How Hearing Happens
☛ The Hair Cell: When a young student heard that the lab next to mine studied frog hair cells, she exclaimed, “Oh? I didn’t know frogs had hair!” Actually, hair cells, so named because of the curious stacked arrangement of hair-like stereocilia emerging from their crowns (image a), are the cells that detect sound. About 16,000 of them line the snail shaped cochlea of our inner ear, picking up sound induced vibrations of the fluid inside our ears of less than 1 nanometer. The remarkable hair cell is what gives us humans the ability to detect sound of frequency ranging from 20 Hz to 20 kHz.
☛ Mechanosensation: How does a hair cell detect sound? The secret lies in the way the stereocilia are stacked. Each one is connected to an adjacent taller ‘hair” by a tenuous thread, known as tip link (image b). At the bottom end, the tip link controls the opening of an ion channel while at the upper end it is held taut by a motor protein (myosin) that moves along tracks (actin) inside the “hair”. When a mechanical disturbance in the surrounding fluid pushes against the hair bundle (image c), the tip link is stretched, yanking open the gate of the ion channel. Calcium ions flood the interior, changing the electrical potential of the cell and triggering a message to the nerves leading away from the cell. Immediately, however, the motor protein slips down, releasing the tension on the tip link and closing the ion channel to end the signal (image d). Later, the motor protein climbs up the cables again to re-establish tension in the tip link.
☛ Deafness, Eugenics and Alexander Bell: It may come as a surprise that the inventor of the telephone also had a profound impact on deaf culture. With both his mother and wife deaf, Alexander Bell became an avid proponent of “oralism” – teaching deaf people to articulate sounds in place of sign language. Given the uniqueness of deaf culture with frequent intermarriage among deaf people, Bell cautioned that the incidence of deafness could rise until there was a separate race of deaf people. Although his ideas on eugenics are not credited now, he was responsible for many changes made to education of the deaf. Deafness is the most common inherited sensory defect at 1-3 births per 1000. Interestingly, the most common inherited form of deafness has actually increased due to assortive mating (this is also seen in other disorders linked to ethnicity or race). Bell’s goals may yet be achieved, not by eugenics but by cochlear implants, which may restore hearing and abolish deaf culture in the future. Will that be a good thing?
Another installment in the #excyting series on cell types.
▶ Cardiomyocyte: http://goo.gl/uBL37G
▶ Adipocyte: http://goo.gl/S4fQFS
▶ Erythrocyte: http://goo.gl/R5R6Y0
▶ Astrocyte: http://goo.gl/SMpXMV
Image and Free Read: Corey, D. (2009) Cell biology of mechanotransduction in inner-ear hair cells.
Through Deaf Eyes (Alexander Bell): http://goo.gl/DY2ouS
try to deal with hearing loss and mind disorders.I don’t know if I’m hearing things or going crazy.
Very good 🙂
Mark Bruce , great question on how this structure evolved. I would have to dig around for this, but the short answer is that every cell has a primary cilium, which in this type of cell is called a kinocilium. Little finger-like microvilli develop around it and grow in size (see http://goo.gl/GmmSvu). Structures like this are seen in the “statocysts” of medusae so they are found in many primitive forms. What is also cool is that many of the same proteins are also used in vision, to hold the photoreceptor cells in place, so mutations in the proteins cause deaf-blindness. If I come up with something specific re. tip links, I’ll let you know.
Randy Bressler, sorry about your hearing loss 😦 There are advances in restoring hearing these days, so I hope you benefit from them.
Great info! Any chance you can explain what causes the different types of deafness? For example if a tip link is broken is that permanent or do they self-heal?
Daniel Carr , the tip link proteins can be lost by harmful mutations causing a form of Usher syndrome (deafness). I believe they can be regenerated if damaged. Very loud sounds (rock concerts!) can damage the stereocilia and cause temporary deafness, but hearing returns after a while, fortunately.
I forgot to mention that there are ~200 genes that can be defective, leading to deafness. The whole apparatus is complex enough that defects in individual parts disrupt the hearing process.
