The inner ear is the portion that actually contains the sensory receptors that are responsible for the detection of sound and are going to be involved in balance. Now, the balance-detecting system is called the vestibular system, and it's basically attached to the sound-detecting system. The vestibular system is made up of these semicircular canals, these three fluid-filled cavities that you can see here, on the inner ear structure. And essentially, fluid is going to move through these canals and stimulate hair cells in them, and that's how we're going to be able to detect rotational motion and get a general sense of balance. This is why, for example, as a kid, you know, when you spin around a lot and you get dizzy, it's because the fluid is still moving through your semicircular canals due to inertia, so you're still getting that perception that you're moving even though you've stopped. Now, the cochlea, this structure is what's going to be responsible for the detection of sound, and it's a spiral-shaped cavity, and it kind of looks like a cone that got coiled up. Now you can see, sort of, a modified version of the cochlea here, which is made to look like what the cochlea would appear as if it were uncoiled. That's why it's all straightened out. However, in reality, its structure is all curled up. So imagine, you know, taking this and then rolling it up like a fruit roll-up or something. Now, the cochlea has a bunch of different structures in it. You don't need to worry about all of this, but I'm just going to give you a general sense of what's going on in there so that we can talk about the part that you do need to understand. So the cochlea actually has three ducts in it, or three fluid-filled tubes. There's this one on top, we call the vestibular duct, there's the tympanic duct on the bottom, and then in the middle we have the cochlear duct. Now, the hair cells in the cochlea that are going to be responsible for detecting sound sit on something called the basilar membrane. This is the membrane, kind of in the middle of the cochlea, that's going to be underneath the cochlear duct and above this tympanic duct. But, really you only need to know about the basilar membrane because that's what the organ of Corti sits on. This is the structure that contains many hair cells. As you can see here, we have our organ of Corti, and all these little projections on top are our hair cells. And, these are the hair cells that are going to be stimulated to perceive sound. Now, it's worth noting that the basilar membrane is different in different regions of the cochlea. It will actually have different regions that vibrate at different frequencies of sound waves. And in this way, we're able to better perceive a wider range of sound frequencies. Now, how these hair cells in the organ of Corti are actually stimulated has to do in part with something called the tectorial membrane. This is going to be a little membrane, you can see it here, that sits above our hair cells, which are here in the organ of Corti. Now basically, when the stapes hits the oval window, it's going to send a wave through the fluid of the cochlea, and that fluid moving is going to vibrate those membranes, and those membranes are going to move when they vibrate. Right? It's kind of what vibrating means. But, that vibration of the basilar membrane is going to cause those hair cells to bend due to their connection with the tectorial membrane. Although, it's worth noting that not all hair cells are connected to the tectorial membrane. And I should point out that this process is not super well understood. So you know, in 10 years' time or something maybe what I'm saying will no longer be held as the standard of the day. So, you know, not totally well understood, but there is an interaction between those hair cells and the tectorial membrane, and it's those hair cells being stimulated that's going to lead to the perception of sound. And again, remember that the amplitude of the wave translates into volume, and the frequency of the vibrations translates into pitch. Now, I also want to point out that there's this structure called the round window, that's kind of all the way at the other end of the cochlea, and its job, it's a membrane-covered opening like the oval window, but its job is to dampen waves and prevent reverberation. Essentially, this is going to allow the waves to sort of diffuse their energy out of the cochlea, and make sure that they're not just reverberating or echoing around inside there. So, with that let's actually go ahead and flip the page.
46. Sensory Systems
Sensory System
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