Anatomical study and HRP identification of electromotoneurons and motoneurons in the spinal cord of Gymnarchus niloticus.

Two types of neurons were identified by light and electron microscopy in the spinal cord of Gymnarchus niloticus following HRP injections at different peripheral sites: one, situated in the mediodorsal region, is labelled after injection into the electric organ; the other, situated in the lateroventral region, is labelled in addition after injections extending beyond to the lateral muscle. The cells of the first type, thus shown to be electromotoneurons (EMNs), are spherical and do not have dendritic processes; some of them are connected by somato-somatic gap junctions. The EMNs are surrounded by dense glial processes and embedded in a network of myelinated axons of large diameter. Synaptic contacts with the EMNs are of axosomatic type, exhibiting mostly gap junctions and, less frequently, mixed or chemical junctions. Some endings establish gap junctions simultaneously, with two electromotoneurons, the somatic membrane of which may be joined. The initial segment does not bear any endings. The second type of cells, identified as motoneurons, are pyriform and have large dendritic processes. The motoneurons are surrounded by myelinated axons of small diameter. They receive axosomatic endings with chemical synapses.