Hearing and balance:

- Sounds create vibrations in the air that beat against the ear drum 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.
- The ear's more vital role is maintaining equilibrium.
- 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 [table slap] sounds different than this [plucked D string] 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.
- 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
ear is key to both hearing and maintaining equilibrium.
- The pinna, or auricle, is the part that we can see, is 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 middle and inner ear.
- Sound waves traveling down the auditory canal eventually collide with the tympanic
membrane, also known as the eardrum. It is ultra-sensitive, translucent, and slightly cone-
shaped membrane of connective tissue that is the boundary between the external and
middle ear.
- When the sound waves collide with the ear drum, 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.
- It amplifies them because the inner ear moves sound through a special fluid, not through air:
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 known as the "labyrinth", tiny, complex maze of structures, safely buried
deep inside the 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 body’s equilibrium.
- 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 cochlea. 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 and is 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.
- When the 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 twenty thousand fibers, and they get longer the
farther down the membrane you go, the fibers near the base of the cochlea are short and
stiff, while those at the end are longer and looser so, the fibers resonate at different
frequencies: 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 we hear and how we 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 we are hearing a high-frequency sound; if it's shaking 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 ----- channels.
- That influx of ---- 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.
- 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.
- 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 structure in the labyrinth: 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, 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.
- Sensory conflict:
- In the case of me spinning around in 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 those two types of movement is why we get motion sickness.