Somatosensory nucleus in the torus semicircularis of cyprinid teleosts

Cytoarchitecture and fiber connections of the ventrolateral region of torus semicircularis were studied in goldfish and carp. Cytoarchitectural analyses indicate that two toral nuclei, ventrolateral (TSvl) and external (TSe) nuclei, are present in this toral region. The TSvl mainly receives lateral line inputs from the medial nucleus of the rhombencephalic octavolateral area, while the TSe mainly receives somatosensory fibers from the spinal cord, sensory trigeminal nuclei, and lateral funicular nucleus. The TSe also receives sparse fibers from the retina and the octavolateral area medial nucleus and sends descending fibers to the lateral funicular nucleus. Both toral nuclei receive projections from the ventromedial thalamic nucleus and the central part of dorsal telencephalic area, although the latter descending fibers terminate mainly in the TSvl. Both toral nuclei project to the rostrolateral region of lateral preglomerular nucleus, restricted to the somata layer of the dorsal zone of the region. Ventrolateral and external toral fibers, however, tended to reach lateral and medial zones of the somata layer, respectively. The present study suggests that there are three functional compartments within the cyprinid torus semicircularis (central nucleus: auditory, TSvl: lateral line, and TSe: somatosensory) and that sensory pathways arising from these toral compartments maintain largely separate ascending systems into the forebrain. The present study also extends knowledge of the organization and connections of the lateral valvular nucleus and nucleus praeeminentialis that project to the cerebellar granular layer and cerebellar crest, respectively.