Lecture 22 - Special Senses - Hearing & Balance PDF

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 22 - Special Senses: Hearing & Balance Video 1 Introduction Welcome to the next online lecture. In this lecture, we're going to finish this special senses by looking at Hearing an...

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 22 - Special Senses: Hearing & Balance Video 1 Introduction Welcome to the next online lecture. In this lecture, we're going to finish this special senses by looking at Hearing and Balance. So we're going to start by looking at the anatomy of the external, middle and inner ear. And then we're going to look at how we can turn a sound wave into an electrical signal for the brain. Then we'll look at structures in the inner ear related to balance. And that gives us information about our heads position relative to the ground and whether or not we're accelerating or decelerating. We'll also look at things like, why do people sometimes get motion sickness when they're reading a book in the car. So let's get started. Slide 1 The material for this online lecture module can be found in chapter 15 of your textbook. Specifically, we're going to be covering Section 15.4 called Hearing and Balance. I just wanted to highlight one of the pictures here on this slide, this one on the right. This is actually a picture of one of the structures that's in the inner ear that's involved in hearing. It's called the cochlea. Now what makes this picture so unique and cool is that the structures of the cochlea are actually deep within the temporal bone. And it's really hard to visualize this particular structure. So what they've done in this images, they've sort of broken away the temporal bone to reveal the thin membranous structures that are found in the space in that temporal bone. So this is a really cool picture of this, and we're going to be talking about what this specific structure does in just a few moments. Slide 2 So before we begin to look at the anatomy of structures for our special senses of hearing as well as balance. I first want to explain some of the terminology associated with hearing. Specifically what is sound? So basically sound is a vibration in air, and what it does is it causes bands of compressed air that are followed by bands of less compressed air. And this results in what's known as a sound wave. So when we look at these graphs here on the right, a sound wave is depicted in these images. Now when we look at the peak of the wave, that represents the compressed air band and the valley of the wave represents the less compressed air. So of course, in these images we have nice perfect sound waves because we have a set pitch and a set volume. However, of course, in the real world, everything's going to be much more muddled than this, and we're going to have a combination of various different compressed and less compressed air bands. So of course, sound has different components to it. The first is going to be our volume. Now volume is dependent upon the wave amplitude. So when we're whispering, basically, the amplitude is going to be lower. So it doesn't go up quite so high. So our compressed band is going to be smaller, basically, and closer to our last compressed band. So this is going to give us sort of the whispering sound. Of course, if we want to have a higher volume, higher volume is going to then have a higher compressed band of air, followed by a larger difference between the compressed and last compressed air. So that's what's creating volume. The other component of sound is pitch, and this is dependent upon wave frequency. So how close the waves are to each other. So if we have low pitch, that means our waves are going to have a lower frequency and are going to be spread farther from each other. But if we have high pitch, this is going to create waves that are closer together or have a higher frequency. So of course, when we combine all of these wave amplitudes and these wave frequencies, it will create all the various sounds that we hear in our environment. Slide 3 So let's now start to look at some of the structures involved in hearing as well as balance. So our structures for hearing can be divided into three main regions. The external ear, the middle ear, and the inner ear. So we're going to work our way through each of these different regions. So in the external ear, our most prominent structure that you can see on the outside of the body that flop, that is collecting the sound waves from our environment is known as the auricle. It's also known as the pinna, P I N N A. So this is basically going to gather sound waves from our environment and direct them in towards this channel that leads to the middle ear. This channel is known as the external auditory canal, and you can see the external auditory canal actually starts as soft tissue with some cartilage imbedded in it, but then it actually becomes a canal or hole in the actual bone. And this bone here is the temporal bone. So this is going to be an actual structure that we'll see when we start to look at the bones a little bit later in the course. Now, in this external auditory canal will have things like ear wax. The role for ear wax is to prevent dust and foreign objects from entering into the structures of the middle or inner ear. It also helps prevent damage from water and keeps things like insects out of our middle and inner ear regions. The anatomical name for ear wax is called cerumen, spelled C E R U M E N. Now the border between the external ear and the middle ear is denoted by this structure called the tympanic membrane. The tympanic membrane is a thin, delicate membrane that vibrates when the waves reach it. So imagine those bands of compressed air and less compressed air are entering into this external auditory canal and then they get to this very thin membrane. Well, when they hit this membrane, it causes it to vibrate. So the tympanic membrane is also commonly referred to as your eardrum. And that's because it vibrates when we had these bands of air hitting it. So this is also the start of the middle ear. And the middle ear region. It's also an air filled cavity like the external auditory canal. And within this region we have three small bones known as the auditory ossicles. The first bone is actually physically connected to the tympanic membrane, and it's called the malleus. It gets its name from the fact that it's shaped like a mallet or a hammer. And in fact, the handle of the mallet is actually attached to the tympanic membrane. Whereas the head of the mallet is forming a joint with the next bone in the series, so the second auditory ossicle, which is called the