Many of my SciBlings have been doing posts on “basic concepts” in their fields of expertise. As I am studying hair cell regeneration as a therapy for hearing impairment, I thought I might do a ‘basic concepts post’ on what hearing is, how humans hear, and why we lose it sometimes.
The most reductionist answer is the ability to perceive sound in an environment. However, to humans, hearing is much more complex than that, as we use sound to communicate to others or to appreciate music. Humans, among many other species, perceive sound waves through holes in our heads (ears). However what we consider “ears,” that is, the outside fleshy part, doesn’t do much except collect these waves and focus them towards the middle and inner portions of the ear. Truly, the goal of this system is to somehow turn the information contained in these sound waves (frequency, wavelength, period, amplitude, and speed) into neural signals which can be integrated in the brain into a bigger picture of what’s going on around us.
The range of hearing varies with the species, and is measured in audible frequencies. Whales’ hearing range is in the very low frequency ranges, while bats (who use echolocation) have a much higher frequency range of hearing. Dogs can hear “dog whistles” although we can’t, but most birds lack the low-frequency ranges that we can perceive. When we are born, humans’ hearing is typically in the 20 Hz to 20 kHz range, although as we age we begin to lose our high-frequency hearing. It is an inevitability, however future treatments (by my lab and others) currently work to correct that fact.
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Hearing in a Nutshell
What is colloquially called ‘the ear’ is a complex organ consisting of outer, middle, and inner portions (see diagram below). Sound waves enter the outer ear (pinnae) and travel through the ear canal to the tympanic membrane (eardrum). This membrane vibrates from the incoming sound waves and transmits these vibrations to the tiny bones in the middle ear.
These three tiny bones (ossicles) consist of the malleus (hammer), the incus (anvil), and the stapes (stirrup). Their purpose is to amplify the sound and transmit it to the inner ear, by a factor of 2 or 3. The stapes makes contact with another membrane, the oval window membrane of the cochlea. The cochlea is a snail shell-shaped coil with is filled with fluid, as well as thousands of mechanosensory cells called hair cells. Hair cells have stereocilia (which look like long hairs) which project into the fluid spaces of the cochlea. As the stapes vibrates against the cochlea’s oval window, the fluid of the cochlea is disrupted causing the stereocilia to bend. As the stereocilia are deflected, ion channels in the hair cells open which cause the release of calcium from the hair cell onto neurons which form the auditory nerve and project to the areas of the brain involved in auditory processing.
The Cochlea and Hair Cells
The cochlea is the most complex part of the ear, and is divided into “turns.” These turns merely refer to the number of coils the spiral of the cochlea has. A guinea pig has four turns, a mouse has a turn and a half, and humans have two and a half. As mentioned above, the spaces in the cochlea is filled with fluids. Two membranes sepearate fluid-filled chambers (making 3 seperate chambers), with most of the action occuring in the space called the “scala media.” The scala media is the portion of the cochlea where the hair cells are located, and where fluid displacement bends the hair cells’ “hairs.” (See stereocilia projecting from the top of a hair cell, below)
All the hair cells sit on top of a firm but flexible membrane called the basilar membrane. As the stapes bangs against the oval window, a wave is transmitted through the basilar membrane. The distance this wave travels (and subsequently, the hair cells that are stimulated) are dictated by the frequency of the sound wave. The basilar membrane becomes stiffer at the top of the cochlea, which allows different parts of the cochlea to correspond to specific frequencies. High frequency sound-specifity corresponds to the base of the cochlea while the top (or “apex”) of the cochlea transduces low frequency sounds. The area on the cochlea where the most hair cells are stimulated during a given sound wave is considered the resonance point, and loudness can be perceived by the number and duration of hair cell stimulation at that point.
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But here’s the bad news: we are born with all the hair cells we’ll ever have (~30,000), and we are constantly losing them due to noise exposure, toxic chemicals, and time. Loud noises such as close-range gunfire can easily kill all the hair cells in your cochlea, rendering you instantly deaf. These cells are never replaced in mammals, hence the ever-worsening hearing that many aged persons experience. In addition, one of the major causes of genetic deafness is being born with no hair cells or faulty hair cells. Birds and reptiles are unique in that they can regenerate hair cells, and regain their hearing fully after being deaf. So, determining why some animals can regenerate hair cells, but mammals can’t, is a HUGE research question in the area of hearing and inner ear biology.
Hearing aids help the situation somewhat by amplifying salient sounds. Salient sounds would be sounds within the common frequencies of speech or music, and some hearing aids are quite good at zeroing in on these sounds and eliminating noise. Another treatment for severe hearing impairment is called a cochlea implant, which is a device which serves to directly stimulate the auditory nerve in response to sounds. Cochlear implants are improving, but are currently so crude as to prohibit the listening of music and voices sound extremely high pitched. Essentially, nothing sounds “normal” but the sounds that are available can be useful after training.
Well, that seems to be enough info for now….maybe I’ll do a follow-up post with more details if people are interested.