All About Hearing- Episode 3
Have you ever wondered about how our hearing works? Did you ever consider all the different parts of the ear? In this episode, we discuss the different parts of the auditory pathway. We discuss the outer ear, the ear drum and ossicles, and the fascinating structure of the cochlea. Hearing starts with our ears and truly happens in our brains, but we’ll discuss that more in future episodes.
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Welcome to All About Audiology, a podcast that’s, well, all about Audiology! I’m your host, Dr. Lilach Saperstein. Last episode, we spoke about what Audiology is, and how varied the field is, and how many different things audiologists do. And today we’re going to talk all about how hearing works.
Anytime I learn about the intricacies of how our body works I get so fascinated and so inspired and then also I’m in awe of just how much there is to understand. So even when I learned about how enzymes work or how our kidneys function and then I realize there are people who study that for decades- just that one element- meanwhile all millions of things are happening within our bodies. All the time! -it’s amazing!
When I started learning about the ear it not only gave me this incredible awe and inspiration about how our hearing works but just overall about how incredible our biology is. And how wonderful it is that when you study it and start to see how wonderful and complex one part is, it even gives you this excitement about wanting to learn more about other parts of the body and other areas of science. So if you share this kind of inspiration and awe, I think you’ll enjoy this episode where we’re taking a dive into the anatomy and physiology of our ear and of our auditory system.
Let’s start with a quick introduction to sound which in itself is so interesting. Sound waves are essentially movement of particles in the air in an organized way- vibrations- and there’s definitely this sense of magic when you think about how it’s literally movement of pieces of matter in the air and that these vibrations we’re able to produce with our mouths to make speech, with instruments for music, or by dropping something on the ground and it makes the sound because you’ve displaced -you physically moved the pieces in the air. When we’re manipulating the air around us and we’re making the sounds, that movement is picked up by the receiving organ in our ear. It takes my breath away!
The ear has three major parts: the outer ear, the middle ear, and inner ear. And the outside of the ear, the piece we just call our ear, that’s called the pinna, and it funnels sound into the ear canal opening. At the end of this canal, we have eardrum; the eardrum is made up of three very fine and delicate layers of skin. The ear drum vibrates in response to the different incoming waves and on the other side of the ear drum we have this middle ear space. This is a place where we have the three tiniest bones of the human body. The first one, the malleus, or the hammer, is connected to the eardrum so any movement that comes in and makes movement of the eardrum also is moving the malleus on the other side of it and that’s connected to the incus and the stapes, the other two little bones. The malleus, incus, and stapes, also known as the hammer, the anvil, and the stirrup, based on their shapes. (So it’s like their Latin name and then just the English equivalent of those names.) These are called “the ossicles” which also is a very cute little word that just means “little bones”- ossicle – like popsicles [laughs]. Okay.
These tiny bones are suspended in this middle ear space by ligaments and muscles. This is a space, this is an air-filled cavity, so the three little bones conduct the sound and the vibration and the movement of the eardrum go on into the cochlea in the inner ear. But let’s just pause here for a second and talk about this middle ear space. There is a connection from the middle ear space to the back of the throat or the back of the nose and that place is called the nasopharynx. Right so, why are there ear-nose-throat doctors? Because actually the ear nose and throat are all connected back in this space. The nasopharynx connects up to the middle ear space of the ear through this small tube called the “eustachian tube.” The eustachian tube is what helps regulate the air pressure in the middle ear space to match your environment.
Think of when you’re in a high-altitude, like on a plane or sometimes if you’re driving in the mountains, and your altitude keeps changing. You might feel your ears pop. What that popping is, what’s actually happening is that the eustachian tube is opening- the muscles around the eustachian tube allow it to open- then the air pressure is regulated between the middle ear space and the rest of your environment. Which is why it can sometimes be extremely painful if you’re also congested because then there’s all this mucus that’s blocking the passage and if eustachian tube isn’t functioning properly for any reason that can be very painful and if it’s so so bad, then it can also sometimes bust the ear drum because the air pressure is so high, it has to release somehow and it will go through the eardrum instead of through eustachian tube.