incus. Now the incus gets its name from the fact that it looks like an anvil. So if you think about a blacksmith or taking a mallet and they're hitting against an anvil. So that's why they get their name. So the malleus is hitting the anvil. The incus is attached to the third of the auditory ossicles known as the stapes. So the stapes gets its name from the fact that it looks like a stirrup. So we'll be able to see it a little bit better in some images, but if you think about the thing that you put your foot in when you're horseback riding, the thing that holds your foot up, that's basically the shape of the stapes. So it has a bit of a foot plate and then it has sort of a loop of bone on the top. Now all three of these bones are connected to each other but very small synovial joints, and we'll be talking about what synovial joints are later in the term, but imagine that they can move relative to each other. So as I already mentioned, this is an air-filled cavity just like the external auditory canal, and this is going to allow those bands of compressed and less compressed air to vibrate the tympanic membrane because we need air on either side of the membrane. The other thing that's important is that we need the pressure of that air to be equal on either side of the membrane. We also need the middle ear region to be connected to our external environment. And we do that via a region called the auditory tube or the eustachian tube. So basically this is actually an opening in the tissue that connects to the back of your throat or your pharynx. And what this is going to allow for is the pressure in this middle ear area to be equal to the pressure that's in the external auditory canal. Now why do we want this to happen? Well, remember our tympanic membrane is a very thin, delicate membrane. So if we add high pressure on one side of the membrane and lower pressure on the other side of the membrane, essentially what it would do is it would push on this membrane and tighten it so that it can't vibrate. So we actually need equal pressure on either side. Now, you might recognize this sometimes happening when you say go on an airplane. So when you go up to altitude, there's some pressure changes, and what happens is because your mouth is closed and you're not talking necessarily, then this auditory tube sometimes can become blocked. When it becomes blocked, the pressure on the middle ear side can sometimes stay at that lower pressure, whereas the pressure and the external auditory canal is rising. So this is what we would see at the plane was descending, but the opposite is also true if the plane was a ascending. And that's because as we go up in altitude, the total atmospheric pressure drops, so when the pressure is higher on one side of the membrane, what happens is this pressure pushes on the tympanic membrane and it gets to the point where it starts actually hurt you a little bit and is really, really hard to hear. That's because the tympanic membrane can't get those vibrations anymore. So what do you do on an airplane? Well, sometimes you chew gum, sometimes you talk, sometimes you swallow, or sometimes you close your nose off, you blow air against your closed glottis, So that closes over your esophagus leading into your digestive tract. So essentially what you're doing is you're increasing the pressure in the oral cavity in your throat, and what that does is it forces the air up into the auditory tube so that you can open up that tube and equalize the air pressure on either side of the tympanic membrane. So essentially that's what is happening, if you don't have equal pressure on either side. We just need to open up this auditory tube and let the air from the external environment equalize. And again, chewing gum can do the same thing I often will yawn, and that will also open up the auditory tube. You'll hear that little pop sound that's when the pressure is equalizing again. Now in some situations, the auditory tube can be problematic, so in certain individuals, sometimes they get some fluid building up into this area or if they say have an infection and it gets up into this area than it can interfere with the ability of the auditory ossicles to do their job, as well, it can interfere with the ability for the pressure on either side of the ear drum or tympanic membrane to equalize. So we want to keep for both of these cavities air filled. Essentially what the auditory ossicles do is they amplify the sound wave. So they take the vibration and the tympanic membrane, they amplify it before they enter into the final region of the ear known as the inner ear. So the inner ear is no longer an air-filled cavity. It's actually a fluid-filled cavity. And the portion of the inner ear that's involved in hearing is known as the cochlea, so this is the structure that we saw on that image on the first slide. So there are two main connections between the middle ear and the inner ear. The first is something known as the oval window, so this is an opening from the middle ear that's actually covered over by the foot plate of this stapes, and this connects to the fluid-filled cochlea. Now, this oval window has a very thin membrane in it because remember this is a fluid-filled cavity and this is an air-filled cavity, so it's keeping the fluid in the fluid-filled cavity. But what happens, remember is these auditory ossicles will move, and this stapes is essentially, we'll bang on the membrane of the oval window. And as it does that it creates vibrations in the fluid of the cochlea. So that's basically how we're going to transmit a sound wave in air into a fluid filled cavity. And again, as I mentioned, these auditory ossicles, their job is to amplify that sound. So it's basically going to allow for the waves to become stronger as they move into this fluid-filled cavity. The way I like to think of this is if you've ever been at someone's house that has a pool and they have their pool cover on, imagine you take your hand and you hit the pool cover over and over again. Well, basically what that does is it creates waves underneath the pool cover and that's exactly what the stapes is doing on this membrane that covers over the oval window. The other connection between the middle ear and the inner ear is the round window, which is very similar to the oval window. It's an opening covered by a membrane. But in this case, that membrane doesn't have any ossicles associated with it. So if we think about the wave's going into the cochlea via the oval window, I'll talk about this more in a moment, but the waves have an exit point through the round window. Now again, if you picture this as yourself at the lake, for example, and