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The cochlea -- its structure and oddities

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Old 07-Aug-2008, 12:05 AM (00:05)     1        21684
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Default The cochlea -- its structure and oddities

This is meant as a companion thread to a thread I will be doing on cochlear implants, and also as general companion thread to yet another one I will be doing on Deaf culture.

The cochlea (IIRC, ancient Greek for "snail", and is the snail-looking thingy in the pic at left) is the organ that actually translates sound into nerve electrical impulses, and is part of the inner ear. The eardrum only amplifies sound; the cochlea amplifies sound, translates it into nerve impulses and also does initial frequency analysis. At left is a pic showing how a cochlear implant is sited; the pic is also good for showing the relevant component parts of the inner ear. You can get a good idea of the make-up of the cochlea by looking over the Wikipedia page on it.

The fact that the cochlea is an active amplifier means it must necessarily also produce sound, a fact about the cohclea predicted by an engineer back in (IIRC) 1947, though it took another couple of decadedfor someone to develop a suitably thin microphone and slide it down the ear canal to pick up the sounds produced by a healthy cochlea. Please note that these sounds have nothing whatseover to do with tinnuitis; in tinnuitis, sounds are not actually produced and do not exist, but are still "heard".

The sounds produced by a healthy cochlear are not heard by the person themselves; either the cochlea itself or the brain filter out such sounds automatically. You can sometimes hear such sounds without a mic and amplifier if you put your ear right next to the ear of a new-born baby, and of course you need to have good hearing yourself, especially for the high ranges; you will (if lucky) hear what sounds like a high-pitched signal.

Inside the cochlea, which has evolved its snail-like spiral shape to do basic frequency analysis, there are what are called (confusingly) "hair cells", so termed because they look vaguely hair-shaped, but they are not at all hairs, but instead cells whose job is either active amplification (the "outer hair cells") or actual transduction of sound into nerve impulses (the "inner hair cells"). You can read up on hair cells and the structures on the relevant Wikipedia page.

One of the most common causes of deafness is dying of the hair cells, caused either by genetic factors, or by infection in the expectant mother * when being borne as an embryo, or infection of the ears in later life, or by physical damage to the hair-cells by too much noise, and so on.

Very interestingly, even though a person may be functionally completely deaf fromm birth on, the hearing nerve itself does not usually die, even though you might expect a nerve to die if it had no input at all. This means that the nerve around the cochlea can often be electrically stimulated and the brain wiill "hear" sounds, even in those deaf from birth; a fact that makes the cohlear implant a possibility, and I will start discussion of the cochlear implant on another, new thread.

* A handy mneumonic I learnt for infections in the expectant mother that can cause deafness in the carried child is STORCH, for:
S - syphilis
T = toxoplasmosis
O = other
R = rubella
C = cytomegalovirus
H = herpes

There are (from my poor memory) around 16 dominant and 212 recessive gene defects that can cause deafness; one of them is Wardeenburg syndrome, which also causes somethig that looks like vitiligo; it was very patiently explained to me twice very long ago just how both the cochlear hair cells and the pigment cells develop from the same source in the embryo, so if a gene defect hits one it hits the other too, but I have completely forgotten the explanation, sorry, and embryology was always a little too complex for me.

Wardeenburg syndrome came up in a very godo albeit pessimistic book by a linguist, who detailed (relatively) largish numbers of deaf people in certain Carribean communities; his main point was that unless there is enough support from (or at the very least, lack of oppression by) the surrounding hearing people, then deaf people could not get Deaf Sign languages developed by themselves even with an otherwise critical mass of deaf people being present, since they were routinely discouraged (by bullying etc.) from doing so. More on that in yet another new thread, that being on Deafness and Deaf culture.

I wish to hell I could remember the name of the book, or the author, but I can't.

There are of course also several books on the much more famous and luckier Long Island Deaf communities.
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Old 26-Oct-2010, 06:50 AM (06:50)     2        41529
Junior Marshwiggle
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I have an implant. Any questions?

Long Island Deaf communities
How about Martha's Vineyard? There was a book about it. Seems that their luck ran out when the inventor of the telephone, Alexander Graham Bell, started helping them. A school for the deaf was started on the mainland, and the deaf were brought off the island, intermarried with the general population, and the recessive genes were lost in the gene pool.

certain Carribean communities
Puerto Rico? There may be others, but I remember particularly that there was a strong group there.

I suspect that your interest in this is due to your linguistic background (I think).
Check out this book written in the 1880's. The Indian Sign Language on It includes deaf signs, also.

Last edited by muddleglum; 26-Oct-2010 at 06:51 AM (06:51). Reason: url
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Old 07-Nov-2010, 12:55 PM (12:55)     3        41730
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Originally Posted by muddleglum View Post
I have an implant. Any questions?
Please shoot me if I get too personal but I'm wondering about when you got the implant and whether it was your choice.

I posted on another thread about learning ASL and seeing hearing parents get very upset when their kid wanted to sign.
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