Afferents to the midbrain auditory center in the bullfrog, rana catesbeiana

Horseradish peroxidase (HRP) histochemistry was used to visualize cells afferent to the bullfrog torus semicircularis. These afferent cells are located in several sensory and nonsensory nuclei. The sensory structures which project to the torus are mainly auditory nuclei, with the major input coming from the ipsilateral superior olive. A very small contralateral projection is also present. In addition, afferents arise from the contralateral, and to a lesser extent ipsilateral, dorsal acoustic nucleus and nucleus caudalis, both primary eighth nerve nuclei. A vestibular input is also apparent in that HRP-positive cells were seen in the magnocellular vestibular nucleus and among elongated bipolar cells at the ventral border of the eighth nerve nuclei. In addition, the torus receives somatosensory input from the contralateral perisolitary band. Afferents from spinal cord cells proved difficult to visualize. Nonsensory areas throughout the brain innervate the torus as well. In the medulla, HRP-positive cells were present bilaterally in both medial and lateral reticular areas. The tegmentum contributes a major input from the superficial isthmal reticular nucleus and a minor input from the tegmental fields. Commissural toral projections are also present. Descending forebrain input arises from the pretectal gray bilaterally, the ventral half of the ipsilateral lateral pretectal nucleus, and possibly, from the ipsilateral posterior thalamic nucleus. HRP-positive cells were also occasionally seen in the posterior tuberculum, ventral hypothalamus, and caudal suprachiasmatic preoptic area. Finally, a telencephalic projection from the ipsilateral anterior entopeduncular nucleus is present.