CDIS 4037 Inner Ear Physiology (KT Done)


Okay, back back to the inner ear. The anatomy of the inner ear I understand the concern some of you might have with respect to its sort of it’s complicated and difficult to picture nature. That is to say all the images of that we have of it pretty much are two-dimensional yet to really understand what’s going on you have to look at it in three dimensions. You have to try to picture it that way because the inner ear mechanism, the cochlea, the hearing part of it, is shaped like a corkscrew and it’s very hard to imagine all of those tiny mechanisms coiled up together in a very small area. And because of that, a lot of times when we try to picture and describe inner ear physiology, we end up kind of unrolling the cochlea and so, that’s what we see here. So, again, just to get oriented, obviously, this is the eardrum, these are the three ossicles which you should be able to label in either the medial to lateral direction or the lateral to medial direction. Stapes foot plate right here fitting into the so-called oval window. We’d mention the round window before it’s it’s just a of adjacent to the oval window more on that later. And then here we’ve got a cochlea that has been unrolled to some extent. That is to say rather than coiling around like a spiral staircase or like a corkscrew, we’ve just kind of pulled the two ends of it apart to kind of unroll it a bit. So here’s the apical section the helicotrema. We recognized in the last lecture that low-frequency sounds would be processed over here. And then this would be the base of the cochlea, this would be the part of the cochlea, again, where the stapesfootplate actually starts the vibration of those fluids that run throughout the cochlea. What we’re going to see now is how that stapes foot plate motion produces a few different events that occur inside the cochlea. These events are going to be related to the activation of the sensory mechanisms inside the cochlea, specifically the hair cells and we’ll see later how nerve impulses result from that. But for now, we’re going to recognize that energy is going to be delivered through this vibration, that the energy delivered is going to move the fluids that fill the cochlea, and that there will be a variety of things that occur after that activation has taken place. Here’s another view of this and again we’re going to we’re going to come back to these two views in just a little while once we kind of make a little headway with respect to the shaking up of these sensory mechanisms. But for now, here again is the oval window here’s the round window energy is going to be delivered here. And as you can see, if we push through the fluids in a fo oval window and push through the fluids in this particular cavity, which again is the scalavestibule, what that wave of pressure is going to do is to actually change the shape of the fluids within this bony capsule. So, you see the arrow, this little cursor running around the the perimeter of this thing. Again, this is just a slice through one of the coils of the cochlea so all of this area here is actually temporal bone and all of the stuff inside here is hollowed out from this temporal bone. So, what we’ve got is a couple of tubes separated from one another by these thin little membranes here’s reissner’s membrane, it’s called the vestibular membrane here but we’ve been calling it reissner’s membrane up until now and that’s where what we’re going to keep calling it. And then the basilar membrane would be down here and notice that if I push fluid through the scalavestibule it’s going to actually shove down reissner’s membrane and physically change its position from here down to here and ,of course, the harder I push, in other words, the greater the sound pressure and the the greater the force with which the stapes footplate drives into this cavity. The further down this is going to actually get pushed, right? And then you notice also how the tectorial membrane is is also going to get displaced up and down pushed up and down and how the baslar membrane will actually bulge down into the scalatympani. Well, that’s going to push on the scalatympani in the same kind of the same way with the same forces of displacement that the oval window was pushed on by the stapes. So, if I push in here, I’m ultimately going to be pushing down here at toward the scalatympani and that’s going to send a similar wave of fluid motion pressure out through the scalatympani and out at the round window. So, if you want to think about it in terms of pressure applied and pressure relieved from this encased in bone organ you’d recognize that when the oval window gets pushed in the round window gets pushed out. Now, the oval window is going to get pushed in only during those time periods, only during those phases of the waveform, you got to think back to you know the first couple lectures now, where the eardrum is getting pushed in and recall that’s what would happen during the condensation phase of a wave form, the compression phase of a wave form. So, that pushing in at the eardrum is going to produce pushing in at the oval window and that’s going to produce pushing out toward the eardrum back out in the