| 00:02 | This is lecture 19 of Neuroscience. today we're discussing hearing in particular the |
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| 00:09 | system and to understand how we perceive , uh we have to first understand |
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| 00:18 | sound is. And as when we about visual system, we said, |
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| 00:22 | talk about some of the properties of and what we can perceive what is |
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| 00:27 | range of visual perception for us, is is 400 to 700 nanometers wavelengths |
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| 00:33 | uh of light. And now we talking about hearing, which is |
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| 00:39 | sound is neural perception of sound energy travels in sound waves. And the |
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| 00:47 | of sound is 343 m a second 767 MPH, the traveling vibrations of |
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| 00:56 | and they consist of al regions of and rare faction of air molecules. |
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| 01:03 | an example here that is given this a region of compression, this is |
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| 01:07 | region of rare faction. So the compression and molecules, air molecules in |
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| 01:12 | air become more dense and obviously in factor there it less dense. So |
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| 01:19 | is a tuning form. So the form is just standing in the |
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| 01:23 | nobody has uh touched it. And tuning fork is gonna have pretty much |
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| 01:29 | the quiet room and even surround the of these air molecules around it. |
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| 01:34 | the minute you tap and this tuning , it starts vibrating and these |
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| 01:41 | what they do is the tuning fork . It produces these waves of rare |
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| 01:49 | and compressed air waves that are the waves that are traveling in air. |
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| 01:56 | the same principle is with the speakers you have in the car in your |
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| 02:04 | or in your ears because ear buds headphones are speakers. And the principle |
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| 02:12 | speakers is that there's a fire and speaker that vibrates and as it vibrates |
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| 02:19 | moves the air molecules, certain amplitudes certain frequencies. And uh this is |
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| 02:26 | recreates the sound, whether it's analog um record uh player or digital form |
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| 02:34 | anything that you typically use as like cellphones and computers and such. |
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| 02:41 | , and then there's a speaker someplace there's a vibration that produces that sound |
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| 02:46 | any one of these listening devices you know, sound devices as you |
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| 02:52 | , uh human audible range. we're limited from 20 Hertz to 20,000 |
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| 02:58 | . 20,000 Hertz is 20 kilohertz. is above 20 kilohertz infrasound is below |
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| 03:06 | Hertz. Of course, there are creatures and animals in this world that |
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| 03:13 | at different frequencies and they have the , we don't have that our ear |
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| 03:20 | systems and the cochlea system is designed perceive the sound within this 20 to |
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| 03:27 | for its range. But other uh , for example, dolphins herring, |
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| 03:35 | can communicate at really, really high like 180 kilohertz. Not something that |
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| 03:41 | cannot perceive. Even if it is us, we don't perceive that |
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| 03:46 | We're just limited to perceiving what we given by the creation of the sensory |
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| 03:55 | in the brain and the brain circuits the systems. That means that there |
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| 04:02 | ultrasound and there is infrasound around us the time. We don't hear |
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| 04:07 | Infrasound. We can actually perceive in ways. Infrasound would be a slow |
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| 04:13 | and car vibrations are slow below 20 . The subwoofer, some subwoofers, |
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| 04:21 | you read, actually, when you purchasing an audio system or a |
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| 04:25 | like as speakers for your computer will some for low range 10 Hertz as |
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| 04:32 | the hearing range, then it goes to four Hertz, some of them |
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| 04:37 | for the speakers as below the hearing . So if you are at some |
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| 04:43 | or concert and there's really powerful you can actually feel the vibrations of |
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| 04:52 | molecules. So you will feel them some matter sensory system, not through |
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| 04:56 | auditory system, but those are really vibrations. And it's just the fact |
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| 05:03 | this world, the way that we this world, visual information, 400 |
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| 05:09 | 700 auditory 20 to 20,000. That's what's in this world. That's what |
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| 05:16 | perceive is this world, this world light and we saw sound outside all |
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| 05:24 | the ranges that we can perceive and there are animals that live in this |
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| 05:29 | that we depend on as a part the ecosystem that perceive this world completely |
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| 05:36 | . We perceive it as heat They might perceive it as high frequency |
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| 05:40 | , That means something that we don't understand what it means. So, |
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| 05:45 | of the sound is loudness and intensity the sound, the sound waves will |
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| 05:49 | reflected in the aptitude are uh in words, this is the volume button |
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| 05:56 | your, in your, in your uh equalizer or in your car stereo |
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| 06:03 | . So the low frequency and the frequency, this is the pitch, |
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| 06:08 | frequency, high frequency. But then know how low frequency, low intensity |
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| 06:13 | low frequency, high intensity, it's same, it's the same pitch, |
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| 06:17 | . So these are the basically properties the sound waves that you're looking |
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| 06:22 | Now, those vibrations in the outside the air, they're designed to enter |
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| 06:29 | the of our air, be directed the auditory canal, also called external |
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| 06:38 | . And the air vibrations are eventually to vibrate the air drum or the |
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| 06:45 | membrane, which is on the outer . As this air drum vibrates from |
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| 06:51 | air vibrations, we have obstacles, smallest bone in the body that are |
