 Welcome back to the lecture series in bioelectricity. In the previous class, we talked about vision and we talked about the retinal processing, the dark current, the rods and the cones. And I highlighted about the work of Mark Homoion, where he has developed or bypassed the whole retina or the eye and develop cameras interfacing with the brain. So, this whole area of neural processes where different kind of electronic intervention or where electronic tools have been used to interface with the brain, it all started with the hearing in the ears, it was not in the visual system. So, initial attempts were made where there are eight electrodes where there are people who had the complete hearing loss and instead of their ear, it was all replaced by with a mic, you know the mic which is interfaced with eight probe electrodes and which could convey the sound information to the brain. So, today what we will do in this class, we will be talking about the basic architecture of the ear and the cells which are responsible for translating the sound waves into electrical signals. So, in the previous class, we talked about the cells which are responsible for translating the light signals into electrical signals. So, it is basically the rods and the cones and we talked about the different cones with which had sensitivity towards different wavelengths of light. So, here we will be talking about a series of cells within the cochlea or the inner ear which are sensitized to different frequency of sounds. So, essentially what happened when a sound wave gets into the cochlea based on the component, what frequency, what amplitude, the sound wave kind of you know get divided or sensitized by different cell types. So, we will be talking about that whole architecture cellular geometry and in the case of prosthesis, how it is being bypassed. So, let us start the lecture 20 with hearing and the anatomy of the ear. So, we are into lecture 20 which is essentially hearing. So, for hearing you have the ears, the anatomy of the ear that is what we are going to study and the function. It is very interesting, it has two function, one is of course the hearing, it is another function, it is function of equilibrium. In the sense, say for example, you are moving along the mountains, you are going up, some people feel the feeling of you know throwing away or throwing out or in the vomiting feeling. Because, when you move up like this or you kind of keep on looking like this or you keep on looking like this, you feel uneasy because your face have to be parallel with the ground. When you look up like this after a fall, after a point you feel dizzy because there is a disequilibrium in your body and this equilibrium is being sensed by a specific part of the cochlea. We will be talking about that in very briefly though and then we will be talking about within this functional system, we will be talking about a specific cell types which in the anatomy will be coming through function by something called hair cells. These are the cells which codes for, let me go to the next slide. These are the hair cells which codes for your sound wave electrical signals or electrical impulses and these hair cells are the ones which constitute or they set inside the, their location is cochlea which is essentially the inner ear. So, these hair cells could sense different frequencies of sounds. So, this is the feature of these hair cells. So, we will be talking about hair cells anatomy, physiology or actually essentially electrophosiology of these cells. So, to start with let us explore the structure of the ear itself. So, the ear is something like this. Now, the structure of the ear is something like, so this is the outer ear which consists of something called a tympanic membrane like this. So, membrane is a structure like this. So, the sound waves are coming like from outside. So, here the sound waves are travelling. So, the first they hit upon the tympanic membrane. After hitting upon the tympanic membrane, they had to interface. So, this is the outer ear. Now, the sound wave moves to the middle ear which consists of three different bones. Those bones are kind of you know arranged like this, malleus, incus and staples. So, this is what is essentially this is bone one, malleus, this is staples. This constitute your middle ear. So, up to this you have the outer ear. This is what constitute the middle ear. After the middle ear, now from here starts the inner ear which is essentially. So, from here is a very interesting spiral structure like this and which has extension out here like this, network of channels like this. This is your inner ear. This is cochlea and on top of cochlea you have something called semicircular canals. The upper part of the cochlea and the semicircular canals are the ones which are involved in the equilibrium and cochlea is the one which is involved in all the sound deciphering the sound. So, basically the sound wave travels along this and at different zone it is being you know perceived at different level. So, this is essentially the overall architecture of the ear how it looks like. Outer ear let us summarize. So, here you have the sound waves, sound waves, second sound waves traveling out here, hit upon the tympanic membrane transmitted through this the vibration within this malleus incus and staples within the middle ear those three middle ear bones it reaches to the cochlea and this vibration now travels all along this spiral flute like structure. You must have seen those the kids have those you know they blow from here it is a structure like this you know the kids blows from here and it flattens out it is almost like a very similar structure like that. We do like this and that whole thing flattened out and