Understanding Ear Anatomy and Function

All right, so let's begin our lecture on the ear. Here we see the entire anatomy of the ear and we divide the ear into three parts. The first part is out here called the external ear, where you have the pinna or oracle of the ear collecting the sound, delivering it into the external auditory. canal or the external acoustic canal or the external acoustic meat is all of those are correct. It's just a channel that is lined with skin and delivers sound waves to the tympanic membrane, which is your eardrum. The tympanic membrane is a very tightly, very thin, very tightly taut collagenous membrane covered over with some epithelial tissue. but it is like a drum very much in the sense that it's very tightly drawn and very sensitive to air vibrations and sound waves are essentially compression waves of the air and they come in a series and the higher the frequency of those waves the higher pitch of sound is registered and the lower the frequency of sound I should say the lower the frequency of air compressions hitting the eardrum, the lower the sound. So high vibrations will give you a high pitch, low vibrations will give you a low pitch, and that's all interpreted here in what is called the cochlea. But I'm getting ahead of myself. So we're talking about the external ear all the way to the sound collector of the collagenous ear, external ear, the auricle. Sound waves travel down the external auditory meatus or canal and then the external ear ends here at the tympanic membrane. Okay, now the second part of the ear is the middle ear and the middle ear consists of this air pocket within the petrous portion of the temporal bone. See, we're in bone here. It's all spongy bone, you see. And there's a hollow space in there called the tympanic cavity or the middle ear cavity. See, is it labeled here anywhere? No. Okay, well that's what it is. It's a cavity and there are three bones in that cavity. One is attached to the tympanic membrane and it is called the malleus, which means hammer, and it articulates, actually forms a synovial joint with the second bone here called the incus, which means the anvil. And the incus then articulates right here with the third bone called the stapes, which literally means the stirrup, because it looks exactly like a horse's saddle stirrup. You see the flat plate right here, and that little flat plate fits in a little oval window, as it's called, and there's a membrane covering that window. And when sound waves come and hit the tympanic membrane, the tympanic membrane vibrates. the malleus is attached to it, so the malleus vibrates, the malleus pushes on the incus, and the incus pushes on the stapes. The stapes then pushes in the oval window, and on the other side of the oval window there is this fluid-filled system that receives the compression of the stapes, and that sets up fluid waves in this structure here called the cochlea, which is the organ of hearing. Okay, so There's other stuff here too, but I'll explain that more later. So that's your middle ear. Okay, so the middle ear cavity, tympanic cavity, three ear bones called the ear ossicles, which transmit the vibrations of the eardrum to the olal window, which then begins this third part of the ear called the bony labyrinth. All of this is the bony, that's all bone. That golden color is all bone, but it's buried in this other spongy bone. Now, One thing I want to point out about the middle ear here is that it opens down to the upper throat, to what is called the nasopharynx. That's the upper reaches of your throat. And this is a channel that allows the oral cavity, this is the back of the oral cavity, the nasopharynx, to communicate with this air pocket, the tympanic cavity. And this is the... thing that is responsible for your ears popping as you go up and down in elevation, because that represents... air either escaping through this tubule or getting in through this tubule. If the air pressure increases outside, like if you're going down in elevation, air will be trying to get into this space because the pressure in this space changes. I shouldn't say it doesn't change, but there's an imbalance that happens between the pressure inside the tympanic cavity and the pressure outside in the outside of the body that is sitting on the outside of this eardrum. And so what happens is if you have a differential of pressure, let's say you have very high pressure here, very low pressure here on the outside, then the air is trying to expand in here and it's starting to push on the tympanic membrane. And if it can't escape, you could actually push so hard. with the air pressure that you could rupture the eardrum. So you need an escape valve and that's what this little tube is called the eustachian tube or the auditory tube and that allows air to escape. Alternatively, if the pressure is getting too high out here, for example, if you've been up at 15,000 feet and now you take a train and you're coming down or you're in a plane and you're coming down to land, the air pressure, you know, 15,000-20,000 feet is far less than it is at sea level. So if you've got air pressure in here equalized at 15,000 feet, it's low air pressure. So if you're at 15,000 feet, it's equilibrated with this eustachian tube allowing air to move whichever way it needs to move. So you're at 15,000 foot air pressure, same out here, but then if you're in a plane, that's the most dramatic example, You're in a plane and the plane now comes down to sea level where the pressure is maybe hundreds of millimeters mercury greater. And so now you have very low pressure in here, much higher pressure outside, pushing in on the eardrum and it can tear the eardrum. I actually have allergies. And so what that causes is a swelling of the mucous membranes of the throat here. And this closes off and it doesn't allow free movement of air in and out. And so. When I used to fly when I was younger and had allergies, I don't have them quite so badly anymore, but when I was younger, I did. And coming down for a landing was excruciating. It actually felt like somebody was driving an ice pick into my ear because the pressure was so great as I came down and it was pushing so hard, pushing inward on this tympanic membrane that it was, I'm sure it was doing, it could have done some damage. So I'm not sure. you have this, when everything's healthy and you don't have swollen membranes closing this off, you have the ability for air to get in or get out to adjust the air pressure in the tympanic cavity with the air pressure outside. And so that's what the Eustachian tube is for. And if you have problems with allergies and stuff, and you do have the same thing with when you're flying, what pilots do who have this problem is that they take all sorts of antihistamines to reduce the swelling. They sometimes take drops down back down their nose and they dribble it back against their back throat. It reduces the swelling and that allows them to then fly up and down and allow air to move through this passage more easily. So that's your eustachian tube leading to the middle ear cavity. When children get earaches, they have essentially what is called otitis media, which is an inflammation of the middle ear chamber and the this bacteria get in here, they get a sore throat or something and the bacteria migrate up in. And it's more common in children because the eustachian tube is very short and it's more horizontal, it's not as vertical. So the bacteria easily get access into the middle ear, set up an infection, very painful for the child and when they're like one or two years old they don't know what's going on. It's very upsetting to parents. And what happens is it fills with blood and fluid and pus. And it can be excruciating. It can even happen to my child when he was five. It filled with so much inflammation and pus and stuff. It caused an erosion of the tympanic membrane and it burst. And when that happened, pus and blood came out of his ear. And I was absolutely terrified. I didn't know what had happened back in those days. learned later that, well, when it did happen, he was all of a sudden able to go to sleep. He had been up all night long. And when it finally ruptured, the pain subsided, all the pressure subsided, and he was able to go to sleep. So I took him to the doctor and they said, yep, he's got a ruptured eardrum, but he'll be fine. It'll heal up. And he may not hear quite as well with that