lateral direction at the round window. And then, of course, most waveforms have a rarefaction phase following the condensation phase that would be the part of the pressure wave or the way from where the particles were pulled further apart and that’s going to make the eardrum kind of bulge outward laterally. Well, that’s going to pull the oval wind the stapes footplate out of the oval window. That’s going to bring all of this pushing and pulling we mentioned earlier, that’s it’s going to change the direction of everything so that’s actually going to be an arrow turned around pointing the other direction which means all of this stuff is going to get pulled up toward the scalavestibule rather than getting pushed down toward the scalatympani and of course that means the round window is going to be distended or pushed inward again in the opposite direction of this arrow. So, given the alternating regions of high pressure and low pressure that you should recall from the first basic terminology lecture, that is to say alternating regions of condensation and rarefaction in a waveform and in the way the air particles are set into motion by the vibrating source, those alternating regions of high pressure and low pressure will alternately push the stapes footplate into the oval window and then pull it back out again from the oval window. Of course, it’s attached there pretty firmly so it’s not like something rips out of your head but you get the idea. Alternating movement back and forth is very similar to what you would imagine if you pictured a sine wave in your mind where the movement of the air particles was essentially back and forth as well. So, like I said, we’re going to come back to this in just a few minutes but I want to kind of set the table for you to recognize that all of the things that we’ve been talking about the entire hearing section of this course still are going to apply to the movement of fluid and the response to pressure waves inside the cochlea just like we saw it had happened in the with the eardrum and the ossicles and the air particles in the external canal and the eardrum and indeed the air particles in the environment that were set into motion when our vibrating source displace those particles. Now, we have already mentioned that the basilar membrane vibrates at different amounts at different physical locations along its length depending on the sound the frequency of the sound that is stimulating it. So, toward the end of this lecture we’ll kind of try to bring that information back in and tie all of that together, but for now let’s look not so much at the frequency organization of the cochlea and the and the frequency organization of the basilar membrane but but recognize, instead, that all of the structures we’ve talked about so far are linked together by virtue of the fact that they all sit on this same membrane the basilar membrane and they are all kind of stuck in between the basilar membrane and the tectorial membrane. Let me jump ahead here momentarily. So, again, we’ve got all of the structures of the organ of Corti that would be the outer hair cells over here and the inner hair cells over here, but you see how they’re stuck between the basilar membrane and the tectorial membrane which sits over the top of the organ of Corti like that. Now, because these hair cells are stuck between these two membranes when those membranes get pushed the hair cells get bent. And specifically, what happens is that the hairs on top of the hair cells are going to get sheared. They’re going to get pushed back and forth. Again, as these membranes move back and forth because the fluid around them is getting pushed. In response to the shearing of those hairs on top of the hair cells, we will see that nerve activity is going to result and that’s what’s actually going to produce activation of the eighth cranial nerve, that is to say the vestibulocochlear nerve, which is the cranial nerve responsible for delivering the nerve activity related to inner ear activation to the central nervous system and essentially, eventually up to the brain. So what we want to recognize is that the movement of these membranes caused by the movement of the fluids, caused by the movement of the stapes footplate inside the oval window is going to be transformed, all of that mechanical energy is going to be transformed into nerve energy or neural activity and it will be the conveyance of that nerve activity up to the central nervous system and eventually the brain that results in the sensation of hearing. So let’s maybe back up for a moment here since we have a fluid-filled tube and a piston that’s going to push the fluid through the tube, we have what would be more typically known as a hydraulic system. So your inner ear, your ear cochlea is a hydraulic system, it’s a mechanical system, and the job of the sensory cells in the inner ear will be to transform hydraulic energy into nerve energy. That’s going to be the job of the sensory mechanism in the inner ear and I think we would all agree that that is a profound change to the physical state of the elements, the mechanisms, the hair cells and the nerve fibers that are involved in hearing. So, again, going back to that first