Basic Concepts: Hearing

i-0cb74a9bd2f92475cde110932bf66099-small ear.jpgMany 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.

(Continued below the fold.....)

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)

i-637439b3b2ed917b62bf5cf775232efc-hair cell.bmp

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.

i-15a0110031318452e6c0c9a8263f45be-cochlea map.bmp (Pop-up for higher clarity: View image)

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.

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I accidentally came upon this post while searching for pics of hair cells for my Biology class presentation. My 5 mo old son is actually deaf. We are planning to get him an implant next summer. I have heard that some people save an ear for possible hair cell regeneration in the future. Do you think this technology will be available to our son in the near future?

the outside fleshy part, doesn't do much except collect these waves and focus them towards the middle and inner portions of the ear

I know this is supposed to be basic concepts, but the pinna is a handy little apparatus in itself (i.e. localisation), as I'm sure you're aware.

Many years ago now I heard Peter Narins lecture on hearing--he was all excited about new data suggesting that only the inner haircell row was sensory, whereas the outer 3 rows of haircells seemed instead to be effectors--that is, nerve impulses made the haircells move, rather than vice-versa. Is this idea current?

Yep, David, the pinna certainly aren't useless. The timing with which sound travels "around the head" to get to the other ear helps us pinpoint a sound's direction. I was just pointing out that in the grand scheme of the hearing process, most of the action takes place in the cochlea which people never see. Human pinna are not nearly as useful as the large pinna of many other creatures like dogs, etc. But you can augment their sound-collecting ability but cupping your hands around them, as we all know. :)

And CCP, yes, thats mostly right. The Inner hair cells 95% project to the brain, while the outer hair cells mostly receive info from the brain. This seems to have the effect of either dampening or amplfying certain sounds.

When I did audition, one of my professors was rather into the idea that reduced hearing in old age is not a fact of life, but a consequence of modern life. Supposedly, in non-industrialized societies you don't observe the same hearing deficiencies in old people, presumably because they live in a society which isn't as hard on their stereocilia.

Maybe a bit outside your area of interest, but what do you make of that idea?

Modern life certainly exacerbates the process, but it would happen anyway since the cells cannot replace themsleves (and really, people weren't meant to live 100+ years from a cellular standpoint). It would just happen more slowly. Guns, rock concerts, jackahmmers, planes, etc all contribute to a larger than healthy level of ambient noise. However old people from all cultures lose their hearing.

On the other hand, modernization has also worked to preserve hearing through medical advancement. We now know to protect our hearing in certain circumstances. Drugs as simple as aspirin have shown to have a neuroprotective effect on hair cells and spiral ganglion neurons (research from UM no less!). Whether its a net gain or loss for your hearing, well the jury's still out on that. But living in the noisy modern world shouldn't negatively impact your hearing as long as you take sensible precautions.

I'm absolutely interested. I have friends whose child suffered a lot of hearing loss from some type of birth trauma which affected his cilia.

I would read with particuarly rapt attention to a series of blog posts which would, one at a time, look at various proposed treatment strategies, how they would (theoretically) work, what the current "blocking issues" are that keep them from working today, and what your educated ballpark guess would be as to the probability and timeframe that such a treatment would become clinically available.

By Jason Black (not verified) on 05 Feb 2007 #permalink

That's an interesting suggestion. Maybe I'll do an all-week posting flurry just on hearing issues and deafness. As for possible treatments, I can offer descriptions and the science of theoretical applications but can make no guesstimation as to timeframe to people. I'm not really in the know about that since treatments for deafness *in any capacity* have only really just begun to be hinted at in animal models. However, there are some promising things that have been shown in animals thus far and I'd be happy to relay that.

The timing with which sound travels "around the head" to get to the other ear helps us pinpoint a sound's direction.

What I was getting at was that the pinna provides cues (augmenting interaural intensity/time differences) so we can tell front from back.

Speaking of basic hearing concepts, I demand you explain the K+ cycle. :D

I was planning on posting that tomorrow. :) Its a long story (and different for IHC and OHC) so I figured it deserved its own post.

Is there anything could be known about the function of the "type II" auditory nerves? Thanks, Incze.

By Incze Lajos (not verified) on 05 Feb 2007 #permalink

Thats the same question that CCP had. Type II neurons connect the auditory processing centers of the brain to the outer hair cells. They are slower and unmyelinated, and are thought to have the function described above.