ear as he does with the other ear, but he'll be all right. And he's done quite well since then. Okay. So that's your middle ear cavity, your tympanic cavity. Then we come to the inner ear, which is this elaborate structure, semicircular canals, vestibule here, cochlear duct here. Semicircular canals, they monitor head rotational movements, rotational movements in the three cardinal planes. And then the vestibule is monitoring head position relative to gravity, also vertical and horizontal accelerations. And then the cochlea, this is where we hear. So let's go through the structures of the inner ear. Also This whole structure is called the bony labyrinth. And let's see, anything on? This is just a more detailed look at the tympanic, I'm sorry, at the tympanic cavity. Notice you got muscles that do attach to the stapes. And what these muscles do is when your ear is being subjected to very high decibel sound, these muscles try to restrain the movement. these ossicles so they don't punch that oval window too severely. So there's some dampening effects of these muscles. Okay, so there is your bony labyrinth and inside we see the membranous labyrinth. This is a system of membranous tubules. Both the bony labyrinth and the membranous labyrinth, bony in the light blue, membranous in the dark blue, both are filled with fluid. Okay, and we know or the fluids are not quite the same. They're clear fluids, very much like lymph, but they are different chemical makeup. Turns out that the fluid inside the tubule system that you see here, all of that is called endolymph, and it has a very high potassium ion concentration. Outside the membranous labyrinth, In the lighter blue areas, we have perilymph, which is a much lower potassium ion concentration. There are other things in there, but for us, the potassium ion is of interest, and you'll see why in a little bit. Okay, so you have now the bony semicircular canals, okay, and there's three of them, the anterior, the posterior, the lateral. So these are the... Semicircular canals. You see this one's horizontal and this was kind of in a frontal and this is in a, let's see, it's really hard. They're not exactly sagittal and horizontal and frontal. The whole thing is situated at an angle, but they are three planes at right angles to one another. And they're monitoring head movements and rotational movements in particular. OK, so that those are your semicircular canals. Inside are your semicircular ducts. Endolymph in the ducts, perilymph outside. Now in the middle here we have the vestibule and in the vestibule are two membrane-filled sacs. One is called the utricle, one is called the saccule. The saccule is more vertical, the utricle is more horizontal. And what these two things do is measure or monitor horizontal and vertical accelerations. monitors the horizontal accelerations, the saccule monitors the vertical accelerations, and they also monitor head position relative to gravity. Okay, so that's what we get in the utricle. And then here's the organ of hearing, the cochlea, which literally means the snail, because of the shape of this structure is very much reminiscent of a snail's shell. So this is the cochlea. So semicircular canals and ducts. the vestibule with the utricle and the saccule, a fluid-filled sacs, and then the cochlea, the snail-shaped bony cochlea with the cochlear duct running up the tubule inside, and that is filled with endolymph. So perilymph out here, endolymph there. Okay, all of this apparatus is connected to the eighth cranial nerve, the vestibular cochlear nerve. Okay, now Notice here in these little swollen areas at the ends of the cochlear ducts and here in the wall of the sacs, the utricle and the saccule, and you're not seeing it, but they're in there. in the tubule system. There's nothing indicating they're there, but they are there. So what are these dark areas and what's not shown but is in this tubule in the cochlea? And the answer is hair cells. Hair cells. You have hair cells here in what is called the cresta ampullaries. I like to call it the cupula. It goes by various names, but this is the actual point where we're going to generate an impulse from fluid movement in here. Same way on the wall of the utricle. This is something called the otolith organ. Again, hair cells there as well. And then there are hair cells all the way along the length of this tubule. Okay, so what's the deal with these hair cells? Well, every hair cell... is connected to sensory neurons that run to the brain. And if they're in the cochlea, they run to the auditory centers of the brain to help you interpret sound. Inside the cell body of the hair cells are little vesicles filled with neurotransmitter. And when the hair cells get stimulated, they release the neurotransmitter onto the sensory neurons. and that stimulates action potentials in the sensory neurons, and they carry an impulse to the brain. Okay, well, what causes the release of neurotransmitter? Remember when I said that the endolymph, and the hair cells are always in the endolymph, when the endolymph has high potassium ion concentration? Well, when you take the hair cells, and you take these little structures called stereocilia, and you bend them, If you bend them one way, like you bend them this way, the little potassium ion channels here have a little hatch on them, and they open. And when they open, potassium rushes in because it's high on the outside, low on the inside, so it follows its diffusion gradient. Potassium rushes in, brings a whole bunch of positive charge in here, which changes the voltage of the membrane, which opens a voltage-gated calcium channel, and calcium goes rushing in and triggers the release of neurotransmitters. neurotransmitter. So bend the stereocilia this direction to the left and potassium ion channels open and potassium rushes in and neurotransmitter is released. Bend the stereocilia the opposite direction, that is to the right, and the little hatch comes down and closes off the potassium ion channels and no positive charge gets in and no neurotransmitter is released. So The hair cells have two positions. The on position, where they release neurotransmitter, impulses go to the brain, and the off position, where no neurotransmitter is released, no impulses get to the brain. So it's on, off, on, off, on, off, depending on how these things are stimulated. When you're talking about the ear, the ear is responding to vibrations of the tympanic membrane. And the vibrations will bend these hair cells first one way, and then the other at whatever the frequency of the sound waves are. So if it's 500 cycles per second, you're bending these stereocilia back and forth 500 times per second. And so you're sending action potentials to the brain with that kind of frequency. So hair cells are the key to stimulating action potentials that communicate to the brain the various things that the inner ear monitor. The most interesting or the most important of which is hearing sound, but the inner ear does more than that. So let's go and take a look at the saccule first. We'll do the easier stuff first. The utricle and the saccule in the vestibule. This whole fat area right here is the vestibule. And so there's perilymph out here and endolymph. in the saccule and the utricle. And you see these dark areas? These dark areas are the otolith organ. Some people or some books call them the macula, but it's better to call them the otolith organ because they are filled with little crystals of calcium carbonate buried in a gelatinous layer. And the gelatinous layer lays over the hair cells. and the hair cells have their stereocilia stuck up into the gelatin. The otoliths, calcium carbonate crystals, add weight to the gel. So if you're looking at the otolith organ in the utricle, and the utricle is monitoring, the utricle monitors horizontal position. It's kind of horizontal here, but in the actual placement in the ear, it's more horizontal. And you go ahead and Tip your head forward like this woman is doing here. Then your horizontal otolith organ is, as she looks down, the orientation of the utricle goes vertical. And now the gravity is pulling on the gel and the gel slides downwards, bending the stereocilia this one direction. That tells the brain the lady is looking down. If she wants to bend her head backwards and look up, then the thing would... tilt the other way and the gel would flow in the opposite direction telling the