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| 06:57 | to the ear drum, but they're in such a way that they can |
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| 07:02 | and they have quite a bit of to move. It's all almost like |
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| 07:06 | jointed together and they have these flexible where they can move. And that's |
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| 07:12 | because then the movement of the air can be amplified through this torque like |
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| 07:21 | of the obstacles and the obstacles which part of the middle here will be |
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| 07:26 | the oval window and that oval the window into the cochlea. And |
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| 07:34 | is a snail like structure looking This is the cochlea and this is |
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| 07:41 | top of it is the vestibular para out of the cochlea, you have |
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| 07:47 | auditory component of the Bulo cochlear nerve the cochlear component of the tibula cochlear |
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| 07:55 | , which is the eighth cranial which you've learned in the second |
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| 08:00 | So this is the information that will processed the sound information in the cochlea |
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| 08:07 | then through the cranial nerve, auditory nerve. And in particular, in |
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| 08:14 | case, the auditory component, auditory is the same as vestibular cochlea nerve |
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| 08:19 | another term for it. Uh But this case, the cochlear component, |
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| 08:24 | is all comprising cochlea and the organ corti what is called the inner |
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| 08:31 | There is also uh this is uh air wax would be here. Uh |
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| 08:38 | also most importantly, this is your tube in here, that kind of |
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| 08:44 | if you can imagine in the, your ear, kind of in |
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| 08:47 | in the back of your throat and station tube is for equalizing, it |
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| 08:55 | into the back of the throat, the bar or equalizing the pressure between |
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| 09:00 | outside and the inside of your And an example is when you're |
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| 09:06 | your ears plug up or they start or whatever you wanna call it, |
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| 09:11 | because of the changes in the air at the high elevation. You don't |
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| 09:17 | have to fly. You can drive on the mountain or drive down the |
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| 09:21 | fast in the car and you'll feel changes. And um, if you're |
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| 09:27 | the plane, you'll see people, know, starting to yawn and move |
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| 09:31 | jaws or blow their ears and that's to trying to equalize the pressure between |
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| 09:39 | air and the outside. And this done through the, with the help |
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| 09:43 | the station tube. And you can if you can equalize the pressure, |
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| 09:48 | can be very painful. It can uh rupture the um, the air |
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| 09:53 | and upset the, the, the, the auditory system for a |
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| 09:58 | . So also it's a site in Two, you will have possibility of |
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| 10:05 | infections. So this is a very site with ear infections in Children that |
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| 10:10 | repeated ear infections, chronic ear So there's a consideration for surgery that's |
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| 10:16 | tubes. So my child got the inserted. You will hear that. |
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| 10:20 | my, somebody's uh, child is tubes in the ears dunked. And |
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| 10:24 | because this becomes a breeding ground for and viruses. It's a mucosil |
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| 10:32 | It's sweat, it's warm, it's the back of the throat. It |
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| 10:36 | uh you know, a good way basically infiltrating him. Now, what |
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| 10:43 | if the child has chronic ear Uh and they have to go to |
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| 10:51 | . We talked about the critical period plasticity. I remember. So early |
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| 10:55 | , early learning is very important. child doesn't hear as well. They |
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| 11:00 | pay as much attention. They don't as well. They don't have as |
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| 11:03 | plasticity in their auditory apoca in their , a poly potentially tube. So |
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| 11:08 | tradeoff is to place these plastic, grade plastic tubes, replace them uh |
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| 11:16 | the natural station tube and circus And that is not a very favorable |
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| 11:22 | for bacteria and viruses to grow. that is a tradeoff and at least |
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| 11:26 | have good hearing and they develop uh . So oss obstacle, as I |
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| 11:35 | , they will amplify the vibrations, air vibrations that are eventually vibrating the |
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| 11:40 | e when it's a panic membrane, you can see these arrows and you |
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| 11:44 | the movement of the OTAs and eventually movement of the obstacles will be moving |
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| 11:51 | oval window. And this oval window is going to move the fluid in |
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| 12:00 | inner ear inside the cochlea. We'll that next obstacles are the smallest bones |
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| 12:06 | your body. And mali as tinkers stabs uh cochlea is about the size |
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| 12:18 | a P like green peak. that's in, in, in, |
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| 12:23 | human cole is about the size of peak. Now, uh these obstacles |
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| 12:31 | also you can see here there's tensor muscle and then there's Sted muscle |
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| 12:42 | So just think about the location, of these muscles. If you contract |
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| 12:48 | muscles, then you're contracting and making space smaller, right? If you |
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| 12:57 | , if you're making the space you're not allowing these obstacles to move |
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| 13:04 | as much torque, eliminating their to . Now, I wrote something down |
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| 13:12 | that's called attenuation response. And I ask students. So if you hear |
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| 13:19 | loud noise or persistent loud noise, is the first thing you typically |
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| 13:28 | What's that? Cover my, cover , cover your ears? Ok. |
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| 13:33 | tied your hands. Um maybe, don't know. I just don't |
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| 13:39 | oh, interesting. We're getting Actually anybody else, John. |
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| 13:52 | Well, both of you are kind like you're doing something different than just |