lecture where we required for a sound to exist a vibrating source a medium through which the energy would be propagated and then a receiver whose physical state would be altered, these hair cells and these nerve fibers are therefore our receivers whose physical state is altered by the presence of that energy. And specifically, in this case, it’s mechanical energy and in the cochlea hydraulic energy. Okay. So, that’s all pretty cool, that’s all good. But now, one of the curveballs that the inner ear is going to throw us has to do with the fact that the hair cells, the two hair cell populations as I indicated last time, the two hair cell populations do not all do the same thing. They don’t their jobs that the two pair cell populations have are not the same. The so-called inner hair cells are responsible for the vast majority of the conversion from the hydraulic energy and the mechanical energy into electric energy and so we can kind of look at that here. Here’s an inner hair cell and you see that there are a lot of nerve fibers attached to each inner hair cell and we’ll will look at that more closely when we look at the auditory nerve itself, but just again, recognize that the vast majority of the conversion from mechanical displacement hydraulic energy into nerve energy is carried out by the activity of the inner hair cells. But, if you had only inner hair cells without outer hair cells, even though you would still have hearing your hearing would not be nearly as sensitive, you would not be able to respond to nearly the the low levels of energy that we can normally respond to if you didn’t have the outer hair cells to in a sense change the mechanical response of the inner ear change, the mechanical response of that traveling wave and by doing so actually make it easier for the inner hair cells to be stimulated. When I say easier what I mean is have those inner hair cells be stimulated at lower signal input levels. So what the inner hair cells do is the transformation of energy from mechanical to electrical, if you will, from hydraulic to nerve. What the outer hair cells do is increase the sensitivity overall of this system and we’ll we’ll see an important element of that is that the outer hair cells actually have to be able to be able to move themselves on their own and and I guess that’s kind of a misstatement. What I mean is the outer hair cells in order to increase the sensitivity of the inner hair cells they’re actually going to change the distance between the tectorial membrane and the roof of the organ of Corti or the tectorial membrane and the hairs, the stereocilia, on those inner hair cells. You see here that the the hairs or the stereocilia of the outer hair cells are actually embedded in the tectorial membrane. Think with me now: if the outer hair cells could actually change their shape, for example, get shorter, then that would bring the tectorial membrane closer to the part of the inner hair cells, the stereocilia, that will be so essential to their transforming of hydraulic energy into nerve energy. So, as we’ll see in a few moments, when these hairs get bent that is what initiates that transduction or transformation process. So if the outer hair cells could actually pull down on the tectorial membrane imagine that that would make the inner hair cells more easily stimulated and indeed that’s exactly what happens and here’s an image of an outer hair cell with an electrode stuck in it. And this outer hair cell is vibrating back and forth or moving back and forth or in fact getting shorter or longer, shorter, longer, shorter, longer, and if this outer hair cell is firmly attached to the tectorial membrane then you can see how it would actually change the orientation of the membranes with respect to the to the hair cells themselves, and in as a consequence that movement of the outer hair cells would actually serve to increase the stimulibility or increase the sensitivity of the inner hair cells and the result is we can hear sounds at low levels lower levels than we would otherwise be able to hear them if we had lost the outer hair cells or if we didn’t have them to begin with. Perhaps we’lll talk more about hearing loss later on in the semester but imagine an ear without outer hair cells, in other words, we wipe out all of these guys. The resulting hearing loss would really only be a mild to moderate hearing loss, but if we wiped out all of these guys the resulting hearing loss would be profound. So the idea then is that whereas inner hair cells are coupled as mechanoelectric transducers, that is to say they they convert mechanical energy into electric energy, the outer hair cells are bi-directional, that is to say they both are mcmechen electric transducers just like the inner hair cells, and kind of like what we would expect from a microphone where a mechanical wave is transformed into an electrical current and sent through a wire. The outer hair cells are also electromechanical transducers which is different from the inner hair cells and but more similar to what a loudspeaker does. So, these electromechanical transducers can be stimulated electrically as we see here with an electrode and in response to that stimulation vibrate mechanically. So