maybe you have a doc that's a little bit solid or a solid surface that's on the side of the water. If you were to create a wave in the water that works towards the doc, then as soon as it hits the dock, it bounces back, and that wave would then interfere with further waves or ripples that are coming towards the dock, so there's sort of competing. So essentially that's what's going to happen in the cochlea as well. So the waves are going to enter in the oval window, and then if they didn't have the round window as an exit point, they would bounce back and that would actually cause an echoing effect. So we're actually going to look a little bit more closely on how this works on an upcoming slide. But that's the function of the oval window, the exit point for the waves in the cochlea. There are some other structures associated with the inner ear, but these are involved in our special sense of balance. So this middle area here is known as the vestibule and that's involved in our static balance or our heads position relative to the ground. And then we have the semicircular canals, which you can see two of the loops here, ut basically there are three loops and they're in all three planes, the x, y, and z plane. The semicircular canals are involved in dynamic or kinetic balance, or basically whether our head is accelerating or decelerating. So we're going to look more at our special sense of balance in some upcoming slides. Finally, the last thing you can see on here is the vestibulocochlear nerve. So of course, this is our cranial nerve number eight, and it's divided into two branches. So we have the cochlear branch, which goes to the cochlea for our special sons of hearing, and the vestibular branch, which goes to the vestibule as well as the semicircular canals, and that's going to be for balance. So when these two branches come together, that's what forms the vestibulocochlear nerve. Slide 4 So this image is really just highlighting the structures of the middle ear so that you can see them a little more clearly. So you can see a little bit more of this stapes here that it looks more like a stirrup with the foot plate there and it's covering over that oval window. But all the other structures that are on this slide, we've already talked about it. So it's just giving you a better view of those structures. So let's take a pause here and we'll do some practice questions and then we'll come back and started to look at the structures that are associated with the inner ear. Video 2 Slide 5 So now we're going to look a little more closely at the structures of the inner ear. So recall the inner ear is made up of three regions. We have the cochlea, which is involved in hearing. We have the vestibule, which is involved in static balance. And we had the semicircular canals, which are involved in dynamic balance. You can also now see the three loops of the semicircular canals in this image, and notice that you have one in each of the planes, x, y, and z. The other things that you can see in this image are the oval window. Now the stapes is removed, so you can see that thin membrane or opening into the inner ear. And you can see the round window, which is the exit point for the waves after they've moved through the cochlea. Now if you remember from the very first slide I showed you that picture of the cochlea where the bone had been chipped away and you could see the membrane that was left. Now often when we look at these images of the cochlea, we're picturing a structure, but the bone is always sort of chipped away in these images, but remember, we actually have some cord out ducks and spaces within the temporal bone itself, and that is basically setting the stage for the structures of the inner ear. And in fact, this space in the inner ear is really divided up by a series of membranes, and in and around those membranes is where we're going to have the fluids and the specialized cells that are going to detect things like hearing and balance. So when we're looking at this structure, the outside region of the inner ear is known as the bony labyrinth. Now if you think about labyrinths, it's like a maze, it's the bony labyrinth because these are all the little corded out regions of the temporal bone itself. That's the outside of the inner ear structure, or at least sets the stage for the outside of the structure. In the bony labyrinth, we have a series of membranes. and in those membranes we're going to have some fluids. So these membranes actually divide up the bony labyrinth into a series of different sections or regions, and in those sections are regions we can contain different types of fluids. Now, the reason we want to have different types of fluids is because some fluids will be high and one ion concentration and some fluids will be high and another ion concentration. And this is kind of important because again, we need specific ion concentrations in order for action potentials to be created. So our membranous labyrinth is going to exist in all of the different regions and it divides up those channels or canals into very specific patterns. So in this particular image at the top here, we're taking a cross section through one of the semicircular canals. So you can see the bone on the outside here, qnd then you can see the bony labyrinth is basically the space that's been cored out of that bone. Inside the bony labyrinth, we have a series of membranes, and these membranes are going to divide the space up so that we can have different types of fluids inside. So this is the membranous labyrinth. In the case of the semicircular canals, it's also known as the dynamic or kinetic labyrinth because this particular region for balance is related to dynamic or kinetic balance, or basically accelerations and decelerations. So in the membranous labyrinth we have some fluids. Now in pink, on the inside of the labyrinth is a fluid known as endolymph, and endolymph contains a very high concentration of potassium ions and a low concentration of sodium ions. The other type of fluid is known as perilymph and perilymph is basically between the bony labyrinth and the membranous labyrinth, so it's filling in the rest of the space inside the bony labyrinth. Now perilymph has the opposite ion concentrations, it's high in sodium concentration and low in potassium concentration. So the way that I like to remember this is that the endolymph is inside the membranous labyrinth, so it's kind of like the inside of a cell where we have high potassium and low sodium. And the perilymph is between the bone and the membranous labyrinth, so it's sort of like it's outside the membranous labyrinth. So it's like the outside of the