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| 13:56 | your hand. So if you cannot your hands to block the sound, |
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| 14:01 | you'll see people, even if they block the sound with their hands as |
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| 14:07 | blocking, they'll also do this like of a squint, right? You |
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| 14:12 | do it like, you know how , you know, but if you |
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| 14:16 | that and squint, you can see the ambient level of sound that you're |
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| 14:21 | in the room changes. Ok? that's because you are contracting these |
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| 14:27 | And this is a protective mechanism for loud noise, not to activate the |
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| 14:35 | as much because that can end up your hearing. It's a protective |
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| 14:40 | a loud noise to stiffen up. the torque movement of these obstacles by |
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| 14:50 | of these little muscles. We take cochlea which looks like a snail here |
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| 14:58 | we roll it out. We have base of the cochlea here where we |
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| 15:03 | the oval window and we have the of the cochlea that is also referred |
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| 15:10 | as helicotrema. Otherwise it would be here. Um If we take the |
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| 15:17 | section of Ronal section through this uncoil , we will see that there are |
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| 15:23 | chambers, scala vestibule. This is one that is the closest on |
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| 15:29 | the closest to the vestibular apparatus, media which is in the middle and |
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| 15:34 | contains the organ of or the hearing that has the hair cells as the |
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| 15:40 | receptor cells and the scala timy at bottom. So there are three fluid |
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| 15:48 | chambers. And so the movement of oval window will result in the movement |
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| 15:59 | here off the fluids in these chambers we're discussing in the cochlea. Before |
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| 16:08 | get there, the three chambers have fluid inside scala vestibule and scala |
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| 16:18 | they have parallel and parallel is very to what we've learned already as a |
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| 16:24 | of a standard cerebrospinal fluid which is in potassium and high in sodium, |
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| 16:30 | in sodium chloride. But Scalia which a stream of muscularis and this stream |
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| 16:40 | Sculli establishes a very high potassium gradient using active transport where endo limb in |
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| 16:50 | is dominated by very high external potassium , 150 milli. Now this is |
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| 16:59 | where sound transduction takes place where this displacement of the fluids and mechanical displacement |
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| 17:07 | the hair cells becomes an electrochemical potential receptor potential. So that suggests also |
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| 17:17 | a lot of it is going to mediated by potassium uh conductance. In |
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| 17:23 | case, another feature of this retina if you unroll it from the base |
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| 17:32 | the apex, what we saw in in the retina in the retina, |
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| 17:41 | saw retina topic map. Uh Remember said that there is a point in |
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| 17:47 | looking over there point is looking over , point is looking over there that |
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| 17:52 | point by point for presentation of the world we had it in the retina |
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| 17:58 | G M all the way into the visual cortex. Here we have |
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| 18:04 | these hair cells that are aligned along extent of the uh scala media. |
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| 18:15 | throughout the whole cochlea and it has outer hair cells and the inner hair |
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| 18:21 | . And these are the auditory receptor . And it turns out that the |
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| 18:26 | cells that are located the closest to base and to that oval window that |
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| 18:33 | being moved, they are most responsive the high frequencies. So 20 |
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| 18:40 | 18 kilohertz, 16 kilohertz in the of the cochlea midrange frequencies and at |
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| 18:49 | very apex uh of the cochlea, have hair cells that are most responsive |
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| 18:58 | low frequencies. Uh one kg Hertz Hertz and all the way to 20 |
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| 19:05 | . One of the analogies that I to use is gymnast ribbon. So |
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| 19:13 | uh Olympic Games uh are on, uses uh uh is uh usually uh |
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| 19:20 | watches uh gymnastics and some of us the ribbons. But uh one of |
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| 19:28 | uh numbers for the gymnasts and one the tools is using this ribbon. |
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| 19:33 | so typically when they move the ribbon this is their hand. So this |
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| 19:37 | a stick that holds the ribbon, ribbon will be flaring out like |
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| 19:44 | So closer, this is not even good drug but closer to the, |
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| 19:55 | to the handle where they're holding they're gonna vibrate or move this up |
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| 20:00 | down like that. But as it out, it's going to be much |
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| 20:07 | and also low frequency weight if you or low frequency. The other way |
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| 20:11 | think about it is when you drop stone in the, in the water |
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| 20:16 | an object in the water, the from that stone first, they're very |
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| 20:20 | together and then later they spread and become slower and spread and spread. |
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| 20:25 | this is sort of an analogy, closer you are to that window, |
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| 20:28 | higher frequencies you will process it. is where the oval window would be |
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| 20:32 | high frequencies and the further away you from that you'll be processing lower frequencies |
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| 20:39 | the sound and that becomes important for cochlear implants. And this is referred |
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| 20:45 | as tonotopic map. Again, this not a map of auditory localization signal |
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| 20:53 | . In other words, we're not signal is coming from from there. |
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| 20:57 | is the frequency map that is of course, there's sound localization map |
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| 21:03 | we can recognize which direction the sound coming from. And we'll talk about |
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| 21:07 | of the physiology behind that in a . But this is the frequency |