This is one of the most common sites of inflammation for children who have ear infections or middle ear effusion (which is a whole episode for us in the future) and anytime there’s a buildup of fluid or mucus in this middle ear space and it inhibits the movement of the ossicles, it can lead to all sorts of issues, medical, but also with the hearing.
So let’s come back to the pathway here. We’ve gone from the outer ear to ear canal. We’ve talked about the three fabulous little bone and then we’re going to come down to the star of our auditory pathway- the cochlea! The cochlea is a snail-shaped bone bony structure and it’s actually part of the temporal bone of the skull on each side of the head. Inside it has these fluid-filled compartments and what happens is the sound- or the movement- the movement of particles- the waves- the sound that’s coming in that was conducted in by the ossicles, gets transformed into movement of this fluid by the footplate of the stapes. The third little bone there, the stapes, or the Stirrup, it has this end piece, a little foot plate and that footplate fits in exactly into a little tiny oval window.
If you think of a paddle or an oar making ripples in the water that’s kind of just a very simple way of seeing how you could deduce the frequency of the rowing from the ripple. You could be watching the water without ever seeing a boat and know how many oars there were and how fast they were rowing, you know, just by simply seeing the ripples. So that’s like a very small way of thinking about how this works. When you think about the incredible complexity of sound and how it’s all decoded from the movement of this fluid, I mean, it’s truly astonishing.
So we have the spiral and all along the spiral, you can kind of think of it like a spiral staircase. Right, so, it’s got like a banister on one side and that’s like the outer parts of the shell and then on the inside, you’ve got like the inside of the spiral staircase. You might have like a pole or something. All along that are these tiny tiny hair cells that are the nerve ending. All the movement of this fluid in the cochlea moves the hair cells and that then triggers the cellular response and nerve impulses. Then all those tens of thousands of messages are being sent up the auditory pathway up to the nerve.
You need to dive into cellular chemistry and gated-ion channels and the way that our neurons are organized. There’s inner hair cells and outer hair cells and all these rows of nerve endings that are organized like an orchestra or a supercomputer! I mean, if you want to go down the rabbit hole of this way that we hear, that our cochleas work, there’s so much wisdom.
When I was in graduate school, I would pipe up during class with an analogy because, for me, that’s one of the biggest ways that I learn and I process information, by having a kind of visual or spatial map of it in my mind. I love examples. I love mnemonics. This is something that I’m into a lot.
Let’s talk a little bit more about the cochlea and I’ll tell you the one that occurred to me during our anatomy course. This imagination of an ice cream cone swirl. Imagine that you have this little glass cup and you’re putting it under the ice cream soft serve and then you get that swirl so if that’s the snail shape, like the cochlea. Then if you were to take this then and take a straw, and put the straw right through the center, from the tip of the soft serve swirl all the way down, you can kind of imagine how the straw is going through all the different stages. We talked already of a spiral staircase and this is another way that I sometimes think of it. If you had the straw going through the center, then you’d be hitting the inner curve along all the different, the two and a half curves, of your ice cream cone or of your cochlea, and that is a way to visualize where the auditory nerve goes into. It has a name, this center space of the swirl is called the “modiolus” and within the modiolus is the auditory nerve fibers, the eight nerve, that we talked about in the last episode. So I hope this is another way to visualize what’s going on in the cochlea.
And I’m always so fascinated to see how everything that we just talked about in a few minutes is just the tip of the foundation of all of this. There’s this saying in Audiology; “We hear with our brains.” Of course our ears and the whole pathway that I’ve been describing until now, are collecting the sounds and then conducting the sound, and then transducing the physical wave, particle movement, into this fluid medium in the cochlea, and taking all of that and making it into an electrochemical signal that then moves into neurochemistry and electricity in our brains. Sound is then mapped and sorted and processed within the auditory brainstem and all the way up to the auditory cortex.
