Projection of brain stem neurons to the giant electromotoneurons in the cervical spinal cord of the electric catfish Malapterurus electricus.

Two giant electromotoneurons located within the cervical spinal cord form the centerpiece of the electromotor system in the electric catfish Malapterurus electricus. The cytoarchitectural organization suggests a high degree of input convergence onto the electromotoneurons. In order to obtain insights into the connectivities of the electromotor system, pre-neurons of the electromotoneurons within the brain stem and the spinal cord were labelled by application of FITC-dextran and horseradish peroxidase onto the surface of a single electromotoneuron. Our results show that the electromotoneurons receive their main inputs from the nucleus profundus mesencephali within the tegmentum and from large neurons of the medial reticular formation. Both nuclei possess an intimate connection to the optic tectum which mediates orientation responses. This pathway to the electromotoneurons could be instrumental in eliciting electric organ discharge during prey catching. The electric avoidance response in turn could be mediated by the Mauthner neurons which are also labelled. In addition to these neurons, cells of the nucleus fasciculi longitudinalis medialis, the descending octaval nucleus and the nucleus funicularis medialis were labelled. As compared to the corresponding neurons in ictalurid catfish, none of these neurons displays any alteration in its general morphology. It is concluded that the evolution of the electric organ from muscle tissue and the development of a central control system of the electromotor response in Malapterurus involved a minimum of alterations in central nervous system circuitry. In contrast to many other electric fishes the electromotor control is mainly accomplished at the level of the electromotoneurons.