brain that the head is looking upwards so they the this is the this is the monitoring of head position now if she's sitting in a Ferrari is you know that's at a stop sign and the the driver accelerates that Ferrari zero to 60 in like three seconds then the acceleration is experienced by the fact that in the saccule, I'm sorry, in the utricle, there is all sorts of endolymph in there, okay, and endolymph is in the fluid state, so it would be washing over the top of this gel. If the endolymph is still, the gel is still, but if you accelerate forward like in your Ferrari, What happens is the endolymph is stationary because objects at rest that aren't fixed remain at rest. But the rest of this is fixed to the bony labyrinth. And when you're accelerating forward, you're moving forward, your skull is moving forward, and all this apparatus is moving forward. But the fluid in that sac is stationary. And so what happens is... You're dragging this gel through a stationary fluid and that pulls the gel backwards, bending the hair cells. So you get the sensation of acceleration. And from the utricle, the sensation you get is horizontal. It's more in the horizontal plane. In the saccule, because it's more in the vertical plane, you get a more horizontal plane. you're getting the sensation of when you go up or down in an elevator, the sensation of going upwards or going downwards. The same thing applies. The gel would be pulled one way or the other through the stationary fluid, and that would give you the sensation of vertical or vertical acceleration or deceleration. Okay. So that's what you're getting out of the otolith organ in the vestibule. So we took care of the vestibule, saccule utricle. Now let's go look at the semicircular canals and the semicircular ducts. Each duct has a little swollen area here and in that swollen area there's a structure that looks like this. It has a variety of names. The whole thing is called the crista, the little little gel-like mound here it's called the cupula. the little mound of hair cells tied to all these neurons. This is the creaste ampullaris, or here the ampullary crest. I just sometimes call the whole thing the cupula, but you don't have to know all these names. Just know that this is the gel is called the cupula, and in the cupula there are hair cells with stereocilia. So here's another view of it. This is the gel. These are the hair cells all tied to their sensory neurons. connected I should say. And then this is the swollen area of your semicircular duct. So we're full of endolymph in here and here you see a semicircular duct okay and that would be in the semicircular canal so we're not seeing the canal. So here you see the canal in light blue but the duct is going through there but we're only looking at the fluid in the duct and you can see the fluid comes around and hits this swollen area, this ampulla as it's called, and This is your ampulla here. Okay, now understand that the three semicircular ducts are oriented in the three cardinal planes. And so it's like, you know, you have three cardinal planes to essentially define three-dimensional space. So any orientation of your head is going to be picked up by these semicircular ducts. to varying degrees depending on how you're rotating your head. But the key is this. Let's say this is a duck that's, let's say the person is looking to the right. So they're facing over here, they're looking over here. So if you're looking at this person, you're looking at the side of their head. So their face is facing this direction, the back of their head's over here, and this is essentially in the sagittal plane of this person's head. Now if the person looks down, if they look down, the head rotates downward. As the head rotates, so does the duct. The duct actually rotates. So the duct with the ampulla rotates upwards. The head is looking down, the head is rotating to the right. As the head rotates to the right, this thing turns downward and this part comes upwards. And then here's what happens. The fluid in there does not move. The fluid in there is stationary. Objects at rest remain at rest. So the fluid is at rest. What you're moving is the duct around the fluid. So when you look downward, the duct rotates, pulls the cupula upwards, and the cupula is being pulled through stationary fluid, which bends the cupula backwards. Bends it backwards because you're pulling this gel through stationary fluid as you rotate this way. The stationary fluid is exerting pressure on this side and the cupula bends downward, bending the stereocilia. That tells the brain, because you're sending signals every time you do that because you're opening potassium channels and firing action potentials going to the brain, it's telling the brain we're looking down, the head is rotating downward. And then if you turn around and look upwards, this thing would rotate the other direction and the cupula would be pulled this way. through the stationary fluid and that would bend the little gel gelatinous cupula the opposite direction and you get the the hair cells probably let's just say they were on when you went this way if you go the opposite way they shut off so that the brain gets the signal that you're looking up now and so then you The horizontal one, when you're shaking your head no and you're looking side to side, that would be the horizontal semicircular duct. And if you're trying to touch the side of your head to your shoulder or your ear to your shoulder, side to side, in a frontal plane, the other semicircular duct would take care of that. So the idea of rotational movements in three-dimensional space are monitored by the semicircular ducts and canals. Okay, so what have we done? We've done the semicircular ducts and canals. We've done the vestibule, the saccule, and the utricle. And now we come to the last part, the organ of hearing, the cochlea. All right, so take your cochlea, cut it in half, pull off one side, take a look at it. Okay, so we're looking like from this direction. We're just looking there. And you see... The cochlea is a series of chambers, but really it's one continuous tube that coils upwards. Okay, one continuous tube that coils upward like a turban and very much like a snail shell or a nautilus if you know what a nautilus is. Have you ever seen one of those? Anyway, if you take one section of the tube, what you see are three chambers. Upper chamber, lower chamber, middle chamber. And the tube goes in and comes back around, cut it there, upper chamber, lower chamber, middle chamber. Tube comes around, goes back in, upper chamber, lower chamber, middle chamber. You see that? Same here and same here. So you're looking at the cross section of the tube and the tube is the same all the way up. The only thing that's happening is the tube gets smaller and smaller. That is the diameter of the tube. Okay, so all you have to do is understand one cross section of the cochlea and you're going to understand. hearing. So here's another view of it. Okay. Here is one turn of the cochlea and upper chamber, lower chamber, middle chamber. The upper chamber is called the scala vestibuli. And the lower chamber is called the scala tympani, scala vestibuli, scala tympani. Both the scala vestibuli and scala tympani are filled with perilymph. In the middle, there is the cochlear duct. It is filled with endolymph. So scala vestibuli, scala tympani, cochlear duct, perilymph, perilymph, endolymph. and it's the same all the way up through that tube. Okay, the business end of this whole thing is right here at the bottom of the cochlear duct. It is what we call the organ of Corti, and here it is in a little more detail. The organ of Corti has in there little hair cells. The hair cells have stereocilia. The stereocilia are sticking up into a little flap of gel. It's not a cone now. It's a shelf, so to speak, that they give an unfortunate name to. They call it the tectorial membrane. It's really a flap of gelatin that spirals all the way up with this tubule as a little shelf of gelatin that lays over the stereocilia of the hair cells. The hair cells are supported by other cells around them. These are just supporting cells. And the supporting cells and the whole shebang is resting on the basilar membrane. Okay. This whole thing is the basilar membrane. Now that is your organ of corti. Basilar membrane, hair cells, tectorial membrane. The hair cells are each connected to sensory neurons. All of them