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| 21:12 | And again, that means that the that are located closest to the oval |
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| 21:17 | , those hair cells are going to most responsive to the higher frequency |
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| 21:22 | Now, this is what's really neat that it when we talked about uh |
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| 21:28 | potentials in the very beginning of the , we said that there are these |
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| 21:32 | that are voltage gated channels. So changes and the channel opens, it |
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| 21:37 | its confirmation and has these voltage sensors these the voltage sensors slide up and |
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| 21:43 | . Then we talked about neural And then we said there are these |
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| 21:47 | gated receptor channels and lien uh you , dupo coupled receptors. And uh |
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| 21:57 | talked about visual system and we talked photoreceptors. And there the stimulus was |
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| 22:04 | . It had the photo pigment and was a change in the G protein |
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| 22:10 | signaling cascade resulted in a change in protection. Now, we're here in |
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| 22:15 | auditory system and the stimulus is But for the hair cells, the |
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| 22:21 | is what they're, they're not out , they're inside in the inner |
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| 22:27 | So they're subjected to the movement of fluid and the displacement of the |
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| 22:33 | And this is let's say a baseline no sound or quiet. And then |
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| 22:39 | the fluid starts moving, it starts this basilar membrane. On top of |
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| 22:45 | vasa membrane, you have the organ corti, the three rows of outer |
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| 22:50 | cells, one row of inner hair . And on top of that, |
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| 22:55 | CIA these are called stereo on top the hair cells are connected to the |
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| 23:01 | membrane, tectum or Tector membrane. , to the roof membrane. And |
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| 23:07 | as the fluid moves in these chambers displaces the basil membrane up and then |
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| 23:15 | and down. And as it displaces basilar membrane, as it illustrated |
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| 23:21 | what it does, it bends the in one direction. Here, it's |
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| 23:27 | stereocilium to the right and here it's stereocilium to the left. OK. |
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| 23:34 | if the membrane is steady, this the stereocilium. If the membrane is |
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| 23:40 | , the stereocilium shifts to the If the membrane is down, the |
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| 23:46 | shifts to the left. And this from what this is doing actually is |
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| 23:56 | are mechanic contained mechanically gated channels. what happens is that this movement to |
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| 24:05 | left or to the right or up down the leg encodes the receptor |
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| 24:11 | So when the hair cell stereocilium moved the right, it actually opens up |
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| 24:16 | of these potassium channels and causes influx potassium and it causes depolarization. So |
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| 24:24 | would be depolarization and then it moves and then there is hyper polarization, |
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| 24:31 | , hyper polarization left hyper polarization depolarization. So essentially the sinusoid wave |
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| 24:39 | the sinusoid sound wave that we saw we have in the air becomes a |
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| 24:47 | wave movement of the fluid and that waves becomes encoded as an electrical sinusoid |
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| 24:57 | and the sense of the receptor So, depolarization or hyper polarization and |
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| 25:04 | how you can have excitation and inhibition the circuit with displacement of one direction |
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| 25:11 | another of the stereocilium. This is microscope image of the stereocilium. You |
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| 25:19 | see how tightly and how well they're in the Tector membrane. So once |
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| 25:25 | basilar membrane moves and you're attached to , you have to move to the |
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| 25:30 | once you the right. But if are attached to that and that will |
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| 25:33 | the signal. It also shows that spiral ganglion cells that comprise the auditory |
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| 25:41 | of the vestibulocochlear ner are mostly inner the inner hair cells and not the |
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| 25:48 | hair cells. So that means that sound processing really the organ of |
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| 25:54 | most of the information is coming from inner hair cells, but you only |
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| 25:59 | one row of inner hair cells and have three rows of outer hair |
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| 26:04 | So for a while, it was that, you know, they're playing |
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| 26:08 | role and call it kind of a up the roof with their stereo. |
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| 26:13 | what's really interesting is that they also these motor proteins. So the outer |
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| 26:22 | cells contain these motor proteins. And these motor proteins allowed to do, |
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| 26:26 | serve sort of like springs, then stretch the lock of the membrane to |
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| 26:32 | . So if there's a displacement of basin of membrane and the stereo cella |
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| 26:36 | bending, then they're sort of like w like a spring exaggerates that uh |
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| 26:44 | movement even further, which amplifies the and the displacement of the basilar membrane |
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| 26:53 | respect to pictorial membrane amplifies the encoding the inner hair cells. Most of |
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| 27:00 | output is being taken from inner hair . So if the outer hair cells |
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| 27:05 | not just supporting that, but they're exaggerating the movement, they're allowing for |
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| 27:10 | inner hair cells to encode that information effectively, the sound information. Now |
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| 27:17 | there is a movement of the stereo , there's going to be an influx |
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| 27:22 | potassium. And the way that these work is that they are mechanically gated |
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| 27:30 | , they're called T R P A or trip A one. And these |
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| 27:35 | gated channels will allow for the influx potassium. Because if you remember endo |
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| 27:41 | loaded with potassium, and it's also that these mechanical gated potassium channels have |
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| 27:52 | protein extensions that are called tip So when the stereocilium moves in this |
