Auditory System: Neuroanatomy Video Lab – Brain Dissections


>>Today we’re going to talk
about the auditory system. It’s just beautiful, but it’s
very tiny and hard to see. But I want to dwell on it,
because many of the problems with the auditory
system have to do with the external
conductive mechanism or the sensory neural hair
cells and nerve and not with central connections. So we’ll start with a
little more detail here. I have a beautiful dissected
and isolated and prepared and painted temporal bone. This temporal bone is from
the left side of the skull, so it would fit in my ear
approximately like this. My external auditory
meatus would correspond to this area here. And let’s now bring it down here to the zygomatic
arch coming forward. And this is all the squamous
part of the temporal bone. I’m going to put it down here on
the table, and now we’re going to look at the details
on the inside that have been prepared for us. What we can see is the imprint
of the middle meningeal artery that really runs in the dura,
but it makes a groove here in the bone, very important for lacerations of
the temporal bone. We can see the blue vein, which
is draining blood from the brain into the internal jugular vein. We can see the internal carotid
artery, which is very important with this relationship. And now if I tip this, you
can see the teeny weeny little cochlea. The cochlea, which is two
and a half snail turns, looks like a seashell,
and you can see one of the painted canals of the semicircular
canals here in green. And back in here we have
the mastoid process. And if we open it up,
we can see the air cells that are inside the mastoid. This is the mastoid process here
where you put your tuning fork to test for conduction. And here are cross-sections of
the facial nerve, which travels through the internal auditory
meatus and exits the brain, separate from the
auditory system. But it’s traveling through
their, and it’s often involved with auditory disease. And here is one of the
semicircular canals. And nicely shown here is
the inside of the eardrum. The external auditory
meatus, which is here. If you look through
it with your otoscope, you will see the other side
of the tympanic membrane. Here two of the ossicles
that vibrate with sound waves are shown here. The incus and the malleus. And now let’s go to a larger
model where we can look at these things in more detail. So this temporal bone model is
from the right ear, the opposite from the last one
we were looking at. And here we have the
external part of the ear, which is gathering sound
waves and funneling them into the external
auditory meatus. A word of warning, when you’re
testing facial sensation, avoid testing the skin around
the ear, because it’s innervated by several cranial nerves,
five, seven, nine and ten. So unless you’re an expert on
embryology, be a little careful and remember that more than
one cranial nerve innervates this area. Now we can look into the
external auditory meatus here, and if we were to have enough
light we would see at the end of the tunnel there is a
membrane, the tympanic membrane. And that is what you look
at with your otoscope. So now let’s look at an even
larger model, and we’re going to take it apart and we’re
going to go into the middle ear. So here’s our even bigger,
better model, mastoid process, right side of the head. And now let’s rotate it
and look at the inside. So now this end is
anterior toward the nose. And this end is posterior
toward the occipital region. And what I want to do
now is, this is going to be the inner ear, but we want
to start with the middle ear. So I’m going to remove this
part with the inner ear. Now we have a really good view
of the tympanic membrane here as seen from the middle ear. And we can see the three
ossicles, the three bones. The one the malleus,
the incus and the stapes that are attached here for
amplifying the sound wave that is coming from
the external ear. And the stapes here,
this blue footplate, is compressing a membrane
called the oval window. And this is the place
where the air conduction of the wave is transferred into
a waterborne wave of sound, because I’m going to show
you in a moment the cochlea and the area where the fluid
compartments will move the sound wave to stimulate different
parts of the cochlea and different hair cells. And of interest is this white
nerve coming across here. This is called the
chorda tympani. It happens to be a branch
of the seventh nerve, a branch that is carrying taste
and going over to the tongue. It’s very incidental,
but it’s very obvious. And the other thing
that I want you to notice is the
Eustachian tube. This is the tube that
connects the middle ear with the back of the pharynx. And so this is how you
equalize the air pressure between the oral cavity
and the middle ear. This channel can easily be
swollen and occluded in any kind of inflammation in the
oropharyngeal cavity. Infections can then build
up in the middle ear. This condition is otitis media. And fluid can accumulate
and press against the tympanic membrane so that the tympanic membrane
can bulge out and can be viewed with your otoscope from the
external auditory canal. Also the relationship to
the mastoid air cells means that the infection could
also move into the bone and become even more serious. So disease of the
middle ear has to do with either the bones not
conducting or moving properly, or fluid or infection are
the most common problems when we deal with
middle ear disease. And when we test for hearing, when we say there is a
conductive loss of hearing, we’re talking about either the
bones are not working right, or there is a problem in
the external auditory canal. And the sound waves are
not getting through. So now let’s look
at the cochlea. The inner ear is sometimes
called the labyrinth. I think that’s an
appropriate name for it. And let’s just remove it. It sits in a bony cavity. So there are two parts
to this labyrinth, and outer bony labyrinth