Fascinating! When I was born the doctors did not know the cause of my deafness. I wonder if it could be discerned today. It is probably one of those 200 genes as there is no family history nor any syndrome-related symptoms.
Daniel Carr , you probably inherited one recessive mutation from each of your parents. You may even have mutations in Connexin26, which is the most common cause of deafness. Check out 23andMe: https://www.23andme.com/health/nshl/
Thank you, this is a great info. Simplistic yet brilliant. Someone has used the exact same method to produce electricity in big noise producing cities. http://inhabitat.com/soundscraper-transforms-vibrations-from-city-noise-pollution-into-green-energy/
Amit Vora , what a fantastic concept. Thanks for the link! Skyscrapers do sway appreciably in the wind as well, perhaps that could be tapped to generate energy. Reading more now….
Great share, Rajini Rao !
Thanks, John Kampsen 🙂
Hmm interesting. Before having kids we got some genetic tests and were told we (hearing wife) were no more statistically likely to have deaf children than anyone else, I can’t remember the percentage I’m afraid but it was negligible, something like 1 in several hundred. Can also confirm daughter is not deaf 🙂 I don’t know if they would have tested all 200 genes though, it may have just been the ones known to pass on deafness through reproduction.
Edit: Suspect you are right only a full rundown via 23andme or similar will solve the mystery!
Daniel Carr , the odds would have been much greater had your wife also been deaf. But with one hearing parent, all it means is that the kids are carriers since the odds of your wife carrying a hearing mutation are low.
Hmm I need to dig out that old test for another look. Thanks for responding!
Thank you for sharing your story, Daniel Carr 🙂
Excellent information. Thanks, Rajini Rao !
Delicious post Rajini Rao Thanks.
Fun article Rajini Rao ! Did you come across any description of how gain, amplification and dynamic range are adjusted at a cellular level? One of the concepts we have explored at the macro level in structures is termed “negative stiffness” – my understanding is that some researchers have attributed the same effect being used in living systems to deal with the wide range of acoustic energy detectable in mammalian hearing systems.
Bill Carter , yes to all three..there is a fair amount known and I was barely able to scratch the surface. For example, the length of the stereocilia is tuned to frequency. The amplification is not linear, with lower intensity sounds being amplified more to give a range of six orders of magnitude. There is a protein called prestin in the lateral wall of the hair cell that causes a change in the length of the cell itself, a process called electromotility (piezoelectric effect). Prestin mutations cause loss of amplification and frequency discrimination. There is also something called hair bundle motility, and I’m not clear how that fine tunes hearing. There is a membrane (tectorial membrane) that rests on top of these hair cells that also contributes to the mechanism.
Ryan Scheidt still interested in hearing research? Here’s one of your fellow Hopkins-ites.
A mix in Social and Science, excelent.
wow, I didn’t know about the hair cells, they look amazing !!!
Jose M. G. Guerreiro , I was quite surprised to learn about Alexander Bell’s role in policies towards the deaf. From reading about it, many find his ideas disturbing and anti-deaf (even if there was some genetic truth to his views). E.E. Giorgi , there are some lovely images of hair cells ..they are great subjects for SEM!
Awesome Rajini Rao ! When I am off mobile I will do some research. Shape and connectivity are great ways of tuning mechanical response. Little mystery that nature uses them fully.
Please share, if you figure it out Bill Carter . The mechanical details went whistling past my ears 🙂
Yes Rajini Rao he was a defender of eugenic policies, in the same line of Francis Galton theories. In vogue at last years of 19 century.
designed by evolution – complicated yet it works good enough
Thanks Rajini Rao. Very thorough. Of interest ’cause I’m losing my hearing.
Fantastic article and excellent write-up, as always! Rajini Rao Thanks for the post!
Rajini Rao I can’t thank you enough. I said good bye to my ‘ear drums’!
