How hearing senses nanometers: auditory hair cells

Hearing relies on a mechanical system that detects nanoscale motion. This video explores how sound is detected and converted into a sequence of electric pulses called action potentials. You’ll see how sound information is encoded and a neural circuit for calculating sound location. Finally, you’ll explore the causes of hearing loss, a hearing test, and how our ears contain amplifiers that can emit sound.
This work is copyright © 2023 by Kerry Kim. All rights reserved. No part of this video can be reproduced or transmitted in any form or by any means without my written permission.
0:00 Introduction
0:57 Sensing nanometers
2:27 Action potentials
5:09 Sound localization
7:26 Hearing loss
9:16 Amplification
11:54 Cells are mechanical
3D coordinate files for proteins:
PDB ID: 6BNO (Actin)
Gurel, P.S., Kim, L.Y., Ruijgrok, P.V., Omabegho, T., Bryant, Z., Alushin, G.M. (2017) Cryo-EM structures reveal specialization at the myosin VI-actin interface and a mechanism of force sensitivity. Elife 6 doi: 10.7554/eLife.31125
PDB ID: (Cadherin)
Narui, Y., Sotomayor, M. (2018) Tuning Inner-Ear Tip-Link Affinity Through Alternatively Spliced Variants of Protocadherin-15. Biochemistry 57: 1702-1710 doi: 10.1021/acs.biochem.7b01075
PDB ID: 7LGU, 7LGW (Prestin)
Ge, J., Elferich, J., Dehghani-Ghahnaviyeh, S., Zhao, Z., Meadows, M., von Gersdorff, H., Tajkhorshid, E., Gouaux, E. (2021) Molecular mechanism of prestin electromotive signal amplification. Cell 184: 4669 doi: 10.1016/j.cell.2021.07.034
PDB ID: 1F88 (Rhodopsin)
Palczewski, K., Kumasaka, T., Hori, T., Behnke, C.A., Motoshima, H., Fox, B.A., Le Trong, I., Teller, D.C., Okata, T., Stenkamp, R.E., Yamamoto, M., Miyano, M. (2000) Crystal structure of bovine rhodopsin. doi: 10.2210/pdb1F88/pdb

[@isaaccunningham2042]

Yes , finally another video!

[@nano_redstone]

So glad to see a new video from you. As always great work !

[@WildEngineering]

hi nano

[@nano_redstone]

@@WildEngineering hello 👋

[@leontedumitru]

I'm so speachless that our bodies are so complex, yet they function so well with every system. As always, great videos. Please make more, I was really sad to think I'll not see any new video from the channel

[@jonahansen]

Yes indeed. But there was a time in the past where everything working perfectly meant the difference between life and death... Now, we fix all sorts of flaws that would have been fatal for a cave man.

[@swaree]

babe new Dr Kerry Kim video just dropped great news to have you back! I was blown away by the coindicence detectors, which I didn't know about. Just fascinating how an AND gate of sorts is used to detect direction of sound.

[@zachreyhelmberger894]

Wow!! Wow!! Wow!! This is the best video I've ever seen on ear function! I am totally deaf in my right ear and have high frequency loss in my left (fetal rubella??). If I cup my ear, I can locate usually crickets better than people with normal hearing (probably due to confusing reflections). 6:15 gives me a sense of HOW binaural direction finding is possible.

[@jonahansen]

Yeah - ears are pretty amazing. You can separate all manner of source with just 30,000 neurons from each ear, whereas vision requires like 4 million per eye. Not only that, the ear processes them so each frequency is separated out and can be heard, as in notes in a chord. In the eye, different frequencies get combined and encoded as one color, and it is impossible to identify exactly what frequencies of light caused the sensation.

[@misastrejckova5389]

THANK YOU SO MUCH OMG!!! After trying to understand the outer hair cells mechanism for like 3 hours I FINALLY undrestand what is going on. Thank you sooo much again.

[@SineEyed]

Biological functions at this level are just bonkers.. 🤯

[@accurrent]

you are an incredible educator. thank you for that!

[@cam6914]

This instills a deep sense of wonder in me. Thank you.

[@raguaviva]

amazing, you are the next veritasium!

[@orianemeijerink6983]

This video is absolute gold! Amazing work! There are several features of the ears mechanisms that i have been trying to understand all day and i finally understand!

[@jonahansen]

This is such a great video - best on KZbin regarding the molecular mechanism of hearing. I can't believe it only has 1.1k views after a day...

[@arseniix]

I'm absolutely astounded how the inner machinery of hearing is so complex. It feels even more profound than the eye. Probably because of differences in nature of light and sound waves. Regarding the thermal motion, I think that the valves are effectively springed by the scaffolding and the cell hair, which makes them oscillate wholly instead of just wiggling around and opening randomly. At least to some degree. But even if some positive charge leaks inside the cell and even triggers some action potential, it's not coherent with nearby cells and doesn't look like a real sound to the brain. Just as it was with cones and rods, where random false detections aren't coherent enough to be treated as a legit photon signal. Otherwise, we usually still hear some constant humming in the ear anyway. The brain just cuts it out from our normal experience, so you wouldn't notice it

[@raulgutierrez4943]

Please make more videos, they are awesome!

