hi everyone welcome to Professor Mullins lectures on anatomy and physiology I'm Professor Bob long if you're watching these videos they are intended for use by my students taking my human anatomy and physiology courses at don't want College anyone else out there in YouTube land finds these helpful by all means use them to your advantage but know that these are designed for my students so we've been doing a series of sensory physiology videos for my part 2a AP class this is the last of the series this is the video on hearing and how hearing works so as you guys know we've been looking at the anatomy of the ear in lab and I'm going to go over some of that anatomy here so when we look at the external part of the ear what we'll see here is we'll see this we'll see the pin on our article which is the part of your ear that we typically call the ear the opening is called the external auditory meatus there's a tube that runs from the external auditory meatus and - up to a structure called the tympanic membrane which is your eardrum okay and then on the backside of the eardrum is a series of small bones called the auditory ossicles that's the auditory ossicles Hickel means little in Oz means bone so the first auditory obstacle or small bone that's going to sit here is going to sit with its base on the back of the tympanic membrane and this bone is called the malleus I'll put an m4 malleus that is connected to a bone called the incus malleus means mallet incus means anvil like you said a horseshoe or sword on an anvil to hammer and then that is connected to a small stirrup shaped bone which is called the stay piece so in the language that these words originated in the e would be pronounced a piece so not stapes so anyway those are the three auditory ossicles now obviously this is not drawn to scale at this size these obstacles would still be very very tiny but they're here to illustrate now I'm going to erase this we know it's malleus incus and stapes in that order when we look at the inner ear where they're connected enlarged area called the vestibule and then over here is the coiled snail shell called the cochlea over here would be your three semicircular canals and I've kind of tilted this back so that we can see it the stay piece sits in an opening over or within the bony labyrinth called the oval window and somewhere over here is another opening called the round window and the membranous labyrinth is also kind of covering that also in lab what you guys should know is if we cut this cochlea right here and look down the tube there's a series of three chambers so we're gonna have that outer bony covering this is another model that we did in lab that will look like this this is all bony labyrinth but within the cochlea the bony labyrinth starts to grow across here the membranous labyrinth comes across forming part of the basilar membrane extends up forming the vestibular membrane and also hugs the narwhal here so in the cochlea the membranous labyrinth is not sitting in the middle like it is in the semicircular canals it's actually up against the bone and seals this off into three chambers as you should have learned in lab this is called the vestibular duct or scale of the stimuli the tympanic I'm starting to cochlear duct or skaila cochleae and the tympanic duct or schema Tiffany now it's in the cochlear duct sitting on the basilar membrane that we have a small amount of cells called the organ of Corti or spiral organ of Corti and there's a membrane hanging over that called the tectorial membrane we have these small groups of cells that have little suya sticking up called hair cells the inner and outer hair cells those cells are going to synapse on some other neurons and they'll send their axons over here to a group of cells called the spiral ganglion and then those axons will go out through the cochlear nerve and join the rest of the vestibulocochlear nerve okay so that's sort of the anatomy now this tube if I extend it this way this tube is - unfolded and I would see these three chambers running all the way through here it's a little bit hard to visualize but nonetheless let's just say that I ran the vestibular membrane all the way down to a certain point and the tectorial membrane all the way down to a certain point I still have this tubular duct cochlear duct tympanic duct and the organ of Corti would extend all the way down the entire length here with all these little hair cells that's why when we look at lab at the models especially the inner ear model when we break this part off you see the three tubes repeating and repeating and repeating us to coil in we've just uncoiled the cochlea and all these little hair cells would be going all the way down much closer together and the tectorial membrane would be hanging over them so it says if we're looking at this on end extend all the way down I hope that and that may make sense to you now this is how hearing worse one of the things that people don't see is that the round window is the opening at one end of this vestibular duct I'm sorry of the tympanic duct the vestibular duct would be here where the oval window is and so technically I have a plate of bone covering this and I have a stay piece sitting here and then I would have the malleus and the incus so the stay piece is sitting over the end of the vestibular duct the fluid in the vestibular duct would be called perilymph so all the way around here I have perilymph and really these two ducts are connected and are the same tube just they named it differently because they first looked at it this way now this would be the cochlear duct and it's filled with endo lymph perilymph and no live period got it so here's how a hearing is going to work when we understand all of this so as sound waves come in and they hit the year one of the things that the pinna does is it sort of acts like a funnel and funnel sound into the external auditory meatus and the external auditory canal if you're facing an opposite direction then it becomes harder to hear that's why we need to turn to someone to hear better to allow our ears to funnel the sound waves the vibrations of air molecules hitting this and those vibrations get funneled in this way and they hit the tympanic membrane the tympanic membrane will begin to vibrate as it vibrates it's going to wiggle the malleus the incus and stapes the function of the tympanic membrane is a convert sound into mechanical movement into physical movement that's its main function converting sound into physical movement now when it starts to move the three bones are going to amplify that movement they act like levers for example if I grab to my elbow and I moved it just a little bit one way or the other the other end of my hand where my hand is would move a great deal so as one men of the malleus moves a little bit with the tympanic membrane the other end moves a lot which causes the incus to amplify the motion and that really causes the state bees to pump on the end of this channel that will begin to create pressure waves of fluid in the vestibular duct when those pressure waves slam into the tectorial membrane that will cost pressure