CDIS 4027 Middle Ear


Hi. So in this lecture, we’ll start talking
about the middle ear. Before we progress with the middle ear, let’s quickly review
some of the anatomnical orientations. So you’ve got superior, with structures
towards, facing towards their head, inferior, away from the head, anterior, towards
your nose, posterior, at the back of your head. Medial is towards midline, lateral is away
from the midline, proximal is close to the midline, distal, away from the
midline. And many of the figures, we might look at them as a sagittal section or
a midsagittal section, where the body is divided into two exact halves. In the
middle ear, lies in between the external ear and the inner ear. And it’s mainly
responsible for conducting the sound energy from the external ear to the fluids in
the inner ear. It’s also known as the tympanic cavity. From an evolutionary
biology point of view, the middle ear actually is considered a signature feature, anatomical
feature, and it’s believed to be what’s made us able to listen to high pitched sounds.
We do know that animals, and fishes in the water, do hear, when they do have an auditory
system, but they’re not as efficient as mammalian auditory system is, essentially
because they don’t hear the high pitched sounds well. But the middle ear apparently has
made us able to hear this high pitched sounds and now we know that we, listening to the high pitched sounds is important
because that’s what enables us to localize, or detect, the direction of the
sound source. Again, an evolutionary basis from a survival point of view that
was important. So the majority of the sounds that we
encounter are traveling through the air medium. And that’s not a problem when
sound travels through external ear, but ultimately for us to hear that, that
sound needs to be converted into a form that the human nervous system can,
can understand. And that’s, that is done by the auditory transduction within the
inner ear. We know that the inner ear is fluid filled. So, we’ve got the air filled
external ear and we’ve got a fluid filled inner ear, where the sound needs to be
conducted in whatever form it can be, can be converted by the auditory nerve
and taken towards the brain for it to be interpreted. Now, the problem comes when
the sound energy travels from an air medium into a more denser medium, like a fluid medium within the inner ear. You probably would’ve experienced this when you’re underwater
and how sound is muffled and even if somebody’s yelling at the top of the
surface of the water. And that’s because when energy travels from a less dense
medium to a more denser medium, much of this is reflected back. In terms of water, which is about five
times more denser than air, almost about 90 percent of this energy is reflected
back. Now, since the dB scale is a log scale, it kind of translates to a 30 dB loss. So this loss of energy, between when sound travels from a less denser medium to a denser medium, is
what we call as impedance mismatch. Now the middle ear’s main role is to overcome
this loss of energy because of this impedance mismatch, to effectively conduct that sound from the external ear to the inner ear. So something, what we call as
impedance mismatch, the loss of energy. Let’s quickly review the anatomy of the middle ear. So the adult middle ear cavity is almost oval shaped and it’s about 2 centimeters cubed in
dimensions. The middle ear is enclosed, it’s completely covered by this important bone of
the skull, known as the temporal bone. And the middle ear cavity is actually lined by
this moist mucous membrane, which, just like the membrane in our nose,
results in this mucus, mucus fluid that is periodically
removed by our intact Eustachian tube. And we’re going to be talking about the Eustachian tube in a bit. The middle ear cavity also encloses
those three small bones that we call as ossicles, that actually connect from
the lateral wall to the medial wall. So the temporal bone is a bone that lies
behind the ears, around the ears, and the part of the temporal bone is this mastoid process, that’s just refers to
the part of the bone just behind our pinna. The mastoid part of the temporal
born, unlike much of the skull, is actually a porous bone. It’s an air filled bone and
there’s a number of functions that’s attributed to that. Because it’s lightest part
of the the skull, it’s believed to, to make our head actually lighter than the size,
enabling us, the head to float when you’re in water. But from an acoustical point of view, the
mastoid air cells play an important role by improving the resonance of sound, so
improving the resonance of our voice. From a clinical point of view actually,
the mastoid air cells can be troublesome because now, an infection. If you’re
having a middle ear infection, resulting in a condition known as mastoiditis,
probably we’ll talk about when we talk about the disorders of the ear. So the middle
ear, or the tympanic cavity is separated from the external auditory meatus of the
outer ear by this tympanic membrane. It’s all separated by, separated from the inner
ear by this oval window and round window, of the two windows, it opens into the
inner ear. And schematically, it’s easier to study the middle ear cavity as a six
sided cube, so it’s got six sides. So if you’ve got the lateral side, lateral wall
over here, which would be majority of that would be the tympanic membrane. In
this illustration, the anterior wall is actually removed; that’s like the front
wall, the side of your face, the side of the face. You’ve got the medial wall, which would be the wall that
separates the middle ear from the inner ear. We’ve got roof, which would be the top and
the floor, the base, and then opposite to the anterior wall, towards the mastoid air cell side would be your posterior wall. In here, you can see the tympanic membrane connected
to the inner ear by this chain of ossicles, beginning with the malleus, then the incus, and the stapes over here. Let’s talk about each of those walls and some of the
important anatomical landmarks in each of those walls. So you’ve got the anterior
or also known as the carotid wall. It’s got one of the important arteries
supplying, actually, for the head with the internal carotid artery. It also has the
origins of one of the two middle ear muscles, the larger of the two middle ear
muscles, known as a tensor tympani. And it also has the origins of the Eustachian tube.