are connected to sensory neurons. All the sensory neurons are running out of the middle of the cochlea and head out as the eighth cranial nerve, the vestibular cochlear nerve, heading out to the brain. Okay, here is another more clear view. So here's our cochlea in cross-section. Here is a magnified scala vestibuli, scala tympani, cochlear duct, organ of core T. And beautifully illustrated for you here is all the parts. Basilar membrane, hair cells in purple here, and the tectorial membrane in green, stereocilia embedded in the gel. All right, how does it work? Well, when vibration, well, let's go back. When vibrations hit the tympanic membrane, the tympanic membrane pushes on the malleus, which pushes on the incus, which pushes on the stapes, which pushes on the oval window. The oval window vibrates, pushing on the perilymph inside the inner ear, inside this bony labyrinth. Now, where does that perilymph go next? Well, here, into the scala vestibuli. Where is that? Where is that? Scala vestibuli, scala vestibuli, the upper chamber, upper chamber. Okay, so that's the upper chamber. And it's scala vestibuli all the way up. And then you get to the top. This is the cochlea sort of unwound, okay? And follow that scala vestibuli all the way up, perilin, perilin, perilin. Around we come, around the top of the cochlea and down. And now we're in the scala tympani and we come all the way down to here and that is the round window. So this is the oval window that the stapes pushes on and when it pushes on the fluid in the scala vestibuli, the fluid waves travel some distance up the scala vestibuli. Because this is all enclosed in bone, when you compress the fluid, the compression wave has to go somewhere. So it travels up the cochlea and ends up pushing out on the oval window. So that's where you absorb the wave. So you push in with the stapes and you, the fluid wave travels all the way through the cochlea and comes down the scala tympani and pushes on the round window. So the stapes goes in, round window goes out. Stabies goes in from 500 times per second the round window goes out 500 times per second depending on the sound frequency sound wave frequency Okay, so let's look at this again. Tympanic membrane, malleus, incus, stapes, oval window. Pushing on the scala vestibuli filled with perilymph. The waves travel. up the cochlea, come around, comes down the sclera tympani. Didn't they label it for us? Yes, they did. There it is. And down it comes to the round window. Okay, so that's how the fluid wave travels. But here's the trick. When the fluid waves come in to the cochlea, here's the cochlea unwound, straight out. And you see, if you are talking about high frequency waves, they press on the vestibular membrane very soon, very near the beginning of the cochlea. If you are looking at low frequency waves, here we're getting as low as 20 cycles per second, there you depress the membrane further up in the cochlea. Here's another view. Fluid waves come down the scale of the vestibule, they press on this membrane. This membrane, you may remember, is the vestibular membrane. Remember this one? Remember this one? Right here? That's the vestibular membrane. Don't I have it labeled somewhere? Ah, all these drawings. This is the vestibular membrane right here, and nobody has labeled it. Oh, they have labeled it here. So there it is, the vestibular membrane. So the sound waves come into the scala vestibuli, and at some point, they push on the vestibular membrane, which pushes downward on the fluid of the cochlear duct, that would be endolymph, and the endolymph pushes downward on the basilar membrane. So you see, fluid wave on the vestibular membrane pushing on the cochlear duct, the fluid in there is the endolymph, which pushes on the basilar membrane. And now what's resting on the basilar membrane? What's resting on the basilar membrane are all these cells, and in particular, the hair cells. So when the vestibular membrane presses in on the endolymph, the endolymph presses downward on the basilar membrane, and the basilar membrane drops down. When the basilar membrane drops down, it pulls the hair cells down. But then there's recoil. and then it bounces back up and pushes the stereocilia of the hair cells into the tectorial membrane. And if it's 500 cycles per second, it does that 500 times a second. So that is how sound is registered. High-pitched sounds register very early on in the cochlea, low-pitched sounds very farther up the cochlea, and all the sound frequencies in between somewhere else along the length of the organ of corti. And it's just like keys on a piano. The highest pitch keys are early on, and then they go progressively down through the lower frequencies until you get down to the lowest pitch keys way over here. So there you go. It's like the keys of a piano laid out along the organ of corti. Here's another view. This one shows you the short wavelengths impinging early on. Vestibular membrane pushing down on the endolymph in the cochlear duct, pushes down on the basilar membrane. And if you're talking about low frequency waves, they're further up, pushing on the vestibular membrane there, basilar membrane there. This is the black line is the tectorial membrane. The little vertical lines are your hair cells. That's what they're trying to indicate there. So this is basically your organ of corti running all the way up the cochlea. And again, the cochlear tube has been straightened out. Fluid waves go in on the vestibular, in the scalar vestibuli, pushing on the vestibular membrane at some point. And then the fluid wave continues on down the scalar tympani and then finally pushes out on the round window. Okay, so that's how we register frequency of sound. Here we see... electron micrograph of the organ of Corti. These are your hair cells here. These are your stereocilia. This is your tectorial membrane. It's been separated in little ways. This is gelatinous. Here you see top of a single hair cell. These are the stereocilia. And you see these little connections? I don't know if you can see the little connections there. Those little connections are real. Go back to this drawing. Those little connections are links to the hatches that open and close the potassium channels. It's really quite remarkable. They are pulled open by a little thread when you bend one way and shut when you bend another way. So let's see where did I have that? Yeah you can kind of see there's there and there and a little bit there, there's one there, one there, one there. Gotta have to look for them one there. But as they bend one way they pull the hatch open then the other way the hatch closes. There are better pictures of this, I just don't have it here. So there are about 15, 12, 15, looking at the surface of the organ of Corti there, 12, 15 hair cells. Here's the healthy organ of Corti, all the hair cells lined up, stereocilia very nicely framed. Here's the damaged cochlea, listening to high decibel sound, chronic, unrelenting sound. The ear essentially suffers repetitive stress syndrome, where by literally you just blow out your stereocilia and they don't grow back. So in this range this person is deaf, can't hear anything. People will have certain ranges of their frequencies just gone. They can't hear those frequencies any longer. The inner ear is connected to the vestibular nerve which connects to the vestibule and to the semicircular canals, and then the cochlear nerve, which connects to the cochlea. Together, they join and form the vestibular cochlear nerve, cranial nerve number eight, and then they go and synapse in the pons. Here's your olives, by the way, and they synapse in the brainstem and travel through the thalamus to the auditory cortex, which is in the temporal lobe of the brain. And so that's where we hear. See, am I at the end? Maybe I am. I think I am. So that's our little lecture on the ear. And we have some beautiful temporal bones in lab. And you want to be sure to look at those. They've been dissected. And you can look in there. and you can see the semicircular canals, you can see the tympanic membrane, and I can show you the eustachian tube or at least the location where it should be. It's pretty good. They're beautiful little dissections so be sure to see those. They're little bony temporal bones that we'll have for you. All right, I think I've just about got it covered. All right. Okay. So we'll call it good. And we'll stop right here.