[@far1ne]

waited so long for a new vid, great to see you back!

[@hnnnnnghhh]

Just as good as the last video!

[@99jdave99]

Huh, I never knew that our ears could *emit* sounds too! It makes sense that a mechanical process in the ear could cause sound emission (though I didn't know that some parts of the ear have such mechanical motion prior to this video; I knew vaguely about the hair-vibration-detection, but not about the sound-amplification part of hearing), but I never would've imagined that such a small scale thing could cause reasonably detectable sound emission *from* the ear; very cool that it can be clinically useful too! Such devices must be *extremely* sensitive, I would imagine! Thank you for the video!

[@piratedgenes]

Glad to have you back!

[@ayzmmo]

This is so detailed! Thanks a lot :)

[@ironlegnebula]

YES! I was waiting for so long, I KNEW you will return

[@marlin3576]

OMG I love your work, you deserve way more subscribers than you already have.

[@3g0st]

Absolutely fantastic presentation

[@alfiebayfieldfarrell4078]

you are fantastic

[@TheAmazingFlint]

Saw your videos on photon detection and absolutely loved the way you visualize and explain things; probably one of the best videos I have ever seen. Been waiting for a new upload ever since :))) Good job, keep it up!

[@Macieks300]

This is so fascinating. Thanks for making this video!

[@happyman6102]

I love your videos! Thank you so much for making them, I am endlessly fascinated by our amazing biology!

[@AKLM24]

super super quality , thank your

[@setharnold9764]

Another excellent video! Thanks

[@user-si2qu3if8c]

I'm so glad about your new video! Thank you very much for your video, it is amazingly beautiful! When my biophysics students and I were studying vision, this was not the first time I gave them a link to your video and said that this is the best video for understanding the basics of phototransduction) And now I will send students to watch your video about hearing! Thank you so much!

[@derek8482]

beautiful work.

[@bencastleberry7591]

These videos are amazing

[@Niclas-dr3qt]

I love this

[@Ambidextroid]

Amazing, I really appreciate the attention to detail, using accurate molecular models and scales rather than abstract shapes

[@daviddong1286]

Wonderful video, I understand how much work those 3D animations take as myself is a 3d artist, maybe you can make some story video to talk about how people discovered those fascinating structures and functions, that will be equally interesting I imagine, and don't need (or you can just reuse) animations. Again, thank you for your wonderful videos.

[@professorchimp1]

Love your amazing content ❤

[@heinrichvonschnellfahrer7895]

more videos pliiiiiiz!

[@EpochSecutor]

Oh! I didn't expect a video but what a welcome surprise! The animations always help so much with visualizing everything.

[@pifibbi]

My boi is back!

[@d0x2f]

Amazing

[@AmbientMorality]

Thanks for the excellent video! It's amazing to learn about the different ways cells work to amplify signals.

[@s0meus3r]

🎉🎉🎉🎉🎉 new video!!

[@ben_g_min]

Your videos are so good. Love seeing these processes at the molecular level and explained in depth, just sad to see you have so few videos, please make more. Also would recommend upgrading the microphone 👍

[@RoopeshChopra]

Wow bro you made us wait for soooo long for another video Pleaae be more regular

[@GeoffryGifari]

Whoa that's cool.... never seen this explained (and animated) in this much detail. There are some thoughts that come up: 1. "Coincidence detector" in directional hearing seems to work like an AND logic gate. I wonder if neurons DO form logic gates. 2. Looking from the perspective of physics, a vibrating string that can produce (and sensitive to) low-frequency sound needs to be longer than a string for high-frequency. If each frequency slot has its own hair cell, do all these hair cells have different sizes? 3. What about frequency sensitivity? Do people who are more sensitive to slight difference in sound frequency have more "density" of hair cells? 4. Stereocilia connects a short hair cell to a taller hair cell. Is this related to the question at the end about noise from heat?

[@KerryKim]

Great questions! 1. Neurons do approximate logic gates in some conditions/circuits. You're right that the coincidence detectors behave as AND gates and other circuits do things similar to XOR, or even NOT. Some neurons receive input from thousands of other cells and do complex calculations doing things like comparing the relative strenth of signals (greater than/less than) or testing against some threshold. 2. There is a lot of tuning of the auditory system I didn't cover, but you're correct that the length of the auditory hair cells changes with their frequency like a string--lower frequency cells are taller. There are also stiffness variations in parts of the inner ear that also help tune different parts of the ear (stiffer areas around the high-frequency-sensitive areas). 3. I'm not sure about whether people better at frequency discimination are born with a greater density of hair cells or if its due to the processing of the sound by the brain (or possibly the structure of their inner ear). 4. Each hair cell has many stereocilia, and the strings between them are the tip links. Each of those tip links (shown as the yellow strings) is attached to an ion channel, and there are many channels (one for each of the many stereocilia) all feeding into the same cell. The question at the end is indeed about this arrangement.

[@MagicToadSlime]

The G.O.A.T. has returned!!

