CDIS 4037 Outer Ear (KT Done)


Now that we have a decent understanding of what sound consists of and and how sounds are made and perhaps propagated from one place to another let’s take a closer look at the perceptual mechanism, the sensory mechanism I should say that will be responsible for number one collecting the sound energy, processing the sound energy, and then eventually converting the sound energy into nerve impulses that may then be sent through nerves, through the central nervous system, eventually to stimulate the auditory cortex. With respect to the ear let’s start at the outside and work our way in, okay? So this is a diagram by Frank Netter the great an animist and illustrator who whose images will be seen throughout this presentation and what Netter shows us basically is not only the outside of the head here but the ear canal and then everything on the inside with the bone essentially stripped away. So, what we’ll do is we’ll start on the outside the or the outer ear a work our way inward and try to keep in mind along the way some of the processing that this pathway will be conducting on the sound energy as the sound energy is radiated from the outside and carried a conducted into the head and into the auditory mechanism. Remember, we talked about filtering last time and we’re going to see how this system performs that kind of filtering with with the sound from the environment. So, for example, with this outer ear, we could see how there is a very small tube right here formed by the ear canal. And what you want to be thinking about is what’s actually being set into vibration when a sound is present in the environment. So my vocal folds move, I shape the sound coming out with my articulators and I actually send a sound wave across the room if we were in a classroom or to the microphone that I have strapped to my head right now and the movement of those air particles is going to ultimately make it into the movement of the air particles ultimately will be seen or processed as a pressure wave inside this small tube. Now the tube that is your outer ear behaves just like the tube that is formed when you have a water bottle filled part of the way up with water and you blow across the top of it and when somebody does that the sound that comes out is fairly tone like it’ll it’ll have a pitch but the idea is the more air there is in that bottle the lower the pitch that will come out of the bottle. Just that I as I had mentioned last time with respect to the difference between a piccolo and a flute. A flute holds more air than a piccolo. When you blow across the mouthpiece of a flute you set that air into motion and it is that vibration that is then transmitted out of the flute and across the room because a piccolo holds less air when you blow across the mouthpiece of a piccolo that air gets set into vibration at a faster rate and even though it’s propagated across the room at the speed of sound just like it would be for a flute. The frequency emphasis is higher in pitch, higher in frequency than it would be for a flute. Again because the plug of air, the mass of air that is being set into motion is lower in the piccolo. Well here’s an even smaller piccolo if you will the outer ear canal and the air that is held in that space is of a very small mass. There’s a very small volume of air and it also happens to be a fairly stiff plug of air mainly because it’s got a wall on one side of the cavity. But you can just remember it or think about it as a very small plug of air. So when a sound wave hits that structure and and sets into motion the air particles in your outer ear canal recognize that those air particles are going to favor vibration at higher frequencies simply because of their physical makeup simply because of their physical composition. We also want to recognize that the outer ear consists of a bony portion and a cartilaginous portion and we’ll talk more about that in just a little while but for now let’s just focus right here on the outer ear space which consists of this pinna, the external canal, and the outer surface the lateral surface of the eardrum.The middle ears primary function is to serve as an amplifier for sound energy and that is because if you look at the configuration of the middle ear and its location you realize that it forms a something of a bridge between the outer ear and the inner ear. And it turns out that the inner ear is filled with fluid and because the inner ear is filled with fluid it’s going to reflect much of the sound energy that would otherwise be going into it. So when we have a sound propagated or sent through the atmosphere that sound energy is reflected off the surface of a fluid. Just the same way that you as a person jumping into water there’s some force that that water exerts trying to keep you out of it, right? The the interface the surface of the water can actually be quite an impediment to your getting into the water, especially if you offer that serve that surface of the water a large surface area of your body. So, if you jump into water feet first and hold your arms down at your sides you can glide into the water pretty easily, but if you try to get into that water belly first and face first you find that the water offers a bit more resistance to to your body and it holds you out pretty well compared to how well it would hold you out if you focus to all of your mass on to a very small surface area like the balls of your feet or your fingertips if you were diving in. So we’ll talk more about that in just a few minutes but the idea is that the middle ear has to focus all the sound energy that was available out here on to a very small surface area in order to drive that sound energy into the inner ear. The other thing about this middle ear space is that these bones are held in place, these are three middle ear bones that we’ll look at in much more detail in a little while and those bones are held in place by these little ligaments, little pieces of tissue that are very stiff and that suspend the middle ear bones from the walls of the middle ear space. Because those bones are very low-mass and because these ligaments are very high stiffness we would expect that that middle ear would have a fairly high resonance frequency which