canal or the external acoustic canal or the external acoustic meat is all of those are correct. It's just a channel that is lined with skin and delivers sound waves to the tympanic membrane, which is your eardrum. The tympanic membrane is a very tightly, very thin, very tightly taut collagenous membrane covered over with some epithelial tissue.

but it is like a drum very much in the sense that it's very tightly drawn and very sensitive to air vibrations and sound waves are essentially compression waves of the air and they come in a series and the higher the frequency of those waves the higher pitch of sound is registered and the lower the frequency of sound I should say the lower the frequency of air compressions hitting the eardrum, the lower the sound. So high vibrations will give you a high pitch, low vibrations will give you a low pitch, and that's all interpreted here in what is called the cochlea. But I'm getting ahead of myself. So we're talking about the external ear all the way to the sound collector of the collagenous ear, external ear, the auricle. Sound waves travel down the external auditory meatus or canal and then the external ear ends here at the tympanic membrane.

Okay, now the second part of the ear is the middle ear and the middle ear consists of this air pocket within the petrous portion of the temporal bone. See, we're in bone here. It's all spongy bone, you see. And there's a hollow space in there called the tympanic cavity or the middle ear cavity. See, is it labeled here anywhere?

No. Okay, well that's what it is. It's a cavity and there are three bones in that cavity.

One is attached to the tympanic membrane and it is called the malleus, which means hammer, and it articulates, actually forms a synovial joint with the second bone here called the incus, which means the anvil. And the incus then articulates right here with the third bone called the stapes, which literally means the stirrup, because it looks exactly like a horse's saddle stirrup. You see the flat plate right here, and that little flat plate fits in a little oval window, as it's called, and there's a membrane covering that window.

And when sound waves come and hit the tympanic membrane, the tympanic membrane vibrates. the malleus is attached to it, so the malleus vibrates, the malleus pushes on the incus, and the incus pushes on the stapes. The stapes then pushes in the oval window, and on the other side of the oval window there is this fluid-filled system that receives the compression of the stapes, and that sets up fluid waves in this structure here called the cochlea, which is the organ of hearing. Okay, so There's other stuff here too, but I'll explain that more later.

So that's your middle ear. Okay, so the middle ear cavity, tympanic cavity, three ear bones called the ear ossicles, which transmit the vibrations of the eardrum to the olal window, which then begins this third part of the ear called the bony labyrinth. All of this is the bony, that's all bone.

That golden color is all bone, but it's buried in this other spongy bone. Now, One thing I want to point out about the middle ear here is that it opens down to the upper throat, to what is called the nasopharynx. That's the upper reaches of your throat. And this is a channel that allows the oral cavity, this is the back of the oral cavity, the nasopharynx, to communicate with this air pocket, the tympanic cavity. And this is the...

thing that is responsible for your ears popping as you go up and down in elevation, because that represents... air either escaping through this tubule or getting in through this tubule. If the air pressure increases outside, like if you're going down in elevation, air will be trying to get into this space because the pressure in this space changes. I shouldn't say it doesn't change, but there's an imbalance that happens between the pressure inside the tympanic cavity and the pressure outside in the outside of the body that is sitting on the outside of this eardrum.

And so what happens is if you have a differential of pressure, let's say you have very high pressure here, very low pressure here on the outside, then the air is trying to expand in here and it's starting to push on the tympanic membrane. And if it can't escape, you could actually push so hard. with the air pressure that you could rupture the eardrum. So you need an escape valve and that's what this little tube is called the eustachian tube or the auditory tube and that allows air to escape. Alternatively, if the pressure is getting too high out here, for example, if you've been up at 15,000 feet and now you take a train and you're coming down or you're in a plane and you're coming down to land, the air pressure, you know, 15,000-20,000 feet is far less than it is at sea level.

So if you've got air pressure in here equalized at 15,000 feet, it's low air pressure. So if you're at 15,000 feet, it's equilibrated with this eustachian tube allowing air to move whichever way it needs to move. So you're at 15,000 foot air pressure, same out here, but then if you're in a plane, that's the most dramatic example, You're in a plane and the plane now comes down to sea level where the pressure is maybe hundreds of millimeters mercury greater. And so now you have very low pressure in here, much higher pressure outside, pushing in on the eardrum and it can tear the eardrum.

I actually have allergies. And so what that causes is a swelling of the mucous membranes of the throat here. And this closes off and it doesn't allow free movement of air in and out.

And so. When I used to fly when I was younger and had allergies, I don't have them quite so badly anymore, but when I was younger, I did. And coming down for a landing was excruciating.

It actually felt like somebody was driving an ice pick into my ear because the pressure was so great as I came down and it was pushing so hard, pushing inward on this tympanic membrane that it was, I'm sure it was doing, it could have done some damage. So I'm not sure. you have this, when everything's healthy and you don't have swollen membranes closing this off, you have the ability for air to get in or get out to adjust the air pressure in the tympanic cavity with the air pressure outside.

And so that's what the Eustachian tube is for. And if you have problems with allergies and stuff, and you do have the same thing with when you're flying, what pilots do who have this problem is that they take all sorts of antihistamines to reduce the swelling. They sometimes take drops down back down their nose and they dribble it back against their back throat.

It reduces the swelling and that allows them to then fly up and down and allow air to move through this passage more easily. So that's your eustachian tube leading to the middle ear cavity. When children get earaches, they have essentially what is called otitis media, which is an inflammation of the middle ear chamber and the this bacteria get in here, they get a sore throat or something and the bacteria migrate up in.

And it's more common in children because the eustachian tube is very short and it's more horizontal, it's not as vertical. So the bacteria easily get access into the middle ear, set up an infection, very painful for the child and when they're like one or two years old they don't know what's going on. It's very upsetting to parents. And what happens is it fills with blood and fluid and pus.

And it can be excruciating. It can even happen to my child when he was five. It filled with so much inflammation and pus and stuff. It caused an erosion of the tympanic membrane and it burst. And when that happened, pus and blood came out of his ear.

And I was absolutely terrified. I didn't know what had happened back in those days. learned later that, well, when it did happen, he was all of a sudden able to go to sleep.

He had been up all night long. And when it finally ruptured, the pain subsided, all the pressure subsided, and he was able to go to sleep. So I took him to the doctor and they said, yep, he's got a ruptured eardrum, but he'll be fine.

It'll heal up. And he may not hear quite as well with that ear as he does with the other ear, but he'll be all right. And he's done quite well since then.

Okay. So that's your middle ear cavity, your tympanic cavity. Then we come to the inner ear, which is this elaborate structure, semicircular canals, vestibule here, cochlear duct here.

Semicircular canals, they monitor head rotational movements, rotational movements in the three cardinal planes. And then the vestibule is monitoring head position relative to gravity, also vertical and horizontal accelerations. And then the cochlea, this is where we hear.