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| 27:59 | and opens this channel, it also in the tip link and encourages a |
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| 28:06 | stereocilium located the passing channel to open addition to just the movement of |
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| 28:14 | So these tip links sort of like chain. If you open one |
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| 28:17 | it's gonna encourage the opening of the channels, not just through the band |
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| 28:23 | the stereocilium, but also through this like connection, the tip links to |
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| 28:29 | adjacent channels on the adjacent stereocilium. the same way as if you close |
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| 28:35 | , then obviously you encourage for all the channels to close. And the |
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| 28:39 | links also to pull the channels to influx of potassium will cause the depolarization |
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| 28:47 | will then open voltage gated calcium That's a scheme that's familiar to us |
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| 28:52 | influx is necessary for neurotransmitter release. cells do not generate action potentials, |
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| 28:58 | generate receptor potentials. Uh and the of glutamate will excite these spiral ganglion |
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| 29:09 | cells in the nerve that will be the action potential. So now, |
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| 29:18 | so potassium influx is here as depolarization by once the depolarization is there opening |
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| 29:26 | voltage gated calcium channels. So potassium are mechanically gated and calcium channels are |
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| 29:34 | gated. Um why is it not actual potential? So, in the |
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| 29:43 | cells, if you recall, it's of a common theme in retina, |
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| 29:48 | discussed that uh this whole circuit in photoreceptors, bipolar cells, they don't |
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| 29:55 | action potentials, they're all graded receptor . And then when you came through |
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| 30:00 | ganglia cells, the nerve, that's we we see the actual potential |
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| 30:06 | So now why it's a it's a deep question why um one of those |
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| 30:15 | could be that uh graded potential also of what analog information is or in |
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| 30:24 | way an analog mode of neuronal Uh they're graded, they change in |
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| 30:31 | . So they represent different amplitudes. potential is all or none. So |
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| 30:37 | zero or 101 01. That is code that's a digital code, |
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| 30:48 | right? So you can view this why is that maybe these receptor cells |
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| 30:54 | processing the world as close as possible this analog graded like information that later |
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| 31:01 | converted into a different all or non , which is more of a digital |
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| 31:05 | representative code OK. So very good . But once again, I have |
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| 31:12 | question here, what is the equilibrium value for potassium? And if you |
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| 31:16 | we talked about equilibrium potentials, talked Nerds equation in the first section of |
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| 31:23 | course. So I'm just reminding it's not really a homework question. |
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| 31:26 | can go ahead and do it. if you're looking at the endo length |
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| 31:30 | you're looking at the current length, equilibrium potentials for potassium is gonna be |
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| 31:33 | different because Ners equation is RT Z log of concentration on the outside versus |
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| 31:41 | inside from that given ion. And obviously here you have very high concentration |
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| 31:48 | potassium. So it's gonna have a equilibrium potential for potassium. Um And |
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| 31:55 | it's going to be influx and potassium than e fluxing potassium like we saw |
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| 32:01 | the action potential generation influx and potassium having a very different reversal or equilibrium |
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| 32:08 | for potassium. Yeah. Uh So the channel is open and then your |
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| 32:14 | is released is that then after that's the action happens and that's how we |
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| 32:18 | time. That's if there is enough this greater potential as for the |
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| 32:23 | which is uh the spiral gang Uh This is here with the spiral |
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| 32:31 | cells that are now a to essentially will generate a action potentials but not |
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| 32:39 | . So this is like an equivalent reno gang cells that you saw the |
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| 32:45 | one is the only output because there's output from these. So there's no |
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| 32:49 | from photo bipolar cells like gang cells the only output and the action potentials |
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| 32:55 | first generated there. This is only from the cop and action potentials are |
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| 33:01 | here. So I'm glad you're asking because hopefully it kind of drives you |
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| 33:05 | some of the what we call canonical of canonical connectivity in these senses sort |
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| 33:09 | systems. So most of that information is coming from inner hair cells, |
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| 33:18 | hair cells of course have the spiral cells connected to them. But as |
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| 33:22 | see, most of their function is amplify the displacement of amplified the encoding |
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| 33:28 | sounds. I'm gonna skip talking about experiment here. Just move to the |
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| 33:33 | slide auditory pathway. It's very there's some similarities with the visual |
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| 33:42 | but also there are clear differences. in particular, you have projections from |
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| 33:49 | , spiral ganglia or auditory nerve will into the dental cochlea nucleus here. |
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| 33:57 | the way that you read these diagrams this is number one. So here |
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| 34:01 | the cut through the lower uh brain area. This is the cut coming |
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| 34:09 | here to midbrain. And number this is a cut that you're looking |
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| 34:16 | . Number three in this thal section through the thalamus in the cortex. |
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| 34:22 | . This is how you read these . So the first one which is |
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| 34:27 | the level of brain stem is the cochlear nucleus. And then these are |
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| 34:33 | lateral projections and then from ventral cochlear , some of them remain eps lateral |