it again comes back to its original position. So, imagine cochlea is a very similar structure like that looks like this now from here what we will be doing we will be moving on to coming back to the next slide. Now, what we will be doing is we will be talking about the structure of the cells which are lining this out here this what is the structure of those cells. These cells are essentially called the hair cells just like. So, your corollary should be that when we talked about retina we talked about the rods and the codes which were translating the light impulse into electrical impulse here we will be talking about the hair cells which will be translating the sound signal into electrical signals and all those hair cells are located all along this track out here this is the track where the hair cells are sitting. So, let us talk about the structure of the hair cells now once again. So, these are the stereocelias and. So, these are the kinasellium and these are the stereocelium these are the nerve endings and just like the retinal pigment epithelial cell in the retina these are sitting on there is a lot of supporting cells which are there which I am showing in green. So, these are the supporting cells. So, this is the overall structure. So, now the most interesting feature out here are those interesting hair like structures and that is why they got their name hair cells and do not mistake them with your hair they are totally different. So, those stereocelia and kinasellia has a very interesting feature if you see the tip if you kind of highlight at this part of the structure it is not that simple this is structure actually is something like this it is a narrow and depending on in which direction the kinasellia is moving all the stereocelia they will move in that same direction. So, for example, if imagine these are the hair like I assume that this is the this is the kinasellia and these are the stereocelia if kinasellia moves like bent like this all the all of them will bend in this direction and if the kinasellia bends in this direction all of them will bend in the another direction it is just like they are all attached as if underneath they are all kind of you know just like a spring they are all attached with each other like this or it is like kind of spring like structure. So, if you give a pull in this side they all will move like you know they all will move like this and if you give a pull on that side they all will move in the reverse direction or in other word actually what happens actually the pull has to be given only on the kinasellia if you move the kinasellia like this or you can move the kinasellia in this direction depending on which direction you are moving the whole thing will move on the that is specific direction and based on the direction it decides whether this is going to conduct electricity or conduct current or not. So, it is purely purely mechanical if all of them move in one direction they will be on and if them all another direction they will be off it is almost like this. So, what we can essentially put out here this. So, what is essentially is happening is that if you had to put it in text then it will be a displacement in this direction say for example, if all of them are moving like this is positive or generate signal and if the displacement are on the other direction say for example, something like this is negative signal or this is saying inhibition and this is excitation. So, what is essentially is happening think of it when they are bending in one direction like this they are sending excitatory signal it means when they bend in one direction as if they are coupled with a series of sodium channels and all the sodium channels open and they bend in the other direction all the sodium channel closes. So, you can add one more component to this if there is a molecular if this is something like this is a spring which is pulling in this direction and this spring move motion once again this spring motion is ensuring the sodium channels to open and the reverse direction when they are moving in this direction these ensure sodium channels are closed these are the closest state the triangle I am drawing the half triangle I am drawing these are the no more sodium is unable to enter sodium is unable to enter sodium is unable to enter. So, depending on the pull of the string they ensure that whether sodium is entering or sodium is exiting. So, this is a very very classic feature of these hair cells and these hair cells if you go back to this picture sorry all these hair cells are sitting out here likewise. So, the way the structure is now I will come to the further microstructure of it if this is the now I will only I will be only drawing the cochlear structure. So, if this is the cochlear structure think of it I kept it simple. So, then the hair cells. So, this is the membrane the one side of the membrane of the tube and this is the other side of the tube. So, the hair cells are sitting something like this. So, these are the locations of the hair cells these are the hair cells are they are just let me draw it and I will explain what is exactly happening. So, likewise this is continuing all the way to this and similarly all the way to this and of course, continuing all the way like this. So, now when the sound wave enters. So, let us the sound wave in place now the sound wave is entering like this. So, this sound wave is creating pressure on these walls. So, sound wave is creating bulge on these walls something like this and these bulge which is creating is deforming these cells either in the direction of this direction or that direction which. So, and this bulge which is creating deformation in these cells are sent as nerve impulses. So, sound wave is