So let's go through the structures of the inner ear. Also This whole structure is called the bony labyrinth. And let's see, anything on? This is just a more detailed look at the tympanic, I'm sorry, at the tympanic cavity.

Notice you got muscles that do attach to the stapes. And what these muscles do is when your ear is being subjected to very high decibel sound, these muscles try to restrain the movement. these ossicles so they don't punch that oval window too severely.

So there's some dampening effects of these muscles. Okay, so there is your bony labyrinth and inside we see the membranous labyrinth. This is a system of membranous tubules. Both the bony labyrinth and the membranous labyrinth, bony in the light blue, membranous in the dark blue, both are filled with fluid.

Okay, and we know or the fluids are not quite the same. They're clear fluids, very much like lymph, but they are different chemical makeup. Turns out that the fluid inside the tubule system that you see here, all of that is called endolymph, and it has a very high potassium ion concentration. Outside the membranous labyrinth, In the lighter blue areas, we have perilymph, which is a much lower potassium ion concentration.

There are other things in there, but for us, the potassium ion is of interest, and you'll see why in a little bit. Okay, so you have now the bony semicircular canals, okay, and there's three of them, the anterior, the posterior, the lateral. So these are the... Semicircular canals.

You see this one's horizontal and this was kind of in a frontal and this is in a, let's see, it's really hard. They're not exactly sagittal and horizontal and frontal. The whole thing is situated at an angle, but they are three planes at right angles to one another. And they're monitoring head movements and rotational movements in particular. OK, so that those are your semicircular canals.

Inside are your semicircular ducts. Endolymph in the ducts, perilymph outside. Now in the middle here we have the vestibule and in the vestibule are two membrane-filled sacs.

One is called the utricle, one is called the saccule. The saccule is more vertical, the utricle is more horizontal. And what these two things do is measure or monitor horizontal and vertical accelerations.

monitors the horizontal accelerations, the saccule monitors the vertical accelerations, and they also monitor head position relative to gravity. Okay, so that's what we get in the utricle. And then here's the organ of hearing, the cochlea, which literally means the snail, because of the shape of this structure is very much reminiscent of a snail's shell. So this is the cochlea.

So semicircular canals and ducts. the vestibule with the utricle and the saccule, a fluid-filled sacs, and then the cochlea, the snail-shaped bony cochlea with the cochlear duct running up the tubule inside, and that is filled with endolymph. So perilymph out here, endolymph there. Okay, all of this apparatus is connected to the eighth cranial nerve, the vestibular cochlear nerve. Okay, now Notice here in these little swollen areas at the ends of the cochlear ducts and here in the wall of the sacs, the utricle and the saccule, and you're not seeing it, but they're in there.

in the tubule system. There's nothing indicating they're there, but they are there. So what are these dark areas and what's not shown but is in this tubule in the cochlea?

And the answer is hair cells. Hair cells. You have hair cells here in what is called the cresta ampullaries.

I like to call it the cupula. It goes by various names, but this is the actual point where we're going to generate an impulse from fluid movement in here. Same way on the wall of the utricle. This is something called the otolith organ.

Again, hair cells there as well. And then there are hair cells all the way along the length of this tubule. Okay, so what's the deal with these hair cells? Well, every hair cell...

is connected to sensory neurons that run to the brain. And if they're in the cochlea, they run to the auditory centers of the brain to help you interpret sound. Inside the cell body of the hair cells are little vesicles filled with neurotransmitter.

And when the hair cells get stimulated, they release the neurotransmitter onto the sensory neurons. and that stimulates action potentials in the sensory neurons, and they carry an impulse to the brain. Okay, well, what causes the release of neurotransmitter?

Remember when I said that the endolymph, and the hair cells are always in the endolymph, when the endolymph has high potassium ion concentration? Well, when you take the hair cells, and you take these little structures called stereocilia, and you bend them, If you bend them one way, like you bend them this way, the little potassium ion channels here have a little hatch on them, and they open. And when they open, potassium rushes in because it's high on the outside, low on the inside, so it follows its diffusion gradient. Potassium rushes in, brings a whole bunch of positive charge in here, which changes the voltage of the membrane, which opens a voltage-gated calcium channel, and calcium goes rushing in and triggers the release of neurotransmitters. neurotransmitter.

So bend the stereocilia this direction to the left and potassium ion channels open and potassium rushes in and neurotransmitter is released. Bend the stereocilia the opposite direction, that is to the right, and the little hatch comes down and closes off the potassium ion channels and no positive charge gets in and no neurotransmitter is released. So The hair cells have two positions. The on position, where they release neurotransmitter, impulses go to the brain, and the off position, where no neurotransmitter is released, no impulses get to the brain.

So it's on, off, on, off, on, off, depending on how these things are stimulated. When you're talking about the ear, the ear is responding to vibrations of the tympanic membrane. And the vibrations will bend these hair cells first one way, and then the other at whatever the frequency of the sound waves are. So if it's 500 cycles per second, you're bending these stereocilia back and forth 500 times per second.

And so you're sending action potentials to the brain with that kind of frequency. So hair cells are the key to stimulating action potentials that communicate to the brain the various things that the inner ear monitor. The most interesting or the most important of which is hearing sound, but the inner ear does more than that.

So let's go and take a look at the saccule first. We'll do the easier stuff first. The utricle and the saccule in the vestibule.

This whole fat area right here is the vestibule. And so there's perilymph out here and endolymph. in the saccule and the utricle. And you see these dark areas?

These dark areas are the otolith organ. Some people or some books call them the macula, but it's better to call them the otolith organ because they are filled with little crystals of calcium carbonate buried in a gelatinous layer. And the gelatinous layer lays over the hair cells. and the hair cells have their stereocilia stuck up into the gelatin.

The otoliths, calcium carbonate crystals, add weight to the gel. So if you're looking at the otolith organ in the utricle, and the utricle is monitoring, the utricle monitors horizontal position. It's kind of horizontal here, but in the actual placement in the ear, it's more horizontal. And you go ahead and Tip your head forward like this woman is doing here.

Then your horizontal otolith organ is, as she looks down, the orientation of the utricle goes vertical. And now the gravity is pulling on the gel and the gel slides downwards, bending the stereocilia this one direction. That tells the brain the lady is looking down. If she wants to bend her head backwards and look up, then the thing would...

tilt the other way and the gel would flow in the opposite direction telling the brain that the head is looking upwards so they the this is the this is the monitoring of head position now if she's sitting in a Ferrari is you know that's at a stop sign and the the driver accelerates that Ferrari zero to 60 in like three seconds then the acceleration is experienced by the fact that in the saccule, I'm sorry, in the utricle, there is all sorts of endolymph in there, okay, and endolymph is in the fluid state, so it would be washing over the top of this gel. If the endolymph is still, the gel is still, but if you accelerate forward like in your Ferrari, What happens is the endolymph is stationary because objects at rest that aren't fixed remain at rest. But the rest of this is fixed to the bony labyrinth. And when you're accelerating forward, you're moving forward, your skull is moving forward, and all this apparatus is moving forward.

