Morphological aspects of the human vestibular nerve.

A quantitative study was made of the number of fibers, their diameters, and distribution in the nerves innervating individual vestibular receptor organs of three human temporal bones. The specimens were obtained from autopsies conducted within 4 hours postmortem. The temporal bones containing the nerves were fixed in 3% glutaraldehyde and decalcified with EDTA until they were soft enough to allow dissection of the nerve branches to the individual receptors. The nerve branches were processed for osmium staining, embedded in plastic, and cut serially in 1-micron-thick sections for light-microscopic histologic evaluation of their fiber composition. Quantitative evaluation of nerve-fiber characteristics was made with the aid of a laboratory computer and programs for graphic representation and statistical analysis. In the nerves to the individual cristae the number of fibers ranged between 1416 and 2335. Fibers with diameters between 2.5 microns and 3 microns were the most numerous. The number of fibers decreased exponentially with increased size. The thickest fibers had diameters up to 11.5 microns. The distribution of fibers in the nerve of each crista was different for the central and intermediate areas of the crista than it was for the ends. Thin fibers with diameters less than 2.5 microns, which represented 36% of the population, projected to the ends of the receptor area. Thick fibers with diameters greater than 4.5 microns, which represented 8% of the fibers, were distributed relatively uniformly except for the extreme ends of the crista, where they were nearly absent. Fibers with intermediate diameters were distributed throughout the crista, although they were more concentrated at the ends. In the nerves to three maculae, the number of fibers ranged from 3744 to 5538. The percentage of fibers of each size, however, was similar to that in the cristae. The greater portion of fibers had diameters between 3.0 microns and 3.5 microns. Because of the anatomical configuration of the maculae, it was not possible to compare fiber diameters from one area to another. The composition and differential projection of fibers to the crista in the human labyrinth were similar to those found earlier in squirrel monkeys and bullfrogs. On the basis of these similarities in the pattern of innervation, it is suggested that the physiological properties in humans must be similar to those in animals.