By Travis SuttleIn lecture a few weeks ago, we learned about the physiology of hearing and I was absolutely fascinated! Though after class, on my walk to get lunch, I distinctly remember contemplating on how the process could be interfered to result in hearing loss. My parents repeatedly told me as a child to turn down my music, but they never explained to me how it leads to hearing impairment (probably because they have no idea). With that said, my investigation into understanding the mechanisms behind hearing loss begins!
Hearing loss or hearing impairment is the partial loss of the ability to hear, or perceive sound by detecting vibrations in the air. To learn how hearing loss happens, it is essential to understand the underlying anatomy and physiology behind hearing. Our ears are comprised of three parts; the outer ear, middle ear, and inner ear. The outer ear consists of the pinna and ear canal, which lead to the tympanic membrane, or eardrum. The outer ear channels sound from the environment into the ear canal in a way that allows the person perceiving the sound to locate its origin vertically. One may question why the physiology of the outer ear only allows one to locate the sound vertically; it is because the ears are paired organs, so together they can interpret the location of the origin of sound horizontally. Therefore, it is necessary that both ears are functional in order to properly interpret the location of the sound origin.
The middle ear is a small air-filled chamber that contains the three smallest bones in the body, the ossicles. The ossicles transmit vibrations of the tympanic membrane to the inner ear. The stapedius and tensor tympani muscles are attached to the ossicles and contract to protect the hearing mechanism. The ossicles transmit sound into the inner ear by vibrating on the oval window, an elastic membrane that separates the inner ear from the middle ear.
Lastly, the inner ear contains the cochlea, a snail-shaped, fluid-filled, organ about the size of an aspirin that translates mechanical waves into neural signals, which are sent via the cochlear nerve to the opposing side of the brain. If you were to unwind and look down the cochlea, you would find about twenty thousand sensory cells, commonly referred to as hair cells. High sound frequencies (such as a minion voice) depolarize hair cells at the entrance to the cochlea, where low frequencies (such as the voice of the guy who makes all the movie trailers) depolarize hair cells deeper within the cochlea. The hair cells themselves do not depolarize, but they release neurotransmitters at synapses with auditory nerves that produce action potentials to transmit information about the frequency of the sound to the brain.
Mechanisms Of Hearing Impairment
There are two kinds of hearing loss, conductive and sensorineural. Conductive hearing loss is when the sound does not reach to the cochlea. It is commonly caused by wax buildup in the inner ear, perforation of the eardrum, or excessive fluid buildup in the middle ear. Sensorineural hearing loss is the most common type of hearing loss in adults. Noise damage, the aging process, or other environmental factors can cause sensorineural hearing loss. About 10% of the world population is affected by hearing loss, which varies from moderate to severe cases. Presbycusis, or the gradual loss of ability to hear high frequencies, is a form of sensorineural hearing loss and is considered a normal process of aging. Presbycusis is caused by hair cell degeneration in the early cochlear canal. This is because the hair cells at the entrance of the cochlea are the first to encounter sound waves. Though hair cells throughout the cochlea can be damaged when listening to loud music. A great analogy for hair cell-induced hearing loss involves a field of grass; when you walk on it, you compress the grass and it bends down over night, but in a few days, it springs back up and is OK again. However, if you continually walk on the grass, you will wear a path in it and it will not grow back. Similarly, briefly listening to loud music will not result in permanent hearing loss, though listening to loud music for extended periods of time will inevitably result in hearing loss. Recently, it has been discovered that the death of hair cells is not the only contributing factor to sensorineural hearing loss.
About a month ago, a study published in Neuroscience revealed new insights into the relationship between noise exposure (or acoustic overexposure, AOE) and hearing loss. Tagoe et al. (2014) discovered that the dysmyelination (or the loss of the extracellular protective sheet) of the auditory neurons might also lead to hearing impairment. Dysmyelination of the auditory nerve axons impairs propagation of action potentials (signals) down the auditory neurons leading to impaired hearing. The graph above depicts decreased excitatory postsynaptic currents (EPSCs) in nerves from the acoustic overexposure (AOE) group. The results of this study are very significant because they uncovered a new mechanism in which hearing impairment may develop. So now that we know how sensorineural hearing loss can happen, what options are currently available for those with hearing impairments?
Assistive Devices for Hearing Impairment
Though no current treatments reverse the underlying cause of sensorineural hearing impairment, there are many devices that can improve ones hearing abilities. The most popular device is hearing aids, which act by amplifying incoming sound to improve hearing ability. Though hearing aids often only provide a limited hearing assistance in the high frequency range. Cochlear implants are a novel device which artificially stimulate the cochlear nerve. Essentially, they mimic the firing of hair cells within the cochlea in order for the recipient to perceive sound. Unfortunately, cochlear implant procedures vary greatly in success. The reasoning behind the varied success of cochlear implants is most likely due to problems in auditory nerve signaling as Tagoe et al. (2014) discovered.
Prevention Of Hearing Impairment
So now that we understand how hearing impairment develops, what can we do on a daily basis to avoid substantial hearing loss in our lifetime (besides not standing behind a jet when it takes off).
2. Wear ear protection!
A common rule of thumb is to wear ear protection if something sounds as loud as a lawnmower. Since noises above 86dB can damage your hearing and a lawnmower is about 90dB, it serves as an identifier of noises that can potentially cause hearing loss. Ear protection is widely available, from 50-cent foam ear plugs to electronic ear muffs that allow you to hear normal sounds but muffle loud noises. It doesn't matter what type of ear protection you use, the most important part is using it in the first place.
3. Turn the music down!
Finally, to answer the question that Lil Jon repeats is his currently #11 iTunes ranked song "Turn Down For What?," you should turn down your music to prevent it from damaging the hair cells within the cochlea and the myelin sheath on your auditory nerve. Remember, like boiling a lobster, damaged ears cannot be returned to their previous state.
P.S. Am I the only one who hates when that song comes on the radio?
Tagoe, T. , Barker, M. , Jones, A. , Allcock, N. , & Hamann, M. (2014). Auditory nerve perinodal dysmyelination in noise-induced hearing loss. J Neurosci, 34(7), 2684-2688.
Matsunaga, T. (2009). "Value of genetic testing in the otological approach for sensorineural hearing loss". The Keio journal of medicine 58 (4): 216–222.