cell where it has high sodium and low potassium. Now, this is part of balance and you can notice the pattern that we see here with the membranous labyrinth is quite different than what we see in other areas of the inner ear. So based on these ducks and tubes that are created by the membranous labyrinth, it will sort of dictate some of the functions of what's happening in these different regions. So when we start to look at the specialized cells found within the membranous labyrinth. You'll see that it's all sort of contained within these regions of pink or where we have endolymph located. So when we take a cross section through the cochlea, which we're going to highlight a little bit more in the upcoming slides, this is what it looks like. So we have the bone, we have the same bony labyrinth, except now our membranous labyrinth is really dividing up this area into three distinct chambers. So we have the two top and bottom chambers which are filled with perilymph, and then we have this central chamber that is filled with endolymph. So this is kind of like taking three of those really long balloons that you make balloons animals with and you take them and you squash them together. So we have one balloon on the top, one balloon in the middle, and one balloon on the bottom. Again, the top and bottom balloons are filled with perilymph, and the middle balloon, where we have our specialized receptors for hearing, is filled with endolymph. Now this is a cross section, but imagine this is one long tube that rolls in on itself. In this region where we have the specialized membranous labyrinth, as I said, we're also going to have our specialized cells that are going to detect our sound waves. These cells are found on a membrane that separates this chamber from the chamber below it, known as the basilar membrane. So we're going to look a little bit more closely at some of these structures in a second. Now also notice that the cochlea, as I said, is really long tubes that role on themselves. So again, inside this single tube we're going to have three separate chambers, but it's rolling in on itself. But eventually it comes to the end, and that point where it's the end of the tube is known as the helicotrema. So we'll see what that looks like on the inside as we go through some of these slides. Slide 6 So now what we're looking at is a cross section of the cochlea. So again, we've cut through the cochlea at one of the regions, you can still see the three chambers, except now we're going to get more and more focused on some of these structures so that we can look at them in more detail. So what do we see here? Well, this is our membranous labyrinth, again, it's basically creating the three separate chambers. The top chamber is known as the scala vestibuli, and this is filled with perilymph, and remember perilymph is going to have high sodium and low potassium. On the bottom part we have this scala tympani and that's also filled with perilymph, are in fact, these two chambers, while they're separated in this particular cross section, when you get to that helicotrema, these chambers actually are connected to each other. So essentially we have really one long balloon and then when we get to the end, it folds back and comes back to form the bottom chamber. So the only one that are technically fully separated is going to be this center region, and the center region is known as the cochlear duct. The cochlear duct, as I said, is filled with endolymph, so again, the scala vestibuli and scala tympani are attached to each other at the helicotrema, but at the other end, the scala vestibuli is attached to the oval window and the scale a tympani is attached to the round window. So you can see how you can start at the oval window, travel all the way down the scala vestibuli, come around back around the helicotrema into the scala tympani, and then the wave would travel all the way out towards the round window, and I'll show you what this looks like on an upcoming slide, but I just wanted to highlight that here. Inside the cochlear duct, we're going to have this specialized cells that are going to receive those waves of information and be detected by the brain. Separating the scala vestibuli and the cochlear duct, we have a very thin membrane known as the vestibular membrane. So this is really just separating these two chambers in terms of their fluid, and it's really thin, so it doesn't really have a functional role aside from creating the two separate compartments. The bottom portion separating the cochlear duct from the scala tympani, we have a thicker membrane known as the basilar membrane, and the basilar membrane is important because this is where our specialized cells are located that are going to be involved in creating that hearing response. This region of the basilar membrane, where we have specialized epithelial cells mixed with our special receptor cells called hair cells, is known as the spiral organ or the organ of Corti. The hair cells, which you can kinda see here in purple, they actually stick up into a gelatinous membrane that sits above them, and that's known as the tectorial membrane. So you'd be able to see that a little bit more on the next slide, but imagine that when you cut through this, this entire thing is stretching or spanning the length of this really long chamber. So this region of cells is really going to go back all the way in and around the whole cochlea. So we're going to blow this up and look at some of these a little bit more in detail in a second. Now, because this is where we have our specialized receptor cells, this is also where our neurons are going to connect to these cells to take the information back to the brain. So this is showing the cochlear branch of the vestibulocochlear nerve or cranial nerve number 8, of course, because this is for our special sense of hearing. Slide 7 So now what we've done is we've taken just the cochlear duct and we've blown it up so we can look at what's happening inside the cochlear duct. So one, this is the duct itself and again it's going to be filled with endolymph. This is our vestibular membrane. So up here we have the scalar vestibuli, and down here we have this scala tympani. So here's the basilar membrane, and you can see on the basilar membrane we have a region of very specialized epithelial cells, and these cells are containing their specialized receptor cells. Just above those specialized receptor cells, you can see another membrane that sits just in the middle of the cochlear duct, and that's the tectorial membrane. So notice it doesn't span the entire cochlear duct it sort of is tethered on one side. And it's a gelatinous