and then inside it, a membranous labyrinth suspended with fluid outside
and fluid inside. So now we are looking at
the membranous labyrinth. And if I rotate it like this, you can see these
two pink spots, the oval window and
the round window. The oval window is where
the stapes footplate was. And so it’s banging against
this membranous window, the oval window, and setting
up sound waves in the fluid in the perilymph canal that’s
inside here that we’ll look at. And the round window,
up here, is the place where the sound wave going
through the fluid is relieved. So pressure at this end and
relief of pressure at that end. Perhaps of interest is the fact that through this round window
is where an electrode is placed when a cochlear implant
is performed in order to stimulate artificially
the nerve that remains after
hair cell loss. Now, let’s go and see how
this fluid affects the cochlea itself. So now I’m taking
apart and we’re looking at a cross-section
of the cochlea. And we’re going to look at this
is more detail in a diagram. But here we have the
three fluid-filled spaces. The upper one is where
the sound wave is coming, and it’s vibrating this
membrane here that’s called the vestibular membrane. And it goes, this sound
wave travels all the way up, all the way up to the top point
here called the helicotrema. And then it goes down. It’s kind of like an
amusement park here. You go down the slide, and then
the sound wave is traveling in this compartment down here. And this is the one that’s
connected to the round window. So the sound wave set up by
the stapes goes up this canal or channel and down this one. And in here, and this is
filled with perilymph. In this channel here,
called the scala media, this is where the hair
cells and the receptors are that perform the transduction. High potassium, unlike
your CSF, is in this space. And on this membrane
called the basilar membrane, sit little hair cells. And the hair cells are moved,
their cilia, their stereocilia as they are called, are
moved, and open channels through which potassium flows
and ion currents are set up that begin the conversion
of a mechanical wave into an electrical signal. Now let’s look at a diagram
of these hair cells in detail. So here we have a very nice
diagram of the basilar membrane with the hair cells
sitting on top of it. It’s these inner hair cells
that are most important because they are the ones that
have most of the sensory endings that are carrying the
information to the brain. The outer hair cells are
thought to have more to do, not with sound transduction,
but with the tension on the basilar membrane. So it’s these 15,000 inner
hair cells that we really want to protect from loud sounds
and from toxic elements, particularly antibiotics. On top of the hair cells sits
another membrane called the tectorial membrane. And it touches the
stereocilia of the hair cells. And when it vibrates,
because it is in the endolymph and the pressure wave causes
vibration of this membrane. Then the stereocilia are
bent, and this deformation of the membrane causes
potassium to flow. And it is amplified
by a similar movement of the basilar membrane,
which also puts pressure on the stereocilia causing them. So there are two mechanisms
for the deformation of the stereocilia,
the tectorial membrane and the basilar membrane. So depending on where you sit
on the basilar membrane, the top or the bottom, the high
tones, the low tones, will determine the kind of
labeled line you are going into the central nervous system. So now let’s take this
information into the brain by looking at the
gross specimen. I’ve selected some
gross specimens that I think will show
you the salient features of the auditory pathway
to the cerebral cortex. And let’s begin with
the skull here. And I have set an axial
section through the cerebellum and medulla here
for you to look at. And I want you to
notice that the seventh and eighth cranial
nerves, the facial and cochlear-vestibular
are coming in or going out, depending on if you’re sensory
or motor, through this hole in the skull called the
internal auditory meatus. So remember seven and
eight go together. So problems with the auditory
nerve often are accompanied by problems of facial paralysis. And let’s move over to
the brain dissections. Here we have the medulla,
and we have the pons. And just a beginning
of the midbrain, the end of the cerebral
peduncles. And the seventh and eighth nerve
come in on the lateral side, right at the level
of the junction between the medulla
and the pons. The seventh nerve
is more medial. And the eighth nerve
is more lateral. And you can even sometimes see
a groove in the eighth nerve. One part is the vestibular part. And the other is
the cochlear part. The vestibulocochlear nerve. Down here you can see
cranial nerves nine and ten. And this area here that
I’m outlining is the cerebellopontine angle. And in this area, there is a
predilection for tumors to grow on the vestibular
or acoustic nerve. These are Schwann cell
tumors, schwannomas. Sometimes they’re
called neuromas. So now let’s move on
to a cross-section. So here we have our two
cerebellar hemispheres. This space here is
the fourth ventricle. This is the vermis of the
cerebellum hanging down into it. And this is right at the
junction between the pons, which is off here to the
side, and the medulla. So this is at the rostral
part of the medulla. Here are pyramids, our olive, our inferior cerebellar
peduncle. And here we have our seventh and eighth nerves coming
in on the side here. And they end, the auditory nerve
ends right here, along here, in a nucleus called the dorsal
or ventral cochlear nuclei. There are actually three nuclei. So all of these auditory
fibers coming from the auditory nerve end
in these cochlear nuclei. And from here now, the
information ascends but makes multiple crossings. And this is an important point. Because of the multiple
crossings, unilateral hearing loss either
