Multiple Neuronal Processes, Including the Mauthner Axon, Form a Multi-Axial Fiber Within a Common Myelin Sheath in the Central Nervous System of Adult Lungfishes, Protopterus annectens, Lepidosiren paradoxa, and Neoceratodus forsteri

Mauthner cells are found in most fish and amphibians. The prominence of their large fiber is commonly used as one criterion to identify the presence of these cells in fish and the largest of these fibers have been reported in lungfish. While some authors believe that Mauthner fibers in lungfish contain a single axon, others report that many processes join the Mauthner axon (M-axon) inside a common myelin sheath to form a “multi-axial fiber.” To distinguish between these two possibilities, we have used light and transmission electron microscopy to determine whether multi-axial fibers exist in African, Protopterus annectens, Australian, Neoceratodus forsteri, and South American, Lepidosiren paradoxa, lungfish. Ultrastructural analysis provides evidence of a multi-axial fiber that contains a M-axon, non-M-axons and glial processes within a common myelin sheath. The glial processes form myelin and paranodal-like structures. Stacked desmosome-like structures have been identified that may be part of Schmidt-Lanterman incisures. We discuss how the electrical activity of a select group of axons may affect that of other axons within a common myelin sheath.