the outer hair cells are very unusual when it comes to the components of a sensory mechanism because not only do they transform the mechanical activation produced by an environmental sound into nerve signals and really that’s what we typically think of as what a sensory system does. The outer hair cells also have the ability to receive signals from the brain or from somewhere in the central nervous system and convert that nerve energy into mechanical motion such as we are seeing right here. So what that means is that you’ve got a sensory system that both that transduces energy in a sense in both directions from mechanical to nerve but also from nerve to mechanical and in doing so that outer hair cell activity seems to sharpen the tuning of the cochlea, makes it more effective as a as a tuned mechanism and we’ll see how that’s important for frequency resolution later on, and it also makes the outer ear or… I’m sorry, the inner ear the cochlea a much more sensitive organ than it would otherwise be. This, by the way, is the link to the film really that you’re seeing here. This is not an animation by the way this is a real hair cell that was pulled out of a living organism, stuck into a dish, and poked with an electrode to demonstrate this motion. Now, again, just to skip up ahead just for a moment. So we’ve got the stereocilia and we’re going to consider these stereocilia to be almost like gates, gating some kind of gating mechanism whereby under normal conditions when there’s no sound the gates are closed and when a sound is turned on, we’re going to see how these hair these stereocilia the hairs on top of the hair cells are going to move and it’s going to be the movement of these stereocilia that produces the transduction event and the eventually the initiation of nerve signals. To be able to grasp what’s going on there however, it’s important to remember that the inner ear is filled with actually two different kinds of fluid there’s the endolymph which is housed in the scalamedia and then there is paralymph. So the endolymph here would be blue, the paralymph would be orange. And the paralymph fills the scala vestibule and the scala tympani. So we’ve got this very unusual fluid high in potassium ions filling the scala media, whose chemical composition is maintained by activity here in the street vascular, and it is separated by these membranes, reissner’s membrane and the basilar membrane, from the paralymph vestibuli and the scala tympani. These are these fluids are very different both with respect to their ionic content, enioplyph high in potassium, perilymph high in sodium. And also, very different from one another with respect to their voltage. We’re not going to go too deep into that the issue of the electrical charge of these cavities, but just recognize that this positive charge, this positive voltage found in the endolymph is the highest positively charged fluid in the body. So endolymph is a very unique fluid for the human body and it is found exclusively in the inner ear and that should remind us of the sort of the uniqueness of this as an organ. It’s kind of put together differently from any other organ in the body. We’ll probably come back to this slide over and over and over again so really make sure that you understand this certainly there will be some identification of those structures on your next quiz. Okay. So getting back now to the matter at hand with respect to transduction. Again we’ve got inner hair cells, we’ve got outer hair cells. Remember there would be rows and rows of these as we look through the coils of the cochlea. Now with respect to Inter hair cells and outer hair cells, the stria vascularis should be out here and the modilous of the inner ear would be in here, right? So, remember, the organ revolves around, rotates around the modiulour region and the stria is on the outside. So, again, modiolus would be here and the stria is out here. That’s how I knew that you’d have outer hair cells here and inner hair cells here, right? And it’s going to be the deflection or the movement of those stereocilia that generate the flow of endolymph from this region here, through the sensory cells, and eventually stimulating the nerve fibers. Now it’s important for you to keep in mind that the fluid inside each of these sensory cells is when the cells are at rest negatively charged. So there will be some push, if you will, kind of like you know opposites attract kind of like negatives attract positive ions or the negative side of a magnet attracts the positive side of a different magnet, there will be some push to get these potassium ions into these sensory cells. These gates, typically remain closed, but when a sound is presented and the the stereocilia start to get pushed against the tectorial membrane the gates open and the ions flow into these sensory cells. The flow of the ions into the sensory cells actually changes the voltage inside the sensory cells and in response to that change in voltage the sensory cells communicate with these nerve fibres and stimulate the nerve fibers thereby generating nerve impulses. So what we need to see, what we’ll need to recognize is how the presentation of a sound modifies the shape of the