The number of fibers and range of fiber diameters in the cochlear nerve of three odontocete species

Abstract

The total number of fibers and the range of fiber diameters in the cochlear nerve of three species of toothed whales were determined. There are 90 672 fibers in Lipotes vexillifer, 82 570 in Neophocaena phocaenoides, and 77 171 in Sousa chinensis. In Lipotes the fiber diameters range from 0.4 to 25.8 μm, with a modal diameter of 9.5 μm, and the largest fiber measures 35.2 μm; in Neophocaena, the diameter ranges from 0.6 to 36.0 μm, with a mode of 9.5 μm, and the largest fiber measures 54.9 μm; in Sousa the diameter ranges from 0.3 to 39.7 μm, with a mode of 8.5 μm and the largest fiber measures 50.3 μm. In Lipotes, Neophocaena, and Sousa, 1.5, 8.5, and 14.2%, respectively, of the cochlear fibers are ≥ 20 μm in diameter. Almost all the fibers are myelinated, and the ratio of the diameter of the axis cylinder to the outer diameter of the fiber in each species averages between 0.6 and 0.7. The high percentage of large fibers and the very wide range of fiber diameters indicate that the odontocete cochlear nerve is specialized for high-speed communication with the brain. The maximum diameters of the myelinated fibers of the cochlear nerve reported in this paper are the largest diameters recorded for the cranial nerve, or any other nerve, among the vertebrates.