Central organization of the electrosensory system in the paddlefish (Polyodon spathula)

The central connections of the electrosensory system were studied in the paddlefish Polyodon spathula by injecting biotinylated dextran amines into the dorsal octavolateral nucleus (DON), the cerebellum, and the mesencephalic tectum. The sole target of primary electrosensory fibers is the ipsilateral dorsal octavolateral nucleus. The principal neurons ascending from this nucleus project to the torus semicircularis, the lateral mesencephalic nucleus, and the mesencephalic tectum. The mesencephalic tectum projects back to the nucleus preeminentialis, which, in turn, projects to the cerebellar auricles and to the DON. The auricles are the main source of parallel fibers in the cerebellar crest ventral to the DON. The DON also receives input from the contralateral DON. These descending feedback loops are very similar to those of other electrosensory fishes. However, the paddlefish is unique in having three mesencephalic targets of electrosensory information. It is the only bony fish known to have extensive projections directly to the mesencephalic tectum and to a lateral mesencephalic nucleus in addition to the torus semicircularis.