In my undertandings there are two *afferent* auditory nerve types. The type I innervates the inner hair cells (one by one, thick, myelinated), the type II the outer hair cells (30-60 cells, thin, unmyelinated). E.g., Introduction:

"Hearing commences when hair cells in the organ of Corti of the inner ear transduce sound energy into electrical signals that cross the recepto-neural junctions to depolarize sensory axons in the cochlear nerve. Action potentials (APs) propagate along axons of bipolar cochlear ganglion cells, forming point-to-point connections between hair cells and the cochlear nucleus (see Fig. 1A,B). There are two types of ganglion cells. Type I is myelinated, innervates one inner hair cell each, and provides rapid discrete coding. Type II is unmyelinated, and each innervates up to 30-60 outer hair cells (Spoendlin, 1973Go; Perkins and Morest, 1975Go; Kiang et al., 1982Go; Ginzberg and Morest, 1983Go; Liberman et al., 1990Go). Efferent fibers, arising in the superior olivary nuclei, innervate either inner or outer hair cells and their sensory endings and may have feedback functions (Brown, 1987Go). However, the function of type II ganglion cells is not very well understood."

By Incze Lajos (not verified) on 05 Feb 2007 #permalink

Very informative! I like you introduce more detail of how cortex receive and mediate information of hearing? Would you? Thank you in advance!

SB: Sure thing!

If we use earplugs to listen music (iPod)everyday , will it create the
damage to our ears and brain at least when we get old?

SB: No, as long as you listen to it as reasonable levels (less that 100dB). For reference, those around you shouldn't be able to hear what you're listening to.)

I am interested in knowing more about this issue of efferent innervation of hair cells. Although I am no physiologist (and, consequently, don't really know my way around the relevant literature), I had occasion to mention the issue in a paper I published a few years ago:

Although some auditory tests (for example for locating a sound source)involve moving the head, and thus the ears, relative to the vibrating air, it may be that other tests are applied within the ear itself, to the vibrational state of the cochlear fluid (itself, of course, a causal product of the vibration of the sound source). The cochlea receives a considerable degree of efferent innervation, which seems to control movements of "hair cells" within its fluid-filled interior. These movements are thought to dynamically regulate the cochlea's sensitivity and frequency tuning, and possibly underlie active discrimination of specific speech components (Dallos, 1992, 1997). [Thomas, 1999 p. 221]

The main focus of my paper was mental imagery, but this passage was part of my attempt to explain an enactive theory of perception along the lines of that since popularized by O'Regan and Noë (2001; Noë,2004) (although they focused almost entirely on vision, I think they believe, and I certainly believe, that their theoretical outlook applies to all the sensory modes).

Although I did look up some other, more introductory, material on hair cells at the time, what I say in the quoted passage relies almost entirely on the cited papers by Dallos (and mostly on the first one). As I understood it, Dallos's idea was that the outer hair cells, controlled by efferent fibers, produce vibrations of various frequencies within the cochlear fluid which can then interact with the incoming sound vibrations, perhaps destructively interfering with some frequencies, constructively interfering with others, and perhaps producing beat frequencies (forgive me if my terminology is not quite right, I hope you know what I mean), and that it is these interference products that actually stimulate the inner hair cells to produce the afferent signals. Thus, the movements of the outer hair cells act to "tune" the cochlea to be more sensitive to some frequencies and less sensitive to others, and very possibly its tuning could be actively and dynamically adjusted depending on what sort of sound one might currently be listening out for. (I do not have the paper in front of me, so I may be extrapolating here a bit beyond what Dallos actually says.)

Anyway, I was a little uneasy at putting this in my paper (although it fitted so nicely with my argument) because, not knowing the wider literature, I could not really tell whether what Dallos was saying (and my gloss on it) was something that would be broadly acceptable to the consensus in auditory physiology, or whether it was excessively speculative and unreliable. In any case, Dallos's article is 15 years old now, so things must have moved on. I would very much like to know what Shelley or any other cochlea experts who might visit her blog might think about this. Is this now a more or less accepted picture of the function of the outer hair cells, or has it been either rejected (or radically modified)or ignored by most researchers? Does anyone know of any subsequent research that either supports, extends, or refutes it? (I would be glad of citations, or, indeed, actual reprints/eprints, if anyone knows of any.)

[If you like, you can email me through my website:]


Dallos, P. (1992). The active cochlea. Journal of Neuroscience, 12, 4575-4585.

Dallos, P. (1997). Outer hair cells: The inside story. Annals of Otology, Rhinology, and Laryngology, 106 (Supplement), 16-22.

Noë, A. (2004). Action in Perception. Cambridge, MA: MIT Press.

O'Regan, J. K., & Noë, A. (2001). A Sensorimotor Account of Vision and Visual Consciousness. Behavoral and Brain Sciences, 24, 939-973.

Thomas, N.J.T. (1999). Are Theories of Imagery Theories of Imagination? An Active Perception Approach to Conscious Mental Content. Cognitive Science, 23,207-245.

Could there be a flat sound whose wave structure is neither high nor low and (IF) so what would its sonic qualia be like perceptually ?

By Jason Leary (not verified) on 03 Feb 2008 #permalink