00:01
Now we've reached the deepest
part of the ear, the inner ear.
00:05
Which we can think of as
having a bony labyrinth
and then an inner
membranous labyrinth.
00:12
We have an oval window,
which interacts with the stapes
from the middle ear cavity.
00:19
A round window.
00:21
A spiral shaped cochlea,
cochlea actually means snail so
it kind of looks like a snail.
00:28
We have these three
semicircular canals,
an anterior, posterior
and lateral canal.
00:36
And then in between
more centrally,
we have this thing
called the vestibule.
00:41
The membranous labyrinth
is membranous tissue that lies
within these bony components.
00:49
Inside the cochlea, we
have the cochlear duct.
00:53
In terms of the
vestibular apparatus,
we have the saccule
and the utricle.
00:58
And then we have these widenings
of the semicircular canals
called the ampulla for each one.
01:05
We also have a
utricosaccular duct
and something called an
endolymphatic sac and duct
that carry something
called endolymph.
01:17
Let's first look at the cochlea.
01:20
The cochlea has an
outer osseous labyrinth
with a space inside,
that's going to be our
spiral canal of the cochlea.
01:30
There's also going to be a tiny
bony projection within this canal.
01:34
That's going to be the
osseous spiral lamina.
01:39
The axis of this spiral is
something called the modiolus.
01:45
At the center of this,
receiving all of
these tiny inputs
is going to be
the cochlear nerve
and that's going to go back into
the brain and perceive sound.
01:57
Here's a cross section through
one of those spiral canals.
02:01
We see that that spiral lamina
connects to the surrounding bone
via the basilar membrane.
02:09
Then there's another
thin membrane
called the vestibular
or Reissner's membrane,
and these two are connected
peripherally by a spiral ligament.
02:21
This encloses a space
called the cochlear duct
or the scalar media.
02:28
On the other side of
the vestibular membrane
is the scala vestibuli.
02:34
And on the other side of the basilar
membrane is the scala tympani.
02:40
Within the cochlear duct side,
on the basilar membrane,
we have something called
the spiral organ of corti
and this is the actual
organ of hearing.
02:51
This cochlear duct
or scala media
is filled with a fluid
called endolymph.
02:58
The scala vestibuli in
tympani, on the other hand,
are filled with something
called perilymph.
03:05
And it's going to be
vibration of this fluid
that gets translated by
the spiral organ of corti.
03:13
That is passed through
a spiral or ganglion
into the cochlear nerve.
03:18
That will be how
we perceive sound.
03:21
And to zoom out
for just a second,
we're going to have
the vestibular nerve
carrying out
vestibular information,
joining the cochlear nerve
to form the vestibulocochlear
nerve or cranial nerve VIII,
which is going to travel
with the facial nerve
through the internal
acoustic meatus.
03:42
So how is hearing happening?
Well, sound is going
to be transmitted
through the external
ear to hit the eardrum
to vibrate those ossicles
ultimately causing vibration
on the oval window.
03:59
And that oval window will
cause fluid to vibrate.
04:04
And that vibration will be
picked up by little cells
on the organ of corti
and transmitted as information
back to the cochlear nerve.
04:14
That round window is going
to basically compensate
for pressure changes
within the fluid.
04:21
So as the oval window pushes in,
a round window will bulge out,
so that things don't burst.
04:31
So this leads us to understanding
certain types of hearing loss.
04:35
So if we were to have damage to
the auditory nerves themselves,
or the cochlea, where we have
the spiral organ of corti,
that will be something called
sensory neural deafness,
meaning whether or not
sound reaches this area,
it can't be perceived.
04:54
Which is in contrast
to conductive deafness,
which is a problem with
getting sound waves
to the sensory neural
apparatus in the first place.
05:05
And that can be external
in the ear canal,
such as some type of
blockage of earwax,
it could be a problem with
the tympanic membrane,
such as a perforation
of the eardrum,
or it could be a filling defect
where there's otitis
media or inflammation,
filling up the
middle ear cavity.
05:27
Now let's take a look at the
other portion of the inner ear,
the portion responsible
for equilibrium.
05:34
What's happening here
on a very small scale,
in areas like the
utricle and the saccule,
is that we have this fluid containing
otoliths or very tiny stones
that can knock over certain
types of hair cells.
05:51
So these are cells
that have stereocilia,
or cilia that don't beat but
are really sensing movement.
05:58
And what happens is
as the head moves,
we have linear acceleration,
causing the otoliths
to tip over these hair cells.
06:08
And in the case of the utricle,
this tells us that
our head is moving
or accelerating in
a linear direction.
06:17
The saccule does something very
similar but in a vertical direction,
such as sensing that you're
moving in an elevator.
06:26
In the semicircular canals,
we have something
similar happening
but for rotational acceleration.
06:33
At the ampulla of the
semicircular canals,
we have these gelatinous
structures called the cupula
and more hair cells
with rotational acceleration.
06:45
It will tip over
these hair cells
providing information
about rotation,
and because the semicircular
canals are in the X, Y, Z axis,
we can tell which axis
the rotation is happening
as well as if it's happening in
a combination of different axis.