Auditory System: Damage Due to Overstimulation

Sound is the stimulus that activates the hair cells in the inner ear and starts the sequence of chemical and electrical processes that activate auditory nerve fibers and ultimately lead to perception. Sound that is too loud and lasts too long can severely damage the hair cells, can reduce or abolish activation of auditory nerve fibers, and can lead to impaired perception. This entry discusses damage to the auditory system caused by over-stimulation.

Noise exposure levels are strictly regulated in the workplace to prevent hearing loss; typically a level of exposure of about 80 to 85 dB(A) is allowed for 8 hours, depending on the jurisdiction, and assumes the remaining 16 hours per day to be noise free. Every 3 dB(A) increase in sound level would halve the allowed exposure time. However, exposure to recreational noise in bars, sport venues, and concert halls may also lead to gradually accumulating hair cell loss and thus to hearing loss. For instance, the average noise levels in a hockey arena can be more than 100 dB(A) during the 3-hour game. Taking the 80–85 dB(A) industrial norm, this would then result in an allowable exposure time of only 5 to 15 minutes. Often, these games are watched in addition to the 8-hour allowable workplace noise, and the cumulative effect of these exposures can result in hair cell damage.

Hair cells come in two types, the inner and outer hair cells. Both inner and outer hair cells function as microphones, converting vibration into membrane potential changes. In addition, the outer hair cells function as mechanical amplifiers that increase both basilar membrane vibration amplitude as well as fluid motion near the inner hair cell stereocilia. Hair cell damage as a result of noise exposure typically starts with the outer hair cells in the high frequency region of the inner ear and, when the overstimulation is even more severe, is followed by loss of the inner hair cells in that same region. Complete loss of outer hair cells leads to the loss of amplification and produces about 40 dB hearing loss, and loss of frequency selectivity; combined, this results in deteriorated (speech) perception. When the inner hair cells are also damaged, the auditory nerve fibers that synapse with these cells commonly degenerate. In regions without hair cell damage, overstimulation also leads to excess inner hair cell release of the excitatory neurotransmitter glutamate, which in turn opens receptor channels in auditory nerve fiber terminals and allows excess calcium to flow into these terminals.

This causes damage to the inner hair cell auditory nerve fiber synapse resulting in loss of stimulation of the auditory nerve fibers and, hence, hearing loss. This neurotoxic process can be [Page 192]reversed under certain conditions, and the synapses may be fully repaired in less than a week. This neurotoxic damage is one of the substrates for the so-called temporary threshold shift following noise trauma. Hair cell damage, however, is irreversible and leads to a permanent threshold shift. Whereas the hearing loss can be compensated by amplification provided by hearing aids, the loss in frequency selectivity that results from outer hair cell loss cannot be ameliorated.

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