Quantitative analyses of the fiber population in rat chorda tympani nerves and fungiform papillae

The numbers of nerve fibers in 44 intact chorda tympani nerves (CT) from 34 rats were counted on electron micrographic montages of cross‐sections of the nerves. The mean number of fibers was 1,096, consisting of 641 myelinated and 455 unmyelinated fibers. There was no significant difference in the mean number of fibers between CT's from right and left sides, nor was there any significant difference in counts between CT segments taken from the middle ear and those taken from just proximal to the region where the CT joins the lingual nerve. The relative contribution of efferent and afferent fibers in the CT was assessed in 13 animals as follows. Intracranial surgery was performed to cut the root of the facial nerve just proximal to its entrance into the internal auditory meatus on the right side. After 19 to 21 days, to allow Wallerian degeneration of the efferent fibers, counts were made as above. The mean number of afferent fibers in the rat CT was 484 myelinated and 122 unmyelinated (606 total); the intact left CT's from the same animals contained a mean of 616 myelinated and 402 unmyelinated (1,018 total) fibers. The afferent limb of the CT, therefore, contains 79% of the total number of myelinated fibers and only 30% of the total number of unmyelinated fibers. Removal of the superior cervical sympathetic ganglion did not significantly affect the numbers of fibers. In order to determine whether the efferent limb of the CT contributes fibers to the fungiform papillae on the tongue, we compared the number of (CT) fibers in fungiform papillae from “de‐efferented” rat tongues with that from papillae of controls. In order to make this comparison, it was necessary to section the lingual nerve, which contributes about 75% of the total number of fibers to the fungiform papillae. This was done in both the control and de‐efferented animals. There was no significant difference in mean number of fibers in fungiform papillae innervated by a de‐efferented CT when compared with those innervated by an intact CT. There is, then, no evidence from these studies to suggest the presence of an efferent CT component within the fungiform papilla.