Fiber grouping in the feline vestibular nerve before and after labyrinthectomy

The vestibular nerve is composed of fibers with a wide spectrum of diameters. The fibers of largest diameter are known to innervate the type I hair cells of the cristae, while the small-diameter fibers innervate the type II hair cells. Midsized fibers (dimorphic fibers) represent neurons that innervate both type I and type II hair cells. Reports by others have commented on the tendency for clustering of fibers with like diameters. Rigorous statistical proof for or against clustering has not yet been presented. The explanation for this is, in part, the mathematic complexity of analyzing clustering in a system composed of three elements. We report a new method for analysis of fiber clustering and apply this method to large-, medium-, and small-diameter fibers in the feline vestibular nerve. The fiber grouping in the caudal and rostral ends of the vestibular nerves of six normal animals is compared to that in similar areas of the nerves of five animals 12 to 17 months after unilateral labyrinthectomy. No statistically significant clustering of fiber types was found in the rostral portion of either the control or the labyrinthectomized animals. In the caudal portion of the control nerves, clustering of the large fibers was demonstrated (p < .005, chi2 test). This clustering was not demonstrated after labyrinthectomy. An explanation of these findings is discussed. The method used in this study to analyze fiber clustering may be applicable to other nerve systems of greater complexity.