[@drake8846]

Fantastic video, there does not seem to be a explanation of how electrical and mechanical signals are translated into modulations of consciousness in the mind. Is the link between the brain and nervous system and the mind still completely unknown? Exploring the senses and imagining how our minds work makes me think that our perceptions of reality that we take to be accurate objective representations of reality are probably very subjective and distorted representations of the actual environment that we have miraculously evolved, more like dream imagery than being in touch with 'reality'?

[@newpractice]

this was incredible man, thank you! I've definitely noticed that my hearing becomes way more sensitive when I am meditating. my next question is, so how does earwax play into all this? 😆 peace!

[@jonahansen]

Earwax is an entirely an outer ear phenomenon, and generally messes things up, save for keeping the dirt out of your ear. Clean out your ears!

[@JamesThigpen]

Ears can produce sound?! That’s wild. I love your videos.

[@TheBlackRock-]

omg what a wonderful Surprise

[@jonahansen]

I was wondering, at about 2:06, you state the positive ions that move in are like potassium and calcium. Generally, the extracellular fluid is sodium enriched, and it is entry of Na+ ions that depolarizes cells with the intracellular fluid potassium enriched. If you mean Na+ is "like" K+ and Ca++ in that it is a positive ion, this could make sense. Am I missing something?

[@KerryKim]

Good question. Auditory hair cells' K+ concentrations are atypical. You are correct that in most neurons, depolarization is through Na+ influx. The auditory hair cells are unusual as their stereocilia are bathed in endolymph, which differs from the typical extracellular fluid for most neurons. The endolymph has a high K+ concentration, so the mechanically-gated ion channel current is primarily carried by K+ ions (not Na+) entering the cell. This is part of a larger system of K+ transport and electrical response where the rest of the auditory hair cell is bathed in a more typical extracellular fluid (called perilymph with low K+) where the potassium exits the cell. For more details, see this review paper: www.ncbi.nlm.nih.gov/pmc/articles/PMC4415853/

[@jonahansen]

@@KerryKim Thank you.

[@nofunsir]

@KerryKim I experience something no one else I've talked to has. I call it Contralateral Tensor Tympani Syndrome. If specific ranges of volume/tone hit my RIGHT ear, while my LEFT ear is prevented from hearing it (such as watching your video while lying on a pillow on my left side), then my LEFT tensor tympani flutters objectively. I can really feel it. This does not happen for my RIGHT ear when my LEFT ear is stimulated. So I might call it "Asymmetrical Contralateral Tensor tympani Syndrome". It was interesting to see your animation of the Action Potentials interacting with Coincidence Detectors. I have no problem with localization. In fact, I would say my hearing is very good. The animation made me wonder what other similar mechanism might be abnormal for me. I'm leaning to it's a brain thing. I wonder if you have any thoughts that could steer me down a research path. Thank you.

[@georgeman27]

So how about tinnitus that is result of mechanical damage? One day I was working with a gravel compactor, and after the work I noticed I have lost around 20% of my hearing, on the quiet range. On top of that I now hear constant high pitched tone for 2 years now. No variation, just constant 14 KHz tone ( I have tried to measure it). I know that there is no cure, but it is driving me insane, and I want to at least understand what happened...

[@BartJBols]

How do my ears know if the sound is straight ahead or straight behind? If i have only 2 ears and my brain calculates sound direction by measuring the difference between when sound arrives at the ears, i would assume i have a disk where the sound origin would arrive at the same time between both ears, and i would be unable to hear if it comes from the front or back. Yet i just tested it, i can definitely hear it if it comes from the front or back.

[@KerryKim]

Great question! The shape of our ears slightly alters the sound depending on the front/back direction, and we learn to recognize these alterations; kind of like how we can hear the acoustics of singing in the shower versus a larger room--the sound is shaped just enough to tell. We can also filter sound to mimic these effects, producing 3-D sound that sounds like its coming from different directions; some computer games and sound systems do this.

[@joalsoal1645]

Was this all made in blender? Including the molecular simulations?

[@KerryKim]

Yes; everything was done in Blender except the DNA logo--that was done in Maya.

[@joalsoal1645]

@@KerryKim thank you !

[@Scalifs]

I'm not even kidding but you can break 1 million subscribers easily

[@s0meus3r]

Too bad I can't like and subscribe twice ))

[@MagarMaharaj]

If its so sensitive why can't it decode other vibrations

[@MyMy-tv7fd]

design and engineering of God-like genius

[@SineEyed]

I think if that were the case, then we wouldn't have things like tinnitus, or astigmatism, or bed-wetting, or autism..

[@MyMy-tv7fd]

anything can go wrong @@SineEyed

[@SineEyed]

@@MyMy-tv7fd maybe... but would so many things go wrong if they were truly designed and engineered by a god-like genius?..

[@MyMy-tv7fd]

literally any car grinds to a halt without lube, the number of things that can go wrong is very great, the wonder is not just in its existence but in its excellence and rarity of malfunction - or we would not be here, would we? @@SineEyed

[@kyhxx]

. 5.7.24 twkn motif- cure s°°n pst chronixx^

[@5piles]

alas no actual instance of sound was explained. decent intro to biological correlation however.