Hear, Hear: A Primer on the Cochlea

If you read the 'About' page, or anything about me, you probably noticed that I work on hair cell regeneration in the cochlea. But, perhaps, some readers are not familiar with the machinations of the inner ear. So, I'll make a quick post with some relavent info to help in understanding future hearing and ear-related postings.

The sense of "hearing" refers to the detection of sound waves in our environment. Humans are able to distinguish a lof of information out of these waves: direction (due to fact we have two ears!), loudness, pitch, timbre, and frequency. Waves are "collected" by the outer ear (what we think of as an ear) and transmitted down the ear canal, resulting in vibrations across the ear drum. The ear drum, in turn, passes vibrations on to the ossicles (tiny bones called the malleus, stapes, and incus) which serve to fine-tune and amplify the sound. The stapes sits atop the round window, which is a small membrane-covered hole in the base of the cochlea.

The name "cochlea" means "snail" and refers to its coiled shaped. (More below the fold....)

Above is an illustration showing the outer and middle ear, and below is one of the inner ear and cochlea.
As the vibrations of the stapes cause it to "hit" on the cochlea's round window, small frequency-specific disturbances are transmitted into the fluid-filled spaces in the cochlea. Each part of the cochlea corresponds to a different frequency range, high frequencies at the base of the coil and low frequencies at the top. The sensory cells of the cochlea are called "hair cells," so named for their hair-like projections which protrude into the fluid filled space (endolymph) in V-shapes.
i-608c3f631e9ea03251b42f20d2bde718-hair cells.bmp
Hair cells are organized into 3 rows of outer hair cells, and 1 row of inner hair cells. Inner hair cells send auditory information to the brain, while outer hair cells act to stabilize or intensify the vibrations transmitted throughout the cochlea. Deafness and hearing loss occurs (most often) in a one-to-one ratio with hair cell loss. Noise, certian chemicals, and age all act to selectively and gradualy kill off hair cells. This is a serious and prevalent disorder in America, and is currently irreversible. For more on the anatomy of the cochlea, please check out a link on my BlogRoll (sidebar to the left) called Promenade 'Round the Cochlea. It contains a lot of helpful information about inner ear biology in a graphical interface.

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It is not clear to me how the IHC and OHC are arranged in the cochlea. Your last picture is nice, but where is what? Is there only one layer of IHC and OHC cells? Where is this (or these) layer situated in the cochlea? Maybe you could provide us with some other pictures showing this?
(I hope that my question is clear.)

can our body survive with out this cell??

I'd like to include a copy of your IHC/OHC micrograph (seen above) in my not-yet-published book "Hearing Aid Help". Can I have your permission - or how do I get that permission.

By Gary Harris (not verified) on 15 Jun 2010 #permalink

FYI the stapes actually abuts the OVAL window, not the round window. That's a prelim question for sure :P

Greetings! Human perception has always interested me, not that my interest has translated to a great amount of retention of anatomy and physiology. What I remember most clearly from my crash course in audition in a "biological bases of behavior" class were the electron micrographs of hair cells, showing what looked like very insubstantial and (to me, whimsical) "tip links" joining these comparatively huge cilia-- an interesting feat of engineering (that is not, of course). I don't remember a great deal other than this, unfortunately. Maybe it will come back to me; I look forward to reading about your research!

It is not clear to me how the IHC and OHC are arranged in the cochlea. Your last picture is nice, but where is what? Is there only one layer of IHC and OHC cells? Where is this (or these) layer situated in the cochlea? Maybe you could provide us with some other pictures showing this?
(I hope that my question is clear.)

The area where the hair cells are located is called the organ of corti, which is surrounded with endolymphatic fluid. There are three rows of outer hair cells and one row of inner hair cells. Here is a picture of a cross-section of the cochlea http://www.laermorama.ch/bilder/modul_hoeren/Corti-Organ.jpg
Hopefully that may help you to visualize the position of the hair cells in the cochlea.

Yes it did, thanks.
But your link Promenade 'Round the Cochlea was also very very good, with an animation that explains a bit how the sound is processed. Very interesting. (And in french, which is good for me :-))
If I understand correctly, there is a resonance when a sound wave arrives, which is located near the base of the cochlea for high frequencies and higher in the cochlea for low frequencies, which let some stuff (cells of the organ of Corti I guess) move (the red arrow in their animation) which leads to the detection of the given frequency.
So now, I guess that a loud sound gives a strong resonance, which generates a lot of movement, which might destroy some cells (or some parts of some cells), is that correct?

I came looking for sparkler recipes, instead finding something much closer to home; to wit, my conductive hearing loss and balance disorder.

I enjoy your graphics and explication. As a blogger, you will know the frustration of discerning an ISP problem from a hardware/driver problem on your PC or Mac. By analogy, I hope you can be the pioneer that stops the shuffling back and forth between Neurology (the ISP in the analogy) and ENT (the hardware) for those of us with hearing and balance problems.

Living in the City of Medicine, I have gotten to debate causal relevance and the finer points of epistemology and neuroscience with some great clinicians. I'm still dizzy. Good luck with the cilia, as well as the great blog and links.

Who complains more of hearing loss?
Scuba divers complaining of their own hearing loss.
Scuba divers partners complaining of their diving partners hearing loss.
Or the divers complaining of their partners complaints of their hearing loss.
You mention that sound, chemicals, age as well as genetic predispositions lead to hearing loss. What about scuba diving?
Your research may not only save hearing it may save marriages too!!!

By richard wollocombe (not verified) on 12 Apr 2007 #permalink

Hi Shelly
I am no scientist but really can appreciate the work you are doing. Although one of the "older" generation I consider myself lucky to have reasonably good hearing, especially as I worked in a very noisy underground environment for best part of 30 years. What has happened though is that I have developed tinnitus. As far as I can see you have not mentioned this anywhere. Is it relevant to your work?

I used to have an African Grey parrot called Lucy. She was bought from a breeder in southern England as a 12 week old chick and as you say, she bonded with just one person. Me. She was not a great talker but excellent with imitating sounds and certainly was unhappy if we were separated, or at least I thought so. On nice days we would spend time together outside in the garden sharing a cup of tea. On 1st of August two years ago she suddenly looked around and just flew away. She stayed in the neighbourhood flying back and forward over what I gradually found to be a regular route. She was seen by lots of neighbours, but when the phoned to tell me that she was in their garden or on their farm, by the time I had got there she had gone. Late summer was a good harvest. There was a lot of food around. Then in November the weather broke. Cold and wet. Towards the end of the month I was given a carrier bag with her little body in it. She had died in a neighbours hay barn. That little body was the first time I had seen her since she had left me. I can only conclude that she had not been so happy with me after all. She took the decision to leave and fly into the unknown. I feel so wicked.

By the way, Haverfordwest is in Wales.

By Alan Taylor (not verified) on 30 Jan 2008 #permalink