[Efferent neurons of the auditory center in the midbrain of the frog Rana ridibunda].

Individual cells which produce projections from the torus semicircularis in the frog have been visualized after injection of horseradish peroxidase (HRP) to various thalamic and isthmal areas. Labeled toral cells were observed if HRP had been injected to the posterodorsal areas of the thalamus or to the isthmal areas where lateral lemniscus fibers and cells of the premature lateral lemniscal nucleus are situated. Medium and large size cells in the rostrolateral torus semicircularis were mainly labeled. Thalamic injections of the HRP produced more labeled cells in the lateral part of the magnocellular nucleus, whereas isthmal injections produced labeled cells mainly in the lateral part of the laminar nucleus. A few HRP containing cells were observed in the principal nucleus of the torus. Specificity of the neuronal organisation of the auditory pathway in amphibians is discussed.