it does. What we’ll see with the inner ear when we get there is that it is kind of where the rubber meets the road, as they used to say in the in the hearing mechanism, because this is where the mechanical energy of a sound, that is to say acoustic energy, is converted into nerve energy. The brain, the central nervous system are able to process are able to work with neural energy but the sound as it exists in the outside world as it exists in the outer ear and as it exists in the middle ear is a mechanical form of energy not an electric or bioelectric or neural form of energy and so that conversion occurs here in the inner ear space. And it is that conversion that makes the organism, makes the human or other animal the receiver of the sound energy because it is in here in this inner ear that the physical state of the organism is modified by the presence of the sound energy. So we had a vibrating source, we had an elastic medium to transmit the sound, and here’s our receiver, the inner ear, the mechanism that whose physical state is altered by the presence of the energy and which converts that mechanical sound energy into a nerve signal that the the central nervous system and that the brain can receive and process. It also turns out that the physical characteristics of this inner ear are very there are somewhat unique in the sense that the mass and the stiffness throughout the length of this organ vary. They are different from place to place really it’s the stiffness that differs from place to place. But that means that different frequencies of energy will actually resonate at different physical locations in the cochlea. And that’s cochlea by the way is the inner ear for those of those of you uninitiated. And this idea that frequencies are processed in different physical places in the cochlea in the inner ear will be a very important idea when we come back and look more closely at the process of hearing itself, but just recognize that these same and stiffness considerations that I’ve been harping on now for the last little bit here are going to be very important when it comes to analyzing the way that the inner ear works. When we talk about audiology and the measurement of hearing we find it convenient to divide the hearing system, the peripheral hearing system that is to say the hearing system that’s kind of on the outside part of the body, into both a conductive system and a sensory neural system. So the conductive system is the part that’s further toward the outside of the body and the sensory neural system is more toward the inside of the body I guess if you want to look at it in a very simplistic way. And so, what we’re going to do is ultimately you’re going to see how there are actually ways that you can measure the whole hearing pathway or other ways that you can bypass this outer and middle ear and stimulate more directly the inner ear in order to tell if there’s a problem with the outer and middle ears during hearing testing, but the line is drawn right here where the stapes footplate which is one of the middle ear bones makes contact with the fluids of the inner ear or the cochlea. If I skip back to here you’d see that same point right there. There’s the last bone in the middle ear chain and you see how it’s going to fit into a hole in the skull and stimulate directly the inner ear mechanism. So we got conductive system on the outside, the inside is called sensory neural kind of an abbreviation for sensory which is the cochlea, and neural which is the nerve and everything else on the way up to the brain. Now, it turns out that there are two pathways that sound energy can take when the sound energy is present in the environment and it encounters a a human listener or mammalian listener or whatever kind of listener you are. The air conduction pathway includes the entire system and really we think of hearing as hearing by air conduction, that is to say there’s an airborne sound, some kind of sound wave out here that displaces air particles in the outer ear that pushes and pulls on the eardrum moving these three bones and as these three bones are moved this last bone the stapes bone, in something of a piston like fashion it’s not exactly like a piston but you can sort of think of it that way, moves in and out of this hole in your head that contains these very interesting sensory organs. And so, the air conduction pathway is the pathway that is stimulated again by an airborne sound wave that initiates inner ear activity as the sound wave is sent through the middle ear space. The bone conduction pathway is arrived at by stimulating directly the inner ear and this is accomplished by stimulating the area, the bone that surrounds the inner ear. So you can think of the cochlea as a container deep inside your skull. It’s about the size of your fingertip, your index finger tip and it’s like I said a hollowed-out portion of the skull. And because it is a hollowed-out portion of the skull you could even shake the skull and in doing so shake the fluids that are inside this hollowed out area much like you would be you could shake a bottle which is a hard-shelled container if you will and the fluid inside the bottle when you shake the bottle the fluid inside moves around. The same kind of thing happens inside here. Shaking the skull and in turn moving the fluids of the cochlea can be accomplished with tuning forks and can be accomplished with a so-called bone conduction vibrator on an audiometer. Again, if we were sitting in a classroom and if we have a study group I’ll bring some tuning forks so that I could illustrate for you how easy it is to stimulate the skull by placing a vibrating sound source directly on it. Recognize that when you do that for the most part you bypass all of this stuff out here, you bypass the conductive system and stimulate directly the inner ear. Now, that’s important because you know suppose I have a kid who comes into the clinic with a raging ear infection in the middle ear space so that they actually have a fairly moderate hearing loss, I want to know whether their inner ear is working properly and so to do that I have to when I do my test bypass this trouble spot and stimulate directly the inner ear and that’s the whole idea behind bone conduction testing