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| 34:38 | others cross over and become contralateral. basically this is being cut off a |
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| 34:48 | bit. Basically cochlea nuclei are ipsilateral all others from superior olive to inferior |
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| 35:00 | and cortex, they already have by inflammation, but it's different. Remember |
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| 35:08 | binocular in information only became binocular in primary visual cortex and only layers to |
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| 35:15 | here by oral information for both ears already being processed at the level of |
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| 35:21 | brain ST uh you recall that this here from superior of the inferior |
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| 35:28 | It's a part of corporate quadri it has superior colliculus for processing sy |
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| 35:33 | movements in the colliculus for processing auditory , superior colliculus and inferior colliculus, |
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| 35:41 | to each other. So at the of the brain stem, there is |
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| 35:44 | visual input coming in here and there's input coming in here. And you |
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| 35:49 | have some basic reflexive auditor and visual processing at the level of the brain |
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| 35:55 | . And that would be reflexive like reflex like because it's not really cognitive |
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| 36:00 | of either auditor or visual signal at level of the brain stem. |
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| 36:06 | brain stem neurons send feedback to the hair cell. So from brain |
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| 36:11 | there is uh input and it goes into the outer hair cells and communication |
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| 36:18 | the auditory nerve and that's very different there's nothing in visual system that was |
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| 36:22 | back into the retina. Yeah. there's almost like a communication from thalamus |
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| 36:29 | the colliculus go uh from inferior colliculus into the thalamus, medial geniculate nucleus |
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| 36:34 | G M. It's medial to the gene nucleus and L G M is |
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| 36:40 | visual information. This is processing Audi and from there it goes into the |
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| 36:44 | auditory cortex. So it's different from visual system which was retina in the |
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| 36:53 | cortex. So you have brain stem colliculus that are all involved. I'm |
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| 36:59 | that information and auditory cortex will talk uh mediagenic nucleus and inferior colliculus and |
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| 37:06 | stem neurons will talk to outer hair . And I had a good question |
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| 37:09 | a student yesterday. What does that ? Does that mean we can somehow |
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| 37:14 | uh tune in to certain sound? I said that's exactly what maybe we |
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| 37:19 | do is that if you, for , are in a room and there's |
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| 37:23 | music and you're talking to somebody, can still almost tune in to that |
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| 37:29 | frequency to that voice. You use of your sensory inputs, auditory and |
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| 37:34 | to, to follow that person uh the opposite when there's so much uh |
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| 37:39 | in the grocery store and things like . But somehow you just just tune |
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| 37:43 | out and, and, and listen somebody talking to you. OK. |
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| 37:50 | tonotopic map that we describe this frequency . And the cochlea, it extends |
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| 37:57 | the spiral gangland, it extends into cochlea nucleon and it extends all the |
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| 38:03 | through the Audi fat into the primary cortex. And if you recall in |
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| 38:09 | primary visual cortex, we have the specific the specific orientation of the bar |
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| 38:14 | or direction. Here you have collections columns and layers of the cells of |
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| 38:22 | regions of this primary auditory cortex that most responsive to 500 Hertz 1,005,000, |
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| 38:31 | and so on. So there's this map that continues all the way into |
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| 38:36 | primary auditory cortex. Are we good sound localization? We are pretty good |
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| 38:42 | sound localization. But we typically use senses also to confirm where the sound |
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| 38:47 | coming from. Uh when the sound our ears, it can hit our |
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| 38:53 | slightly different. So if the sound coming from the right, it's gonna |
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| 38:57 | the right ear first and then some of millisecond, maybe half a millisecond |
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| 39:03 | , it's gonna hit the left ear same because it has to travel |
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| 39:07 | It's a movement of air molecules. different angles is coming from the front |
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| 39:12 | gonna reach the side, it's gonna the back some fraction of milliseconds. |
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| 39:17 | , we can recognize where the sound coming because the sound wave is coming |
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| 39:21 | the right, we'll first hear it the right ear and also our left |
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| 39:25 | is gonna be in the sound so it's not going to be as |
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| 39:28 | . So we know that the source sound is from the right. If |
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| 39:31 | coming from the front or the it's likely gonna hit our ears at |
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| 39:35 | same time. Uh And, and there's no difference there and if |
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| 39:41 | from the front, then the shadow going to be in the back. |
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| 39:43 | it's from the back, the shadow from the front. And this is |
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| 39:47 | ale it's a sound shadow, our uh built in such a way that |
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| 39:53 | we all have different, you looking ones, small, large different |
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| 39:58 | . So they kind of uh inside anatomy is built to direct those air |
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| 40:03 | of sound into the channel here into pathway into the external law. To |
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| 40:12 | when you talk about sound localization, do you encode whether the sound is |
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| 40:17 | from the right and to the left a cellular level or uh a physiological |
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| 40:23 | ? And this is an example where have sound from the left side |
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| 40:28 | So sound is coming from the initiates activity in the left cochlea nucleus |
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| 40:33 | is then sent to the superior and you have this axon with cole nucleus |
