So we're continuing with our lecture on the middle ear anatomy and function. So the
middle ear includes these three small bones that we call as ossicles, namely the malleus, the incus,
and the stapes. The malleus is attached to the tympanic membrane on the lateral
wall of the middle ear cavity, while the stapes is attached to the, to the oval
window on the side of the inner ear. And the incus attaches the malleus to the
stapes. And all these three bones are dangling within the middle ear space by
and attached to the different walls by five ligaments. These ligaments do such an
incredible job, such that the
movement of these, this chain of ossicles is almost negligible in inertia. In other
words, no energy is lost as ossicles are being displaced or moved by the
tympanic membrane. The malleus is the largest of the three ossicles. It has a head, a neck,
and it's got three processes, the largest of which is the manubrium. And then it has
a anterior and lateral, an anterior and lateral process.
The lateral tip, the end of the manubrium is what's attached to the tympanic membrane,
at the point that we call as a the umbo, retracting the tympanic membrane inwards,
giving it its conical shape.
The incus consists, it's kind of a triangular shaped bone with a long process. So that's the
triangular shaped part of the body and then it's got two crura. And the end of the
long crura is the lenticular process and it's the point of the incus that
articulates with the stapes.
The stapes, the smallest bone human body, consists of the head and neck and it's got two crura,
that attaches to an oval-shaped base and it's a space that fits right in,
it's of the same shape of the oval window and it fits right snugly into the
oval window. The head of the stapes has a depression and that's what is connected
to the lenticular process of the incus. And as I said, the base is what's fixed
to the oval window and normally, it gently articulates with the oval window. One
of the significance of this junction of the base of the stapes and the oval
window is some condition, a condition on as otosclerosis. This is where there's a
spongy bony growth. And this condition, otosclerosis, may run in families and you
probably might have heard somebody having to have a middle ear surgery, and typically
it's for this condition where they actually removed this stapes and it's normal
bony growth and put a prosthesis instead of that. Something that we might touch
on when we talk about the disorders of the ear.
There's two muscles within the tympanic cavity, the tensor tympani and the
stapedius. The tensor tympani is the larger of the two, still about only about
2.5 centimeters in length. It originates from the anterior wall of the middle ear
cavity, just above the Eustachian tube. And it, it attaches itself to the
manubrium of the malleus. Whenever we're talking about a muscle, we need to talk
about what nerve innervates that muscle. In the case, of the tensor tympani, it's the
trigeminal, or the fifth cranial nerve. So when the fifth cranial nerve is
activated, it actually contracts the tensor tympani muscle. When it does
that, it draws the tympanic membrane inwards and thereby increasing the
tension of the tympanic membrane. The second middle ear muscle is the stapedius.
And it is, again, it's the smallest muscle in the human body, with the length of
about only seven millimeters. It originates from the posterior wall, the
mastoid wall. And the stapedius muscle is innervated by the facial nerve, which is the seventh cranial nerve. Again
it's important to note, it's two cranial nerves traversing through the middle ear
space, namely the trigeminal and facial nerve. Both of these nerves have
important functions in controlling your facial muscles and taking sensory
information from the facial, from the face area. The clinical significance of
that is, if there is a longstanding middle ear infection, there is a chance
that it might result in paralyzing these nerves and hence, one of the things
that as an audiologist you would notice when you initially, you doing this
physical examination of the ear
is to directly assess the intactness of these nerves by asking the person
to smile. And you're looking at the symmetry of the facial muscles. Going
back to the stapedius, the stapedius muscle, when it's activated by the
seventh cranial nerve, or the facial nerve, it pushes the stapes into the oval
window. And again it tenses this ossicular chain and that plays an important role
actually plays a protective role. The acoustical stapedial reflex is when
the stapedius muscle is activated and in this case, it's activated by relatively
loud sounds. If the sound is about, more than moderately loud, like 70 to 90 decibels
above one's threshold, the stapedius muscle is activated, stiffening the ossicular
chain. What that does is, it serves as a protective function by reducing the
amount of sound entering the inner ear. And the reduction of sound can be as
much as 5 to 10 decibels, especially around frequencies, at frequencies below
about 1.2 kilohertz, especially reducing those low frequency sounds. And the
recent understanding is this stapedial reflex primary plays a role in reducing one's
own voice, the volume of one's own voice, especially for the lower frequencies. So
on the other hand, if there's a clinical condition that affects the stapedial reflex
contraction, apparently one's own voice would sound more boomy, in other words, there's going to be
more amplitude in the lower frequencies.
This stapedial reflex does have a protective function, it reduces the amplidute of the
sounds that are allowed, but it does have about 60 to 120 milliseconds delay. So
wouldn't be effective in reducing those impulsive sounds, like a loud sudden
explosion or that from a gunshot. Also this reflex is sustained only for
about two minutes. After that, it's relaxed. In other words, you lose a
protective function, so if you're having longstanding noise, like factory noise,
then you need to wear ear protection. The stapedial reflex might not be enough.
Nevertheless, the acoustical stapedial reflex is a reflex, it's automatic and
in audiology, it's used as an objective tool to assess the one's hearing. It is an
important test within the middle ear immittance battery, something that we will
be talking about the next lecture because it's reliable and it's, again, it's
objective, in other words, it doesn't require the participation of the patient. So often,
it's used as a tool to directly assess one's hearing levels and it's very
worthwhile doing that for difficult to test populations, like for instance, children,
where we want an objective means of assessing the hearing.
Again,
just to stress, I've got a number of videos, many of them are YouTube videos that are linked to the
PowerPoint slides. You may not be able to open them from the video lecture, but
if you were to open and view through the PowerPoint slides, you should be able
to link that and that'll take you to different websites and different videos.