mass basically that sits over top of the spiral organ region where we have these specialized cells, and in fact the tops or tips of these specialized cells are actually embedded in that tectorial membrane, and I'll talk about what it does in a second. So much like what we've seen with other special senses there supporting epithelial cells around our specialized receptor cells. And then we have the specialized receptor cells, which in hearing are known as hair cells, and technically they're also hair cells in balance as well. So we'll see those a little bit later. So these are our hair cells. Now, if you notice, our hair cells are organized into rows. So these rows would extend all the way along the entire cochlear duct. Remember, this is on the inside of one of our really long balloons. So we're just looking at a cross section here. So there are three rows of outer hair cells and one row of inner hair cells. Now this row of inner hair cells are actually the hair cells that are going to detect sounds. So this is going to be our perception of sound. The outer hair cells, they're job is actually to regulate the tension on the basilar membrane. So as we'll see in a moment when we talk about the functioning of these cells, what's going to happen is that when the waves are coming into this area, they move the basilar membrane, and when we move the basilar membrane, we're going to move these hair cells that are within the basilar membrane, but the tops of the hair cells are actually embedded in this tectorial membrane. So we're basically moving the bottom of the cell relative to the top of the cell, and that's what's going to help to open those mechanoreceptors and create the electrical signal. So depending on how much the basilar membrane moves, will regulate the amount of sounds where getting or the tension on the basilar membrane helps regulate sounds or dampens sounds if they're too loud. These are just gathering sensory information so that we can control how tense or loose this basilar membrane is, and it's the inner hair cell row that's actually creating the sounds that we're interpreting. Now all of these things together, as I mentioned, make up that spiral organ or the organ of Corti. So if we look a little bit more closely at our hair cell, this is what it looks like. So this is the bottom of our hair cell, and it's connected directly to the nerve endings of the cochlear nerve, So kind of like the gustatory cells, there's no axon in the hair cell, we're simply going to depolarize the cell that will release neurotransmitter and then that signal can be sent off to the brain. Now notice on the top of our hair cell, we have something known as a hair bundle. And in the hair bundle we have microvilli, and specifically the micro villi that stick up on the top of our hair cells are known as stereocilia. So it is kind of funny that it's called stereocilia, it's not silly at all, it's microvilli. But notice or hair bundle is kinda fashioned in an organized way and that these microvilli, sort of go up from short to tall as we move across the hair bundle. That will be important when we start to talk about how we depolarize this region of the hair cell. So here you can see those nerve connections on the basis of the hair cells and those are gonna be carried back to the brain. And there's the cochlear nerve right there. So again, the way that this is going to work, when the stapes is going to hit on that oval window, it creates a wave in the top chamber because that's actually connected to the oval window. So that chamber remember was the scala vestibuli or vestibuli. So when the wave above here goes through, it creates a wave in the vestibular membrane with which I said is a really thin membrane that will then transmit the wave into the cochlear duct, which is also filled with endolymph. So now we've gone from perilymph into endolymph, and now the wave is going to move through the cochlear duct and cause a vibration of the basilar membrane. This vibration in the basilar membrane is going to move this spiral organ and the bottoms of the hair cells relative to the tops where the stereocilia is located. Now these stereocilia are actually embedded in this tectorial membrane. So if you move the bottom relative to the top than it actually tips these stereocilia over, and that's what's going to help create the action potentials. So we're going to look a little bit more at how we can open the mechanoreceptors in this stereocilia region. But before we do that, let's stop here and try out some more practice questions. Video 3 Slide 8 Welcome back, now we're going to take a little bit of a closer look at those microvilli of our hair cells. So again, at the top of each of our hair cells, we have bundles of microvilli, and these microvilli are actually arranged in such a way that they get progressively taller as you move from one side of the cell to the other side of the cell, and you can sort of see that in this image. As I mentioned on the previous slide, the microvilli are actually referred to as stereocilia, but in fact they're not cilia at all, they're microvilli. What makes these stereocilia unique is there's a physical connection between each stereocilia and the stereocilia next to it, which is taller. These connections are known as tip links, and essentially it's kind of like a little spring that's attached to the ion channel gate. When you move the stereocilia relative to each other, you physically open the gate, allowing the ions to rush into the cell, causing depolarization. So these specialized mechanoreceptors are known as tip links or sometimes referred to as gating springs. Slide 9 On this slide, you can actually see these things in action. So in the resting condition our microvilli will be in the upright position. All of the microvilli again are attached to each other via these gating springs, and the microvilli, will become progressively taller as you move along the hair cell. So you can see here the gating spring is attached to the actual gate, and the channel is a potassium channel. So when we have a wave of the basilar membrane, it's going to move the bottom of the hair cell, and this top region again is going to be connected via the tectorial membrane. So as the bottom moves, what happens is this stereocilia will move towards the taller stereocilia. So though progressively move towards the next taller stereocilia. As this occurs, the gating spring will pull, open the gate, and because we have a very high concentration of potassium inside the endolymph region within the cochlear duct, the potassium will rush into the cell, and that's what actually causes the depolarization. So this is