moving here and along this tube it is creating inflation like this as it is creating inflation underneath you have these sensor cells or the hair cells which are all sitting underneath it as it is moving like this it is creating those bulge and those bulge are deforming these hair cells which are just sitting underneath that membrane. So, essentially these hair cells are outside the cochlea it is kind of they are lining the whole cochlea, but there is a thin membrane which is ensuring they are not directly getting hit. So, it is basically the bulging of the membrane like this sound is moving and the bulging of the membrane which is like this sound is moving sound wave and it is bulging the membrane and you have the hair cells like this and based on that that movement the hair cells changes their shape. So, in order to top the seriously either move in the direction or moves in the other direction and that generates and if I have to just have a slightly more cleaner picture out here. So, something like this here is the tube and here you have the deformities taking place within the tube as the sound wave is moving through and on both the walls what I was trying to animate and show you this membrane is kind of you know changing its micro feature with respect to time as the sound wave is travelling. So, this is the sound wave which I am drawing now and here is the direction where it is moving and underneath you have under this membrane you have these cells which are sitting which are connected to the nerve on both sides you have to realize that this is a three dimensional structure. I am just drawing in a two dimension and these are all connected with nerve endings like this like this. So, whatsoever bulging taking place this is resulting in a motion of these either in this direction or this direction depending on which direction they all will be moving and this movement is essentially generating a signal or not generating a signal depending on and these impulses are all eventually reaching to the brain. This is the overall functioning just the way when you talked about the rods in the cones here the sodium channels are regulated by the mechanical motion of the stereo cilia and the kind of cilia and the stereo cilia in which sort of direction they move depending on the direction all of them will move and that will either will open up all the sodium channels or will close them and will not send any further signal. So, this is how this whole architecture works, but there are few other features what which I wanted to add now on that on top of this some of the features what has to be added now is something to do with one more thing I have to tell you which is just slightly of the track coming back to this structure when I talk to you that it involves in the equilibrium out here there is one more feature let me add that and let me come back to the sound processing what is happening at this in those cells in those cells again they have the hair cells sitting like this and there is a membrane on top of it, but out here there are some wonderful three-dimensional cube like structures which are sitting there and this is all to do with the equilibrium and those are essentially called autolith these autoliths or these are gelatinous material these gelatinous material depending on whether a person is looking up like this or a person is looking down like this or a person is in equilibrium like this depending on with respect to the azimuth or with respect to the plane these autoliths all this small gelatinous matrix on top of these hair cells either become like this or become like this and these are of course all connected to the brain and this is what regulates your equilibrium. So, whenever there is a ear damage or something there are people who had problem in understanding the equilibration with respect to the body and the azimuth and the plane where they are looking at. So, coming back so this was a little detour for this for the sound thing because I this is what I tell you and this autolith is basically autolith sorry this autolith is basically a gelatinous matrix which is present on top of instead of the sound waves they are basically what you are having is complete matrix and this matrix and these these cubes actually the way these cubes moves say for example, if this is a rectangular cube sitting here. So, if the person is looking down like this they will create they will attain a shape like this or they may attain a shape like this and underneath this is the normal situation now here you can understand these cells will be will be tending to move like this or these cells will be tending to move like this depending on the position of those small small nano cubes which are present there these all this gelatinous matrix will either move like this or will move like this and that will create a different varying pressure on this membrane and this pressure on this membrane is going to decide in which direction the kynosilia or the stereosilia is going to move and that will ensures the corresponding electrical impulse which will be sent to the brain which will ask the body to compensate according to the plane where we are standing. So this is one small detour to tell you that here also has a role in the equilibration with respect to the ground. Now, coming back to the next slide the next slide will be talking about some of the features which are needed to be added out there about the cochlea. So within the cochlea if you look at it so if this is the cochlear structure just if I straighten it up instead of you know instead of having the spiral structure if I just demonstrate it with respect to like you know if I straighten up the whole semicircular structure so you will see in the beginning