CDIS 4027 Inner Ear Pt 2


So the cochlea of the inner ear kind of spirals around the central modiolus. But if you were to look inside the tube, it actually has three compartments: namely the scala vestibuli, the scala tympani at the bottom, and the scala media. So it’s much like the inner tubing of say a car tire. The scala media runs along the length of the cochlea, and the scala media compartment is what we call as a membranous labyrinth, kind of floating within the bony labyrinth around it. So, the scala vestibuli and the scala tympani are compartments of the bony labyrinth while the scala media is a part of the membranous labyrinth. So there’s 2 important membranes that separate the scala media from the scala vestibuli at the top and the scala tympani at the bottom; namely the Reissner’s membrane that separates the scala vestibuli from the scala media and then the basilar membrane that separates the scala tympani from the scala media. This basilar membrane is actually an important anatomical structure, especially because the Organ of Corti lies on the top of the basilar membrane. And it’s in this Organ of Corti where those sensory, end organs, or structures that we call the inner hair cells and outer hair cells, lie. So a little bit more about this basilar membrane. It’s about 35 mm, or 3.5 cm, long but since it spirals around the modiolus, it’s much smaller, or it looks much smaller. It’s a very thin membrane, and, in terms of growth(?), it is actually narrower at the base and it gets wider as you go towards the apex, so in this figure, illustration, you can see that this is the base and this is the apex, or the top of, the cochlea. I know that this is the base because this is where the oval window and round window is, so the stapes actually sits on the oval window right there, and that’s where the stimulus comes into the inner ear, so when the ossicular chain is moved by the sound hitting the tympanic membrane, the stapes footplate moves in and out of the oval window, moving the fluids inside the inner ear compartments. And the pressure that is created by this actually goes along the cochlea until the very end, which we call as the helicotrema, that opening that connects the scala vestibuli and the scala tympani, and that pressure is dissipated by the round window here. So while it’s taking, traveling, I should say, along the length of the inner ear, it stimulates different parts of the cochlear partition on the basilar membrane. As I said, the basilar membrane is arranged such that it’s narrower at the base and it gets wider as you go to the apex. This is in contrast to the actual size of the cochlear duct. If we were to look at the bony labyrinth, you would actually see that the cochlear duct appears wider at at the base and it gets narrower as it go to the apex. That’s an interesting contrast, but it has a physiological significance that we will be talking about in just a bit. It’s this varying length of the basilar membrane that helps in separating the frequencies of the incoming sound. When we talked about, in acoustics, how length and stiffness affects, if you recollect, we talked about how as length increases, the frequency, the resonant frequency, actually decreases. In terms of stiffness, as the stiffness increases, actually the resonant frequency increases. So the best frequency that this body can vibrate in increases. A little bit more about the anatomy of the basilar membrane. This basilar membrane is actually head by bony projections in this cochlear partition. You’ve got the spiral lamina towards the modiolus side holding, or kind of pinching, this basilar membrane, while we’ve got the spiral ligament at its outer periphery holding the basilar membrane. It’s the spiral ligament and the spiral lamina that kind of changes in length to make the basilar membrane narrower at the base and wider as it goes to the apex. As I said, situated at the top of the basilar membrane, or superior to this basilar membrane, is this important Organ of Corti. Here is another illustration about how the stapes is attached to the oval window, which opens into the scala vestibuli at the top. The scala vestibuli at the top and the round window opens into the scala tympani at the bottom, while the scala media lies in between the scala vestibuli and tympani and is separated by this Reissner’s membrane and the basilar membrane. Again, in this illustration also, you can see how the basilar membrane, that middle, striated membrane over here, varies in length or (breath?); it’s more narrower towards the base and it gets wider as it goes towards the apex. So, here, again, at the base it’s narrower and stiff, so we know that that makes it responsive to the higher frequencies. So here, as the length, length is inversely proportional to frequency, so narrower membranes or narrower objects best respond to higher frequenices. While stiffness