James C. Saunders

The anatomical consequences of acoustic injury: A review and tutorial.

The anatomic consequences of acoustic overstimulation are explored in this presentation, and attention is directed toward issues where improvements in technology and empirical observation are needed before further advances in our understanding can be achieved. Gains have been made in the last decade in appreciating sound-induced cochlear injury, but there is now a need to evaluate not only cochlear pathology but also the functional state of the surviving structures. There is a wealth of information about the susceptibility of inner or outer hair cells to acoustic injury; however, the etiology of this injury is not yet fully understood. In addition, current ideas concerning the effects of noise on hair-cell stereocilia, hair-cell synapses, the cochlear vascular supply, and the central auditory pathways are in a state of flux and are either undergoing revision or emerging. Other issues, such as the basis of temporary or permanent threshold shift at the cellular level, and the individual differences in susceptibility to injury are in need of a fresh approach. It would seem that the time is now ripe to review our knowledge, recognize its gaps, and develop testable hypotheses concerning the mechanisms of acoustic injury to the ear.

Recovery of auditory function following intense sound exposure in the neonatal chick

We report the changes in auditory function that occurred at selected intervals following exposure to an intense pure-tone stimulus. One day old chicks were exposed to a 0.9 kHz tone for 48 h at 120 dB. At 0, 1, 3, 6, 9, 12 and 15 days after exposure, cochlear nucleus sound-evoked potentials were used to assess threshold sensitivity and frequency selectivity. Immediately after removal from the pure tone a threshold shift of 60 dB relative to age-matched controls was measured. The sharpness of tuning curves, as measured by Q10 dB, decreased by over 50%. By post-exposure day 15, near complete recovery of function was seen, with the greatest recovery occurring within the first three days. We relate these results to recent reports of structural recovery on the basilar papilla of the chick.

Middle-Ear Development V: Development of Umbo Sensitivity in the Gerbil

PURPOSE The development of the umbo response in the gerbil was studied in order to further elucidate the contribution of the middle ear to the development of auditory function. MATERIALS AND METHODS Laser interferometry was used to study the development of umbo velocity in Mongolian gerbils between 10 days after birth and maturity. RESULTS Before 15 days after birth, immaturities in the middle ear prevented any reliable measures of middle-ear motion. However, between 15 and 20 days after birth, a 10 dB improvement in umbo velocity was noted in the low-frequency (0.5 to 2.0 kHz) region of the umbo response. This improvement in sensitivity was correlated to an increased admittance due to an expanding bulla volume. Interestingly, umbo velocity remained relatively constant in the mid- and high-frequency regions of the response curve between 15 and 42 days after birth. The umbo response in the adult gerbil was decidedly different when compared with the response at 42 days after birth. CONCLUSION We speculate that a decrease in bulla volume along with increased ossicular mass contributed to the changes in the adult umbo response. When the maturation of the umbo response was compared with more central ontogenetic measures, it became apparent that structures more central to the middle ear continued to develop well past the time the middle ear was structurally and functionally mature.

Hair cell damage produced by acoustic trauma in the chick cochlea

Examination of pure-tone acoustic damage in the chick basilar papilla revealed that the location and extent of hair cell damage was a function of both the stimulus intensity and the age at which the chicks were exposed. Scanning electron microscopic evaluation of noise-exposed cochleae at post-hatching days 1, 10 and 30 permitted the identification of discrete regions of damage, including hair cells with stereocilia injuries as well as those lost from the epithelium. The hair cell damage was tonotopically distributed along the cochlea according to frequency. However, for each exposure frequency two distinct sites of damage were often produced, and their locations were correlated with stimulus intensity. At low intensities, a longitudinal strip of hair cell damage ran along the superior edge of the basilar papilla. As exposure intensity increased, a second damage site developed along the inferior edge of the basilar papilla, distal to the longitudinal strip. This second type of damage initially took the form of a series of laterally-oriented wedges, but at higher intensities, the wedges coalesced to form a large crescent-shaped patch of damage. The location of the damage sites for each frequency did not shift with age. However, there were differences in the extent and position of the damage which could be correlated with stimulus intensity and with changes in middle ear admittance during development [(1983) Development of Auditory and Vestibular Systems, pp. 3-25. Editor: R. Romand. Academic Press, New York]. These results suggest that developmental changes in the location and extent of hair cell damage depend on the effective stimulus intensity reaching the cochlea, rather than on alterations in the frequency coding of the hair cells.

Changes in the organization of actin filaments in the stereocilia of noise-damaged lizard cochleae.

Alligator lizards exposed to 105 dB broadband noise for 24 h showed a 33 dB loss in hearing which was almost completely recovered 11 days after removal from the noise. Two lesions were found in the actin filament organization which could affect the rigidity of the stereocilia and thus could account for this hearing loss. These lesions preferentially affect the tallest stereocilia. The more common one is a depolymerization of the actin filaments at the base of the stereocilium where it makes contact with the cuticular plate. This results in a displacement and detachment of the stereocilium from its rootlet, thereby affecting the orientation of the tallest stereocilium. The other lesion involves a loss in crossbridges between adjacent actin filaments in the stereocilium. We demonstrate that such a loss will dramatically affect the rigidity of the stereocilia.