But the fluid in that sac is stationary. And so what happens is... You're dragging this gel through a stationary fluid and that pulls the gel backwards, bending the hair cells.

So you get the sensation of acceleration. And from the utricle, the sensation you get is horizontal. It's more in the horizontal plane.

In the saccule, because it's more in the vertical plane, you get a more horizontal plane. you're getting the sensation of when you go up or down in an elevator, the sensation of going upwards or going downwards. The same thing applies.

The gel would be pulled one way or the other through the stationary fluid, and that would give you the sensation of vertical or vertical acceleration or deceleration. Okay. So that's what you're getting out of the otolith organ in the vestibule. So we took care of the vestibule, saccule utricle.

Now let's go look at the semicircular canals and the semicircular ducts. Each duct has a little swollen area here and in that swollen area there's a structure that looks like this. It has a variety of names.

The whole thing is called the crista, the little little gel-like mound here it's called the cupula. the little mound of hair cells tied to all these neurons. This is the creaste ampullaris, or here the ampullary crest. I just sometimes call the whole thing the cupula, but you don't have to know all these names.

Just know that this is the gel is called the cupula, and in the cupula there are hair cells with stereocilia. So here's another view of it. This is the gel.

These are the hair cells all tied to their sensory neurons. connected I should say. And then this is the swollen area of your semicircular duct. So we're full of endolymph in here and here you see a semicircular duct okay and that would be in the semicircular canal so we're not seeing the canal. So here you see the canal in light blue but the duct is going through there but we're only looking at the fluid in the duct and you can see the fluid comes around and hits this swollen area, this ampulla as it's called, and This is your ampulla here.

Okay, now understand that the three semicircular ducts are oriented in the three cardinal planes. And so it's like, you know, you have three cardinal planes to essentially define three-dimensional space. So any orientation of your head is going to be picked up by these semicircular ducts. to varying degrees depending on how you're rotating your head.

But the key is this. Let's say this is a duck that's, let's say the person is looking to the right. So they're facing over here, they're looking over here. So if you're looking at this person, you're looking at the side of their head. So their face is facing this direction, the back of their head's over here, and this is essentially in the sagittal plane of this person's head.

Now if the person looks down, if they look down, the head rotates downward. As the head rotates, so does the duct. The duct actually rotates. So the duct with the ampulla rotates upwards.

The head is looking down, the head is rotating to the right. As the head rotates to the right, this thing turns downward and this part comes upwards. And then here's what happens. The fluid in there does not move.

The fluid in there is stationary. Objects at rest remain at rest. So the fluid is at rest.

What you're moving is the duct around the fluid. So when you look downward, the duct rotates, pulls the cupula upwards, and the cupula is being pulled through stationary fluid, which bends the cupula backwards. Bends it backwards because you're pulling this gel through stationary fluid as you rotate this way.

The stationary fluid is exerting pressure on this side and the cupula bends downward, bending the stereocilia. That tells the brain, because you're sending signals every time you do that because you're opening potassium channels and firing action potentials going to the brain, it's telling the brain we're looking down, the head is rotating downward. And then if you turn around and look upwards, this thing would rotate the other direction and the cupula would be pulled this way.

through the stationary fluid and that would bend the little gel gelatinous cupula the opposite direction and you get the the hair cells probably let's just say they were on when you went this way if you go the opposite way they shut off so that the brain gets the signal that you're looking up now and so then you The horizontal one, when you're shaking your head no and you're looking side to side, that would be the horizontal semicircular duct. And if you're trying to touch the side of your head to your shoulder or your ear to your shoulder, side to side, in a frontal plane, the other semicircular duct would take care of that. So the idea of rotational movements in three-dimensional space are monitored by the semicircular ducts and canals.

Okay, so what have we done? We've done the semicircular ducts and canals. We've done the vestibule, the saccule, and the utricle. And now we come to the last part, the organ of hearing, the cochlea. All right, so take your cochlea, cut it in half, pull off one side, take a look at it.

Okay, so we're looking like from this direction. We're just looking there. And you see...

The cochlea is a series of chambers, but really it's one continuous tube that coils upwards. Okay, one continuous tube that coils upward like a turban and very much like a snail shell or a nautilus if you know what a nautilus is. Have you ever seen one of those?

Anyway, if you take one section of the tube, what you see are three chambers. Upper chamber, lower chamber, middle chamber. And the tube goes in and comes back around, cut it there, upper chamber, lower chamber, middle chamber. Tube comes around, goes back in, upper chamber, lower chamber, middle chamber.

You see that? Same here and same here. So you're looking at the cross section of the tube and the tube is the same all the way up.

The only thing that's happening is the tube gets smaller and smaller. That is the diameter of the tube. Okay, so all you have to do is understand one cross section of the cochlea and you're going to understand. hearing. So here's another view of it.

Okay. Here is one turn of the cochlea and upper chamber, lower chamber, middle chamber. The upper chamber is called the scala vestibuli. And the lower chamber is called the scala tympani, scala vestibuli, scala tympani.

Both the scala vestibuli and scala tympani are filled with perilymph. In the middle, there is the cochlear duct. It is filled with endolymph.

So scala vestibuli, scala tympani, cochlear duct, perilymph, perilymph, endolymph. and it's the same all the way up through that tube. Okay, the business end of this whole thing is right here at the bottom of the cochlear duct. It is what we call the organ of Corti, and here it is in a little more detail.

The organ of Corti has in there little hair cells. The hair cells have stereocilia. The stereocilia are sticking up into a little flap of gel.

It's not a cone now. It's a shelf, so to speak, that they give an unfortunate name to. They call it the tectorial membrane.

It's really a flap of gelatin that spirals all the way up with this tubule as a little shelf of gelatin that lays over the stereocilia of the hair cells. The hair cells are supported by other cells around them. These are just supporting cells. And the supporting cells and the whole shebang is resting on the basilar membrane.

Okay. This whole thing is the basilar membrane. Now that is your organ of corti. Basilar membrane, hair cells, tectorial membrane.

The hair cells are each connected to sensory neurons. All of them are connected to sensory neurons. All the sensory neurons are running out of the middle of the cochlea and head out as the eighth cranial nerve, the vestibular cochlear nerve, heading out to the brain.

Okay, here is another more clear view. So here's our cochlea in cross-section. Here is a magnified scala vestibuli, scala tympani, cochlear duct, organ of core T. And beautifully illustrated for you here is all the parts.