is directly proportional to frequency, so as an object gets stiffer, it best responds to higher frequencies. So at the base, since it’s narrower and stiffer, the higher frequencies are best represented at the base, over here. At the apex, where it is wide and more slack or less stiff, it best responds to those lower frequencies. And those frequencies that we are talking about is that whole range of frequencies that the human auditory system can listen to, in other words 20 Hz – 20,000 Hz. So, technically, the highest frequency we can respond to, the 20,000 Hz, is represented at the very tip of the basilar membrane, the basal tip of the basilar membrane, while the lowest frequency that we can hear, 20 Hz, is on the apical tip of the basilar membrane, and all the other frequencies fall in between , progressively as we go from apex to the base, we move from low frequencies to high frequencies. Again, in some of the discussion I saw on the decibel, we talked about the wide range of hearing and some had noted that 20 Hz – 20,000 Hz, just to remind you that 20 – 20,000 Hz refers to the frequency range that we can hear, but even within that frequency range each of those frequencies, we can hear a wide range range of intensities, or amplitude, and that’s where 10 to the power of 14 times come into play, and the need for to compress that using the logarithmic, decibel scale. Just a point to remember. So the frequencies actually, the 20 Hz to 20,000 Hz, and we also talked about how we actually hear even better at those mid frequencies. So, in other words, we are more sensitive to those mid frequencies than we are for very high and very low frequencies. Here’s an interesting animation that talks about this frequency organization along the basilar membrane. A concept that we are going to be talking about in a little bit later is known as tonotopicity. So tonotopicity refers to that frequency organization within any given structure within the auditory system, and it’s kind of one the more important themes of the auditory system. It seems like nearly all the structures within the auditory system, there’s this tendency for the frequencies to be distributed spatially or separated along any given structure. And that’s what we call as tonotopicity. So the inner ear is such a fascinating anatomical structure. We talked about how it’s as small as the nail of your pinky finger, but what’s more important is the stucture even much smaller in dimension, so here the cochlea might be in dimensions of [millimeters], but then the structures that lie within the turns of the cochlea are of the size micrometer. But the actual magic happens in structures even smaller than that, namely those hair cells and at the top of the hair cells are structures that we call as stereocilia, which are in the dimension of nanometers. Nanometers are 10 to the power of -9 meters, while micrometers are 10 to the power -6 meters, so micro is one-millionth and nano is almost one-billionth of a meter, so we are talking about real small structures, and that explains why we had to wait for technology to develop so we could look into these structures and understand these structures. So here is a scanning image of the cochlea. This is probably a mammal cochlea because it has a lot more turns than what we expect to see in humans. I think that this is a guinea pig’s cochlea. Nevertheless, you see that the structures are quite similar to what you see in a human cochlea. So we are seeing those three compartments, scala vestibuli, tympani, and scala media. We are also seeing that central core, known as a modiolus; the modiolus is a perforated structure because the auditory nerve fibers kind of enter through the modiolus, and they serve the different turns of the cochlea kind of extend into those hair cells and connect to those hair cells. Again, we are seeing the Reissner’s membrane separating the scala vestibuli from the scala media. Then at the bottom, we’ve got the basilar membrane separating the scala tympani from the scala media. Another illustration of this cochlear partition. And the Organ of Corti sitting on the top of the basilar membrane. *I’m sorry, pardon my puppy, barking his head off over there.