Degeneration and distribution of efferent nerve fibers in the guinea pig organ of corti. A light and scanning electron microscopic study

After brain stem lesions interrupting the efferent innervation to the inner ear in guinea pigs, the degeneration and the distribution of efferent nerve fibers in the organ of Corti were studied in surface preparations stained by the zinc iodide-osmic acid (ZIO) technique. Light microscopy, transmission electron, and scanning electron microscopy were used to examine tissue from animals with postoperative survival times of 1-41 days. Following section of the crossed efferent bundle alone, degenerative changes were first evident in the outer hair cell (OHC) innervation as early as 1 day after placement of the brain stem lesion. Degeneration in the inner spiral (ISB) and spira l tunnel (STB) bundles was seen after about 3 days. After transection of both components of the efferent nerve supply all neural elements stained by the ZIO method degenerated and disappeared. This finding indicates a high specificity of the stain for the efferent nerve fibers. It was found that both radially and spirally oriented efferent fibers innervate the OHC. The spiral fibers follow long, complex courses directed toward either the base or the apex of Corti's organ. These fibers predominate in the upper cochlear turns where they sometimes meander into the region of Hensen's cells. Radial fibers comprise the major efferent supply to the OHC in the basal portion of the cochlea. Fibers coursing toward both the base and apex were seen in the inner spiral and spiral tunnel bundles. A close structural relationship was noted between these two bundles, suggesting that the STB may, in part, serve in an accessory capacity to the ISB. Single nerve fibers were often seen contacting the supranuclear portions of both inner and outer hair cells. These climbing efferent fibers were particularly numerous in the upper turns of the organ of Corti.