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Anatomy of the Cochlea and Vestibular System: Relating Ultrastructure to Function
Published in John C Watkinson, Raymond W Clarke, Christopher P Aldren, Doris-Eva Bamiou, Raymond W Clarke, Richard M Irving, Haytham Kubba, Shakeel R Saeed, Paediatrics, The Ear, Skull Base, 2018
The stria vascularis (SV) is a strip of tissue 150–300 µm wide (depending on location and species) lining the lateral wall of the scala media and running along its entire length (Figures 47.1a and 47.2b). It is responsible for the production and maintenance of both the high endolymphatic K+ concentration and the EP.1 The SV encloses a complex capillary network and is composed of three cell types (Figure 47.8b): marginal cells that line the endolymphatic compartmentintermediate cells in a discontinuous layer enclosed entirely within the body of the epitheliumbasal cells that separate the SV from the underlying spiral ligament. The SV is reputed to have the highest rate of oxidative metabolism (i.e. where oxygen is used to generate energy from carbohydrates) in the entire body, most likely due to the huge energy demand resulting from the mass of active ion transport that takes place in this tissue. The EP provides a source of energy or ‘battery’ to drive the cochlear amplifier.131 As would be expected, any loss of EP therefore results in significant hearing impairment.
A Biomorphic Active Cochlear Model In Silico
Published in Iniewski Krzysztof, Integrated Microsystems, 2017
Advances in physiological measurements and mathematical modeling in the last few decades have revealed the cochlear mechanics to some extent, although much is yet to be uncovered. In the mammalian cochlea, the nonuniform basilar membrane (BM) vibrates in response to fluid pressure, providing broad frequency tuning that is then enhanced and reshaped by the hypothesized cochlear amplifier. Bridging the gap between the dead cochlea’s response and that of living cochleae, the postulated cochlear amplifier refers to the selective amplification process occurring within the cochlear partition (CP) that enhances cochlear behavior: Exquisite sensitivity, remarkable frequency selectivity, and nonlinearities. In short, the BM preliminarily detects and filters sound signals and the cochlear amplifier selectively amplifies the BM’s responses, resulting in nonlinear active cochlear behavior.
Cochlear mechanisms and processes
Published in Stanley A. Gelfand, Hearing, 2017
The cochlear amplifier enhances the stimulus delivered to the inner hair cells, resulting in greater sensitivity, a wider dynamic range, and sharper tuning (as well as being involved in the production of otoacoustic emissions, discussed below). The compressive nonlinearity of the cochlear amplifier is illustrated by returning to the curves in Figure 4.32. The slopes are steeper for lower stimulus levels (up to about 40 dB) and then become progressively flatter for higher stimulus levels, which reveals that the cochlear amplifier provides more gain for weak stimulus levels and progressively less gain for stronger stimulus levels. Because of this compressive nonlinearity, the cochlear amplifier improves hearing sensitivity by enhancing weak signals without also boosting higher-level signals (which are already strong enough to be heard without amplification). The same mechanism also improves the dynamic range of the auditory system by compressing the very wide scope of audible sound levels into the narrower range that can be accommodated by auditory nerve fibers (Chapter 5). That the cochlear amplifier also provides for sharper tuning is illustrated by referring back to Figure 4.31, which shows that basilar membrane tuning curves are very narrow for low stimulus levels (5 and 10 dB at the top of the graph) and get progressively wider as stimulus levels get higher (toward 80 dB at the bottom of the graph). The cochlear amplifier enhances the stimulus delivered to the inner hair cells, resulting in greater sensitivity, a wider dynamic range, and sharper tuning; also, it is involved in the production of otoacoustic emissions (discussed below).
Detection of middle-ear muscle reflex activation using changes in otoacoustic emission stimulus amplitude versus absorbance: an initial investigation
Published in Hearing, Balance and Communication, 2022
Cochlear amplification provided by the outer hair cells can be modified via the medial olivocochlear reflex (MOCR) reflex, which involves neurons that originate in the medial superior olive and synapse on the outer hair cells [1]. MOCR activation reduces cochlear amplification and can reduce masking of brief sounds in background noise [2,3]. The reduction in cochlear amplifier gain may have benefits such as improved speech perception in noise [4] and protection from noise-induced hearing loss [5]. Assessment of the MOCR has potential uses in the audiology clinic. It could serve as part of a diagnostic assessment in patients with difficulties understanding speech in noise [6] and may predict risk for noise-induced hearing loss [7], which could be useful for counselling patients.
Lack of association between contralateral inhibition of otoacoustic emissions and vowel formant discrimination in noise
Published in Hearing, Balance and Communication, 2020
Ian B. Mertes, Kristin M. Johnson
Our finding of a lack of significant correlation between MOC reflex activity and formant discrimination thresholds at any SNR is in contrast to the findings of Hienz et al. [19] who found that surgical lesioning of the MOC bundle caused a significant increase in discrimination thresholds in cats at poor SNRs. Animal models allow for such experimental control over physiologic processes, whereas we were limited to a correlational investigation. It is possible that the MOC reflex contributes to vowel discrimination in noise but that there is not a monotonic relationship between a TEOAE-based metric of the MOC reflex functioning and the formant discrimination threshold. It is also possible that a relationship may have been apparent had we included a larger range of hearing levels (e.g. listeners with slight or mild sensorineural hearing loss) that would increase the variability in both MOC reflex strength and discrimination thresholds. It must also be considered that activation of the MOC reflex can broaden cochlear tuning due to a decrease in cochlear amplifier gain [30,31], which could be detrimental to the formant discrimination task. If this were the case, it could be expected that stronger MOC reflex activity would be associated with larger discrimination thresholds (i.e. a positive correlation would be present). One explanation for the lack of correlation in the current study is that the increased antimasking may be effectively cancelled out by broadened cochlear tuning, at least for this particular task.
Effects of noise exposure on auditory brainstem response and speech-in-noise tasks: a review of the literature
Published in International Journal of Audiology, 2019
Noise exposure can result in stereocilia damage or, in more severe cases, mechanical trauma to the OHCs or the organ of Corti itself (Henderson, Hamernik, and Sitler 1974; Henderson and Hamernik 1986; Wang, Hirose, and Liberman 2002). Noise-induced damage to the OHCs compromises threshold sensitivity; the electromotile action of the OHC population provides up to 40 dB of gain and the OHCs are therefore labelled the “cochlear amplifier” (Dallos and Evans 1995; Dallos, Zheng, and Cheatham 2006; Ashmore et al. 2010). If the OHCs are damaged, the loss of the cochlear amplifier will result in a reduced input to the IHCs. The IHCs have typically been documented to be less vulnerable to noise injury than the OHCs (Wang, Hirose, and Liberman 2002; Chen and Fechter 2003); however, Mulders, Chin and Robertson (2018) recently argued that noise-induced injury to the IHCs plays a significant role in the Wave I amplitude reductions that are labelled hidden hearing loss. OHC loss shows only a moderate correlation with PTS, at least in part because OHCs may be present (living) but impaired (Chen and Fechter 2003).