The human ear is a complex and wonderful
instrument to say though that all it does is
to enable us to hear is to really understate the things
that is capable of.
It is first of all,
a collector of sound waves,
it catches the sound waves and directs them down into the
external canal so that they can find their way on
further into the inner ear where they can be received and
interpreted.
It is also an energy transducer that is,
it changes the acoustic energy of the airborne
signal to mechanical energy to
hydraulic energy and finally to electrical energy.
It is also a sound amplifier that
is,
it actually increases the loudness of the sound
after it reaches the ear.
It's also a sound analyzer that is,
it allows us to be able to interpret the
complex noise that we call speech.
And finally,
it contains the mechanism that
enables us to be able to maintain our balance
and to stay in an upright position.
Now,
I'd like to tell you some things about the ear.
But before we do that,
I'd like to review with you some of the basic notions
of sound and sound.
Waves,
how they're generated where they come from
and how they travel through the air
sound sources can be of varying kinds.
Some examples are the reed of the clarinet,
the bell,
the human voice and a tuning fork.
All of them produce sound in the same basic
way.
Let's review then the four essentials that we need for a sound to be
created and to be heard.
First of all,
we need a vibrator such as a tuning fork.
Secondly,
we need some force to set the vibrator in motion.
Thirdly,
we need a medium such as the air to convey the
wave motion created by the vibrator.
And finally,
we need the hearing mechanism itself which can receive
and interpret the sound.
I think the best example to explain how a sound
wave is generated is the tuning fork.
If you were to squeeze the tides of the tuning fork together
and then let them snap apart,
you would of course generate a pure tone.
The reason why the tone is generated is
because as the time springs back
past its original resting position,
it pushes the molecules of the air together and
causes them to bunch up,
it condenses them.
Then as it springs back again,
in the opposite direction,
it creates a vacuum or a rare faction
where there are fewer molecules than there should be.
This constant repetition of condensation
and rare faction increase in pressure,
decrease in pressure is the basis for the sound
wave.
Now,
it's important to remember that it's not the molecules in
the air themselves that travel.
But rather the pressure changes that travel through the
air to the ear.
There are two basic characteristics of a sound wave that I
think you should be familiar with.
The first is frequency,
frequency is analogous to pitch.
And it's defined as being the
number of times the
repetition of condensation and
rare faction occurs in some unit time.
For example,
in 1 2nd,
the fewer number of times in a second
that the conversation and rare faction cycle
goes on.
Then the lower the pitch,
the greater the number of times that this condensation
rare faction cycle goes on.
Then the higher the perceived pitch,
if you will look at the screen,
you will be able to see a representation of
a pure tone that's been created by an
oscilloscope.
The uppermost portion of the sound wave is that
represents the condensation portion.
The lower most portion represents the rare
faction portion.
You can see that these waves are some
distance apart and probably
represent a tone with a frequency of about
250 cycles per second
as we increase the tone so that the
frequency is close to 1000 cycles per
second,
the waves come closer together.
And we see more a greater number of
repetitions of condensation and rare faction.
In a second.
The other aspect of a sound wave that I would want you to be
familiar with is amplitude.
Amplitude simply relates to the fact that
the strength of the particle or molecular energy
is greater.
And so we have a louder sound.
This is represented on the screen by the
sound wave increasing in amplitude,
you can see that the envelope becomes wider as the sound
becomes louder.
Then as the sound becomes softer,
the envelope of the sound wave decreases and
becomes smaller.
Now,
this example,
from the tuning fork is a fairly simple
representation of a sound wave and it's
normally not one that we run into every day.
I better a different example which I would like
to show you is what the human voice would look
like if you were to put it on a
screen as I'm talking.
Now,
you can see the jumble of lines and angles on the
screen representing my voice as it comes
to you.
It's,
it's really a wonderful thing to consider the human ear that it
can take such a jumble of noise and
make sense out of it.
Now,
let's take a look at the year just how it's made and just
how it works.
