Auditory structure - part 1

Voiceover: In this video, we're gonna talk about our sense of sound. Our sense of sound is also known as audition. In order for us to hear anything, two things have to happen. First, there has to be some sort of stimulus. In the case of sound, that stimulus is something known as a sound wave - so, a pressurized sound wave. That's the first thing that needs to be present in order for us to hear anything. The second thing, aside from the stimulus, is some sort of receptor that's sensitive to the stimulus. In the case of audition, that receptor is something known as a hair cell, and the hair cell is a specialized receptor that's found within the cochlea. We'll discuss this in further detail later in the video. Let's take a look at these two things. What exactly is a pressurized sound wave? Let's look at an example. Whenever you clap your hands, you have learned from experience that people, when they clap their hands, it makes a sound, and it makes a very distinct sound. Let's imagine that these two lines right here are your hands. When you clap your hands, the lines move towards each other, so your hands are moving towards each other. They're moving towards each other fairly quickly. In between your hands are a whole bunch of little air molecules, which I'm drawing. These air molecules, which I'm drawing right now, let's imagine that they're these little purple dots. So, in between your hands are a whole bunch of these air molecules. They're just floating around doing their thing. Then all of a sudden, the hands are moving towards each other, and all of a sudden, this space that these air molecules occupy gets a lot smaller. A little bit later in time, as the hands are moving towards each other - so here we are just drawing the hands almost about to touch. What happened was all these air molecules that are just floating around, they had all this space.. Now all of a sudden, they're really compressed, so they're really, really close together, and they're super-compressed. They're very compacted now. You can imagine that as your hands are even closer together, that the air molecules get even more compacted. Basically, what is effectively going on is the air molecules here are getting pressurized. As you bring your hands together, you're actually adding all the molecules up, and it creates this pressure. This area of pressure actually tries to escape. It tries to escape and it kinda goes this way. It tries to escape out wherever it can. As it's escaping, it creates these areas of high and low pressure. That's what I'm representing here by these lines. These areas of high and low pressure are known as sound waves. We can have different types of sound waves. We can have sound waves that are really, really close together, or really far away from each other. If we draw this graphically, it might look something like this. Basically, what I'm drawing here is, up here would be an area of high pressure. Over here would be an area of low pressure. Basically, there are just areas of high and low pressure. How close these peaks are together is the frequency. If I clap my hands even faster together, or if there's something else that's a higher frequency, a higher-pitched sound, the sound waves would be closer to one another, and it would look something like this. Depending on the frequency of the sound wave, it's perceived to be a different noise. Let's imagine that this sound wave right here is F1, and that this one over here is F2. Sound waves of lower frequency actually travel further. This actually happens in the ear, so these lower frequency sound waves actually travel further, and they actually penetrate deeper into the cochlea, which is a structure that we'll talk about in a little bit. If you look at these two different sound waves, they both have different frequencies, but you might have noticed - let's imagine that this frequency, F1, is generated by hands clapping and F2 is generated by somebody talking. You can actually listen to both somebody talking and clapping their hands at the same time. What that would look like, if we were to add these two frequencies together, would be something kinda weird. It might look like this. It's not very uniform. If you were to add the two frequencies together, you'd get this really weird jumbled frequency, which we'll label F3. Your ear has a very difficult task now, because it has to actually break this frequency, F3, down into the two simpler frequencies, F1 and F2. Your ear is actually able to do that, and it's able to do that because the sound waves actually travel different lengths along the cochlea. Now that we've talked about what a pressurized sound wave is, let's look at the hair cells. In general, let's look at the anatomy of the ear. Here we have a diagram of the ear. Like I mentioned before, there are sound waves that will be coming into the ear. Imagine that I clap my hands. Sound waves are gonna travel through the ear, and they're gonna go towards your head. The very first thing that they hit will be this outer visible part of the ear. When you look at someone's year, this is what you see. This outer visible part of the ear is something known as the pinna. These sound waves get funneled by the pinna, down into this smaller structure known as an auditory canal. This is also known as an external auditory meatus. I'll write that down here, so external auditory meatus. These sound waves travel down the external auditory meatus, and the next thing that they hit is the eardrum, or tympanic membrane. So, the next thing that they hit is the eardrum. What the eardrum does is it actually starts to vibrate. As this pressurized sound wave hits the eardrum, the eardrum starts vibrating back and forth. When it's vibrating back and forth, it actually causes these little bones - there are three little bones here, one, two and three - it causes these three little bones to vibrate. The first bone is known as the malleus. The second bone is known as the incus, and the third little bone over here is known as the stapes. Let's just recap real quick. The sound waves come in, get funneled by the pinna into the external auditory meatus, otherwise known as the auditory canal, then hit the eardrum, otherwise known as the tympanic membrane, and the eardrum starts to vibrate back and forth. This vibration causes three little bones, known as the malleus, incus and stapes to vibrate back and forth accordingly. The next thing that happens is the stapes is attached to this oval window over here. It's known as the elliptical window, which I'm underlining here. It's also known as the oval window. This oval window starts to vibrate back and forth as well. The next thing that happens is there's actually fluid, so this structure that the oval window is attached to is known as the cochlea. This round structure right here is known as the cochlea. Inside the cochlea is a bunch of fluid. As the oval window gets pushed inside and outside of the cochlea by the stapes, it actually pushes the fluid. It causes the fluid to be pushed this way, and causes the fluid to go all the way around the cochlea. It keeps going all the way around the cochlea, until it reaches the tip of the cochlea. When it reaches the tip of the cochlea, what does it do? The only thing it can do is go back. Now the fluid is gonna have to go back. Let's just follow this green line over here. The fluid moves back towards where it came, but it actually doesn't go back to the oval window. It actually goes to this other window known as the circular, or round, window. Let me just fix that, so it goes to this circular or round window. It causes the round window to get pushed out. This basically keeps happening, so the fluid moves all the way to the tip of the cochlea, all the way back out, and back and forth, and back and forth, until the energy of this sound wave - eventually the fluid stops moving - all that energy is dissipated. Meanwhile, hair cells inside the cochlea are being pushed back and forth, and that transmits an electrical impulse via this auditory nerve to the brain. The reason that the fluid doesn't move back to the oval window when it goes to the very tip of the cochlea is because in between, in the very middle of the cochlea, is a membrane. Let me use a different color. There's actually a membrane. I'm gonna use this black line to demarcate the membrane that runs along the length of the cochlea. This membrane is something known as the organ of corti. Let me just write that down here - organ of corti. This organ of corti is actually composed of two different things. It's composed of something known as the basilar membrane, and another membrane known as the tectorial membrane. tectorial membrane. One final thing that I just want to touch upon is a general classification of the different parts of the ear. This outermost part of the ear, from the pinna including the external auditory meatus, so all the way to the tympanic membrane, including the tympanic membrane or eardrum, is known as the outer, or external, ear. Next, from the malleus all the way to the stapes, so these three bones, from the malleus, incus and the stapes, that is known as the middle ear. It looks like there's an overlap, but I'm just trying to include this word over here. The middle ear is actually this region. From here to here is the middle ear, whereas the external ear is the region from here all the way out here. The third section includes the cochlea and something known as the semicircular canals, which we didn't talk about. That is known as the inner ear. This is the inner ear. Those are the three different general classifications that we could break the different structures of the ear down into. Next, what I wanna do is look at the cochlea a little bit more carefully, and look at how the fluid motion inside the cochlea actually causes hair cells to fire a neural impulse to the brain, which can then be interpreted as sound.