It’s wildly enchanting and there’s still so much that we don’t even understand about it all but when you kind of look at the overview- the outer ear the middle ear the inner ear- and then up to the brainstem and beyond, up to the cortex, and you see that there’s all this pathway. There’s so many different things that need to go right, so many different places where if there’s an issue, or any kind of blockage or broken piece in the chain, we can understand why hearing loss is so complex and we’ll talk about that in our next episode.
To illustrate this complexity that we’re talking about, let’s just look at one concept. The concept that we have “auditory streams,” so this means that let’s say you’re in a room, you’re listening to a lecture so you listen to the speaker you can also be aware of the humming of the air conditioning and maybe there’s also traffic noise outside and also there’s these two people whispering the whole time, like, two rows back and you actually have all these different streams of auditory input and we have the ability- our auditory system and our cognitive abilities that allow us to filter the unwanted sounds. You know, we’re not paying attention to the humming of air conditioning; it’s just going like once we realized it’s air conditioning, we kind of just filter that out and not only that we’re sorting out the different sounds. We can identify them; we can localize them. We can do all these different things with the many different sounds in our environment and we can even flip back and forth- our attention- between the different streams. Maybe their conversation is more interesting than the lecture.
So if there’s all these people at a cocktail party having small conversations of two, three, four people and you’re having a conversation with one person, but all around you all these other conversations are taking place and you’re able to dial in on the one person’s speech that you’re interested in. You can try this one time when you’re maybe on a bus and there’s all kinds of different conversations going on around you. See if you can focus your attention on one and then focus your attention on another.
In research and in some auditory processing testing, there’s this soundtrack that we use for background noise and it’s meant to simulate this kind of taxing of our auditory system and it’s called “multi-talker babble.” If you ever get a chance to hear this recording- basically like the background hum of a cocktail party but when you put that on in the background and then try to focus on whatever task is being given to you, to pay attention to words, repeat back words, or anything like that, you see just how incredible we are at filtering out sounds. This is just kind of one piece of the puzzle of what our auditory systems do, demonstrating the complexity of it all.
This entire beautifully organized complex processing is the tiniest beginning step of all of our higher order processing. That we need to understand speech and make sense of the sounds that we hear, and to learn. So this is why, as we’ll talk more later on, getting a hearing aid or a cochlear implant, is really a drop in the bucket in our entire process of hearing. It’s a prerequisite that we need to be able to collect the sounds, funnel the sounds, conduct the sounds, and all of that to get the input it, but when we would recognize that this is really the beginning of the process of hearing. Our cognitive level and our language exposure and our neuroplasticity. These are all big big ideas that play a huge role that can’t be discounted.
It’s a big part of why I’m enjoying making this podcast and why I love talking about audiology so much is because it’s not just about our ears, not just about children with ear infections and how that affects their hearing, or a simple thing of ‘oh, if a child is born deaf, then we give them a cochlear implant and their problems are solved.’ There’s so much complexity around all these things and I’m so glad you’re here to explore them all with me.
Thank you so much for joining me for this episode of All About Audiology where we talked all about hearing. And thank you very much for being a listener and supporter of the podcast. I hope you learned something today and I invite you to let me know what that was. What’s something that you didn’t know about hearing before or something that you’d like to know more about now?
In the next episode, we’ll be diving into different kinds of hearing loss, what’s meant by the different levels of hearing loss. We’ll talk about deafness and we’ll also touch on the different causes of hearing loss. So be sure to tune in for the next episode “All About Hearing Loss” and I’ll be very happy to read your comments, questions, or feedback. Send me an email, a DM on Instagram, or a review, and it might be in the next mini-episode “All About You”- your comments and your experience with Audiology. I’m Dr. Lilach Saperstein and you’ve been listening to “All About Audiology.”