I hope your ear drums are in fine shape, R Prakash Prakash. Unless it is convenient to not hear when your spouse/family want some work done around the house 😉
Another good write up Rajini Rao
I have severe tinnitus (ringing of the ears) caused from (House Ear Clinic says) strong drugs I took for my arthritis. They did not help. ;-(
The interesting thing is that I also have very acute hearing, no loss at all and I worked around loud machinery much of my career.
Normally with tinnitus caused by loud noise you also lose some hearing, but because mine was a chemical thing my hearing is excellent. I just constantly hear a siren, very high pitched over the top of everything else.
It is something that I had to adapt to, but I wondered what went wrong in those little hair cells.
An aside, normally I just ignore it but when I talk or write about it, like now, it gets incredibly loud. The mind is strange.
And folks, turn down that music, and wear hearing protection around loud machinery, tinnitus is incurable, and some do not adapt.
Gary Ray R , thanks for the wise words of caution. For those who may not know, tinnitus is a “phantom auditory perception-it is a perception of sound without corresponding acoustic or mechanical correlates in the cochlea”. I believe there is no medication that provides relief. This paper has a good summary of causes and treatment options if anyone is interested: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2686891/
My sympathies, Gary Ray R . I can only imagine how annoying and stressful it must be.
Rajini Rao I meant, I knew only till ear drums, until I read this.
Ear drums or no ear drums, the spouse will ensure I get the message clear and also loud, occasionally 😀
Haha! By the way, the ears are also important for balance. I’ll plan to post something on how we know up from down hopefully soon. Cheers!
Thanks Rajini Rao
Thanks for the kind words. Yes, it is incurable. One does learn lots of tricks and ways to deal with it. But before I did I almost went mad.
If anyone out there has tinnitus, seek professional help. You can lead a complete normal life, and learn how to get some sleep, that is the one thing that almost drove me over the edge, could not sleep because of the siren always going off.
Now it is just an annoyance.
Thanks for the paper, but I’ll have to read it later as it is a bit loud in my head right now. ヅ
My tinnitus is thankfully very mild. At worst slightly annoying, although I do worry some what that it will get worse as I get older. I also lost some hearing while in the service, but my VA swears my hearing is normal, even though a civilian audiologist said I was a couple of years from needing a hearing aid, four years ago.
I hear you… but I am not listening lol
Thanks for an excellent post. I didn’t know about hair cells, truly fascinating 🙂
I have tinnitus and got it from playing drums since high school and it can be annoying at times, but its only worse when I think about and I focus on it. I can fall asleep with no problem. Maybe because I’m too tired to think about it or I’m just used to it. I protect myself from making it worse. I don’t use inner ear buds to listen to music. I now use earplugs at concerts. Maybe someone will figure out how to stop it. That would be nice, because I don’t know what silence sounds like. Great article Rajini Rao
Thanks, Rajini Rao ! The hair cell is probably my favorite sensory transducer.
Absolutely fascinating! Since part of my job involves speaking about disabilities including hearing loss and being deaf, I must be very careful to distinguish between Big D Deaf and little d deaf. Most people do not realize that the majority of people who are deaf adults have acquired it as adults, there is much confusion on causes, treatments and politically correct terminology. One of the toughest things to do is speak about this topic and also not offend people who are Deaf, who do not consider themselves to have a disability.
I see the posts here so far include those who are little d deaf; they did not grow up in the Deaf community and likely do not use sign language. I hope Google+ members who are Deaf will also come forth when they see this article and the amazing links you’ve provided. I plan to study this material with your inspiration and I thank you very much for sharing it.
Rajini Rao , thanks for the post! I always wanted to know about the mechanism of hearing. It can’t be more simply explained!
A very concise and informative article to enlighten awarenes!
This post got me so excited that it should be tagged nsfw, 🙂
Thanks for keeping things interesting
Oh, wow, the length of the hairs controls frequency perception? I have trouble telling notes apart. Now I know to blame the hair length.