* So in this illustration, again, you are seeing a breaking down of the inner ear cochlea, so if you were to focus on each turn of the cochlea, you’re seeing this three compartment with this Organ of Corti lying on top of the basilar membrane, and if you were to look more closely into this Organ of Corti, that’s where we would be seeing those hair cells, namely the inner hair cells over here that lie close to the modiolus, or the central core, over here, so we’ve got the inner hair cells right there, and then we’ve got 3 or 4 rows of those outer hair cells, lying towards the outer edge of this cochlear partition right over there. So this is an image from what we call as an electron-microscope, where we are actually seeing how the outer hair cells actually look like, so you’re seeing those hair cells over here; these are the inner hair cells, those rows of inner hair cells, in humans, there is only one row of inner hair cells, along the length of the cochlea While we have 3 or more rows of the outer hair cells that are towards the outer edge of the cochlear duct. In this illustration, you are also seeing this membrane that seems to have curled up, but in reality, this membrane kind of lies on the top of the hair cells, and this membrane is what we call as a tectorial membrane. And it also plays an important role in the physiology of the inner ear. The tectorial membrane, on the undersurface of the tectorial membrane, the outer hair stereocilia are connected while the inner hair cells lie just below that tectorial membrane. The inner hair cells and the outer hair cells are separated by this tunnel known as the tunnel of Corti, and actually the hearing nerves, the auditory nerves, travels within this tunnel of Corti and then they serve those different hair cells as they go to the apex. Another illustration of the inner ear and the hair cells and the Organ of Corti. The reason why I provide so many illustrations is to kind of help you guys make an idea of how the inner ear looks like and it always helps to look at different views and kind of breaking it down to understand the organization of the inner ear. So here, you’re actually seeing a blown-up of the inner hair cell. Over there, and at the top of the inner hair cell lie these even smaller hair cells; these are the ones I was talking to you about in the dimensions of nanometers, or one-billionth of a meter So these are rows of ever smaller hair cells sitting on top of the inner and the outer hair cells; they are known as the stereocilia. They are the ones that actually pick up the fluid movement, and those hair cells convert that into a language that the nerves understand, namely converts it into electrical energy that conducted by the nerves, and which goes higher and ultimately towards the brain. The stereocilia, ‘stereo’ referring to sound and then ‘cilia’ referring to hair structures. So I know we talked about how the fluid of the bony labyrinth is the perilymph and the fluid within the membranous labyrinth is endolymph. So the scala tympani and the scala vestibuli have this perilymph – ‘peri’ meaning outside, while the scala media, which lies between those 2 compartments, has this endolymph. The endolymph and the perilymph are separated by these membranes, the Reissner and the basilar membrane. And they have different chemical compositions, and because of the different chemical compositions, they also have different voltages to them; they have a different direct current. The endolymph is high in potassium and it has less sodium in it, giving it a small, positive voltage, namely that plus 80mV. And relative to that, the scala vestibuli and the scala tympani have very low voltages. The perilymph has low potassium content and hence it has a low positive voltage. So the endolymph is similar to the fluid that bathes the central nervous system, namely the cerebrospinal fluid, while the perilymph is similar to the fluid that lies in the rest of the body. This is called extracellular fluids. So as I mentioned earlier, the endolymph has a strong positive potential (+80 millivolts) That’s what we call as a endolymphatic potential, while the scala tympani and the scala vestibuli, which has the perilymph, has a much lesser voltage to it. So while the scala vestibuli, has a +3 millivolts, a small positive voltage, the perilymph of the scala tympani has almost 0 millivolts And all the structures surrounding these compartments have a negative potential. As I mentioned earlier, the Organ of Corti has those inner hair cells, outer hair cells, and supporting cells that hold these hair cells in place. A few of the important supporting cells are those inner and outer rods that actually form that tunnel of Corti that separates the inner hair cells and outer hair cells. Here, in this cross-section of the Organ of Corti, you can actually see the inner hair cells over here and then the outer hair cells. You can also see the tectorial membrane that lies or hovers on the top of this Organ of Corti, and it’s only recently that they found out that the stereocilia that sits on the top of the outer hair cells is actually embedded in this undersurface of the tectorial membrane, while the stereocilia of the inner hair cells are not, and that apparently has a protective function; so when the basilar membrane moves because of the fluid movement, which is, in turn, because of sound impeding on the or eventually reaching the stapes and moving the fluids through the oval window, it’s the stereocilia of the outer hair cells being deflected directly, but the inner hair cells are being