Morphological correlates of functional differentiation of nodes of ranvier along single fibers in the neurogenic electric organ of the knife fish Sternarchus

Electric organs in Sternarchidae are of neural origin, in contrast to electric organs in other fish, which are derived from muscle. The electric organ in Sternarchus is composed of modified axons of spinal neurons. Fibers comprising the electric organ were studied by dissection and by light- and electron microscopy of sectioned material. The spinal electrocytes descend to the electric organ where they run anteriorly for several segments, turn sharply, and run posteriorly to end blindly at approximately the level where they enter the organ. At the level of entry into the organ, and where they turn around, the axons are about 20µ in diameter ; the nodes of Ranvier have a typical appearance with a gap of approximately 1 s in the myelin. Anteriorly and posteriorly running parts of the fibers dilate to a diameter of approximately 100µ, and then taper again. In proximal and central regions of anteriorly and posteriorly running parts, nodal gaps measure approximately 1µ along the axon. In distal regions of anteriorly and posteriorly running parts are three to five large nodes with gaps measuring more than 50µ along the fiber axis. Nodes with narrow and with wide gaps are distinguishable ultrastructurally ; the first type has a typical structure, whereas the second type represents a new nodal morphology. At the typical nodes a dense cytoplasmic material is associated with the axon membrane. At large nodes, the unmyelinated axon membrane is elaborated to form a closely packed layer of irregular polypoid processes without a dense cytoplasmic undercoating. Electrophysiological data indicate that typical nodes in proximal regions of anteriorly and posteriorly running segments actively generate spikes, whereas large distal nodes are inactive and act as a series capacity. Increased membrane surface area provides a morphological correlate for this capacity. This electric organ comprises a unique neural system in which axons have evolved so as to generate external signals, an adaptation involving a functionally significant structural differentiation of nodes of Ranvier along single nerve fibers.