Fiber composition of the superior laryngeal nerve in rats and guinea pigs.

The fiber composition of the superior laryngeal nerve (SLN) which is served as the laryngeal afferent pathway was clarified in rats and guinea pigs. The proportions of the number of myelinated and unmyelinated fibers in the SLN were almost the same in both rats and guinea pigs. The unmyelinated fibers show the peak distribution of axon diameter ranging from 0.79 to 1.00 micron in both species, whereas the peak distribution of myelinated fibers ranged from 2.51 to 3.16 microns in rats and from 3.98 to 5.01 microns in guinea pigs. The mean axon diameter of unmyelinated fibers was significantly larger in rats (mean: 1.12 microns) than in guinea pigs (0.96 micron), whereas that of myelinated fibers was significantly larger in guinea pigs (4.04 microns) than in rats (3.30 microns). Such findings would reflect the cardiopulmonary reflexes elicited from the larynx in these animal species.