The year is first of all located in the petrus
portion of the temporal bone and this is
the hardest bone in the body and nature designed
it that way to be sure and protect the inner ear from
damage.
As a matter of fact,
the word petrus derives from the Latin word
petrus,
which means rock,
the outer ear is connected to the skull by a
series of muscles ligaments and other
tissue.
Now,
most of the muscles that attach the outer
ear to the skull
are vestigial.
That is at one time,
we used to use them the same way an animal uses
them now to turn his ears to localize
sound.
But uh they are no longer of any
real use to us at this time.
The outer ear is the first thing that
receives the sound.
It's ovoid in shape and contains a
number of depressions and humps.
Its function is to collect the
sound waves as they hit it and direct them
down and into the external canal.
It provides us with about 10 decib of
amplification.
Now,
this amount of amplification is really not
significant.
As a matter of fact,
if we were to lose the outer ear,
it would not cause any real loss of
hearing for us.
So even the outer ear itself is not that useful
to us today and serves primarily a
cosmetic function.
Now,
as we move from the outer ear
into the external canal,
remember now that the sound waves coming through
the air are collected by the outer ear
and directed down into the canal where they
finally will hit the ear drum.
The external canal is about an inch long and
it's about a quarter to a half inch in diameter.
The tissue that uh is made that makes
up the wall of the external canal is the same
tissue that is connected with the tissue
that makes that up that tissue of the ear,
the hand,
the back and so forth.
There are two things about the external canal that I think you should be
familiar with.
One is that it contains a number of hairs
along this portion here.
And the function of those hairs is to protect it
from insects intruding into the external
canal.
In addition to that,
there are also in here,
a number of glands which are called saruman.
It's glands and they will
produce saruman or what's more
popularly known as earwax.
Now,
this earwax or saruman is very bitter
tasting and repugnant material.
And it also serves the function of protecting the
ear from the intrusion of insects.
Now,
the sound wave comes down
from the article into the external
canal and finally,
it comes up to the middle ear,
the middle ear is made up of the ear
drum,
The articular chain which is made up of three
tiny bones called the,
asse.
the Incas and the stay
peas.
The function of the middle ear
is to transform the
acoustic energy into mechanical
energy and also to increase or
amplify the amount of sound that that
is reaching the ear.
And it does this by means of what is
called the a real ratio.
That is if we take the ratio of
the area of the Eardrum to the
ratio of the area of this foot plate.
Here,
we find that the increase in sound is
about 18 times.
Now,
it's necessary for this to occur because
the sound after it leaves,
the middle ear will go into the inner ear,
which is filled with fluid.
And it's necessary to overcome the impedance which
is offered by the fluid,
the fluid in the inner ear.
There are two muscles in the middle ear,
which we should be familiar with.
And those are called the tensor tympani.
And the step PDS muscle,
the tensor tympani originates here and comes
across the middle ear space to the handle of
the malas.
The step pds muscle originates on the
posterior wall behind these bones
and it inserts right here in the head of the stay peas.
These two muscles serve a protective function
that is whenever a sound which is loud enough to
cause damage to the ear occurs,
they contract very quickly and they
pull the articular
chain off its normal course of action.
And so it protects the inner ear from uh
from damage.
Now,
when the sound wave it comes
down into the external canal and it hits the
eardrum,
it causes the eardrum to vibrate at the same
frequency and amplitude as the sound as the
original sound.
The articular chain is connected to the eardrum.
And so that also vibrates at the
same frequency and amplitude.
And so the sound wave is now conducted across this
middle ear space up to the inner
ear as as of yet
though no,
no hearing has taken place.
The only thing that's really happened is that the sound
wave has been conducted up to the inner
ear.
The other part of the inner ear that you should be familiar
with is something called the U station
tube.
And this is located in the
anterior inferior portion
of the middle ear space.
The station tube runs from the middle
airspace down to the nasal firing.
So to the back of your throat And it
serves two functions.