one of those rare instances where it's not sodium that causes depolarization. It's actually potassium causing depolarization, and thats simply because the concentration of potassium is so high in the endolymph relative to the inside of the hair cell itself. So this will allow for the depolarization to occur. So in the unstimulated situation, the gating spring is relaxed, as soon as the stereo cilia bend towards the next taller stereocilia, when the basilar membrane is moved, the gating spring will stretch and this will physically open the ion gated channel, allowing for potassium to move into the cell and cause depolarization. So this is how those mechanoreceptors work. Slide 10 So let's put all this physiology together now so that we can actually visualize what's happening as a sound comes into our external ear, moves through the middle ear, into the inner ear. Note that all of the numbers listed on this slide correspond to points made on the next two slides so that you don't have to take extra notes. So first, we have our sound wave entering into our external auditory canal. Again, this is bands of compressed air followed by bands of less compressed air. This will vibrate the tympanic membrane, and as the tympanic membrane vibrates, it will move the malleus. When the malleus moves, it will move the incus, which it forms a joint with. Once the incus moves, it will move the stapes, the final of the three auditory ossicles found within the middle ear. The foot plate of the stapes is found over top of the oval window. So as the stapes moves, it bangs on the membrane covering that oval window and creates a wave. This wave will then enter the fluid of the perilymph found within the scala vestibuli. The wave can move all the way down the length of the entire cochlea, and in this image you can see the cochlea has kinda been unraveled so that you can see how the wave is moving. So the wave will move down the length of the scala vestibuli until it reaches the helicotrema, which is the endpoint of the scala vestibuli where it connects to the scala tympani. So the wave will continue around the helicotrema and enter into the scala tympani, and at this point it will travel back towards the round window. In the mean time, the waves are also vibrating or moving the vestibular membrane, which is very thin membrane between this scala vestibuli and the cochlear duct, because it's a thin membrane and allows for the fluid within the cochlear duct, the endolymph, to also move. When the endolymph moves, it will move the basilar membrane. When the basilar membrane moves and vibrates, it will move the hair cells within the basilar membrane, and specifically it will move the cell body differently than it will move the hair bundles of stereocilia which are embedded in the tectorial membrane. Because they move differently, this will allow for those tip links to open those ion gated channels and allow for the depolarization to occur. So the round window acts as an exit point for these waves. If we didn't have the round window, the waves would bounce off of the hard service and go back into the scala tympani, and that would move the basilar membrane again and we would get another signal. So the round window acts as an exit point for these waves. Now how do we differentiate between high pitch and low pitch? Well, it's based on which hair cells are activated. The basilar membrane is tighter closer to the oval window, and it's much looser or more able to vibrate closer to the helicotrema. So high pitch waves will stimulate hair cells close to the oval window. Whereas low pitch waves will stimulate the hair cells closer to the helicotrema. When it comes to volume, higher volumes or louder noises will stimulate more hair cells versus quieter sounds will stimulate less hair cells. So depending on which hair cells are stimulated, where they are located along the length of the entire basilar membrane, and how many hair cells are stimulated, that allows us to collect both pitch and volume information. Slide 11-12 So again, this is all of the things that I just mentioned, matching up each of those numbers as you move along the physiology of hearing. Slide 13 So once our hair cells have been depolarized, they send neurotransmitter to the cochlear nerve, but where does this signal go? The signal can go to the cochlear nuclei that are part of the medulla oblongata. Here, this is involved in sensations of pitch perception as well, the medulla oblongata will use some of this information and send signals back to the basilar membrane to help dampen some very loud sounds. So it may change the muscles that work the auditory ossicles so that they're not hitting the oval window with as great a force to help dampen loud sounds. We will then send signals on to the superior olivary nucleus, which is part of the medulla oblongata. Again, this is also where we're going to be using proprioceptive information in order to make subtle changes like dampening of sound. We can also send some information on to the inferior colliculus, which is part of the midbrain, and if you recall back to our brain lecture, this is the region where we have auditory reflexes. So we would send some information to this region as well. From the midbrain, we send the information to the thalamus and specifically the medial geniculate nucleus, which we spoke about when we talked about the brain. From there, we can finally send the information to the cortex. The primary auditory cortex is found in the temporal lobe, ao this is where we would actually have our conscious perception of sound. So let's take another pause here and we'll watch an animation describing the physiology of the movement of the sound waves through the inner ear. And then we'll come back and talk about balance. Animation No caption file Video 4 Slide 14 So that finishes off our look at hearing and now we're going to move on to the structures of the inner ear that are involved in balance. The first area that we're going to look at is this region between the cochlea and the semicircular canals, and that's known as the vestibule. The vestibule contains two specialized structures that are involved in balance, known as the utricle and the saccule. Specifically the utricle and the saccule are involved in static balance or static equilibrium. When we talk about static equilibrium, we're talking about our heads position relative to the ground. So is our head upright? Is it tilted to one side? Is it tilted forward? Is it tilted backwards? The utricle is gathering information typically in a horizontal plane, and the saccule is gathering information typically in a vertical plane. So within these two