Identification of spinal motoneurones in the weakly electric fish, Eigenmannia virescens

The motoneurones of the spinal cord in the weakly electric fish, Eigenmannia virescens, were investigated by light microscopical observations. For identification of motoneurones HRP injections were made: (1) in the electric organ, (2) in different parts of the trunk muscle, and (3) in the muscle of the anal fin. When the spinal cord was considered in transverse section: (1) labelled cells (EMN), 20-25 micron, were observed in the first case in the dorsal grey between the central canal and dorsal border of the spinal cord; (2) in the second case, labelled cells (MN1), 35-40 micron, were found in the central grey ventro-laterally, in close vicinity of the ependymal canal; (3) in the last case, labelled cells (MN2), 18-20 micron, were seen in the ventro-lateral part of the anterior horn. When the spinal cord was considered in longitudinal sections, EMNs and MN1 constitute a continuous column along the spinal axis, whereas MN2 show a metametric distribution. The 3 groups of motor cells can be identified in the spinal cord for the whole length of the anal fin, whereas the MN1 and MN2 disappear at the level of the caudal peduncle. The 3 cell types bear dendritic processes; the EMN thin ones and the MN1 and 2 rather large ones. The results indicate that distinct cell groups innervate the electric organ (EMN), the large muscles of the trunk (MN1) and those of the anal fin (MN2).