Medullary and mesencephalic pathways and connections of lateral line neurons of the spiny dogfish Squalus acanthias.

The neuronal connections of the electrosensory dorsal and the mechanosensory medial octavolateralis nuclei of the spiny dogfish Squalus acanthias were studied by horseradish peroxidase, autoradiographic and axonal degeneration methods. Efferents from each nucleus, in addition to extensive commissural components, give rise to ipsilateral and contralateral lemnisci that ascend to midbrain levels and terminate among the cells of the lateral mesencephalic nucleus (LMN). Within LMN, electrosensory and mechanosensory neurons distribute dorsolateral and ventromedial in position, respectively. Ascending fibers of both modalities also terminate within the central zone of the optic tectum. The LMN of spiny dogfish sharks that possess a primitive pattern of midbrain organization is homologous to parts of the lateral mesencephalic nuclear complex of batoids that possess a more derived pattern of midbrain organization. Other fiber connections of the dorsal and medial octavolateralis nuclei appear to differ from each other, indicating that electrosensory and mechanosensory lateral line information is carried over separate pathways at least to midbrain levels of the brain stem. For example, nucleus B, a feedback center, occupies a position in the descending lateral line pathways of sharks and skates similar to nucleus praeeminentialis of many electrosensory teleosts. The dorsal octavolateralis nucleus of sharks and skates receives afferents from nucleus B but there is no evidence that nucleus B directly feeds back to the medial octavolateralis nucleus of the spiny dogfish. Moreover, unlike the dorsal nucleus, the medial nucleus of Squalus is reciprocally linked with the octaval system.