Hearing and Balance or the
Auditory and Vestibular Systems.
So when you here the structure function
relationship that you’re most concern with
is encoding soundwaves into electrical
impulses that the brain can understand.
So its converting soundwaves to
electrical waves, not too unlike
what your radio does or even
what the cellphone does.
What are the different waves in
which we need to take into account?
Well, first you need to have some sort of
pressure disturbance, some a sort of an event
to start the wave occurring.
The wave will have a certain amplitude,
which is the height of the wave.
A frequency, which is
how often the wave occurs.
But when you’ll think about complex sounds,
you have things travelling to you at different phases.
What we mean here is you’ll
have different frequencies going on
and those would be
offset from one another.
Kind of like an echo, echo, echo.
That response will occur at
different, different wave durations.
If you think about what
structure and function is.
You need to think about the
three different portions of the ear.
You have the external ear, which
is basically funneling in that sound.
You have the middle ear, which is
involve in amplifying the signal.
And then the inner ear is where you’re going
to get the actual transduction of that soundwave
into an electrical signal.
So think about it as, funneling, amplifying
and then sensing a particular signal.
What are the some of the key structures?
The key structures here involve
things like the tympanic membrane,
which is the first spot in which the inner ear
can start amplifying the effects of the sound.
You have three different bones:
the malleus, incus and stapes
that will help transduce that soundwave
from the tympanic membrane to the oval window.
So if we take a greater look at this response.
You have a soundwave that’s entering the ear.
It gets funnel down towards the tympanic membrane.
At this particular point, the
tympanic membrane will start to vibrate.
As it vibrates, the various ossicles, which
are those inner ear bones will also vibrate.
As they vibrate, they can then stimulate
the oval window to start vibrating.
So basically, you’re moving in
that soundwave via vibrations.
Now, are there any ways to
modulate the amount of vibration? Sure.
This transduction can be regulated
by dampening in a response.
How do you dampened that?
By causing muscle contraction to prevent
some of that vibration from moving through.
The different muscles that you could use to do this
are the tensor tympani as well as the stapedius.
How do we get that soundwave into the inner ear?
Through this process, we really want to
think about, where that last ossicles vibrated on.
It was the oval window. And then, how that’s
going to be transfer it down to cochlea?
The other item to think about for the cochlea
is it’s actually divided into three different chambers.
You have first have a top chamber.
And this top chamber is filled with perilymph.
You have a middle chamber which has your
Organ of Corti in it, and this has endolymph in it.
And finally, the bottom most layer
which is also filled with perilymph.
The importance of these
three chamber component,
is that sound is gonna travel
through the places with perilymph
and it’s gonna be sense
at the places with endolymph.
So the actual sense process
occurs within this inner chamber
not that outermost or
the bottommost chamber.
If we roll out the cochlea,
you can probably see this better.
So you’re going to have a oval window
in which the stapes is going to vibrate against.
It’s going to travel down that upper
component, across and down the bottom.
You can see this as is footplate
vibrates against the oval window.
The sound is then transferred all
the way to the apex of the cochlea.
And then it transfers a crossed the apex
and down along the basal chamber.
If you think about,
what is it vibrating against?
That is the organ of Corti or
where the hair cells are located.
So one is the transmission of the sound.
The other is the sense organ.
You noticed that there’s also a round window
at the end of the perilymph field vesicle.
This round window allows for vibrations
to occur within the whole fluid field system.
If you didn't have a place to relieve that vibration,
you would not be able to transmit a signal into it.
You need to have that ability to both vibrate one
side, as well as have the opposite vibrate
so that you can transmit
that vibration or soundwave.
How these works? Is the sound causes
the stapes to move against a window
and the pressure travels down
against the organ of Corti.
So by pushing down in certain spots,
think of it like, kind of like the waterbed,
where you’re pushing down on the certain part,
You’re be living that up. It has then been sensed
as a pressure change in the membrane below.
As it travels around the apex and back,
you’ll be pushing now up against that organ of Corti.
So you have now thoughts of as a
wave enters in gone from the oval window.
There’s certain spots that will hit on,
then it will travel around the apex. And then,
there’s certain spots it will hit on the way back
to the round window.
It is this resignating portion of where the soundwaves
hit will allow for which particular hair cells to fire.
So there gonna be hair cells along the whole distance
from the base to the apex in the organ of Corti.
Hair cells are very specific in how they work.
So let’s go through this process now.
You have hair cells that are located on an inner.
And then hair cells are located
on the outer portion of the membrane.
Here the inner cell once, are the once that fire
to tell you what particular frequency is soundwave is.
The outer hair cells although they’re more numerous,
are there to provide additional information.
Such as, maybe a description or a
ability to have better acuity of the sound.
But the inner hair cells tell you
what frequency the response is.
So you may ask, how can a membrane,
this tactile membrane that’s floating down
over these hair cells cause a response?
Well, if it’s pushing down the tactile membrane
you can think about that pushing against the hair cells.
If it’s coming all the way back around
the apex back towards the round window,
it’s pushing the membrane up
against the tactile membrane.
So whether the tactile membrane
is being push down
or the whole basilar membrane
is pushing the hair cells up,
either way you can get stimulation
of these particular hair cells.