Chapter 27 Auditory

So chapter 27 is split up into 2 parts- auditory and the vestibular system. We’re gonna start with the auditory, and before we get to the specific details of the structures within the hearing system uh, we’re just gonna talk a little bit about what sound is. And sound is… the- any audible vibration of molecules. So it’s the actual movement of molecules from the environment, through the cochlea- which is the hearing organ- and then into its transduction into the brain All right, and so if you remember all the other types of receptors that we have talked about or we’re gonna talk about- you have proprioception uh, photoreception, chemoreception, chemoreception… those are all uh, determined by the stimuli, or the type of stimuli, that excite those organs. And just like with touch um, hearing is based on movement If you remember in touch which is a mechanoreceptor, we talked about meisners and porcinian corpuscles, all right. so those things are reliant on the actual movement of the sensing organ. And hearing is exactly the same way. And the actual sound or what we hear is dependent on some of these basic fundamental concepts that make up sound, and so those wheel all around wavelength, frequency, amplitude… and the way our hearing organ is designed, it’s actually capable of picking up a wide range of sounds. So sounds that vary in amplitude, vary in wavelengths, vary in frequency… we’re capable of sensing them at the same time. It all comes down to the way that the structures are organized. It’s very similar to a harp in the sense that you can- each kind of little hair connecting is gonna play a different… you’re gonna- a different sound is gonna emanate from that, but you can play some at the same time, you can play a multitude of different times, and you’ll get different sounds, you’ll be able to perceive or create different sounds from uh, strumming different things at the same time. It’s very similar in your ear where if those vibrations strum different things, different structures at the same time, you’re gonna be able to hear this wide, rich uh, sound. And you may be familiar with uh, a dog whistle, right? So we can’t- most humans can’t hear dog whistles and it’s because that whistle uh, elicits a pitch that’s higher than 20,000 Hz, which is the maximum capacity more or less of our hearing capabilities. And, and so it’s not that those vibrations aren’t coming into the ear, per se. They’re entering, we just don’t have the structures capable of being stimulated that and then transmitting that information to the brain. So the way the ears- we’re gonna talk about the ear, we’re gonna break into 3 different regions- the outer ear, the middle ear, and the inner ear. And the outer ear’s really involved about focusing the sounds, middle ear is really dealing with transmitting those vibrations, and then the inner ear is about transforming those vibrations from actual movement to an electrical signal. So the structures of on the outer ear that you need to be aware of are called the pinna and the auricle. All right, so this is- they’re bothly- basically the same thing and they’re talking about exterior part of the ear, the part that’s made up of elastic cartilage, and again, they’re all involved with focusing the sound. And there’s also this role of protection- and a lot of those things, external parts of our body are involved with protection and uh, the outer ear and the ear canal are uh, no excep- uh, no exception. So the ear canal, again is gonna be focused with uh, is gonna be focusing that sound, or those vibrations in towards the eardrum. They’re gonna be passing through that external auditory meatus and that internal auditory meatus right through that canal- that ear canal that’s passing that hole in the temporal bone. The ear canal actually has something special about it as well: it has these modified sebaceous glands called ceruman, all right and you remember we talked about sudoriferous and sebaceous glands- sebaceous glands usually secrete sebum… in the case of your ear, they secrete cerumen, which again is involve- uh, it’s involved to trap dirt by foreign particles and kind of protect the ear drum- or the tympanic membrane- same thing. All right, so those are the basic structures that are involved with the external ear. The middle ear- is a couple of bones that exist inside of it. All right, so it’s got these things called ossicles and if you look at the root of that word- “ossciles”… ossteon… bone… all right, same thing. They’re the stapes, the incus, and the malleus um, they get their name because stapes looks like a stirrup uhh, the incus looks like an anvil, and the malleus looks like a hammer or a mallet, so that’s one easy way to identify which is which. And they’re all about transmitting those vibrations that come through the ear canal, hit the tympanic membrane- or the ear drum- and they’re transmitting it to what’s called the uh, an opening called the oval window. And the oval window exists basically at the entrance at the cochlea of the vestibular cochlea uh, complex. All right, and so the only other structure