sensory cells, modifies the fluid flowing through the cochlea to the extent that the pressure waves through the fluid force these stereocilia against the tectorial membrane, and in doing so depolarize or change the potential change the voltage of these sensory cells. So out here you see the plus sign by the chemical symbol for potassium and it will be these movement of these ions into the sensory cells that change the physical state of the sensory cell thereby producing the transduction of that mechanical energy into nerve energy. You’ll see I just kind of skipped through these. The reason I did that is because I’m going to actually come back I don’t want to go on too long here. I’m going to come back and actually show you these animations on the next lecture as a way to indicate specifically how it is the gating of those stereocilia works. Okay. So now get back to a slide we looked at from the inner ear anatomy lecture and this one again just shows us that the size of the traveling wave is going to be different at different physical locations in the cochlea depending on what sound was presented. So if I have a wave that peaks in one specific area much more than it is displaced in other areas then these stereocilia will be shoved with more force at that one location against the tectorial membrane, which means that the nerve fibers hooked up to the hair cells at that one physical location will receive stronger signals than the nerve cells that were hooked up over here or the nerve cells that were hooked up over here. In other words, the nerve activity that results from the traveling wave will be greatest in that region where the traveling wave was the biggest and for high-frequency sounds that’s going to be close to the base or the stapes footplate and round window and all that. As frequency is generally as gradually or lowered the peak of the traveling wave occurs in a different location, okay. So, now with a lower frequency of stimulation our traveling wave was biggest right here and of course then that means that the hair cells in that region of the cochlea get shoved up against the tectorial membrane with more force than they did over here or even over here. And as we go to lower and lower frequencies, the peak of the travelling wave moves closer and closes closer to the helicotrema, to the apex of the cochlea, and again, that’s where the stereocilia will be shoved with the most force against the tectorial membrane here. Just as another image of that that you’ve already seen. Again, different frequencies being processed in different physical locations. And now maybe this makes more sense because, again, with this mapping of the frequency on the basilar membrane you can see how a sound with a frequency of close to 6,000 Hertz will produce more displacement in this physical location than it did in this physical location and as a consequence, again, the stereocilia will be pushed accordingly. Now, it’s kind of interesting to think also about what would happen if the system was if a frequency was present in the environment that human beings were not particularly sensitive to. So, keep in mind, the human audible range is not infinitely wide. We can detect sound energy from about 20 Hertz up to about 20,000 Hertz when we’re young and have normal hearing. As we get older, we we tend to lose that high-frequency sensitivity first. And for all of you who throughout your middle school and high school or maybe even grade school or college careers have set your phones to provide a high frequency beep sound when a text is received a sound that would be high enough in pitch that you could hear but your teacher could not, keep in mind that you’re not getting any younger and soon you will be surrounded by young people who are doing the same thing to you, and I don’t know what else to tell you except that you all have it coming if that’s something that you actually did. LOL, JK, whatever, but just recognize that as you get older it is inevitable that you will lose some of that high frequency sensitivity. Alright. Well anyway, not meaning to ridicule the audience here. What I was going to say is that imagine now that there was an environmental sound out there that was of oh I don’t know a frequency of about five Hertz, that means the stapes footplate is vibrating back and forth in the oval window five times per second. Now remember, the frequency organization of the cochlea is maintained by the mass and stiffness of the fluids and the membranes and all the structures that make the organ up. And we know that the lower the frequency of sound that stimulates the ear, the lower the frequency of sound in the environment the further… excuse me, the further toward the helicotrema that sound is going to actually move these structures around. So, imagine then at about oh I don’t know 50 Hertz this fluid wave would go through the scala timpani and would bulge out, if you will. This region right here right where I’ve got the cursor going like up and back and when I when I say would bulge it out it would actually create a traveling wave of displacement that would peak, it would be biggest right about there. If I went to 100 Hertz it would be right about there, if I went to 