Some of those videos might be the content might be redundant, but they stress
on different aspects, relevant to this lecture. So, I urge you guys to visit those
videos for more information.
Earlier, I mentioned about some ligaments. There's about five ligaments that
actually hold these three ossicles and those ligaments, you can see over here, that
these are like, there's three ligaments attaching the malleus to the walls of the
middle ear space. And there's one each for the incus and the stapes.
Again, that, those are the ones that are responsible for kind of, holding the
ossicular chain, dangling within this air filled middle ear space, with a minimal amount of
inertia.
Going back to the function of the middle ear, the beginning of the previous
lecture, we talked about how the main function of the middle ear is to
compensate for this loss of energy when sound travels from an air
medium in the external ear and when it tries to reach into the inner ear, which is having
a fluid medium. So sound, for that matter any form of energy, when it travels from
a less dense medium, like air, to more denser medium, like a liquid medium, a
majority of the energy is being reflected back. So, in our case, it actually might
be as high as 99.9%, and when we convert that into a decibel, a nonlinear decibel
scale, that accounts to about
30 dB loss. So the middle ear's main function is to kind of match this
impedance so that we don't lose energy when sound travels from the external
ear, reaching into the inner ear. It does so by two main mechanisms, one would
be the area ratio, another would be the lever action. The area ratio refers to the
difference in the surface area of the tympanic membrane, in relation to the
surface area of the oval window. We know that pressure refers to force per unit
area. In an earlier lecture, we talked about, there's two different ways of
increasing pressure. You can either increase the force or you can decrease
the surface area. So in this case, the force remains the same but surface area is
much reduced when you're moving from this much wider tympanic membrane to the
smaller oval window. In fact, there's about of 14 fold increase in pressure
and when that's converted into decibels, that amounts to about 25 dB increase in the
amplitude.
Again, just thinking about the analogy of how the needle of a synringe, because
it's sharp, results in more pressure, such is the case over here, when the same force is
applied to a smaller area, there's more pressure. So increasing the, the
loudness by 25 dB. Another way where this impedance mismatche is compensated, is
by the lever action. The sound from the external ear, converted to mechanical energy by the
tympanic membrane because it
ossiclates, it vibrates, depending upon the pressure and the frequency of the sound.
That, in turn, moves the ossicular chain. As it moves through the ossicular chain, it moves
through the longer malleus to the relatively shorter incus. So the malleus is
about 1.3 times longer than that of the incus. And we know, based on lever action, that
if you put a force in the longer arm of the lever, that results in more force in
the shorter arm of the lever. And in this case, that increases the intensity,
or amplitude, by two more, two more decibels. Thus, out of the 30 dB that we
lose when energy travels from an air to a liquid medium, about 27 dB is compensated
by this middle ear actions. And it seems that this middle ear actions is more,
it's greatest for the lower frequencies. In other words, the lower frequencies
gain is much better than the higher frequencies.
To summarize, in the middle ear, we have three bones, the three ossicles, two muscles, our
tensory tympani and stapedius, each of them are innervated by one cranial nerve. The
tensor tympani innervated by the trigeminal cranial nerve, or the fifth cranial
nerve, while the stapedius is innervated by the seventh, or the facial nerve. We've got two windows, we've got the oval window
and then the round window. The stapes footplate, the end of the stapes, is attached
to the oval window, while the round window's purpose seems to be to dissipate
the pressure that's created when the stapes moves in and out of this bony
inner ear compartment. It's got one opening, the Eustachian tube. Again, in a normal cases, normal adults,
the Eustachian tube remains closed, but it's only active, it's open when you
sneeze or when you swallow. And optimally, it needs to be closed,
the Eustachian tube needs to be closed, to maintain a constant air pressure within
the middle ear space. That's the reason why when it's open, actually, those intermittent
times, it actually might reduce the performance, in a way, of the middle ear
ossicles, the middle ear movement. The reason why sound, the speech sounds muffled or it's difficult to
hear when you're chewing, swallowing, or yawning. Again, to review the functions of
the middle ear, the main function is this middle ear transformer action, where
you compensate about 27 dB of the 30 dB loss on average by the lever action, by
the area ratio. Then you have the acoustical stapedial reflex, which is an
important physiological reflexive action of the stapedius muscle.
It seems to have a protective function, protecting the delicate inner ear
structures from loud sounds. But for us and clinical audiology, it also serves as
an important objective diagnostic tool, because if you see the reflex, in other words,
if you see, you give a loud sound and if we see stiffening of the tympanic membrane, that
kind of tells us that the pathway, this reflex pathway that includes the inner
ear, the hearing nerve, and the stapedius muscle is intact. So it lets us
objectively assess all the structures within the middle ear and the inner ear
for that matter. Again, something that we're going to be talking about the following
lecture, we'll talk about the middle ear assessment. The Eustachian tube is a tube that connects the middle ear space,
it originates from the anterior wall of the middle ear, leading into the
nasopharynx. Its purpose is to equalize the pressure on both sides of the tympanic
membrane, the middle ear and external ear pressure. And that's important to
maintain an optimum vibrating surface of the tympanic membrane. And it's used, in
cases, in clinical cases, where that's affected, like say if you have an
upper respiratory tract infection, when the Eustachian tube, it's not doing it's job and
actually results in a negative pressure within the middle ear. And that actually
might progress into a middle ear infection, resulting in hearing loss, what we call as a conductive hearing loss. And
we'll talk a little bit more about that when we talk about pure tone audiometry
and conductive hearing loss.