| Abstract: | The sources of the descending spinal tracts were identified in the teleost fish Gnathonemus petersii by retrograde HRP transport. HRP injections were made at two spinal levels, either at level of the caudal end of the dorsal fin, anterior to the electric organ, or at the pectoral fin. In both cases all labeled cells were found in the rhombencephalon and the mesencephalic tegmentum. No labeled cells were observed either in the cerebellum and lateral line lobes or in the dorsal mesencephalon i.e. torus semicircularis and mesencephalic tectum or in the telencephalon. Following caudal spinal injections, the majority of the labeled cells were grouped in a median and a ventrolateral column of the rhombencephalic reticular formation. The latter is composed of three parts corresponding to the nucleus reticularis inferior, medius and superior. Both Mauthner cells, all the cells in the medullary relay nucleus controlling the electric organ discharge and a few cells in the posterior part of the magnocellular octaval nucleus were labeled. In the mesencephalon, four nuclei were identified by HRP labeling: the nucleus of the medial longitudinal fasciculus, the nucleus reticularis mesencephali and the anterior and posterior tegmental mesodiencephalic nuclei. The rostral injections revealed several additional spinal projections from the descending vestibular and tangential nuclei, from the medial part of the magnocellular nucleus and, finally, from the rostral periventricular gray of the mesencephalon. Also, after such injections, a greater number of cells were labeled in the reticular formation, especially in the median column and in the inferior reticular nucleus. The results suggest that the rostral spinal cord has a larger connection with the acoustico-vestibular area and the medullary reticular formation than the caudal spinal cord. In contrast, the mesencephalic nuclei, probably linked to the mesencephalic tectum and the pretectal area, appears to be a coordinating apparatus between the visual system and the trunk/tail musculature. Thus, it appears that teleost fish possess the same basic equipment of descending spinal pathways as higher vertebrates. |