areas, the utricle and the saccule, we have some regions with very specialized epithelial cells, and these regions are known as the maculae or macula (singular). Basically the macula contains specialized hair cells, and these hair cells are very similar to what we see in the cochlea surrounding the hair cells, just like we've seen in many of our other special senses, we have supporting cells that are holding those hair cells in place. They also have hair bundles on the top of the hair cells, just like we saw with the cochlear hair cells, except the microvilli are slightly different and we'll look at those a little bit more in a second. In this case, our hair bundle is embedded within another type of gelatinous membrane, and this is called the otolithic membrane. Within the otolithic membrane, we have some crystallized structures called otoliths. And really the job of the otoliths is to create mass or weight for this otolithic membrane, because essentially what we want to do is take these hair cells and move this otolithic membrane relative to the base of these hair cells so that it opens those tip links and you'll see how that is demonstrated on the next slide. Of course, when we do stimulate those hair cells and we get depolarization, this will synapse with some of the vestibular branches of the vestibular cochlear nerve to send those signals back to the brain. So if we look more closely at the hair cells in this example, you can see they're structured very similarly to what we saw in the cochlea, our hair bundle is made up of stereocilia, and those stereocilia are progressively taller as we move across the hair bundle, except one difference about this type of hair cell compared to what we see in the cochlea, it also has a very tall cilia structure known as the kinocilium. So the stereocilia are actually microvilli and we have one single cilia known as the kinocilium. Basically they work in the same way, they have tip links attaching each of the successive stereocilia all the way up to the kinocilium, and if we move or tilt this hair bundle then it will physically open those ion gated channels. We also have endolymph in this region because it's inside that membranous labyrinth where we have endolymph, also known as the static labyrinth in this region. So it will open up potassium channels and potassium will rush in to cause depolarization. Slide 15 So how does all of this work? Basically, you can think of this otolithic membrane as sort of a gelatinous mass that's going to move with gravity because of the weight of those otoliths. So if we're looking at this component over here, it's a flat sort of horizontal position when our head is upright. So you can imagine this is the macular region of the utricle for example. So now our hair cell bundles are not activated because the otoliths are keeping them in the upright position. But when we tilt our head forward what happens is the otolithic membrane because of the weight of the otoliths, pulls those hair bundles and tilts them over to one side, that will open those tip links and cause depolarization. Of course again, we depolarize our cell, the neurotransmitters get sent to the neurons, which then can act to the vestibular nerve and can send that information back to the brain. So thats static equilibrium, the hair cells are stimulated by the otolithic membrane and specifically the otoliths moving with gravity in that otolithic membrane, and as those otoliths move in response to gravity, we get a certain pattern of action potentials, and that's what's being sent back to the brain. This allows for subconscious perception of where our head is relative to gravity and allows us to make subtle adjustments to the muscles of our neck and our back so that we can keep our head in an upright position. Slide 16 The next structure that we're going to look at is involved in dynamic equilibrium or dynamic balance. Basically what we're referring to in this case is whether our head is accelerating or decelerating and in which direction it may be moving. So instead of just a static position relative to gravity or your heads basic position, where is our head moving relative to things around us. The structures of dynamic equilibrium are the semicircular canals, and specifically that enlarged region at the bottom of the semicircular canals, known as the ampulla or ampullae for we're talking, but all three of them. Remember we have semicircular canals in all three directional planes, and those canals are filled with the membranous labyrinth that contains endolymph. So within these ampullae we had the semicircular ducts containing endolymph, and it's within these ducts where we have this very specialized endothelial region known as the crista ampullaris, or simply just crista or cristae (plural). Because the semicircular ducts move in each of the directions, as fluid moves in each of those directions, we can detect that as movement in either the x, y, or z planes, and depending on how much movement we have in each of those planes, our brain will interpret that as which direction our head is actually moving. So this is the structure, the specialized receptor cell structure that's located in that ampulla region of each and every one of the semicircular canals, and it's known as the crista. So the crista is a curved epithelial layer that has very specialized hair cells within it, as well as those supporting cells that we've seen already. So the hair cells in this case are very similar or basically exactly the same as what we just saw on the macular regions of the saccule and the utricle, except now they're located in this curved region in the semicircular canals. The one thing that's different is what the hair bundles are embedded in. In the macula, they were embedded in the otolithic membrane. In the crista, they're located in something known as the cupula. So the cupula basically acts as a float inside this membranous labyrinth filled with endolymph. So you can imagine again, it's sort of like a mass that's floating in a fluid. Basically, what's going to happen is as the cupula moves in the fluid within this membranous labyrinth of the ampulla, in their semicircular canals, tt will tilt those hair bundles and open up those tip links, allowing for potassium to move into the cell and depolarize those hair cells. Again, if we get depolarization, it will synapse with these nerves, which will join up with the vestibular nerves, and the vestibulocochlear nerve eventually, and send that information back to the brain. So as the cupula moves, depending on which one of the three semicircular canals that's located in, will dictate which direction our head is moving. Slide 