Hearing sense and equilibrium

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.