waves in the endolymph and the cochlear duct and the basilar membrane and all the membranes will start to vibrate it's almost as if I have two water balloons or three water balloons like long tubes next to each other really tight if I bump one then the fluid and one is going to cost ripples and the others or vibrations and waves and the others what happens then is that as these membranes are vibrating it will jam these hair cells into the tectorial membrane these hair cells in the cochlea are mechanoreceptors they have ion channels in the hairs and if we bend them that starts to open sodium or potassium channels altering resting membrane potential of these cells they'll send an action potential out to the spiral ganglion out to the cochlear nerve into our brain and tell us that we're hearing so those are the steps of addition there on the bottom of page 19 in my note said that the pinna of the ear will channel sound waves and vibrations into the external auditory canal and vibrate the tympanic membrane the vibrations of the tympanic membrane will cause the malleus to start to to start to amplify the sound it'll start to vibrate or wiggle so to speak and then because of the way the malleus is attached the malleus the incus and the stapes will all begin to vibrate together amplifying that motion all right because the state piece is attached here it will begin pumping and creating waves and the vestibular duct okay those pressure waves in the vestibular duct are in the fluid called perilymph that perilymph will cause all of these membranes to start to vibrate and it will cause the hair cells to begin to jam in and start a receptor potential or an action potential traveling out to the cochlear nerve to our brain so you can kind of see the steps in order we funnel sound into the tympanic membrane the tympanic membrane vibrates and starts to wiggle the malleus the malleus will move the button of incus and stapes that creates pressure waves in the pyramid for the vestibular duct that will cause the other two membranes with the endolymph and Perryman to all start vibrating jamming hair cells into the tectorial membrane altering resting membrane potential and they would all be sending axons out through the spiral ganglion out the cochlear nerve so you should know those steps they're written on the bottom of page 13 okay now here's something that's interesting we were talking about wavelengths when we talked about photons of light well when we make sounds with our voice if you were to sit there and hum in a certain tone like a really low sound for example if someone had a bass drum the bass drum will vibrate and the sound waves will all have the same wavelength the wavelength usually tells us the pitch of a sound the amplitude or the height of the we'll tell us about the volume so you can see if I took this wave here and I took another wave of the same wavelength they cycle at the same time they would have the same pitch they might both be a bass drum or a very low hum except for one would be very quiet the other one would be very loud so the wavelength tells us about the pitch of the sound the height of the wave called the amplitude tells us about the volume I'm going to talk a little bit more about that in a second so a very high pitch sound will have a smaller wavelength or a shorter wavelength low pitch sounds have a longer wavelength it turns out that the closer to the oval window that we stimulate the higher the frequency of sound we're hearing so very high frequency vibrations here cause the waves to slam in here triggering these cells if I stuck an electrode into these neurons and caused them to fire even if you weren't listening you would hear a very high-pitched sound like a cymbal or a piece of glass breaking very low frequency vibrations will stimulate much deeper or more distal to the oval window so depending on where I stimulated the hair cells along this passageway determines the pitch of the sound close to the oval window high pitch further away is a low pitch sound okay and that's based on the frequencies of vibrations now I'm gonna erase this I'm going to cover one last thing which is actually the first part of hearing on this page but I hope you have the anatomy and physiology all linked together so now I'm going to talk about pitch and volume and then I think we're done with a lot of this stuff there's a lot more we could cover but there's a way too much information to cover on one semester if I cover everything you know nothing so now when I talked about the wavelength of sound or the what we can do is we can do this I didn't draw this very well because you know but let's pretend that these waves are the exact same length from this wave to this one to the next one so if I have a certain frequency that means how frequently does this wave crest if I measure that over a period of time now we call from going from top to bottom to top one cycle if I measure for one second then this might be one two three cycles per second so that would be the frequency how frequently does it peak so when we talk about the pitch of a sound whether it's high pitch or low pitch we can talk about cycles per second which is also referred to as the frequency and that is measured in a term that we call Hertz you hear about Giga Hertz and mega Hertz well the term Hertz refers to the cycles per second and there's a scientist named Hertz that figured all this stuff out now the human ear can hear somewhere between about 20 Hertz and 20,000 Hertz or 20,000 cycles per second so if someone broke some glass or hit a cymbal then that would be a much more frequent sound much higher Hertz 20,000 Hertz would be like you know glass breaking and then thunder at a distance would be around 20 Hertz okay something really lower low Rumble so that tells us about the pitch of the sound as I said before if I have the same pitch but I can change the amplitude of the wave how tall the waves are then this would be louder a greater volume so when we measured the volume of the sound volume is measured in the term amplitude or the height of the wave and the person who described this was a guy who made I believe the terms after himself called decibels so we can hear a certain number of decibels as how loud it is okay that's pretty much all that I want you guys to know about that you shouldn't oh that the pitch of the wave is measured in cycles per second the shorter the wavelength the higher the pitch the shorter the wavelength the more cycles per second it is the longer the wavelength the fewer cycles per second or the lower the pitch and we measure that in Hertz 20 Hertz or the lower the Hertz the lower the two the tone or the lower the pitch for volume we measured in decibels but and higher the number of decimals the larger the waves the louder that it is anyway I hope you learned something I hope you found this helpful you guys keep doing this stuff until you can't stand it and then do it three more times Master this be able to teach it to someone else and you'll be ready to make your a in the test all right I hope you had as much fun as I did I'll see you all for the next round