Efferent Feedback Minimizes Cochlear Neuropathy from Moderate Noise Exposure

Abstract

Although protective effects of the cochlea’s efferent feedback pathways have been well documented, prior work has focused on hair cell damage and cochlear threshold elevation and, correspondingly, on the high sound pressure levels (> 100 dB SPL) necessary to produce them. Here we explore the noise-induced loss of cochlear neurons that occurs with lower intensity exposures and in the absence of permanent threshold shifts. Using confocal microscopy to count synapses between hair cells and cochlear nerve fibers, and using measurement of auditory brainstem responses and otoacoustic emissions to assess cochlear pre- and post-synaptic function, we compare the damage from a weeklong exposure to moderate-level noise (84 dB SPL) in mice with varying degrees of cochlear de-efferentation induced by surgical lesion to the olivocochlear pathway. Such exposure causes minimal acute threshold shift and no chronic shifts in mice with normal efferent feedback. In de-efferented animals, there was up to 40% loss of cochlear nerve synapses and a corresponding decline in the amplitude of the auditory brainstem response. Quantitative analysis of the de-efferentation in inner vs. outer hair cell areas suggested that outer hair cell efferents are most important in minimizing this neuropathy, presumably by virtue of their sound-evoked feedback reduction of cochlear amplification. The moderate nature of this acoustic overexposure suggests that cochlear neurons are at risk even in everyday acoustic environments, and, thus, that the need for cochlear protection is plausible as a driving force in the design of this feedback pathway.

Human Balance Control: Dead Zones, Intermittency, and Micro-chaos

“…The nervous system can set the value of the threshold by using efferent feedback. For example, hearing loss to sound intensities that normally would not produce deafness can arise because of a failure of efferent feedback in the cochlear nucleus [59]. The observation that the detection threshold for ankle proprioception depends on muscle tension in the calf muscles [79,80] raises the question as to whether detection thresholds for balance can be similarly adjusted by adopting different postures while standing.…”

Physiology, Psychoacoustics and Cognition in Normal and Impaired Hearing

“…The MOCR may be essential for normal development of cochlear active mechanical processes (Walsh et al 1998), and/or to minimize deleterious effects of noise exposure on cochlear function (Maison et al 2013). A more widely accepted role is that the MOCR possibly facilitates understanding speech in noisy environments.…”

How can the auditory efferent system protect our ears from noise-induced hearing loss? Let us count the ways

“…Furthermore, vulnerability of the mouse cochlear nerve to TTS increases when the LOC efferents are removed [10]. An antiexcitotoxicity role of the LOC system has been suggested [10,38,46]. It is difficult to measure the LOC, but there is some speculation that MOC and LOC strengths are related in individuals [35], so LOC strength might be inferable from MOC strength.…”

“…Recent work has shown that another effect of noise damage, even at moderate noise-exposure levels that do not result in permanent hearing loss, is a permanent cochlear-nerve deficit affecting high-threshold, low-and medium-SR fibers [15,29], which are important for supra-threshold temporal processing [2]. The underlying cause presumably is glutamate excitotoxicity [38]. Why it preferentially affects low-and medium-SR fibers is not known [2] but may be due to low mitochondrial content [38].…”

“…The underlying cause presumably is glutamate excitotoxicity [38]. Why it preferentially affects low-and medium-SR fibers is not known [2] but may be due to low mitochondrial content [38]. The MOC reflex is protective for this cochlear neuropathy [38].…”