to allow the examiner a window if you will to view the function of the inner ear regardless of how well the rest of the system is working. I mean if I put a finger in my ear I give myself a hearing loss but my inner ear is still working fine and that’s the idea of being able to test both of these different sections sort of apart from one another so that we can determine where specifically a person a patient’s problem is arising from. So that’s the air conduction and bone conduction pathways. Now, this outer ear, as I indicated before, consists of the visible parts of the year the pinna that’s the you know kind of floppy things sticking off of the side of your head. Some people have muscle control over the pinna that’s great for a joke at a party or something like that but not much else. Not because moving your pinna is has no value, I mean if you look at a dog or a cat or a bat or a lot of other bidders you know that their movements of their pinnas are very important for their ability to localize and identify sound. The same thing would be true for us we’re just not very good at moving our pinnas and they’re not shaped in a way that would allow us to use them for localization as well as a dog or a cat does or a bat. We use them for some other stuff though and in addition to just like holding sunglasses on your head, pinna can actually provide some help to localization just not as much as with some other critters. External auditory meatus that’s just your ear canal. The lateral surface of the eardrum that’s the outer skin layer of the eardrum, and as indicated earlier that ear canal consists of a cartilaginous portion and a bony portion. Here it is again this area here the lateral one-third of the canal is the cartilaginous portion and on netters diagrams this is what cartilage looks like. The bony portion occupies the medial two-thirds of the canal. So the ear canal is about an inch long. About two-thirds of that on the inside part has a bony infrastructure and the outer one-third consists of or has a cartilaginous infrastructure. Earwax is produced in this outer one-third of the canal by the some glands in the skin here it’s not produced on the inside part. Earwax can typically migrate out of the canal on its own unless you push it in too far with a q-tip or something in which case it will eventually sit in there and harden and form perhaps even a blockage or an occlusion that can affect a person’s hearing. Not all pinnas look the same. Sometimes individuals are born with pinnas that and ear canals that never really even opened properly. Other times there may be a pinta that looks a little unusual but it leads to a normal ear canal and everything on the inside is normal it’s just that it looks a little strange on the outside. Alright. What about the what about the ear canal? Well here are some of the dimensions let me just point a few things out. By horizontal diameter we mean from left to right and I can’t really picture that here because I don’t have that dimension here but imagine if this picture was rotated 90 degrees so you were looking straight down at the eardrum you would see that the diameter was about six millimeters which is about a little bit less than a quarter of an inch across. The vertical diameter is about a third of an inch all together and that’s this dimension here. So it’s a little bit taller, it’s an oval-shaped canal that’s a little bit taller than it is wide. Here are some of the other things that I’ve already mentioned. Ear wax or cerumen really is something that you need to leave alone. It’s good for your ear canal for a variety of reasons, it’s a nice moisturizer, it’s a nice insect repellent, I know that doesn’t really sound too nice but pretty much any audiologist who’s looked in ears on a consistent basis has seen at one time or another insect legs coming out of a pile of earwax and since there’s no insect in the ear canal we assume that that bug whatever it had on its mind got a little discouraged when it lost a leg or two in that pile of earwax. Again, I don’t mean to be grotesque, recognize that keeping insects out of your ears as a pretty a pretty valuable element of that cerumen. We find that the ears of small infants are not quite as osseous, in other words the osseous portion here remains cartilaginous up through the first couple years of life and as time goes on the canal gets bonier and stiffer. The ear canal itself is pretty malleable that is to say its shape can be changed pretty dramatically especially in this cartilaginous portion which can change shape actually enough so that individuals wearing hearing aids notice that when they chew food the hearing aids whistle or even work their way out of the ear canal. So, it depending on a person’s jaw and how how normal the temporomandibular joint is this cartilaginous portion can really actually undergo a pretty substantial change in shape. Here’s a larger picture of it again here’s the bony or the osseous portion, here’s the cartilaginous portion, again the pinna would be right out here and the outer ear then consists of everything from the pinna through the ear canal to this lateral surface of the eardrum. So what about the eardrum? Well it kind of is this doorway except hopefully it doesn’t open, but it does help transmit sound energy from the outer ear into the middle ear. And what we’ll see is that the eardrum actually passes that sound energy passes that vibratory energy through the bones of the middle ear into the inner ear space and the primary function of the eardrum and middle ear system is to overcome what we will see as the loss of energy that of sound vibration encounters as it travels from the air-filled medium of the atmosphere into the fluid-filled medium of the inner ear. Here’s a photograph of an eardrum, a pretty good looking one I’ll say. You can actually see through it. So there’s a there’s a membrane here covering all this stuff. You can kind of see how the one of the ossicles here, this one turns out to be the malleus or the hammer, actually kind of protrudes at these two locations out into the eardrum and especially down here this is a very firm attachment between the eardrum and the malleus it’s called the umbow and we see a few other structures in here as well. This