within here is called the eustachian tube- or the auditory tube- and that’s a dealing with uh, maintaining ear pressure or air pressure so uh, if you go drive down to Phoenix or you take a flight, you hold your nose closed and you force- and you kinda force air back, right? All right, and so what you’re doing is forcing air into this auditory tube, which equalizes the pressure um, in the ear canal with the inner ear. And the inner ear is composed of 3 separate regions: the cochlea, the vestibule, and the semicircular ducts. So you can guess where it gets its name, or the vestibule and cochlea gets its name from this these structures. And we’re just gonna talk about cochlear in part 1 and part 2 will be focusing on uh, the vestibular system. So the cochlea- which means snail- is basically this curly cue of a structure that winds all the way around. And there’s fluid within this structure um, that kind of takes those vibrations and then move it to a structure called the tectoral membrane, which it then gets transmitted to an electrical signal. All right, and so once those hair cells are set in motion, and which hair cells are set in motion, that’s how information is relayed to the brain. And we’ll talk about this in a little bit more detail using that schematic right there. So, these thruid- these fluid-filled chambers have this kind of this gelatinous material. The middle 1- the scala media- is filled with endolymph. The other 2 are filled with perilymph… and the way that I remember that is that para- “peri” like perimeter… perimeters around, so the perilymph is in the 2 outside uh, chambers And if you look at that image – there’s 3… you can see 3 chambers on it right off the bat, right? So you have the scala vesitbuli on top, the scala media in the middle, and the scala tympani on the bottom. And I always use the scala media as my orienting point because it has all the garbage in it, has all that stuff in there: it has the techtoral memory, has all the hearing, your hearing organ is really based in there. The scala media also has the name cochlear duct- you’ll see both of those used. And it really it just winds around like a a snail shell with the scala vestibuli above it, and the scala tympania below it. The way- we’ll just kinda work our way through some of the structures here The- what separates the scala media and scala vesitbuli is a structure either called the vestibular membrane- or the reissner’s membrane- either are acceptable. It’s not gonna have a lot of attachment points, not gonna have a lot things attached to it- it’s a really thin line for most of it. On the other end, you have the basliar membrane… and the basliar membrane- like basement, the bottom membrane- it separates the scala tympani from the scala media, or the cochlea duct. And the basliar membranes go one that has the organ of corti uh, the tectoral membrane’s gonna attached to it through hair cells, it’s kind of got the whole action part to it. All right, and so you imagine that vibrations come through, they’re gonna move the tectoral membrane, they’re gonna affect it in different spots and in the tectoral membrane, that diving board looking thing, and that’s gonna move hair cells uh, at different rates and different intensities and that’s how we get those complex- the ability to hear those complex sounds. And if you have ever have gone to an AC/DC concert, or Justin Bieber or Selena Gomez or Modest Mouse- whatever you guys are listening to these days- and just rocked out way too hard, you stood by an amplifier, and you lef- you leave there and you just hear that ***wooooooo*** y’know, that ringing sound in your ear, what’s what’s happening is effectively… scientifically, you’ve rocked too hard. You’ve taken those inner hair cells- those cilia that attach the tectoral membrane- and transmit that information- the wav- y’know the soundwave information- and you’ve damaged them. So they’re just- they’re bent over, and they’re sending information, they’re transmitting information, but just because they’re damaged- but there’s no kind of richness to it, there’s no complexity, there’s no fine-grain uh, info. So it’s just that ringing sound- that something’s happening, we don’t know what it is, but there’s information being passed along. And that information is that in the long run, is that you damaged those hair cells. So here’s a slide- this is very typical of what you’re gonna see in class when we look at the slides of the cochlea. You can see the cochlear duct um, with the tectoral membrane. In this case, the tectoral membrane is actually attached to the vestibular membrane. In most cases, it won’t be uh, and you really can get a sense of how to organize, right? And you can see here it’s labeled with endolymph uh, perilymph in the scala vestibuli and the scala tympani. And really the only difference uh, is that the concentration of potassium more or less that’s- there’s really not too much um, major differences in the actual gelatinous material filling them, it’s just the relative uh, concentration of different chemicals.

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