International journal of developmental neuroscience : the official journal of the International Society for Developmental Neuroscience
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Int. J. Dev. Neurosci. · Jul 1997
Discharge properties of pigeon single auditory nerve fibers after recovery from severe acoustic trauma.
The time course of recovery of compound action potential (CAP) thresholds was observed in individual adult pigeons after severe acoustic trauma. Each bird had electrodes implanted on the round window of both ears. One ear was exposed to a tone of 0.7 kHz at 136-142 dB SPL for 1 hr under general anesthesia. ⋯ However, the onset of recovery is delayed and the time course is slower than after destruction of short (abneural) hair cells alone. Also, recovery is incomplete, both functionally and morphologically. There is residual permanent hearing loss, and regeneration of short (abneural) hair cells is incomplete.
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Int. J. Dev. Neurosci. · Jul 1997
Time course of nerve-fiber regeneration in the noise-damaged mammalian cochlea.
The time course of events which are essential for nerve-fiber regeneration in the mammalian cochlea was determined using a group of chinchillas that had been exposed for 3.5 hr to an octave band of noise with a center frequency of 4 kHz and a sound pressure level of 108 dB. The animals recovered from 40 min (0 days) to 100 days at which times their inner ears were fixed and the organs of Corti prepared for phase-contrast and bright-field microscopy as plastic-embedded flat preparations. ⋯ Outer hair cells degenerated first followed by outer pillars, inner pillars, inner hair cells and other supporting cells; (2) Myelinated nerve fibers in the osseous spiral lamina became fragmented, starting at the distal ends of the fibers. This degeneration gradually extended back to Rosenthal's canal; (3) Fibrous processes, originating from Schwann-like cells in the osseous spiral lamina, extended laterally on the basilar membrane; (4) Schwann cells lined up medial to the habenulae perforata in the areas of severest damage, apparently ready to migrate through the habenulae onto the basilar membrane; (5) Schwann-cell nuclei appeared on the basilar membrane beneath the developing layer of squamous epithelium which was in the process of replacing the degenerated portion of the organ of Corti; (6) Regenerated nerve fibers with thin myelin sheaths or a simple investment of Schwann cell cytoplasm appeared in areas of total loss of the organ of Corti; and (7) The myelin sheaths on the regenerated nerve fibers gradually became thicker.