hello! i’d like you to think about how i’m doingthis right now. not why i’m doing it, because of course,i’m doing it because i like music and i like science and i like to do both those thingsat the same time. but how can i play music? how can i be hearingit right now? and how can i walk around and play my guitarat the same time without falling on my face? and what is even sound anyway? these are all good questions. let’s startwith the last one, first. the basic answer to “what is sound?†goeslike this:
sounds create vibrations in the air that beatagainst the eardrum, which pushes a series of tiny bones that move internal fluid againsta membrane that triggers tiny hair cells -- which aren’t actually hairs -- that stimulateneurons, which in turn send action potentials to the brain, which interprets them as sound. but there’s a lot more to our ears thanallowing us to experience the pleasure of birdsong, or the pain of grindcore. the ear’s often overlooked, but even morevital role is maintaining your equilibrium, and without that, you wouldn’t be able todance or strut or even stand up. and you definitely could not do this!
at least not without throwing up. in order to really get to the nitty-grittyof how your ears pick up sound, you’ve got to understand how sound works. the key to sound transmission is vibration.when i talk, my vocal folds vibrate. when i slap this table top, or strum a guitar,those vibrations cause air particles to vibrate too, initiating sound waves that carry thevibration through the air. so this, sounds different than this, becausedifferent vibrating objects produce differently shaped sound waves. a sound’s frequency is the number of wavesthat pass a certain point at a given time.
a high-pitched noise is the result of shorterwaves moving in and out more quickly, while fewer, slower fluctuations result in a lowerpitch. how loud a sound registers depends on thewave’s amplitude, or the difference between the high and low pressures created in theair by that sound wave. now, in order for you to pick up and identifysounds from beeping to barking to beyonce, sound waves have to reach the part of theear where those frequencies and air-pressure fluctuations can register and be convertedinto signals that the brain can understand. so once again, it all boils down to actionpotentials. but, how does sound get in there?
your ear is divided into three major areas:the external, middle, and inner ear. the external and middle ear are only involved with hearing,while the complex hidden inner is key to both hearing and maintaining your equilibrium. so the pinna, or auricle, is the part that you can see,and wiggle, and grab, or festoon with an earring. it’s made up of elastic cartilage coveredin skin, and its main function is to catch sound waves, and pass them along deeper intothe ear. once a sound is caught, it’s funneled downinto the external acoustic meatus, or auditory canal, and toward your middle and inner ear. sound waves traveling down the auditory canaleventually collide with the tympanic membrane,
which you probably know as the eardrum. this ultra-sensitive, translucent, and slightlycone-shaped membrane of connective tissue is the boundary between the external and middleear. when the sweet sound waves of your favoritejam collide with the eardrum, they push it back and forth, making it vibrate so it can pass thosevibrations along to the tiny bones in the middle ear. now, the middle ear, also called the tympaniccavity, is the relay station between the outer and inner ear. its main job is to amplifythose sound waves so that they’re stronger when they enter the inner ear. and it’s gotta amplify them, because theinner ear moves sound through a special fluid,
not through air -- and if you’ve ever goneswimming you know that moving through a liquid can be a lot harder than moving through air. the tympanic cavity focuses the pressure ofsound waves so that they’re strong enough to move the fluid in the inner ear. and it does this using the auditory ossicles-- a trio of the smallest, and most awesomely named bones in the human body: the malleus,incus, and stapes, commonly known as the hammer, anvil, and stirrup. one end of the malleus connects to the innereardrum and moves back and forth when the drum vibrates.
the other end is attached to the incus, whichis also connected to the stapes. together they form a kind of chain that conductseardrum vibrations over to another membrane -- the superior oval window -- where theyset that fluid in the inner ear into motion. the inner ear is where things get a little complicated,but interesting and also kind of mysterious. with some of the most complicated anatomyin your entire body, it’s no wonder it’s known as the labyrinth. this tiny, complex maze of structures is safelyburied deep inside your head, because it’s got two really important jobs to do: one, turn those physical vibrations into electricalimpulses the brain can identify as sounds.
