DESCENDING CONNECTIONS FROM THE BRAIN-STEM TO THE SPINAL-CORD IN THE ELECTRIC FISH EIGENMANNIA - QUANTITATIVE DESCRIPTION BASED ON RETROGRADE HORSERADISH-PEROXIDASE AND FLUORESCENT-DYE TRANSPORT

The descending connections from the brainstem to the spinal cord in Eigenmannia sp. were demonstrated using the horseradish peroxidase (HRP) technique. The spinal cord was transected and HRP crystals were deposited in the cut. The point of transection was located at varying distances from the head in different specimens. In all experiments, cells were labeled in both the rhombencephalic and mesencephalic tegmentum. No labeled cells were found in the cerebellum, the lateral-line lobes, the torus semicircularis, the tectum mesencephali, the hypothalamus, the diencephalon or the telencephalon. Labeled neurons were found in the ventrolateral column, nucleus formatio reticularis (NFR) inferior, NFR medius, NFR superior pars superior and pars suprema, NFR tegmenti mesencephali lateralis, nucleus vestibularis magnocellularis and nucleus fasciculi longitudinalis medialis. Furthermore, the Mauthner cells and the neurons of the pacemaker nucleus were filled with HRP granules. The neurons labeled were predominantly the large ones of more than 25 μm in diameter which are very conspicuous along the brainstem. The number of these neurons in the different nuclei varied from animal to animal, however, the number of labeled neurons increased monotonically at a similar rate in all brainstem nuclei with more rostrally located transection sites. In a second series, the number of neurons terminating in a small number of segments independent of absolute position along the body axis was assessed using two different fluorescent dyes. Within tolerable statistical limits, this number was found to be constant, corroborating the data obtained with HRP. A possible interpretation of the data is placed in the context of physiological data previously presented.