Middle ear structure in the chinchilla: A quantitative study

The anatomic features of the chinchilla middle ear were identified and various aspects of the conductive apparatus were measured in a number of specimens by different methods. These aspects included area measures of the tympanic membrane, stapes footplate, oval window, and round window; middle-ear volume; dimensions of the ossicles, the length of their rotational axes as well as the malleus to incus lever ratio. We also weighed the ossicles. The findings are discussed with reference to their possible significance for auditory signal processing in the chinchilla.

A Comparison of Monopolar Electrosurgery to a New Multipolar Electrosurgical System in a Rat Model

Objectives/Hypothesis The purpose of this study is to compare collateral tissue damage and wound healing in incisions created by electro‐dissociation and conventional electrosurgery. Conventional electrosurgery has been used as an alternative to the scalpel to improve hemostasis. However, the heat generated by this instrument can cause tissue damage surrounding the incision, limiting its use around nerves and large blood vessels. A new technology, Coblation (Arthrocare Corp., Sunnyvale, CA), uses “electro‐dissociation” to achieve similar results by creating charged particles from a conductive medium to make an incision while simultaneously achieving hemostasis. This new approach to electrosurgery may reduce soft tissue damage.

Evidence That Rapid Vesicle Replenishment of the Synaptic Ribbon Mediates Recovery from Short-Term Adaptation at the Hair Cell Afferent Synapse

We have employed both in vitro patch clamp recordings of hair cell synaptic vesicle fusion and in vivo single unit recording of cochlear nerve activity to study, at the same synapse, the time course, control, and physiological significance of readily releasable pool dynamics. Exocytosis of the readily releasable pool was fast, saturating in less than 50 ms, and recovery was also rapid, regaining 95% of its initial amplitude following a 200-ms period of repolarization. Longer depolarizations (greater than 250 ms) yielded a second, slower kinetic component of exocytosis. Both the second component of exocytosis and recovery of the readily releasable pool were blocked by the slow calcium buffer, EGTA. Sound-evoked afferent synaptic activity adapted and recovered with similar time courses as readily releasable pool exhaustion and recovery. Comparison of readily releasable pool amplitude, capture distances of calcium buffers, and number of vesicles tethered to the synaptic ribbon suggested that readily releasable pool dynamics reflect the depletion of release-ready vesicles tethered to the synaptic ribbon and the reloading of the ribbon with vesicles from the cytoplasm. Thus, we submit that rapid recovery of the cochlear hair cell afferent fiber synapse from short-term adaptation depends on the timely replenishment of the synaptic ribbon with vesicles from a cytoplasmic pool. This apparent rapid reloading of the synaptic ribbon with vesicles underscores important functional differences between synaptic ribbons in the auditory and visual systems.

The structural and functional aspects of hair cell regeneration in the chick as a result of exposure to intense sound

This paper summarizes the structural and functional damage caused by intense sound exposure in neonatal chicks. Scanning electron microscopy has been used to follow the structural changes to the papilla and their subsequent repair. Pure-tone exposures produced a localized lesion consisting of tectorial membrane destruction, changes in surface organization of the papilla, and hair cell loss. The papilla underwent significant repair following the exposure and new hair cells could be identified on the sensory surface after 4 days of recovery. In addition, various evoked-potential methods provided an objective assessment of auditory function and demonstrated that the peripheral ear was severely impaired by overstimulation. Auditory function returned to near normal levels within 3 days postexposure. The inescapable conclusion from these observations was that hair cell regeneration had little to do with the functional recovery observed during the first 3 days. Tectorial membrane regeneration and the restoration of cochlear micromechanics were combined to form a hypothesis to account for the restoration of auditory function.

Chick cochlear hair cell exocytosis mediated by dihydropyridine‐sensitive calcium channels

1 A semi‐intact preparation of the chick basilar papilla was developed to study calcium‐dependent neurotransmitter release by tall hair cells (avian equivalent of cochlear inner hair cells). 2 Tall hair cell depolarization resulted in changes in cell membrane capacitance (ΔCm) that reflected cell surface area increases following synaptic vesicle exocytosis and provided a surrogate measure of neurotransmitter release. Both calcium current (ICa) and ΔCm were reversibly blocked by cobalt, and exhibited a similar bell‐shaped dependency on voltage with a peak response around ‐10 mV. 3 Pharmacological agents selective for L‐type calcium channels were employed to assess the role of this channel type in neurotransmitter exocytosis. Nimodipine, a dihydropyridine (DHP) antagonist, suppressed ICa and blocked ΔCm. Conversely, the DHP agonist Bay K 8644 increased both ICa and ΔCm amplitude nearly 3‐fold. These findings suggest that chick tall hair cell neurotransmitter release is mediated by calcium influx through L‐type calcium channels.