deflected by the fluid that the outer hair cell stereocilia pushes. Why a protective function? Because now since the inner hair cells are not directly connected to the tectorial membrane, if there is a really loud sound, it doesn’t actually result in directly moving the stereocilia of the inner hair cells, but it’s by indirect means of the fluid movement. So that’s the reason why when we see individuals with what we call noise induced hearing loss, when they are exposed to a loud sounds, you see that the outer hair cells are the ones that are affected first, and when we talk about the disorders of the ear, I actually will show you some images where you can see after acoustical trauma, or noise induced hearing loss, you will see the stereocilia of the outer hair cells are often missing or distorted, while the inner hair cells are only destroyed if the noise exposure was for a prolonged period. So there’s all this in-built, protective functions within the architecture of the inner ear, which is fascinating. So here’s another illustration of the inner hair cells (IHC) and the outer hair cells (OHC). Here, again, you can see that there is only one row of the inner hair cells, while there is 3 or more rows of the outer hair cells. Here, you’re seeing this basilar membrane (BM) over here, and we can see this tunnel of Corti that separates the outer hair cells from the inner hair cells. So situated on the top of the basilar membrane, as I said, are those 3 to 5 rows of outer hair cells, so in total, on average, we have about 12,000 to 15,000 outer hair cells. While we only have about 1 row of inner hair cells, about 3,000 inner hair cells. So we’ve got this tunnel of Corti separating the inner and outer hair cells, with the inner hair cells being towards the medial end of the tunnel of Corti, in other words, it’s towards that central core, or modiolus. Just imagine that another way of thinking about this cochlea is like a spiraling parking garage, so towards the center of the spiral is those inner hair cells, and towards the periphery, or outer edge of the spiral, is your outer hair cells. This is the reason why it’s called inner hair cells and outer hair cells. So these hair cells are really small structures; they are only about 0.01 millimeter long and about 1000 of a meter in diameter. But even smaller structures sit on the top of those hair cells, what we call as the stereocilia. These inner hair cells and outer hair cells differ in terms of their shape, the manner of innervation (how the auditory nerve innervates these hair cells), and the organization of the stereocilia that sit on the top of those hair cells. So in this scanning electron microscope, again, you can see the Organ of Corti, the inner hair cells, the outer hair cells over here. So you’ve got the inner hair cells over here, towards the medial end, then you’ve got the outer hair cells over here, towards the outer or periphery of the outer edge of the cochlear duct. And you can notice some difference in shape of the outer and inner hair cells. The outer hair cells are more like test tube-shaped, while the inner hair cells are more like vase-shaped. So the inner hair cells are round shaped, while the outer hair cells are longer and thinner and more like test tube-shaped. The inner hair cells stand alone, while the outer hair cells, if we were to look at the figure again, you would see that it’s embedded within one of the supporting cells that we call as the cells of Deiters. Also, there’s a difference between the organization in the stereocilia that sits on the top of the hair cells, for the inner and outer hair cells. The inner hair cell stereocilia are in a single row, while those of the outer hair cells are actually ‘V’ or ‘W’ shaped in some cases. And apparently, that has physiological significance also. When we talk about the physiology of the inner ear, we will talk about how the outer hair cells are responsible for stimulating the inner hair cells, because the outer hair cell stereocilia are those that are actually in contact with the tectorial membrane that sits on the top. and the inner hair cells are stimulated by the fluid movement that is caused by this outer hair cells. Now, with outer hair cell stereocilia shaped like a ‘V’ or ‘W’, it’s almost like cupped-shaped, so it’s more efficient in pushing the fluid towards the inner hair cells, much like what we do when we are trying to scoop water from the surface of a table; we kind of cup our hands, or palms, to kind of effectively push the fluid, and apparently that is what is happening when those hair cells are being stimulated – the outer hair cells move and effectively push the fluid towards the inner hair cells. So here’s an actual scanning microscope view of the top of the hair cells, so here, you’re seeing the inner hair cells, kind of when most of the stereocilia are in a straight line, while those