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  • Hello!

  • I’d like you to think about how I’m doing this 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 things at the same time.

  • But how can I play music? How can I be hearing it right now?

  • And how can I walk around and play my guitar at the same time without falling on my face?

  • And what is even sound anyway?

  • These are all good questions. Let’s start with the last one, first.

  • The basic answer toWhat is sound?” goes like this:

  • Sounds create vibrations in the air that beat against the eardrum, which pushes a series

  • of tiny bones that move internal fluid against a membrane that triggers tiny hair cells -- which

  • aren’t actually hairs -- that stimulate neurons, which in turn send action potentials

  • to the brain, which interprets them as sound.

  • But there’s a lot more to our ears than allowing us to experience the pleasure of

  • birdsong, or the pain of grindcore.

  • The ear’s often overlooked, but even more vital role is maintaining your equilibrium,

  • and without THAT, you wouldn’t be able to dance 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-gritty of how your ears pick up sound, youve 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 the vibration through the air.

  • So this, sounds different than this, because different vibrating objects produce differently

  • shaped sound waves.

  • A sound’s frequency is the number of waves that pass a certain point at a given time.

  • A high-pitched noise is the result of shorter waves moving in and out more quickly, while

  • fewer, slower fluctuations result in a lower pitch.

  • How loud a sound registers depends on the wave’s amplitude, or the difference between

  • the high and low pressures created in the air by that sound wave.

  • Now, in order for you to pick up and identify sounds from beeping to barking to Beyonce,

  • sound waves have to reach the part of the ear where those frequencies and air-pressure

  • fluctuations can register and be converted into signals that the brain can understand.

  • So once again, it all boils down to action potentials.

  • 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 covered in skin, and its main function is to catch

  • sound waves, and pass them along deeper into the ear.

  • Once a sound is caught, it’s funneled down into the external acoustic meatus, or auditory

  • canal, and toward your middle and inner ear.

  • Sound waves traveling down the auditory canal eventually collide with the tympanic membrane,

  • which you probably know as the eardrum.

  • This ultra-sensitive, translucent, and slightly cone-shaped membrane of connective tissue

  • is the boundary between the external and middle ear.

  • When the sweet sound waves of your favorite jam collide with the eardrum, they push it

  • back and forth, making it vibrate so it can pass those vibrations along to the tiny bones in the middle ear.

  • Now, the middle ear, also called the tympanic cavity, is the relay station between the outer

  • and inner ear. Its main job is to amplify those sound waves so that theyre stronger

  • when they enter the inner ear.

  • And it’s gotta amplify them, because the inner ear moves sound through a special fluid,

  • not through air -- and if youve ever gone swimming you know that moving through a liquid

  • can be a lot harder than moving through air.

  • The tympanic cavity focuses the pressure of sound waves so that theyre 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 inner eardrum and moves back and forth when the

  • drum vibrates.

  • The other end is attached to the incus, which is also connected to the stapes.

  • Together they form a kind of chain that conducts eardrum vibrations over to another membrane

  • -- the superior oval window -- where they set 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 anatomy in your entire body, it’s no wonder it’s

  • known as the labyrinth.

  • This tiny, complex maze of structures is safely buried deep inside your head, because it’s

  • got two really important jobs to do:

  • One, turn those physical vibrations into electrical impulses the brain can identify as sounds.

  • And two: help maintain your equilibrium so you 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 two layers -- 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 labyrinth that basically follows its shape.

  • Now, the hearing function of the labyrinth is 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 chambers that run all the way through it, separated

  • by sensitive membranes.

  • The most important one -- at least for our purposes -- 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 within the range of human hearing -- and communicating

  • it immediately to the nervous system, because right smack on top of it is another long fixture

  • that’s riddled with special sensory cells and nerve cells, called the organ of corti.

  • So when your cute little ossicle bones start sending pressure waves up the inner fluid,

  • they cause certain sections of basilar membrane to 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 are longer and looser.

  • And, just like harp strings, the fibers resonate at different frequencies.

  • More specifically, different parts of the membrane vibrate, depending on the pitch of

  • the sound coming through. So the part of the membrane with the short fibers vibrates in

  • response to high-frequency pressure.

  • And the areas with the longer fibers resonate with lower-frequency waves.

  • This means that, all of the sounds that you hear -- and how you recognize them -- comes

  • down to precisely what little section of this membrane is vibrating at any given time. If

  • it’s vibrating near the base, then youre hearing a high-frequency sound. If it’s

  • shakinat the end, it’s a low noise.

  • But of course nothing’s getting heard until something tells the brain what’s going on.

  • And the transduction of sound begins when part of the membrane moves, and the fibers

  • there tickle the neighboring organ of corti.

  • This organ is riddled with so-called hair cells, each of which has a tiny hair-like

  • structure sticking out of it. And when one is triggered, it opens up mechanically gated

  • sodium channels. That influx of sodium then generates graded potentials, which might lead

  • to action potentials, and now your nervous system knows what’s going on.

  • Those electrical impulses travel from the organ of corti along the cochlear nerve and

  • up the auditory pathway to the cerebral cortex.

  • But the information that the brain gets is more than just, like, “hey listen up.”

  • The brain can detect the pitch of a sound based 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 action potentials.

  • So the cerebral cortex interprets all those signals, 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 were not done with you yet -- we gotta talk about equilibrium. The way we maintain

  • our balance works in a similar way to the way we hear, but instead of using the cochlea,

  • it uses another squiggly structure in the labyrinth that looks like it’s straight

  • out of an Alien movie -- a series of sacs and canals called the vestibular apparatus.

  • This set-up also uses a combination of fluid and sensory hair cells. But this time, the

  • fluid is controlled not by sound waves but by the movement of your head.

  • The most ingenious parts of this structure are 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 its base into sac-like structures, called the

  • utricle and saccule, which are full of hair cells that sense the motion of the fluid.

  • So by reading the fluid’s movement in each of the canals, these cells can give the brain

  • information about the acceleration of the head.

  • 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 and down head movement, which then send action

  • potentials along the acoustic nerve to my brain, where it processes the fact that I’m bobbing my head.

  • And, just as your brain interprets the pitch and volume of a sound by both where particular

  • hair cells are firing in the cochlea and how frequent those action potentials are coming

  • in, so too does it use the location of hair cells in the vestibular apparatus to detect

  • which direction my head is moving through space, 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 sitting on a rocky boat, can make you sick because

  • it creates a sensory conflict. In the case of me spinning around on my chair, the hair

  • cells in my vestibular apparatus are firing because of all that inner-ear fluid sloshing

  • aroundbut the sensory receptors in my spine and joints tell my brain that I’m

  • sitting still. On a rocking boat, my vestibular senses say I’m moving up and down, but if

  • I’m looking at the deck, my eyes are telling 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 were ending with barf.

  • But, we are. Today your ears heard me tell you how your cochlea, basilar membrane, and

  • hair cells register and transduct sound into action potentials. You also learned how different

  • parts of your vestibular apparatus respond to specific motions, and how that helps us

  • keep our equilibrium.

  • Special thanks to our Headmaster of Learning Thomas Frank for his support for Crash Course

  • and for free education. Thank you to all of our 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 this one -- and get some extra special, interesting

  • stuff -- you can check out patreon.com/crashcourse

  • Crash Course is filmed in the Doctor Cheryl C. Kinney Crash Course Studio. This episode

  • was written