And now that everyone is talking about tinnitus, I can ‘hear’ a slight whine (high frequency noise). Rats. I can also hear the fan (which I totally ignored till now) and other sounds. The mind is good at filtering out sounds when it wants to concentrate on G+ posts! 🙂
Interesting. I don’t think a cochlear implant would help my case (no hearing whatsoever in my left ear, attributed to nerve damage early in life) but a very interesting summary. Thanks Rajini Rao
Cochlear implants are thought to be a “solution” by hearing people but the reality is, they do NOT make you “hear” like other people and it takes a very long process and much work to make sense of the sounds when getting used to an implant. But it is very good that there are becoming more and more developers and inventors who continue to work on such assistive devices.
John Christopher , you bring up a good point: for cochlear implants to work, there has to be a functional auditory nerve. Do you know what caused the damage to your nerve ..early childhood infection?
BJ Bolender thanks for the clarification and all your interesting comments. As you note, the cochlear implant corrects for mechanical sound. The brain still needs to retrain to interpret the sounds properly. That’s why they work best in people who have recently lost hearing or very early on in children born without hearing.
Exactly. That is our best guess, too.
Interesting how the ion channels are still unknown. It’s this way for quite a few senses, it seems: the gating of Drosophila Trp channels still remains elusive in phototransduction, and here (or “hear”), the ion channels themselves aren’t clear.
Schrodinger’s ion channels: you can know their gating or their structure, but not both!
Adam Gill , TRP channels are the top candidates “hear” too, but not persuasively so. It is possible that some new class of ion channel will emerge, in much the same way as the Orai channels were found to be the store operated or CRAC channels. Given how intensely studied this field is, it is odd that the mechanosensitive channel has not been conclusively identified. Something to look forward to!
Hi friends h r u all friends hapy new year
Hi. My apologies for this late addition to this thread, I’ve only just seen it.
In my case I had severe mumps when I was 4, it ‘killed off’ the cillia in my left ear so that I have no hearing in that ear at all. Kids are so resilient and so I just adapted.
Nowadays as an adult I don’t remember what it was like to ever have two working ears. Even though most people cannot tell I am partially deaf I have still learnt British sign language, in case I can ever help interpret for someone or get the chance to communicate by signing.
Now my question rajini rao, is do they know what part of the mechanical structure is affected by mumps and also could this be reversed by stem cells?
Brilliant and informative paper Btw.
Simon Clark I’m so glad that you found this post informative. Thank you for sharing your experience. Now that the incidence of mumps has fallen from vaccinations, most people have forgotten the damage caused by mumps.
Deafness from mumps (also measles and rubella) is thought to be the result of viral infection of the inner ear, known as viral labyrinthitis. The virus destroys components of the inner ear: hair cells, supporting cells of the organ of Corti, cochlear neurons, etc. Depending on the extent of damage, hearing loss can be reversible or permanent. Ultimately, if we could figure it out, stem cell therapy could be used to repopulate hair cells. I don’t think we are anywhere close, though.
Simon Clark I’ve about 60 dBA hearing loss on my left side because of a chronic ear inflammation when I was a kid, damaging ear drum, hearing bones, and a lot of mastoid air cells. I had radical surgerey. Recently I learned there is a plan B for my case as my bone hearing is still reasonable: a Cochlear Baha. So depending on the kind of hearing loss you have, there might be solutions. Even for my plan B, I’m not sure I’m going to do it. The chance hearing will be close to the level of my right ear is not high, and even if it was, it will take a lot of time for my brain to adjust to it. I see plan B as a last resort in case the hearing on my right side becomes really bad.
Rajini and jeroen, thanks to you both for taking the time to respond. I’m glad I found this post. As I said before hardly anyone realises I’m deaf because I’ve adapted so well to it and like you Jeroen, to suddenly have my hearing returned to binaural would be too much of a shock so I think unless I need to have it I’ll stay with good old monaural for now!
Thanks to you and Jeroen, too. I learned much more from the comments on this post than the actual writing of it 🙂
Rajini Rao for me it was the other way around. How these “hairs” work was totally new for me. Thanks a lot: it makes much more sense now, both in the way they work (nature again has made a brilliant solution) and in the delicacy of it (I now get why for instance motor cyclist often have hearing loss, or why virus infections can easily damage such microscopic structures with ease).