Does electrical stimulation of the crossed olivo-cochlear bundle produce movement of the organ of Corti?

Low-frequency microphonic waveforms have been recorded in the basal turn of the guinea pig cochlea with and without electrical stimulation of the crossed olivocochlear bundle (COCB) at the floor of the fourth ventricle. Stimulation of the COCB increased the amplitude of the microphonic waveforms as described previously, but did not alter the shape of the waveforms markedly. The changes observed with COCB stimulation are consistent with a reduction in the impedance of the basolateral wall of the outer hair cells by about 50%, and possibly a 20% increase in the vibration of the organ of Corti at low frequencies, but suggest little or no change in the operating point on the transfer curve relating deflection of the hair bundles to the receptor current through the hair cells. It therefore seems that if slow contraction of the outer hair cells occurs during acute efferent stimulation in vivo, then it produces only a small deflection of the outer hair cell stereocilia, equivalent to a transverse displacement of the organ of Corti of less than 1.5 nm.     

Functional properties of the crossed part of the medial olivo-cochlear bundle

The efferent innervation of guinea pig cochleas was sectioned medially, at the level of the floor of the fourth ventricle, to study the effects of the crossed part of the medial efferent pathway on the compound action potential (CAP) masking phenomenon. Sectioning reduced CAP masking for a masker level varying with the frequency of the masker and the time elapsed between the masker onset and the probe onset. Functional properties of the crossed part of the medial efferent tracts: latency, thresholds and frequency selectivity, could be deduced from these data.     

The effects on auditory function of damage to the pontine olivo-cochlear bundle in man

A patient with multiple sclerosis (MS) in whom a lesion developed in the pons between successive nuclear magnetic resonance image scans (MRI) is described. The patient developed intolerance of loud sounds, with distorted perception of speech and music, and abnormalities of stapedius reflex threshold, masked speech audiometry, and masking level difference assessment in the presence of an unchanged pure tone threshold. These abnormalities partially resolved over a number of months. It is postulated that the efferent olivo-cochlear bundle was involved on one side of the pons and the abnormalities are interpreted in terms of bilateral removal of inhibition from the hair cells of the cochlea.     

The effects of crossed olivocochlear bundle section on transient evoked otoacoustic emissions

The purpose of this study was to investigate the effect of sectioning the crossed olivocochlear bundle (COCB) on transient evoked otoacoustic emissions (TEOAEs) in anesthetized adult chinchillas. Of particular interest is the role of cochlear efferents to the outer haircells (OHCs) and how they control mechanisms responsible for otoacoustic emissions. Specifically the experiment addressed whether a tonic level of inhibitory control is reduced by COCB section. The nonlinear component of TEOAEs was measured before and after COCB section. Analysis was made of the 1, 2, 3, 4, and 5 kHz frequency components and of the total emission, as quantified by fast Fourier transform (FFT) of the raw (time domain) response. After COCB section, the amplitude of the total response and of the 2, 3, 4, and 5 kHz components increased whereas the amplitude of the 1 kHz component decreased. The results indicate that COCB section reduces inhibitory control of the OHC mechanisms responsible for nonlinear TEOAE generation. It is not clear whether the nerve section eliminates a spontaneous level of activity in COCB efferents, or whether it results in the interruption of a stimulus-evoked feedback loop.     

The protective effect of the sympathetic nervous system against acoustic trauma

Objective: the cochlea is innervated by the sympathetic nerve fibers. However, the functions of those fibers in the cochlea are still controversial. The present study was designed to determine whether the sympathetic nervous system (SNS) exerts a protective or enhancing effect on acoustic trauma. Methods: acoustic overstimulation (either of 110, 115, or 130 dB SPL for 10 min) was performed in guinea pigs during electrical stimulation of the ipsilateral cervical SNS, after its surgical elimination or in the non-treated condition. The threshold shift of the compound action potential (CAP) from the pre-exposure value was measured at 1 h and at 1 week after acoustic overstimulation. Two-way analyses of variance (ANOVAs) were completed for the SNS conditions and the frequencies. Results: although no significant difference was found at 1 h after overstimulation among these three groups, the CAP threshold shift at 1 week (110 and 115 dB SPL) was significantly smaller in the SNS stimulation group than in the other two groups. Conclusion: a protective effect was observed in the SNS stimulation group 1 week after the exposure to acoustic overstimulation of moderate intensity (from 110 to 115 dB SPL for 10 min).     

