- In this video, I'll be talking about two special senses, hearing and equilibrium. Hearing, of course, you're all familiar with. Equilibrium is the state of maintaining balance. The reason why we are talking about these together is because both of these occur in the ear. So hearing and equilibrium, it's what your ears do. First, some general anatomy. The ear can be broken down into three parts. You have the external ear, the part that you actually see visibly, including the auricle, or pinna, which is what we think of when you think of what an ear is. The pinna funnels sound into the external acoustic meatus where it passes to the middle ear. And the middle ear is where you have the tympanic membrane and your auditory ossicles which take that sound and pass it into the internal ear where you have your cochlea and your semicircular canals. I want to talk very briefly about what sound is. If you've ever heard of sound waves, sound waves are what your ear detects. And sound waves are basically vibrations in air. You may think of air as empty space, but it's actually filled with lots and lots of particles, lots of molecules. And when something vibrates-- like here we're looking at a tuning fork. When you have a rapid vibration, that vibration pushes the molecules into waves so that they have the molecules all clumping together as the tuning fork vibrates outward and then a space where there's not as many, and then they're clumped together again, not as many, clumped together again. So you have these changes in air pressure that form a wave. So where the molecules are all clumped together, you have a rise in air pressure and then a drop and then a rise, a drop, rise, drop, rise. This rise from the top of the crest to the bottom of the trough is referred to as the amplitude. Changes in amplitude is what we perceive as loudness. So a low amplitude sound wave is going to be quieter than a high amplitude sound wave. The frequency of the wave, so basically how close together these crests are, is perceived as changes in pitch. So high frequency waves coming from a rapid vibration where the crests are close together, this is perceived by our brain as a high-pitched noise. Low frequency waves, where the waves are further apart, is perceived as a low-pitched tone. Those sound waves that we were just talking about, those periodic increases in air pressure due to vibration of some other object, they pass through the external acoustic meatus to the middle ear where those beats in air pressure hit the tympanic membrane. And as they hit the tympanic membrane, they cause the tympanic membrane to vibrate, just like beating on a drum with a stick. In fact, the tympanic membrane is more commonly known as the ear drum. The tympanic membrane is attached to the auditory ossicles. So when the tympanic membrane starts to vibrate, it pulls on the malleus, the first auditory ossicle, and sets up a vibration in the malleus. The malleus then beats on the incus-- oops-- the anvil of the inner ear, transmitting that vibration to the incus. The incus pulls on the stapes, pulling the stapes back and forth. One other anatomical feature of the middle ear that I haven't mentioned yet is the auditory tube. You may have heard of it as the Eustachian tube, same thing. The auditory tube connects the middle ear with the nasal cavity. And it opens up during swallowing and yawning, which keeps the pressure in the middle ear equivalent to atmospheric pressure. This is important, because if you have an increase in pressure in the middle ear, the tympanic membrane isn't going to vibrate as much, the auditory ossicles aren't going to vibrate as much and you're going to have difficulty hearing. This is why sometimes kids, if they have a lot of sinus infections, they may have a delay in acquiring speech because their auditory tube is closed off, there's elevated pressure in their middle ear and they can't hear as well. This is also why, if you have a cold, sometimes things will sound a little bit fuzzy. Because if your sinuses are stuffed up, if your nasal cavity is stuffed up, the pressure increases in your middle ear and dampens out sound detection. Now, back to the transmission of sound, the stapes pulls against a membrane called the oval window. The oval window is the connection between the middle ear and the inner ear. So as the stapes vibrates against the oval window, it sets up a vibration in the fluid in the inner ear. Now let's take a closer look at that. Here's your stapes and the oval window. So the middle ear is back here. In the inner ear, you have two distinct structures. They are these membrane-lined openings within the skull. You have the cochlea and you have the semicircular canals. These are both in the temporal bone. The cochlea is critical for hearing. The semicircular canals are critical for equilibrium. Both of these structures are filled with endolymph. Endolymph is a viscous fluid contained within the membranes that line these bony structures. And endolymph is absolutely critical for both equilibrium and hearing. As the endolymph sloshes around, that contributes to the detection of equilibrium in the semicircular canals. Also, as endolymph vibrates in response to the vibration of the stapes at the oval window, that vibration of the endolymph contributes to hearing in the cochlea. Let's talk about the cochlea first. The cochlea is the snail shell-shaped structure in the inner ear. Snail shell-shaped. Snail shell-shaped. It's actually rather hard to say quickly. If you take a cross-section through a single turn of the cochlea, this is a cartoon version of what you'd see. You can see the bony structure. You can see the membranes around it. You have the scala vestibuli and the scala tympani, both fluid-filled cavities. But in the middle ear, this is really where hearing happens. And this is the scala media containing the endolymph. It also contains the spiral organ, or the organ of Corti, which is where sound detection actually happens. The basilar membrane forms the floor of the scala media. The vestibular membrane forms the roof of the scala media. And the tectorial membrane sits on top of the organ of Corti, or the spiral organ. Now, when you have sound waves passing into the ear, as we mentioned, they vibrate the tympanic membrane, which sets up a relay vibration through the auditory ossicles that ends with the stapes pulling against the oval window. And that stapes pulling back and forth against the oval windows sets up waves of pressure in the scala vestibuli. Now, these waves of pressure, if they are in the right wavelength, the wavelength that we can actually hear, vibrate the basilar membrane. Remember, the basilar membrane forms basically the floor of the scala media. So you've got the scala media is this tube running around along here. Sound waves of a frequency that we can actually detect vibrate the basilar membrane. Sounds with frequencies that are too high or too low for us to actually detect, they pass through and they don't have a huge effect on the basilar membrane. To understand what happens when that basilar membrane is vibrated by sound waves that have been transmitted through