The spinal cord of Gymnotus carapo: the electromotoneurons and their projection patterns

The electric organ of Gymnotus carapo lies parallel to the spinal cord and extends from the pectoral girdle to the tip of the tail. The spinal electromotoneurons are distributed in a relatively consistent pattern: there is a peak at 17% of the fish's length and an irregular distribution beyond 25%. Horseradish peroxidase injections into the electric organ not exceeding 5% of the fish's length labeled electromotoneuron arrays occupying 20% of the fish's length. Injections made in four discrete rostrocaudal electric organ regions resulted in labeled electromotoneurons distributed along four sequential but overlapping arrays. Since the caudal portion of the spinal cord lacks electromotoneurons, there is a shortened representation of the electric organ. The electromotoneuron population is not homogeneous: there are small neurons (somata 25-40 microns) and large neurons (somata 45-60 microns) unevenly distributed along the cord. Small neurons occur at more rostral spinal cord segments, while large neurons lie in more caudal segments. Both kinds of nerve cells coexist in the intermediate regions. Overlapping of subsequent neuronal arrays favors synchronized firing of electrocytes. The presence of two neuronal populations differing in size and projecting to opposite electrocyte faces may account for the timed excitation of the electrogenic surfaces. Taking into account these new findings a comprehensive explanation of the activation sequence along the spinal cord and the electric organ is proposed.