Basilar membrane, hair cells in purple here, and the tectorial membrane in green, stereocilia embedded in the gel. All right, how does it work? Well, when vibration, well, let's go back. When vibrations hit the tympanic membrane, the tympanic membrane pushes on the malleus, which pushes on the incus, which pushes on the stapes, which pushes on the oval window.

The oval window vibrates, pushing on the perilymph inside the inner ear, inside this bony labyrinth. Now, where does that perilymph go next? Well, here, into the scala vestibuli. Where is that?

Where is that? Scala vestibuli, scala vestibuli, the upper chamber, upper chamber. Okay, so that's the upper chamber. And it's scala vestibuli all the way up.

And then you get to the top. This is the cochlea sort of unwound, okay? And follow that scala vestibuli all the way up, perilin, perilin, perilin.

Around we come, around the top of the cochlea and down. And now we're in the scala tympani and we come all the way down to here and that is the round window. So this is the oval window that the stapes pushes on and when it pushes on the fluid in the scala vestibuli, the fluid waves travel some distance up the scala vestibuli.

Because this is all enclosed in bone, when you compress the fluid, the compression wave has to go somewhere. So it travels up the cochlea and ends up pushing out on the oval window. So that's where you absorb the wave. So you push in with the stapes and you, the fluid wave travels all the way through the cochlea and comes down the scala tympani and pushes on the round window. So the stapes goes in, round window goes out.

Stabies goes in from 500 times per second the round window goes out 500 times per second depending on the sound frequency sound wave frequency Okay, so let's look at this again. Tympanic membrane, malleus, incus, stapes, oval window. Pushing on the scala vestibuli filled with perilymph. The waves travel.

up the cochlea, come around, comes down the sclera tympani. Didn't they label it for us? Yes, they did.

There it is. And down it comes to the round window. Okay, so that's how the fluid wave travels. But here's the trick. When the fluid waves come in to the cochlea, here's the cochlea unwound, straight out.

And you see, if you are talking about high frequency waves, they press on the vestibular membrane very soon, very near the beginning of the cochlea. If you are looking at low frequency waves, here we're getting as low as 20 cycles per second, there you depress the membrane further up in the cochlea. Here's another view.

Fluid waves come down the scale of the vestibule, they press on this membrane. This membrane, you may remember, is the vestibular membrane. Remember this one? Remember this one? Right here?

That's the vestibular membrane. Don't I have it labeled somewhere? Ah, all these drawings.

This is the vestibular membrane right here, and nobody has labeled it. Oh, they have labeled it here. So there it is, the vestibular membrane.

So the sound waves come into the scala vestibuli, and at some point, they push on the vestibular membrane, which pushes downward on the fluid of the cochlear duct, that would be endolymph, and the endolymph pushes downward on the basilar membrane. So you see, fluid wave on the vestibular membrane pushing on the cochlear duct, the fluid in there is the endolymph, which pushes on the basilar membrane. And now what's resting on the basilar membrane?

What's resting on the basilar membrane are all these cells, and in particular, the hair cells. So when the vestibular membrane presses in on the endolymph, the endolymph presses downward on the basilar membrane, and the basilar membrane drops down. When the basilar membrane drops down, it pulls the hair cells down.

But then there's recoil. and then it bounces back up and pushes the stereocilia of the hair cells into the tectorial membrane. And if it's 500 cycles per second, it does that 500 times a second. So that is how sound is registered.

High-pitched sounds register very early on in the cochlea, low-pitched sounds very farther up the cochlea, and all the sound frequencies in between somewhere else along the length of the organ of corti. And it's just like keys on a piano. The highest pitch keys are early on, and then they go progressively down through the lower frequencies until you get down to the lowest pitch keys way over here.

So there you go. It's like the keys of a piano laid out along the organ of corti. Here's another view. This one shows you the short wavelengths impinging early on. Vestibular membrane pushing down on the endolymph in the cochlear duct, pushes down on the basilar membrane.

And if you're talking about low frequency waves, they're further up, pushing on the vestibular membrane there, basilar membrane there. This is the black line is the tectorial membrane. The little vertical lines are your hair cells. That's what they're trying to indicate there. So this is basically your organ of corti running all the way up the cochlea.

And again, the cochlear tube has been straightened out. Fluid waves go in on the vestibular, in the scalar vestibuli, pushing on the vestibular membrane at some point. And then the fluid wave continues on down the scalar tympani and then finally pushes out on the round window.

Okay, so that's how we register frequency of sound. Here we see... electron micrograph of the organ of Corti.

These are your hair cells here. These are your stereocilia. This is your tectorial membrane. It's been separated in little ways. This is gelatinous.

Here you see top of a single hair cell. These are the stereocilia. And you see these little connections?

I don't know if you can see the little connections there. Those little connections are real. Go back to this drawing. Those little connections are links to the hatches that open and close the potassium channels.

It's really quite remarkable. They are pulled open by a little thread when you bend one way and shut when you bend another way. So let's see where did I have that? Yeah you can kind of see there's there and there and a little bit there, there's one there, one there, one there.

Gotta have to look for them one there. But as they bend one way they pull the hatch open then the other way the hatch closes. There are better pictures of this, I just don't have it here.

So there are about 15, 12, 15, looking at the surface of the organ of Corti there, 12, 15 hair cells. Here's the healthy organ of Corti, all the hair cells lined up, stereocilia very nicely framed. Here's the damaged cochlea, listening to high decibel sound, chronic, unrelenting sound. The ear essentially suffers repetitive stress syndrome, where by literally you just blow out your stereocilia and they don't grow back.

So in this range this person is deaf, can't hear anything. People will have certain ranges of their frequencies just gone. They can't hear those frequencies any longer. The inner ear is connected to the vestibular nerve which connects to the vestibule and to the semicircular canals, and then the cochlear nerve, which connects to the cochlea. Together, they join and form the vestibular cochlear nerve, cranial nerve number eight, and then they go and synapse in the pons.

Here's your olives, by the way, and they synapse in the brainstem and travel through the thalamus to the auditory cortex, which is in the temporal lobe of the brain. And so that's where we hear. See, am I at the end?

Maybe I am. I think I am. So that's our little lecture on the ear. And we have some beautiful temporal bones in lab.

And you want to be sure to look at those. They've been dissected. And you can look in there.

and you can see the semicircular canals, you can see the tympanic membrane, and I can show you the eustachian tube or at least the location where it should be. It's pretty good. They're beautiful little dissections so be sure to see those.

They're little bony temporal bones that we'll have for you. All right, I think I've just about got it covered. All right.

Okay. So we'll call it good. And we'll stop right here.

Transcript for:Understanding Ear Anatomy and Function

Transcript for:
Understanding Ear Anatomy and Function