the middle ear and through the oval window, we need to take a closer look at the spiral organ. So this is like a zoomed-in, again, cartoonish look at the spiral organ. You can see the basilar membrane here forming the floor. You can see some supporting cells sitting on top of that. But the ones that I really want you focusing on are these hair cells. They're called hair cells because they have long processes called stereocilia. It's a bit of a misnomer. They're more like microvilli than like cilia. So think of them as these long microvilli. And the stereocilia are embedded in the tectorial membrane. It's like they're stuck in the tectorial membrane, right? Now, these hair cells, these are actually the sensory receptors of the ear. They're not actually hair. It's just the appearance of stereocilia. What happens when the basilar membrane vibrates is that it pulls the hair cells up and down against the tectorial membrane, but the top of the hair cells are embedded in the tectorial membrane. So imagine, if you hold your hand up, grab onto the fingers of your hand and now try to move your hand up and down but your fingers are anchored. So what happens is that your hand stretches a bit. The skin deforms a bit. This vibration, since the stereocilia are held in place, when the basilar membrane moves, it pulls cation channels open, causing a depolarization of the hair cell. So what we're looking at is we're looking at mechanoreceptors. We have mechanoreceptors in the stereocilia that are just physically pulled open when the basilar membrane vibrates. So step-by-step, vibration of a basilar membrane pulls the hair cells down and up and down and up. The stereocilia of the hair cells are embedded in the tectorial membrane. They can't move, So the cell gets stretched. And that triggers mechanoreceptors, opening cation channels. Remember, cation channels are channels that allow positively charged ions to pass through. So when those cation channels are pulled open, you get depolarization of the hair cells. And that depolarization is passed through your sensory nerve fibers attached to the hair cells up through the vestibulocochlear nerve to the brain. So that's how the brain gets that auditory signal. The inner hair cells, it's actually the inner hair cells that send auditory signals to the brain. The outer hair cells affect the basilar membrane, kind of adjusting responsiveness in our hair cells, tuning what you detect so that, for example, a loud noise, if it's constant, you become less sensitive to it. Some of you may have heard of a condition called tinnitus. Tinnitus is the ringing of the ears. It's when you hear a more or less steady ringing in your ears that has no relationship to any sound that's actually happening outside of your brain. Tinnitus can have a variety of causes. It's usually more a symptom of some underlying cause, but it can be caused by damage to the hair cells of the inner ear. If these hair cells get bent or broken, the membrane gets leaky and you get random depolarization, which means that you have these random action potentials sent to your brain and your brain interprets that as the ringing noise. OK, let's move away from hearing to talk about equilibrium. Equilibrium, remember, is detected in the semicircular canals. And you have two types of structures that contribute to equilibrium in the semicircular canals. First you have the maculae. And the maculae detect the tilt of your head. Basically they detect changes in your head's position in relation to gravity. So if you would like to experience the function of your maculae, turn your head to the side and notice how that doesn't confuse you if your maculae are working normally. The crista ampullaris detect rotational motion. So if you're spinning, the crista ampullaris become active. You have two maculae in each inner ear. So you have two maculae in your right semicircular canals and two maculae in your left semicircular canals. Macula is the singular version. Maculae is plural. These maculae, they're oriented perpendicular to each other. One is nearly horizontal and one is nearly vertical. The maculae are a membrane with supporting cells and hair cells, again, not actual hair, just cells with these long, extended stereocilia. And the stereocilia are embedded in a membrane called the otolithic membrane. The otolithic membrane has otoliths which are little crystals of calcium carbonate that sit on top of that membrane, giving it a little bit of weight. So when you tilt your head, the otolithic membrane slides a bit to one side, which bends the stereocilia and modifies neurotransmitter release by the hair cells. So it's that slipping of the otlithic membrane that causes the mechanical activation of the hair cells. With the crista ampullaris, you're detecting rotation. And this is an actual histological view of the crista ampullaris. You have the crista there and then the cupula sitting on top of that. This structure is within a fluid-filled cavity, so there's endolymph surrounding this structure in the living inner ear. When you rotate the endolymph swirls, which bends the cupula and there are hair cells, you can almost vaguely see their stereocilia, hair cells embedded in the cupula. So if you're just standing still, the cupula is standing upright. It's not deflected at all. You're looking at the cupula here. You can see the hair cells there. But when you start rotating, when you start spinning, the endolymph sloshes around in the semicircular canal, bends the cupula, which triggers that mechanical excitation or activation of the hair cells. As you slow down, endolymph starts to move in the opposite direction, inhibiting the hair cells. Between this detection of rotational motion of the crista ampullaris and the detection of head tilt that the maculae do, a large part of our sense of equilibrium and our sense of balance comes from that. You also get visual cues to equilibrium and balance. If you've ever noticed that it's more difficult to, say, stand on one leg with your eyes closed, that's because you need those visual cues to help out with balance. But the crista ampullaris and the maculae play enormous parts together with vision. And oftentimes, when you have vertigo, a sense of dizziness, it has to do with what's going on in your inner ear, what's going on in the semicircular canals. There's a condition called benign paroxysmal positional vertigo that many people develop. And that can happen when the otoliths and your maculae, some of them become detached and they are sending random signals that your brain doesn't really know how to interpret. Your brain thinks that it means you're spinning even though you're not. All right, after studying this video, you should be able to do a pretty good, pretty thorough job of describing the structure of the ear. You should be able to explain what sound is and how that sound wave is passed from the air to the inner ear. You should also be able to talk about stimulation of the cochlea to transmit sounds to the brain. Just be able to talk a little bit about equilibrium in the semicircular canals, including the functions of the maculae and the crista ampullaris.