17 So here's an example showing how this works. So here's the ampulla region, here is our crista, and here's the cupula on top of our hair bundles. When our head is in a still position, the fluid inside this membranous labyrinth is not moving, so the cupula is standing upright. However, if we move our head, we start to get fluid movement within the semicircular canals. So our head is moving towards the right in this example, and our cupula would therefore move in the opposite direction. Now this may sound a little confusing. But if you think about a fluid in a cup, imagine you have a coffee or a drink in your car. When your car is parked, the fluid is not moving relative to the cup, it's just nice and flat. But when you start to accelerate, the cup is moving forward, but the fluid takes a little bit longer to get moving forward. So what happens is the fluid will actually tilt backwards towards you because it's not up to speed yet, like the cup is, which is attached to the car in your cup holder. So in this case, the cup is moving in one direction and the fluid within the cup is moving in the opposite direction. So that's exactly what we're seeing here, the direction of the head is in one direction, but the fluid is moving in the opposite direction. Because the fluid moves in the opposite direction, it pulls along with it, the cupula because the cupula is floating in that fluid, and that's what's going to bend or open those tip links in those hair cell bundles. Now eventually your fluid will catch up and it will get to speed and the cupula will come back up to a settled position, just like your coffee in your cup, when you're driving, when you get up to speed and you're driving in a constant speed again, it will stop tilting within your cup and it will flatten out again. Now what happens when you stop the car abruptly? Now the fluid continues to move even though your cup has stopped. So now the cup is stopped, but the fluid continues to move in the direction you were just moving. So in this case, our direction or movement of our head in this case would be not moving, but the fluid continues to move. So again, we would perceive that as a deceleration because the cupula is now moving in the direction that we were just moving, as opposed to the opposite direction like we had when we were accelerating. So as the head rotates in one direction, the cupula is dragged through the endolymph and bent in the opposite direction. That's when we're accelerating. When we're decelerating, the cupula is moving in the direction we were just moving, and our head is now still, so that's sensed as deceleration. Slide 18 So again, the cupula moves in the opposite direction of the head when we're moving, but the stimulation will stop when the fluid in the canals catches up with the cupula. So again, depending on which way we're moving will move fluid in one of the three different canals and it will move it to a different extent depending on which plane we're actually moving in. When the movement of the head ceases, the endolymph continues to move in the same direction that we were just moving, and the cupula will also continue to move, and that will be perceived as a deceleration compared to when the cupula was moving in the opposite direction of the head, which would be perceived as an acceleration. So I mentioned earlier we would talk about motion sickness. Now some people suffer from motion sickness when they're in a car being driven by someone else, and it particularly gets bad when they're doing something that they're looking at stationary objects inside the car, like reading a book, as opposed to looking out the window. Now why do people sometimes suffer from motion sickness? Well, one of the causes is the fact that the information being sent to the brain by the vestibular system doesn't match other pieces of sensory information coming into the brain. So for example, if you're reading something in your hand while you're in the car, your visual system doesn't gather sensory information about your body moving. However, because of the way that your vestibular system works, you are gathering information about accelerations and deceleration. So your brain is actually getting conflicting information from those two pieces of sensory information, that the visual system doesn't match what's happening in the vestibular system, especially in those semicircular canals. So when there is these mixed signals in the brain, sometimes the brain creates that motion sickness response where you feel nauseous, you feel sick because it's unable to interpret these two conflicting pieces of sensory information. Slide 19 So what happens when we take all of this balance information? Where is it going? So we're creating action potentials on the vestibular nerve, and the vestibular nerve is going to send information to the vestibular nuclei. So remember that balances are very, very complicated process and it's going to involve many different systems, so it has a very complex neural pathway and this is a very simplified version of that. So the first region it's having a synapse is the vestibular nuclei, which is in the medulla oblongata. Some of that information will be sent to the cerebellum because remember the cerebellum needs to make subtle adjustments so that we can maintain our posture as well as contract our muscles in a nice coordinated fashion. So it's going to need some balance information. We can also send some of that information to the motor nuclei that control the movement of our eye muscles, because if our head is tilted relative to gravity or is moving, we need to move our eyes relative to that so that we can maintain focus on objects. And then of course, some of the information is going to go through the thalamus, if we want to send it to the cortex. The cortex is going to receive information in the vestibular area of the cortex, and that just happens to be in the post-central gyrus. So any information that the thalamus deems necessary for the cortex to interpret will be sent to this vestibular area of the cortex. Conclusion So that's what we're going to end today's online lecture. Today we looked at hearing by starting to look at the structures of the external, middle and inner ear. And then we started to talk about how we can take sound waves and turn them into electrical signals that are created in that inner ear and sent to the brain. We then looked at balance and the structures of balanced found in the inner ear. We talked about static balance as well as kinetic balance. In the next online lecture, we're going to start to look at bone and specifically the components of bone tissue. So until then, take care.