First of all,
it provides a constant source of oxygen
to the middle ear space.
The second thing it does is to is to provide an
equalization of pressure whenever there are
changes in altitude.
If you were to experience a
severe change in altitude,
for example,
the pressure on the outside of your ear
would be less than the pressure on your,
on the inside of your ear.
And it would force the ear drum outward.
If you yawn,
then you open the station tube and it allows the air
to come up into the middle ear.
And so it equalizes this pressure.
I'm sure if you've flown in an airplane,
you have probably experienced this and have had
to equalize that pressure in order to get rid of the
uncomfortable or stuffy feeling that you find in your
ear.
Now,
once the sound wave again,
has crossed the middle ear,
it enters the inner ear
it is in the inner ear that the essential organ of
hearing is contained.
The inner ear contains also the balance
mechanism.
And this mechanism is made up of the three
semicircular canals,
the sack you'll and the you trickle.
And this mechanism,
all of these parts working together provide
us with a sense of balance to keep ourselves upright.
This yellow line that you see here is the
vestibular portion of the auditory nerve.
The other portion of the inner ear or the labyrinth is
called the cochlea.
And this is a spiral or snail shaped
bony labyrinth within which the
essential organ of hearing is located.
This portion of the inner ear is called the
vestibule.
And you can see that there are three channels here.
This is the scale of vestibular or the
vestibular channel.
The scallop media or the middle channel and the
skeleton pony or the tim panic channel.
This area in here is filled with a fluid
which is called parallel.
And this middle channel floats
in that parallel and it protects it from
bumping up against the walls of the bony
uh capsule.
The fluid which is inside the scallop
media is called indolent.
And this is much different than the parallel
and arises from somewhere near the cortex.
Now,
when,
when we hear
the acoustic signal,
which has been changed to uh mechanical
energy now has to come into the inner
ear and be somehow changed into hydraulic
energy and then into electrical energy.
Now,
what I would like to do is to show you a
more detailed version of what the cochlea
looks like and just what it's made up of.
Now,
this picture over here is a much
more detailed version of the cochlea.
Right here.
We have the auditory branch
of the 8th cranial nerve coming on
up into the cochlea and branching
off at every level.
You can see the snail shaped appearance
of the cochlea.
And this picture shows it as though it had been
cut straight down the middle.
The arrows that you see indicate the
direction uh the force,
the direction of the force of the
energy which is created by the sound wave,
it moves on up and then at the
top turns around comes back down.
Again.
Now,
here's an even more
detailed
picture of of the cochlea.
This is just one section of it.
And again,
you can see a fiber from the eighth cranial
nerve coming on through the spiral lamanna to
connect with the essential organ of hearing.
This is the scalar vestibular,
the scalar tympani and the
scalar media within
the scalar media contains,
as I said,
the essential organ of hearing,
the scalar media is bounded by
prisoners membrane here,
the base layer membrane here
and the spiral ligament
here
within the scalar media,
we find the sectorial
membrane
and the organ of corti,
the pink area
in the organ of corti.
You'll see hair cells
located along its whole length.
Each of these hair cells is connected
to the nerve of hearing
when they say no
comes from the middle ear into the inner ear.
A number of things happen.
First of all,
the stay peas moves inward
as it does,
it compresses the fluid in the inner ear.
Now that pressure has to be released somewhere.
And right down here in the skeleton pony is something called a
round window.
The staples moves in through the over window,
compresses.
The fluid causes it to move.
That pressure is released at the
round window.
So the signal again comes
by way of the article to the external canal.
It has changed to mechanical energy and
amplified across the time panic membrane of
secular change structure.
And now it is causing the fluid of the inner ear
to move.
And so now it has been changed into hydraulic
energy.
You can see the dotted lines here
and they indicate that as that fluid
is moving through the inner ear,
it causes this membrane to bulge
inward.
That membrane causes the fluid in here to
move.
That in turn causes the tech tutorial membrane to
bend downward.