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| 40:40 | it's going to excite the cell one it is. And just when it's |
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| 40:51 | to hit cell three, that same that came from the left within a |
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| 40:57 | of millisecond later, just as this from the left is to hit cell |
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| 41:02 | very soon as the sound reaches the here, initiating activity in the right |
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| 41:06 | nucleus. And now the two will neuron number three. But this nucleus |
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| 41:14 | going to know that my, I excited in this order 12 and then |
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| 41:21 | . And that means the signal is from this side, it was coming |
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| 41:24 | the other side and it would be and it would converge on one and |
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| 41:29 | coming from the front or the it would be 12 or 32 and |
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| 41:33 | would converge in the middle. So is how this structure knows where the |
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| 41:37 | is located on the left or the . And both of these symbols can |
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| 41:42 | reach the uh the synaptic potentials and eight in exciting the cell number three |
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| 41:50 | will produce an action potential. And how it will know that the sound |
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| 41:54 | coming from the left versus the This is a sound localization versus a |
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| 42:01 | recognition which basically tonotopic map is a or frequency recognition, sound localization map |
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| 42:07 | where it is the sound coming Uh Let's talk a little bit about |
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| 42:14 | impairments which most of the hearing impairments uh conduction impairments are probably more common |
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| 42:24 | sensory neural impairments. Conduction impairments have you know, have everything to do |
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| 42:39 | you're mechanics. Mhm So ruptured eardrum probably one of the most common problems |
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| 42:50 | hearing, uh calcified obstacles, Uh Something is wrong with attachment of |
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| 42:58 | obstacles. Anything that has to do changing the mechanical, the conduct of |
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| 43:06 | mechanical stimulus, which is air Ok. And has to do with |
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| 43:14 | conduct dysfunctions, sensory neural dysfunctions have do with the death of hair |
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| 43:24 | the loss of human. So no sensory neural perception, it's not mechanical |
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| 43:32 | , not obstacles, not e it's loss of uh hearing. It's |
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| 43:38 | of hair cells. When you lose cells, it's not like anosmia and |
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| 43:45 | regenerate the factor receptor neurons. When regain the sense of smell and |
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| 43:52 | if you lose hearing, it doesn't back. So if you have |
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| 43:58 | that means that you've killed off some cells. And the very common symptom |
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| 44:06 | partial loss of hearing is not only hearing things. Well, sometimes it's |
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| 44:11 | a specific frequency. The way our is built and twisted and the snail |
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| 44:17 | structure is human. Voice frequencies are perceived and protected, has lost frequencies |
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| 44:25 | be protected and perceived because that's very for our survival. And quite often |
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| 44:31 | damage may be at the high frequencies are very close to the oval window |
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| 44:36 | that movement of membranes is exaggerated. therefore, a possibility for exci of |
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| 44:42 | much excitation and excitotoxic or death of cells with calcium overload and glutamate |
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| 44:50 | That we talked about in the air . So then another symptom of uh |
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| 44:56 | hair loss is not only saying, , what can you repeat the question |
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| 45:00 | myself? Uh I can't hear you can ring, it's called tinnitus and |
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| 45:06 | can, you can hear it when ears ring and I can hear it |
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| 45:10 | I lost a hearing partially in one . And I went to uh a |
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| 45:17 | that had very loud music. It very high pitched frequency that was turned |
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| 45:21 | loud. Even commented to our friends we were too close to the |
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| 45:25 | I came home, you know, you come home and your ears are |
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| 45:28 | up and you say, oh, ok, I'm gonna go to bed |
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| 45:30 | wake up. It's gonna be It wasn't fine. I still couldn't |
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| 45:34 | , it was, felt like I under water still with a, you |
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| 45:38 | , like a, a snorkeling And, um, I said, |
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| 45:43 | , well, I'll wait another day a week later, it still wasn't |
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| 45:47 | away. So I went to see audiologist specialist for hearing and they tested |
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| 45:53 | hearing and they said, sorry, lost partial hearing and high frequencies |
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| 45:58 | in one ear. So I went uh Simpson mode into mode. |
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| 46:03 | no way you cannot be, you , like you, you have to |
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| 46:06 | me again. He says, I'm not gonna do that. |
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| 46:08 | I'm gonna come back two weeks later he says, well, you |
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| 46:11 | but you should maybe come back a later, see if there's any progress |
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| 46:14 | went back a month later. And was like, hm, maybe, |
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| 46:18 | 3% improvement. But it's, it's going there. So they offered me |
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| 46:23 | expensive, uh, uh, ear , $150 earplugs. It's like, |
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| 46:27 | really cool and then I lost them week later and you cannot do |
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| 46:33 | Is hearing, uh this tinnitus is is persistent and sometimes it's like uh |
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| 46:39 | it's bad when I'm tired, I'm up. I have coffee, I |
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| 46:42 | up, it's like ringing so It's like it's a, you |
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| 46:46 | K GB torture chamber of pi pitch . So it, it, |
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| 46:51 | it's true. Now, why would be happening? Right. Why is |
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| 46:55 | , why can we hear the ringing look at that again? I'm gonna |
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| 46:58 | you of the, of this and this image here. The anatomy look |