The Eustachian tube, or the auditory tube, is a tube that connects the middle ear space to the nasopharynx. And we’ll be talking about the Eustachian tube in a bit. Opposite to
the anterior wall is your posterior wall. It’s the wall that separates the middle ear space from the mastoid process and the mastoid air cells. One of the important anatomic landmarks of this posterior wall is this pyramidal eminence, which is but a small spike, a small bony spike from
which the stapedius muscle, the second middle ear muscle and smallest of the
muscles, well actually, the smallest the muscle in the human body, is connected
to. The stapedius muscle connects the posterior wall to the stapes ossicle,
while the tensor tympani muscle connects anterior wall to the, to the
malleus, the larger of the three ossicles. Then we’ve got the medial wall, also
known as a labyrinthric wall. It’s the wall that separates middle ear from the inner
ear. And some of the important landmarks in this medial wall is the oval
window, to which the stapes is connected to and below the oval window is your round window. There is a bony, almost like, shell that separates the oval window and the round
window and it’s known as a promontory. And the promontory is actually caused by
the first turn of the cochlea. Towards upper side of the labyrinthic wall, of the
media wall is the prominence of the facial canal, which is actually kind of a
duct through which the facial nerve, the seventh cranial nerve, travels. Here’s a schematic
illustration of the tympanic, or the middle ear cavity, as a six sided cube.
So let’s look at this figure first. So here, this is a view of the middle ear
cavity, with the anterior wall, the wall where, well, the face is removed and. So here, you
can see the tympanic membrane and this you’re seeing the promontory. So, to the tympanic membrane, we’ve got the
malleus, connected to the incus and in turn, connected to the stapes, which
ends in the oval window at the top of promontory. So here, you’re seeing the
prominence of the facial canal through which the facial nerve travels. In this illustration, you can also see the tensor tympani muscle, which actually
originates from the anterior wall that’s missing in this figure, while the stapes
would be the one that’s connected to the, the stapedius, pardon me, is one that’s
connected to the stapes and that originates from the posterior wall, the wall that’s opposite over here. In this
illustration, we’ve got the tympanic membrane removed and the tympanic
membrane, which would be a lateral wall is removed, so if the tympanic membrane was to be there, would be somewhere like that. That, in turn, it is connected to the
malleus, incus, and stapes, which are also removed in this
illustration. Here would be your anterior wall and that would be your posterior wall. So the anterior wall, you’re seeing the Eustachian tube, and then you’re seeing the
regions of the tensor tympani muscle over here. In the posterior wall, you’re seeing the pyramidal
imminence from which the stapedius muscle originates. So the medial wall, we
have this two windows, oval window and round window. The round, oval window is
occupied, actually by the base, the platform of the stapes and it’s kind of
surrounded by a ring of bone, just like the tympanic membrane is surrounded by a
ring of bone. The round window is situated below the oval window and it’s separated by this
rounded bone, known as a promontory. The round window is also known as a secondary tympanic
membrane, because it also has three layers, just like the tympanic membrane does. And the main
role of this round window is to dissipate pressure ccaused by the oval window moving
in and out. And again, the oval window moves in and out when the ossicles are
moved by the tympanic membrane. So in other words, when sound results in
oscillating the tympanic membrane, the oval window is pushed in and out and almost out of
phase, the round window pops out and in kinda to dissipate this pressure because
the fluid movement within the cochlea is the bony compartment. So whenever any
pressure is made, it has to be released and that seems to be the main purpose of the
round window. So this promontory is a prominence created by the first turn of
the cochlea and it’s what separates the oval and the round window. As I said,
the seventh cranial nerve, or the facial nerve, traverses at the superior part of this
medial wall and those are important landmarks, especially for surgeons who are trying
to do surgical procedures within the middle ear or within the inner ear, such as
when they’re putting a prosthesis for a stapedectomy, or when they are
inserting the electrodes for a cochlear implant. So these important landmarks
needs to be considered when doing those procedures. Here, as an illustration, again
with their anterior wall being removed, the tympanic membrane, the ossicles, with the
middle ear muscles, the larger of the middle ear muscles would be your tensor
tympani, that originates just above the opening of the Eustachian tube and then on
the other wall, opposite wall, the posterior wall, you’ve got the stapedius muscle. You can see
that it inserts into the stapes. The stapes, stapedius muscle actually
has an important function that we’re going to be talking about when we talk about the acoustic reflexes. An important landmark in the anterior wall would be an important
structure, an anatomical structure would be your Eustachian tube, or the auditory tube. It’s a channel that
connects the middle ear cavity to the nasopharynx, upper portion of your oropharynx. It’s about three and a half centimeters
in length and it’s oriented such that it moves downwards and forward and medial-wards. In other
words, it moves from the ears to the nose on both sides. It’s got two parts, the Eustachian tube, the part
that’s close to the middle ear is, is ossoeus or bony, and then the part
that’s towards the nasopharynx is made up of cartilage, that’s the one that’s pliable.
