Hearing & Balance: Crash Course A&P #17

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 to “What 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, 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 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 they’re 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 you’ve 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 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 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 you’re
hearing a high-frequency sound. If it’s shakin’ at 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 we’re 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 around — but 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 we’re 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 by Kathleen Yale, edited by Blake
de 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 Café.


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