with the outer hair cells are actually more ‘V’ or ‘W’ shaped in some cases. Again, the inner hair cells are more medial while the outer hair cells are more lateral on the top of the basilar membrane. Another illustration of the same point. So here, the stereocilia of the inner hair cells are in a straight row, while those with the outer hair cells are ‘V’ shaped over here. Again, here, you’re also seeing that the overall shape of the outer hair cells is more like test tube shaped; they’re longer and thinner, and actually the inner hair cells are more like vase-shaped. Actually at the top of the hair cells, below the stereocilia, there’s this lamina or membrane, known as the reticular lamina, It’s a delicate membrane, and it’s on the top of the hair cells, but below the stereocilia, so the stereocilia are actually embedded on the top of the reticular lamina, like that. Then we talked about the tectorial membrane. The tectorial membrane actually covers, or kind of hovers above, the stereocilia, and it’s only recently that they found that the inner hair cell stereocilia is not in contact with the tectorial membrane, while the stereocilia of the outer hair cells are actually embedded in the undersurface of the tectorial membrane, right there. So the outer hair cell stereocilia are actually embedded, while the inner hair cell stereocilia are just below the tectorial membrane; there is a gap over there. So here, you’re seeing the tectorial membrane coming from the medial edge and kind of hovering at the top of this Organ of Corti. And it’s much more labeled in this schematic illustration over here. Let’s talk a little bit about the innervation of the cochlea. The cochlea is innervated by the 8th cranial nerve, or the auditory-vestibular nerve, and the auditory-vestibular nerve is, for the majority, an afferent nerve, or it’s a sensory nerve, in other words, it’s a nerve that carries information towards the brain, and that is it’s main purpose because the inner ear, for most purposes, is a sensory organ, so taking information towards the brain. What was surprising is when Thomas discovered that there is a small set of efferent nerves, or motor nerves, that actually bring in information to the inner ear. So while we have a majority of afferent nerves, or sensory nerves (about 30,000), there’s a small subset of efferent nerves (about 1800). For a long time, anatomists didn’t understand what was the role of the efferent or motor nerves in the special sensory organ like the ear, it’s only recently they understood that it plays an important role in the physiology of the inner ear that we are going to be talking about later. There is a difference between this innervation of the outer hair cells and inner hair cells. The inner hair cells receive a majority of the afferent innervation, about 95% of the afferent innervation comes from the inner hair cells, so obviously we can see that the inner hair cells play an important role in cochlear transduction because they are the ones that ultimately take the information to the brain, but the outer hair cells seem to receive a majority of the efferent innervation, or the motor innervation, and again, it was only recently that we understood what role those outer hair cells play, so it’s always baffled anatomists why we have a lot more outer hair cells than we have inner hair cells, while the inner hair cells are the ones receiving the majority of the afferent innervation or sensory innervation. So, the role of the outer hair cells has always been debatable, but now we’ve got a better understanding of what it does, and that’s something that we’re going to be talking about in the physiology, in the next lecture. Also, not only do the inner hair cells receive the majority of the afferent innervation, it also receives a condensed or intense innervation. A number of the afferent nerves innervate each of the inner hair cells, so it receives what we call as many-to-one innervation; almost about 20 afferent nerves innervates each of the inner hair cells, so it receives dense innervation, while only one afferent nerve innervates about 10 or more outer hair cells, so relative to afferent innervation, the outer hair cells receive much lesser innervation. So the inner hair cells is innervated by about 20 afferent nerves, so if you were to describe that, we would call that as a cell to neuron ration, so each cell receives about 20 afferent nerves, while a single afferent nerves innervates about 10 outer hair cells, so if we were to calculate the cell to neuron ration for outer hair cells, we have 10 to 1, in other words, 10 afferent nerves innervates each outer hair cell. But again, the outer hair cells receive the majority of the motor or efferent innervation. Again, that plays an important role in the physiology or the fine tuning of the traveling wave.

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