So it, it has sphincters that the closes the Eustachian tube. The diameter of the Eustachian tube is
not uniform, it’s almost like a funnel-shaped on both sides, while
there’s a narrower part in between which we call as the isthmus. In adults, the tube, the Eustachian tube is normally kept closed by an action of three sets of muscles from the
nasopharynx. One of those muscles is this tensor veli palatini, which is also an
important muscle for the soft palate. And these muscles contract and open up the
Eustachian tube when one is yawning, sneezing, or when you’re swallowing, or when there’s excessive pressure on either side, from the nasopharynx or in the middle ear. There’s a difference between the
Eustachian tube that you see in infants, especially younger than 6 months, and those in
adults. In infants, the Eustachian tube, for one, is less inclined. it’s more straight
than, than what you see in adults. And also the muscles that are in the nasopharyngeal end of this tube are not completely developed, so it remains mostly open. And
that’s quite important clinical significance and it’s the reason why
mothers are advised to hold and feed their child in inclined position because there’s
more chances of any fluid, or milk in this case, entering into the middle ear cavity because
the Eustachian tube is not completely developed. And that’s also one of the
reasons why infants are more prone to middle ear infections, because of
bacteria easily entering into the middle ear cavity until the Eustachian tube develops and matures. And the Eustachian tube plays an important role towards middle ear function,
optimal middle ear functioning. So for the middle ear to function optimally, you need a mobile, efficient tympanic membrane that’s
sensitive to even small pressure fluctuations created by sound. So for
that to happen, the air pressure on both sides of the middle ear, well both sides of the tympanic membrane, must be equal. In other words, the pressure in the
middle ear space needs to be the same as the pressure in the atmosphere, or the pressure within the external ear. If
there is a difference between the pressure on both sides of the tympanic
membrane, in this case, the medial side of the tympanic membrane is where the middle ear is,
and lateral side of the tympanic membrane is the external ear, this
difference in pressures is gonna stiffen the tympanic membrane, thereby reducing
the efficiency in which sound is transmitted inside into the middle ear. So, if the middle ear
pressure’s less than that of the external environment, in other words, if you have
a negative middle ear pressure, the ear drum, or the tympanic membrane, is going to be kind of
sucked medial-wards, or in. On the other hand, if you have a positive middle ear pressure, in
other words, if the middle ear pressure is, is higher than the external environment’s
pressure, then the tympanic membrane is going to be kind of bulging out, or more lateral. In
either situation, the tympanic membrane mobility is gonna be restricted, or
reduced, thereby reducing the efficiency of sound traveling inwards. So, this is
where the Eustachian tube comes into play. The main role of the, the Eustachian tube
is to kind of equating the pressure on both sides of the tympanic membrane. And
it does so by periodically opening up the Eustachian tube and letting air in, or
in some cases, out. So, the Eustachian tube is responsible for
optimizing this middle ear function by ventilating the middle ear cavity. It’s main function, again, is ventilation
or equalibrating air pressure of the middle ear the same as that or as close
as possible to that of the external environment, or the pressure in the external ear.
Even within a normal middle ear system, what happens is the middle ear tissues, which are mucus
lined, continously absorb air. So, if not for the Eustachian tube, you actually might develop a
up the middle, a negative middle ear pressure. So that Eustachian tube needs
to periodically open to kind of equate the pressure on both sides of the tympanic membrane. And
one can do that manually also, by this Val Salva maneuver, where we pinch
and, and blow up our cheeks, pinch our nose and blow up our cheeks and trying to push air,
open the Eustachian tube, push air into the middle ear. Another function of the Eustachian tube is periodically draining this middle ear secretions into the
nasopharynx. So it moves fluid from the middle ear outside into the nasopharynx. In the other direction, it also protects the middle ear from
potentially infectious organisms that can travel from the nasopharynx
upwards to the middle ear space. So that’s why it’s periodically, when it’s
closed in adults and preventing this infections entering it.

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