and two: help maintain your equilibrium soyou are continually aware of which way is up and down, which seems like a simple thing,but it is very important. to do this, the labyrinth actually needs twolayers -- the bony labyrinth, which is the big fluid-filled system of wavy wormholes-- and the membranous labyrinth, a continuous series of sacs and ducts inside the bony labyrinththat basically follows its shape. now, the hearing function of the labyrinthis housed in the easy-to-spot structure that’s shaped like a snail’s shell, the cochlea. if you could unspool this little snail shell,and cut it in a cross-section, you’d see that the cochlea consists of three main chambersthat run all the way through it, separated
by sensitive membranes. the most important one -- at least for ourpurposes -- is the basilar membrane, a stiff band of tissue that runs alongside that middle,fluid-filled chamber. it’s capable of reading every single sound withinthe range of human hearing -- and communicating it immediately to the nervous system, becauseright smack on top of it is another long fixture that’s riddled with special sensory cellsand nerve cells, called the organ of corti. so when your cute little ossicle bones startsending pressure waves up the inner fluid, they cause certain sections of basilar membraneto vibrate back and forth. this membrane is covered in more than 20,000 fibers, and they get longer the
farther down the membrane you go. kind of like a harp with many, many strings,the fibers near the base of the cochlea are short and stiff, while those at the end arelonger and looser. and, just like harp strings, the fibers resonateat different frequencies. more specifically, different parts of themembrane vibrate, depending on the pitch of the sound coming through. so the part of themembrane with the short fibers vibrates in response to high-frequency pressure. and the areas with the longer fibers resonatewith lower-frequency waves. this means that, all of the sounds that youhear -- and how you recognize them -- comes
down to precisely what little section of thismembrane is vibrating at any given time. if it’s vibrating near the base, then you’rehearing a high-frequency sound. if it’s shakin’ at the end, it’s a low noise. but of course nothing’s getting heard untilsomething tells the brain what’s going on. and the transduction of sound begins whenpart of the membrane moves, and the fibers there tickle the neighboring organ of corti. this organ is riddled with so-called haircells, each of which has a tiny hair-like structure sticking out of it. and when oneis triggered, it opens up mechanically gated sodium channels. that influx of sodium thengenerates graded potentials, which might lead
to action potentials, and now your nervoussystem knows what’s going on. those electrical impulses travel from theorgan of corti along the cochlear nerve and up the auditory pathway to the cerebral cortex. but the information that the brain gets ismore than just, like, “hey listen up.†the brain can detect the pitch of a soundbased solely on the location of the hair cells that are being triggered. and louder sounds move the hair cells more,which generates bigger graded potentials, which in turn generate more frequent actionpotentials. so the cerebral cortex interprets all thosesignals, and also plugs them into stored memories
and experiences, so it can finally say oh,that’s a chickadee, or a knock at the door, or the slow burn of an 80s saxophone solo,or whatever. so that’s how you hear. but we’re not done with you yet -- we gottatalk about equilibrium. the way we maintain our balance works in a similar way to theway we hear, but instead of using the cochlea, it uses another squiggly structure in thelabyrinth that looks like it’s straight out of an alien movie -- a series of sacsand canals called the vestibular apparatus. this set-up also uses a combination of fluidand sensory hair cells. but this time, the fluid is controlled not by sound waves butby the movement of your head.
the most ingenious parts of this structureare three semicircular canals, which all sit in the sagittal, frontal, and transverse planes. based on the movement of fluid inside of them,each canal can detect a different type of head rotation, like side-to-side, and up-and-down,and tilting, respectively. and every one of the canals widens at itsbase into sac-like structures, called the utricle and saccule, which are full of haircells that sense the motion of the fluid. so by reading the fluid’s movement in eachof the canals, these cells can give the brain information about the acceleration of thehead. so if i move my head like this, because i’m,like, super into my jam, that fluid moves
and stimulates hair cells that read up anddown head movement, which then send action potentials along the acoustic nerve to my brain, whereit processes the fact that i’m bobbing my head. and, just as your brain interprets the pitchand volume of a sound by both where particular hair cells are firing in the cochlea and howfrequent those action potentials are coming in, so too does it use the location of haircells in the vestibular apparatus to detect which direction my head is moving throughspace, and the frequency of those action potentials to detect how quickly my head is accelerating. but things can get messy. doing stuff like spinning on a chair, or sittingon a rocky boat, can make you sick because
it creates a sensory conflict. in the caseof me spinning around on my chair, the hair cells in my vestibular apparatus are firingbecause of all that inner-ear fluid sloshing around — but the sensory receptors in myspine and joints tell my brain that i’m sitting still. on a rocking boat, my vestibularsenses say i’m moving up and down, but if i’m looking at the deck, my eyes aretelling my brain that i’m sitting still. the disconnect between these two types of movement,by the way, is why we get motion sickness. it doesn’t take long for my brain to get confused,and then mad enough at me to make me barf. aaand i’m sorry that we’re ending withbarf. but, we are. today your ears heard me tellyou how your cochlea, basilar membrane, and
hair cells register and transduct sound intoaction potentials. you also learned how different parts of your vestibular apparatus respondto specific motions, and how that helps us keep our equilibrium. special thanks to our headmaster of learningthomas frank for his support for crash course and for free education. thank you to all ofour patreon patrons who make crash course possible through their monthly contributions.if you like crash course and want to help us keep making great new videos like thisone -- and get some extra special, interesting stuff -- you can check out patreon.com/crashcourse crash course is filmed in the doctor cherylc. kinney crash course studio. this episode
was written by kathleen yale, edited by blakede pastino, and our consultant is dr. brandon jackson. our director is nicholas jenkins,the script supervisor and editor is nicole sweeney, our sound designer is michael aranda,and the graphics team is thought cafã©.
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