have recipr ocal connections with the ventral thalamus, and all three nuclei of the tor us receive descending input from the anterior entopeduncular nucleus. The l aminar and magnocellular nuclei also receive descending input from the preo ptic area. Based on our identification of toral nuclei and these results we assign a major function for the detection of water-borne sounds to the lam inar nucleus and a major function for the detection of near field disturban ces in water pressure to the principal and magnocellular nuclei. J. Comp. N eurol. 438:148-162, 2001, (C) 2001 Wiley-Liss, Inc.