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| 47:04 | going along the, from the Uh holding up the tutorial mere, |
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| 47:11 | ? And now this is my hair here. OK. This is my |
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| 47:17 | number and now I lost his hair . What do you think happens with |
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| 47:22 | curl membrane? It's loose. So it's loose within the frequency that I |
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| 47:28 | the hearing or somebody else is high . And that means that this uh |
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| 47:34 | is almost like unattached fluttering. Um and, and constantly not being able |
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| 47:41 | destabilize properly and fluttering creating this high oscillation basically and high frequency pitch noise |
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| 47:50 | my ear. So we call, that's the reason for our tinnitus. |
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| 47:56 | . Uh Now a lot of times people have hearing loss, partial hearing |
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| 48:03 | , you will see people using hearing . A hearing aid is a big |
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| 48:08 | put in your ear. It just the sound and we're gonna talk about |
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| 48:14 | implants, cochlear implants are not hearing , cole implants. If you lose |
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| 48:19 | hearing completely, it's not that you partial loss or suppression of hearing across |
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| 48:24 | of the frequencies within one given frequency don't have hearing at all. And |
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| 48:30 | cochlear implant is still a, you , it's a complicated procedure. It's |
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| 48:33 | developing procedure, but where you would receiving antennas for sound that are typically |
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| 48:40 | underneath your skin here and they're sending information that they're receiving. So outside |
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| 48:46 | is coming and your ear is not working. So this is sort of |
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| 48:50 | your sound receiver and it processes the of sound. And this receiver is |
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| 48:56 | to an electrode and this electrode is here into the cochlea OK. Through |
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| 49:03 | round window here, it's inserted and electrode is wound up inside the |
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| 49:09 | So it's uh this is a, is a cochlea, let's say, |
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| 49:16 | then you basically insert the electrode right the lines of the cochlea along the |
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| 49:23 | of this electrode that surrounds the you'll have many different stimulating electrodes that |
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| 49:30 | coming off here. Uh Now what are you stimulating hair salce are |
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| 49:38 | . There's no hair, no stimulation foul gang. So, so all |
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| 49:43 | still be one, remember the tonotopic . So now you can say, |
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| 49:48 | , if this is the base, is the base and this is the |
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| 49:54 | that means that the base I'm gonna all of these, you know, |
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| 50:01 | very high frequencies. And here I'm have very low frequencies. So when |
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| 50:06 | receiver is gonna hear high frequencies, gonna send the information to the electrodes |
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| 50:13 | are located in this region of the to stimulate the spiral gang cells that |
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| 50:19 | be receiving high frequency information. And it is receiving mid frequencies at the |
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| 50:25 | time or low range frequencies, I'm gonna stimulate these different areas of the |
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| 50:31 | and basically trying to reproduce the So you're converting this through like a |
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| 50:38 | a listening device, a microphone sound , you have multiple, multiple hundreds |
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| 50:47 | little electrodes along this one electrode that insert inside the cochlea and each one |
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| 50:52 | them will be assigned a specific So if they hear that frequency of |
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| 50:58 | kilohertz boom, this electrode gets activated kilohertz boom, this electrode gets activated |
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| 51:05 | that's the cochlear plant. That's not hearing. Uh Now for the last |
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| 51:12 | minutes don't have much time today, I'd like to share this video with |
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| 51:19 | and this is one of the best for our localizing sound. That's barn |
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| 51:28 | . It tunes into the empty The noise of wind and snowfall is |
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| 51:34 | out. It's rustling that the owl interested in a false alarm but it |
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| 51:45 | have to wait long from deep under snow. A Leming transmits a high |
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| 51:53 | rustle and around here, the penalty rustling is death. The signals are |
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| 52:02 | weak for our hearing. But this has the ultimate amplifier. Its face |
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| 52:08 | like a satellite dish. The dish formed by a ring of stiff |
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| 52:15 | They collect and channel sound inwards, eyes look central but the dish actually |
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| 52:24 | on the ears. They are on side of the tiny skull next to |
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| 52:28 | eyes. The dish is divided by lane of bristles giving stereo sound. |
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| 52:41 | like having a giant cupped hand behind ear to pinpoint the lemming, the |
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| 52:56 | must tune its receiver. The dish moved, the eyes automatically follow too |
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| 53:06 | . Then back again, the lemming now being totally reckless sound and eyes |
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| 53:17 | now focused from this point on. won't look away until the lambing is |
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| 53:24 | its talons. The sorting begins, approach is absolutely silent, soft velvety |
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| 53:43 | have serrated edges that simply caress the and behind each ear, the dish |
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| 53:50 | moved too far. Again, the and its I thought there was an |
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| 54:00 | of the of the house, not head on and the talons are raised |
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| 54:15 | the line of claws on each talon extended two above and two below. |
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| 54:27 | for catching cylindrical prey. The lemmings is up. Even if the lemming |
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| 54:35 | moving, the owl can compensate. owl hovers. It checks signal |
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| 54:42 | the body twists and the talons are . Yeah. Right. When you're |
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| 54:53 | with senses, this sharp and specialized defenses, like snow can be |
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| 55:00 | Pretty amazing. Simple |
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