Identification of spinal electromotoneurons in the ray Raja clavata (Rajidae).

Retrograde transport of horseradish peroxidase injected ipsilaterally in several parts of the electric organ and light microscopy were employed to study motoneurons and electromotoneurons of the spinal cord in the weakly electric ray Raja clavata (Rajidae). The horseradish peroxidase-labelled electromotoneurons form a chain of cells located ventrolaterally in the gray matter of the spinal cord segments adjacent to the electric organ. The diameters of cells in the electromotor nucleus vary from 70 to 110 microns. When comparing other motoneurons of the spinal cord with electromotoneurons, the latter reveal no segmentation, whereas motoneurons yield accumulations which are close to metametric ones and are essentially smaller in size (averaging 33 microns in diameter).