1,000 Hertz it would be right about there, if I went to 5,000 Hertz it would be right about there, right? But right now we’re looking at 50 Hertz and it’s going to bulge the system out there. In a way then, that fluid wave kind of crosses over this middle part right at about this location for a very low frequency sound. At a slightly higher frequency, the energy crosses over that middle section there. At a slightly higher frequency crosses over right there. And what is in this middle section? The organ of Corti. So when the energy traverses, if you will, goes across the organ of Corti, that’s when the stereocilia get disturbed, that’s when the transduction occurs. But look, if I’m only at five Hertz it’s going to the energy is going to cross over here at the helicotrema. And guess what, it’s not going to activate at all any of this middle cavity. The energy is going to go in one side, around the helicotrema and back out the other side. So, yeah. We’ll have oval window movement back and forth like this and round window movement back and forth like this but you’re just pushing on this part of the tube sending the energy all around the helicotrema and then back out here and because none of the stereocilia are activated no sound sensation occurs for the listener. The frequency of stimulation has to be in that audible range for these mechanisms here to be stimulated in order for the sensory event to be detected by the brain. So that’s what happens if you try to stimulate with too low a frequency sound. If you try to stimulate with too high a frequency sound in a way these fluids are just too massive for that high frequency sound to displace them. Remember, the relation between displacement and frequency is an inverse relation such that at higher frequencies energy displaces the whatever vibrating source there is by a small amount, smaller and smaller as you go higher and higher in frequency. At some point, the mass of these fluids just is too great for that high frequency energy to displace. And so, not energy above about 20,000 Hertz is unable to displace these fluids in here with enough amplitude to actually share the sensory cells. And again, as we get older, progressively lower and lower frequencies of instead of 20,000 Hertz maybe 16,000 or 14,000 Hertz will suffer the same fate, that is to say they will not be able to displace this fluid by a large enough amplitude to produce a detectable sound or a sound sensation. And again, this image is just kind of showing us a cross-section of that same event. Just recognize here that the size of the traveling wave which will be greatest at one specific place is also therefore going to move the organ of Corti by the greatest amount at that one place. I did want to mention very briefly here the consequence of damage to your sensory mechanism and so here we’ve got some normal-looking hair cells here, obviously we have some damaged hair cells. Notice, how much more damage occurs to the outer hair cells as the result of a loud sound versus the inner hair cells. And this is because the outer hair cells have that physiologic vulnerability that high metabolic activity as a result of or that kind of assists that bi-directional coupling and as a consequence they’re much more easily damaged. The inner hair cells by comparison are quite a bit more robust in the sense that they’re harder to damage, which is good I mean this is why even when you’re around loud sounds you maintain some hearing just not all of it and you lose some because of this damage here. A lot of people I think these days it’s very popular to blame loud music for damage to outer hair cells and the resulting hearing loss and change in the tuning characteristics of the cochlea. Let’s not forget that music for a lot of people is also very relaxing and let’s also not forget that a lot of kids, a lot of young people these days don’t just listen to loud music but they fire guns, they ride ATVs, they ride dirt bikes without hearing protection often and these these noise sources are equally, if not more damaging than music. So let’s not always blame music for all the hearing problems in the world. There are a lot of other a lot of other things that contribute to that also. What we’re going to do in the next section after I the next lecture that I’m going to post is actually going to be one of looking more closely at what physically is going on in the cochlea, but let’s just take a brief look ahead and see that the nerve fibers that carry the information from the cochlea out to the central nervous system are going to be traveling through this auditory nerve or this vestibulocochlear nerve. You see the facial nerve is in there with it. You see, again, how all of this is hollowed out from the temporal bone of the skull all of this area here is bone. But, what we’re going to see eventually is how that nerve functions, how it propagates the energy from the inner ear area into what we will see as the brainstem and then through the brainstem pathway up to the cortex. Alright. So the next lecture is going to be just focused on those three animations that I’ve got embedded in here and I’ll look at those more closely.

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