Hearing research
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Scanning electron microscopy was used to examine the basilar papilla of the granite spiny lizard. The papilla contains three distinct hair cell populations: an apical and a basal population with free-standing cilia, and a central population with a tectorial membrane. In the free-standing populations, stereocilium length decreases towards the ends of the papilla. ⋯ The low CF fibers are associated with the tectorial membrane hair cell population. Fiber CF correlated with hair cell cilium length, not position on basilar membrane, for hair cells with free-standing cilia. Tonotopic organization of high CF fibers could be predicted reasonably well from the histogram of fiber CFs.
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Damage to the tectorial membrane caused by acoustic trauma was examined with scanning and transmission electron microscopy immediately after exposure and at selected time points over a 10 day recovery period. At 0 h of recovery the structure of the tectorial membrane overlying the region of hair cell damage was severely disrupted and connections between the membrane and the basilar papilla were lost. By 24 h of recovery, regeneration of the tectorial membrane was evident in the secretion of new matrix materials by the supporting cells of the basilar papilla. ⋯ However, the regenerated segment included only the honeycomb-like structure of the lower layer of the normal tectorial membrane. The laterally-oriented fibers which form the upper layer of the membrane were not regenerated over the damaged region. These findings indicate that the tectorial membrane is regenerated in parallel with the hair cells during recovery from acoustic trauma but the full extent of this recovery and its effect on cochlear function are not yet clear.
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Examination of pure-tone acoustic damage in the chick cochlea revealed a significant amount of hair cell recovery over a 10 day period following the exposure. The recovery included both a regeneration of stereociliary bundles to replace those that were lost and a reshuffling of the mosaic pattern of the hair cell surfaces that survived. Ten-day-old chicks were exposed to a 1500 Hz pure tone at 120 dB SPL for 48 h and their cochleae were processed for scanning, transmission and light microscopy at 0 h, 24 h, 48 h, 4 d, 6 d and 10 d after exposure. ⋯ After 48 h of recovery, new hair cells were identifiable in the region of hair cell loss and with time they underwent a progressive maturation of their stereociliary bundles. The surviving hair cells showed a dramatic rearrangement and expansion of their surfaces but exhibited no repair of the damaged stereociliary bundles. These results suggest that the chick cochlea is capable of a significant amount of recovery and regeneration following acoustic trauma.
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Recent anatomical evidence has cast doubt on the interpretation of the neural elements involved in past experiments in which efferents were electrically stimulated. To separate effects produced by medial olivocochlear (MOC) efferents from effects produced by lateral olivocochlear (LOC) efferents, MOC efferents were selectively stimulated by an electrode in the region of the MOC cell bodies in cats. For comparison, efferents were also stimulated with an electrode in the fourth ventricle (OCB stimulation, previously called COCB stimulation). ⋯ Fourth-ventricle OCB stimulation typically produced an N1 sound-level shift in both ears of 19-22 dB which is approximately the sum of the crossed and uncrossed MOC shifts. Considering also that many uncrossed-MOC fibers course close to the midline (i.e. near the stimulating electrode), it seems likely that fourth-ventricle OCB stimulation excites both crossed and uncrossed MOC efferents. Referring to such stimulation in the cat as 'COCB stimulation' is therefore inaccurate and may lead to wrong conclusions about the functional role of various components of the olivocochlear fibers.
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After a permanent threshold shift was induced by exposing guinea pigs to a 1 kHz pure tone at 105 dB(A) for 72 h, light microscopic observations of freshly dissected and stained tectorial membranes showed an increased waviness and clumping of the fibers of the middle zone. Hensen's stripe was not seen as a continuous dense structure running through the middle zone but was at times discontinuous and curved. As measured from cross-sections of the cochlea, the thickness of the tectorial membrane was decreased after acoustic overstimulation. ⋯ Visual detection levels of threshold of tectorial membrane movement was determined by stimulating the marginal zone of the tectorial membrane of isolated cochlear coils by an oscillating water jet. After acoustic overstimulation the tectorial membrane became more complaint. The tectorial membrane abnormalities were restricted to the regions of the cochlea that demonstrated a 40-50 dB hearing loss.