And now you see the hair cells here which are connected
between the organ of corti and the tech
tutorial membrane.
When the tech tutorial membrane bends downward,
it distorts or bends these hair
cells out of shape as soon as it does
that,
that triggers off an electrical
signal,
Which is then picked up by the fibers
of the 8th cranial nerve and
carried on up through the central nervous system to
the cortex.
The movement of the fluid,
of course,
being pressure continues on.
And you can see the basilar membrane has
bent down also
that causes this fluid to continue
moving.
And if we can go back over here again for a minute,
it would be released here at the oval
window.
Let's take a look at
schema ties version of
the inner ear.
Remember the inner ear is coiled
snail shaped of 2.5 turns.
And so we've schema ties it a little bit here in order to
show you in some detail,
just how it works.
The sound comes down through the the external
canal hits the
eardrum that is connected to the articular
chain.
Now,
the foot plate of the stay peas moves into
the the vestibule and
it compresses this fluid in here.
Now,
you can see that at this point,
this continuation or movement of fluid
through the inner ears taking place,
risers membrane is bending downward.
The fluid continues to compress
it causes the basal membrane to bend
and comes right on down through and finds its release
here at the round window.
These arrows here indicate
the pressure which is not
necessary here or not,
uh which is not necessary at this
point going on up around through the
whole system and eventually coming out through the round
window.
The basil er end of the
inner ear contains those
uh hair cells and fibers
which are responsible for low pitched
sounds.
I beg your pardon for high pitched sounds.
The a pickle end up here is responsible
for those uh hair cells and
fibers which are responsible for low
pitched sounds.
Once the the
hair cells have been excited and trigger off this
electrical response,
they will excite the fibers of the eighth
cranial nerve And they all come
together in a bunch
of,
of nerve fibers,
which is called the spiral ganglion.
And then they find their way on up through the
central nervous system to the cortex where the
sound is interpreted.
Now,
what we've been discussing so far is the way we hear
by what is called air conduction.
That is the signal travels from the air
directly into the ear canal into the inner
ear.
This is the way that we normally hear in most everyday
situations.
There is another way though that we can hear and it's called hearing by
bone conduction.
Now,
if we can take a look at this chart,
again,
recall that the inner ear is
encased in a capsule of bone and that bone is
part of the skull.
When a sound wave is strong enough to cause the
bones of the skull to vibrate,
those vibrations can cause the
fluid in the inner ear to
move and to carry out the
same essential process as
happens by air conduction.
Now,
this is normally not important for us as long as
the outer ear and the middle ear are intact.
But for those persons who have some
pathology of the outer and middle ear,
who may perhaps need a hearing aid,
we can provide a hearing aid that would put a
vibrator right here on the skull and
that would cause the bones of the skull to vibrate
at an in an amplified way,
excite the inner ear.
And so allow him to be able to hear
normally.
Now,
let's just take a look again at the
whole process of hearing by air conduction.
Remember that we have a signal which is generated,
for example,
by a tuning fork,
it generates a number of compensations and
rare factions changes in
pressure.
Those pressure changes find their way through the
air are collected by the external
ear and forced down into the ear canal.
They reached the eardrum secular
change structure where they are changed to mechanical
energy and amplified.
They then find their way into the inner ear when the
foot plate of the staples moves in and out in
time to the signal that came into the year
originally.
And that causes the fluid to move,
causes the membrane structure to move.
And so excites the nerves of hearing from
there.
They then find their way on up through the central nervous system
to the cortex where they're received
analyzed and interpreted.
Now,
that's essentially how the ear works.
Uh it's come a long way since the time when it
was a simple system for that in a
fish millions of years ago.
So that the fish could detect the pressure changes in the
water and maintain an upright and balanced
position.
We've learned a lot about it since the early days of,
since 1950,
with,
with the invention of the electron microscope,
but we still have a lot more to learn and I'm
sure that in the next few years we'll,
we'll see a lot more being added to our store of knowledge
about the year and how we hear.