DESCENDING NEURAL PROJECTIONS TO THE SPINAL-CORD IN THE CHANNEL CATFISH, ICTALURUS-PUNCTATUS

Retrograde transport of horseradish peroxidase was used to determine t he descending projections to the spinal card in an otophysan fish, the channel catfish, Ictalurus punctatus. The majority of cells projectin g to the spinal cord are located in the reticular formation, which. is organized into rhombomeric segments. Vestibulospinal neurons are loca ted in the descending, magnocellular, and tangential octaval nuclei, a s well as in the medial octavolateralis nucleus of the lateral line sy stem. Cells in the facial lobe project to the spinal cord. Additionall y, axons of cells of the trigeminal system and the nucleus of the late ral lemniscus project caudally into the spinal cord. In the midbrain, descending spinal projections arise from cells of the medial longitudi nal fasciculus and the red nucleus. More rostrally, cells of the ventr olateral thalamus, dorsal periventricular hypothalamus, central pretec tal and magnocellular preoptic nuclei also project to the cord. The re sults of this study indicate that there are a number of homologies in the descending systems of bony fishes and other vertebrate taxa, inclu ding tetrapods. We also provide further evidence that a red nucleus is present in the brains of bony fishes and is therefore a primitive ver tebrate character antedating the evolution of tetrapods. Anat. Rec. 25 2:235-253, 1998. (C) 1998 Wiley-Liss, Inc.