The effect of caffeine on hearing in a guinea pig model of acoustic trauma

Objective

Caffeine is a widely consumed substance affecting the metabolism of adenosine and cellular metabolism of calcium. Noise also affects these metabolic pathways while inducing hearing loss. The aim of this study was to determine the effect of daily intake of caffeine on hearing loss after an episode of acoustic trauma in guinea pigs.

Materials and methods

In this pilot study, forty guinea pigs were randomly divided into four groups: group I (control, n = 10) received intraperitoneal saline, group II (n = 10) received intraperitoneal caffeine (120 mg/kg/day) for 14 days, group III (n = 10) was exposed to noise (tone of 6 kHz at 120 dB for one hour) and group IV (n = 10) was exposed to noise as group III and received caffeine as group II. Auditory brainstem responses were measured at four different frequencies (8, 16, 20, and 25 kHz) prior to and at intervals of 1 h, 3 days, 10 days, and 14 days after the initial treatment. On day 14, morphological analysis was performed to assess the effects of caffeine on acoustic trauma.

Results

Aggravated hearing loss was observed in group IV after 10 days of follow-up. After 14 days, one of the four frequencies (8 kHz) tested showed statistically significant greater impairment in hearing (8.2 ± 3.6 dB, p = 0.026). Auditory hair cells showed no difference while spiral ganglion cell counts were diminished in group IV (p < 0.05).

Conclusion

These findings indicate that caffeine may have a detrimental effect on hearing recovery after a single event of acoustic trauma.     

Negative effect of immediate hyperbaric oxygen therapy in acute acoustic trauma.

OBJECTIVE: The functional evaluation of the effect of the hyperbaric oxygen therapy (HBOT) onset time on cochlea by using distortion product otoacoustic emission. STUDY DESIGN: Animal study. METHODS: Twenty-four Wistar albino rats were divided into six groups and their right ears were directly exposed to a 110-dB sound pressure level (1-12 kHz) white noise for 25 minutes. The first group was considered the control group. HBOT was started at 1 hour postexposure for the second group, at 2 hours postexposure for the third group, at 6 hours postexposure for the fourth group, at 24 hours postexposure for the fifth group, and at 48 hours postexposure for the sixth group. Signal-to-noise ratios (SNRs) were recorded before the noise exposure; immediately after the noise exposure; and on the 3rd, 7th, and 10th day of postexposure. RESULTS: SNRs at 6 to 8 kHz were significantly decreased after the acoustic trauma. The evaluation on the third day of postexposure showed that recovery begun in all groups except the group in which the HBOT was started at 1 hour postexposure. SNRs in the control group and HBOT groups were back to the preexposure levels at 10 days postexposure, except the 1- and 2-hour postexposure groups. However, in the group in which the HBOT was started at 1 hour postexposure, distortion product otoacoustic emissions were lost except at 4 kHz. The recovery of the SNRs in hyperbaric oxygen administration at 2 hours postexposure almost completed on the 10th day after noise exposure. CONCLUSION: Immediate HBOT in acoustic trauma treatment is not necessary; on the contrary, it has an adverse effect.     

Impedance audiometry in the squirrel monkey. Effect of transection of crossed olivocochlear bundle.

Acoustic reflex thresholds in squirrel monkeys did not show any statistically significant change following midline section of the crossed olivocochlear bundle. Lack of agreement with the results reported by Borg in rabbits is most probably due to the species difference.