Differential innervation patterns of three divisions of frog auditory midbrain (torus semicircularis)

The connectivity pattern of the laminar, principal, and magnocellular nuclei of the frog torus semicircularis (TS) was investigated. A small amount of horseradish peroxidase was injected focally into individual divisions of the TS and anterograde and retrograde transport patterns were observed. Results of our tracing study showed that these divisions of the TS possessed distinct innervation patterns. The principal nucleus appeared to be the primary input port of the TS receiving extensive inputs from all caudal brainstem auditory nuclei bilaterally, but especially from the contralateral dorsal medullary nucleus and the ipsilateral superior olivary and lateral lemniscus nuclei. Descending projection to this nucleus was limited to that from the posterior thalamic nucleus. In contrast, the laminar nucleus, but even more markedly the magnocellular nucleus, received extensive descending inputs from numerous structures in the dorsal thalamus and less pronounced ascending afferents from caudal brainstem auditory nuclei. Similar to the afferent connection patterns, the efferent projections originating from these three toral divisions differed substantially. The principal nucleus gave restricted ascending projections, limited mainly to the caudal region of the posterior thalamic nucleus, a region important in processing spectral information of complex sounds, and the pretectal gray. Its descending projection was also somewhat restricted, being limited to the superior olivary and lateral lemniscus nuclei. The laminar nucleus and especially the magnocellular nucleus gave robust descending as well as ascending projections; these nuclei serve as the main output paths for the TS and provide the main routes by which auditory input reaches the central thalamic nucleus, a structure previously shown to be involved in temporal information processing.