Morphology of spinal electromotor neurons and presynaptic coupling in the gymnotid Sternarchus albifrons

Spinal electromotor neurons in the gymnotid Sternarchus albifrons were studied by light and electron microscopy. In this species, the electric organ discharge, which is of high and relatively constant frequency, is generated by specialized axons which arise from the spinal electromotor neurons. The cell bodies are located in medial regions of the spinal cord. They are round to ellipsoid in outline and dendrites are not present. The initial myelin segment often extends partially over the cell body. Fine glial lamellae are interposed between closely adjacent cells, and somato-somatic gap junctions are not observed. The large majority of axosomatic synapses are characterized by gap junctions. Single axons are commonly found to establish gap junctions with two adjacent neurons. Only a few synapses have the characteristics associated with chemically mediated transmission. The morphological data provide evidence for electrotonic coupling between electromotor neurons by way of presynaptic fibres. The absence of dendrites in these neurons may provide a morphological correlate for their simple relay function.