turns out to be the second middle ear bone the incus. This is actually an opening into the cochlea called the round window more on all of these things in just a little while. But, recognize that the normal looking eardrum is translucent that is to say you can see that it’s there but you can see through it. I think you’ll find just about everything in the previous image labeled right here so feel free to kind of do that back and forth in order to see what’s what. Don’t worry about this anterior mallear fold but you will need to worry about the incus, you will need to worry about the pars flaccida, the lateral process of the malleus the pars tensa, the manubrium of the malleus, and the umbo. The count of light is just kind of a land mark. It’s just a reflection of the light from your otiscope looking into the canal but I’ve highlighted for you the things that you need to worry about don’t worry about the posterior or the anterior mallear folds at this time. The eardrum is angled obliquely. Oblique just means that it’s not either parallel to the ground or perpendicular to the ground but rather it’s somewhere in between 55 degrees if we drew a line that was parallel with the ground here and that’s again on average, but recognize that the that angle of the eardrum changes as one goes through life. It starts off much more parallel to the ground and ends up kind of lifting itself up off the floor of the ear canal as we get older. The eardrum is not composed of the same tissue throughout its surface area. We see that there are actually two parts the pars tensa and pars flaccida both of which were mentioned right there. So you see the pars tensa here on the inferior section of the drum and the pars flaccida on the superior section of the drum. And the pars tensa contains all of these different tissue types in its surface area. So, that means this area here consists of three different tissue types three different layers of tissue. An outer outer layer of epidermis and this epidermis is the same skin that same type of skin that lines the ear canal itself. Then there’s this mucosal layer on the inside on the medial surface which is consistent with is the same tissue type that lines the middle ear space. And in the pars tensa there are some other fiber types of other fibers that are on the inside of these two layers forming sort of three different layers of tissue. The pars flaccida only contains these two layers the epidermis and the mucosal layer without all of this stuff in the middle. What that means is that oops the pars tensa is much stiffer or gee tense then the pars flaccida which is much floppier or flaccid. So, owing to the composition of these tissue types and the sort of the way they’re laid out on the basal on the tympanic membrane it is the pars tensa that contributes by far the most to the transmission of sound through the eardrum and into the ossicular chain or that chain of bones that forms the middle ear system. Again, the healthy tympanic membrane or TM is translucent. You can actually tell which ear you’re looking in just by observing the orientation of the malleus. So in this year the malleus is at one o’clock we’re looking at a right ear. Another way that I like to remember it is I am just imagine this is somebody’s nose so their nose would be over here and they would be looking that direction which means that this must be their right ear. In the left ear this would actually be flipped over to the other side sort of at eleven o’clock which would mean their nose was over here and there they’d have an eyeball right there I guess and they’d be looking in that direction so this must in that case be there left ear. Now, as I indicated before, the structures in the outer here do actually change their orientation to one another the canal shape changes. Primarily what happens is that as an individual ages and as that outer ear matures the bony port the sum of the cartilaginous portions become bony and therefore stiffer which will change the resonance frequency of that outer ear system if everything is getting stiffer then that must mean that the resonance frequency is getting higher. We also know that it’s easier to visualize the eardrum during otoscopy by pulling the pinna upward in order to kind of straighten out that ear canal and allow you to stick a light in there to be able to see the tympanic membrane and some of those middle ear structures. For those of you that are interested in looking in ears, if you want, you can stop by and I can show you how to do that how to do otoscopy at least to give you a little bit of an idea of what you might be looking at. I could also I guess bring a nota scope to a study group. But, the main idea here, and this is something that you know already, is that a cartilaginous structure can be tugged on, it can be pulled, it can actually have its shape changed you know and what happens when you let go of that thing? Well it bounces back to its original shape so it obviously it has stiffness and elasticity and we’re going to use that idea to make it easier to actually observe physically some of the structures that are contained inside the ear canal or beyond that your canal through the ear drum. Here’s a coronal section, I’m sorry a transverse section, through the skull. So here’s the person’s nose, here’s their temporal bone, the mastoid process of the temporal bone is the area that actually houses the structures that we’ve just been talking, about the ear canal is right there and everything runs deep into the skull, this mastoid bone and the temporal bone actually is a plug that runs kind of toward the center of the of the head. And what we’ll look at in the next lecture are the structures that are found inside this area, inside that middle ear space that make up what’s known as the the tympanum and that facilitate again the flow of sound energy from the outside through the eardrum into the cochlea. So the next lecture will be on the structures of the middle ear space. Again, we’ll be looking in here. Three bones, a couple of ligaments and tendons, this eustachian tube right here, and then a lot of other important structures that exist in the area immediately surrounding this space.

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