of it it could sense all those hair cells could sense signal of around very high sixteen thousand hertz and in the center it could sense signal around six thousand hertz and as you go up deep inside it sense signals of thousand hertz. So essentially what is happening here is something like this if you look at this structure so out here you have the thousand hertz out here you have six thousand hertz and out here in the beginning you have the thousand hertz. So as I was telling you it has a different kind of the different hair cells now if you go back into the hair cells structure was trying to draw the hair cells. So these hair cells just very similar to the cells of the cones in the retina which could sense different wavelengths of light it could be blue cone it could be green cone it could be red cone the hair cells could sense different hertz it could sense sixteen thousand hertz it could sense six thousand hertz it could sense thousand hertz. So they based on that their ion channels are coded so there will be cells if they receive ten thousand hertz they are going to respond that many those ion channels are regulated which are kind of you know coded with which are mechanically regulated ion channels sodium channels will open. So you realize that how much improvisation biology has done in a simple process like hearing where it has this whole tube which has very different kind of so if you look at it now in terms of the functionality if we had to look at the tube. So this tube is something like this so out here all those so you could really divided it up to different parts it is almost like you have seen those harmonica or harmonium or something like that it is almost like the harmonium beads you can pretty much play it like you know different beads are going to come like this and it can go all the way say from sixteen thousand hertz to you know thousand hertz and you can pretty much play it is just like a harmonium board where you really can you know explore all these things and the nerve endings which are kind of taking to this there is something within the brain called cochlear nucleus which is pretty much present in the somewhere over half of the brain cochlear nucleus in the cochlear nucleus the signal is sent from the cochlear by a series of nerves called vestibulo cochlear vestibulo cochlear nerves. So depending on whether it is coming from the high frequency sound or the low frequency sound this vestibulo so there are specific high frequency and low frequency coding nerves which are coming from both the years and being sent to the auditory cortex from here it reaches the auditory cortex out in the brain where the further processing takes place and within the auditory cortex also there are zones which are for the high frequency and the low frequency there are different areas within the brain within the auditory cortex where you have especially it is in the temporal low actually where you have the low frequency sounds are coded different at the different part of the brain within the auditory cortex of course the medium frequency are processed at another part of the brain and the high frequency are coded another part of that and they are very close to each other but they are anatomically distinct zones within the auditory cortex. So now think of a situation which we where we actually so it is coming back to the overall structure. So if somebody is this structure is has gone wrong this person is not receiving any further signal from the sound waves are not reaching the brain. So here you have the here and sound waves are coming but somewhere or other signal is not reaching the brain x y z region technically it is supposed to reach like this which is not happening. So one of the options is that you replace this ear with say a mic and you interface these electrodes to the vestibular cochlear nerves which I have just talked in the previous slide you see this vestibular cochlear nerves. So you implant the electrodes like this and this is connected to a here a mic. So what is the sound information are coming like this are traveling all along and from here they will be reaching the cochlear nucleus from the cochlear nucleus they will be reaching the auditory cortex. Of course assuming that you know the auditory cortex and the vestibular cochlear nerves and the cochlear nucleus is all functioning right. So the whole area of neural engineering on neural prosthesis actually has its origin in the cochlear implants where basically where essentially the cochlear was damaged and they had to replace it with some hearing device which they were interfacing with the 8 electrode probe to the vestibular cochlear nerves and the message was being sent to the cochlear nucleus from the cochlear nucleus depending on the coding whether it is a high frequency sound or a low frequency sound or a medium frequency sound this was coded by different regions of the brain. So this is where it originated and now it has moved all the way into the visual cortex where basically the retinal prosthesis taken place. So this gives an overall idea about the different cell types in the ear in the eyes which are which helps in processing. Next in the next class what we will be doing we will be talking about the cell types present in the alpha action in the nose which is very well developed in insects I will give you some certain examples in the case of insects and we will be talking about the tongue you know what we taste the gustated receptors. So and the electrical signals which are travelling through this. So I will close in here so in the next class we will be talking about these two senses and that will pretty much will cover most of our special senses which helps us in all our survival modes thank you.