has to be the hair cells, the nerve, or these cochlear
nuclei right here if it is of the sensorineural type. In other words it either has to be the hair cells
or these axons. Because after this
information is crossed and you do not get a
unilateral hearing loss. So now we’re going to
proceed up the brain stem. And now we’re at the
level of the pons. We still have the
fourth ventricle. Here’s your typical
crisscrossing fibers of the pons. That’s where the
basilar artery would be. And we have on this specimen, obviously different
from the last. We still have a bit
of the eighth and seventh nerves remaining. And we can see the deep
cerebellar or nuclei and the cerebellar
hemispheres out to the side. And our fibers are
traveling on either side. There are some important
connections made for sound localization
in the pons, in the superior olivary nuclei, that help us localize
sound out in space. So we can tell if sound’s coming
from the right or the left. And we can tell within
a very few degrees. Now as we continue
farther up the brain stem, I’m going to split
the cerebellum here. So you can look down into
the fourth ventricle. I’ve split the vermis here. So we’re looking into
the fourth ventricle. And at the top of the fourth
ventricle, the ventricle ends and becomes the aqueduct. And at this point we have
two bumps, the colliculi, the superior and the
inferior colliculi. And the auditory system relays
in the inferior colliculus. This bump right here. This irregular cut here
it shows the superior. They’re involved with vision, but the inferior colliculi are
what we say helped us orient. When your head moves with
a quick sound to one side or the other, you are
not only localizing it, but you are responding in an
almost reflexive way to a sound, whether it be threatening or
just environmental surprise. That is the inferior colliculus. Again, there are
multiple crossings in the auditory pathway
as it ascends. From the inferior colliculus, we’re going to move
up to the thalamus. So this is a nice section that shows you the
colliculus and the aqueduct. So this piece here is midbrain. And then as I tip this, I hope you can see here is our
temporal lobe on either side. It’s a little bit
broken on this side. And thereÕs a band of fibers
that come from the colliculus over to a nucleus on the
side and down in the back of the thalamus called the
medial geniculate nucleus. And it’s hard to identify. But like all other sensory
systems, except the olfactory, audition has a relaying
nucleus in the thalamus, and that is the medial
geniculate. And I’ll show it to you
now in an axial section. Here is an axial section. This is the occipital lobe. And this is the frontal lobe. Here’s a bit of the cerebellum. And this is the beginning of the
thalamomesocephalic junction. So this is the end of
the superior colliculus. And out here is our
thalamic nucleus, the medial geniculate nucleus. Right next to the lateral
geniculate, which is associated with the visual system. So medial music, lateral light. Medial geniculate is the
relay nucleus for sound. And then this tonotopically
organized labeled lines goes through this back part of the
internal capsule and out here and over to the temporal lobe. Because we have a
primary auditory cortex, a primary auditory
cortex over here in the temporal lobe called
the transverse temporal gyrus. And now we’ll look at
that in a whole brain. So here I have a left
hemisphere to be oriented. This is the frontal
lobe, parietal lobe, occipital lobe and
temporal lobe. And so primary, or the
main auditory cortex, is in the temporal lobe. And it’s buried in
this lateral fissure. And there are two gyri called
the transverse temporal gyri buried deep in there. And I’m going to
pull them apart. In this lateral fissure
you can see two transverse temporal gyri. I’m going to put my finger
over it and remove it. And that is primary
auditory cortex. Deep in that lateral fissure
you can see multiple branches of the middle cerebral artery. Most important branches. And you can also see
beneath them some cortex that is insular cortex. That is the insula. And now as I close this I
want to point out to you that there is again a tonotopic
organization along those transverse temporal gyri with
high tones being in one area and low tones in another. And if you were to electrically
stimulate this cortex in an unanesthetized
patient, they would report to you certain sounds. More important perhaps is
this superior temporal gyrus. The temporal lobe has three gyri
that you can see, the superior, the middle and the inferior. And the back part,
the back third or half of this superior temporal gyrus
is called association cortex. And it’s very much involved with
the interpretation of sounds. This is where we put the sounds
together and make language. And damage to this area results in people having no hearing
loss, but not understanding, or at least in part
not understanding, what they are hearing or
being able to repeat things. And so we have a language
disturbance called a aphasia, and this area was
described by a Dr. Wernicke, and so often this is
referred to as Wernicke’s area and extends back and around
in this area a little bit. Around the end here of
the lateral fissure. So Wernicke’s area in the LEFT
hemisphere, which is dominant in 97% of people,
language is lateralized to the left hemisphere. Even if you are left-handed. Because 10% of the people are
left-handed, but only about 3% of the people have language, lateralized to the
right hemisphere. So we want to summarize
in saying that primary sensory
cortex is deep in the lateral fissure
surrounded by the superior temporal
gyrus, which is important for the interpretation
of language. And with that, let’s talk about how we come to
test hearing loss. So the first test we do is
the Weber by putting the base of the tuning fork when
it’s vibrating in the center of my head and finding out
if I hear it more in one ear or the other or equally. Normal should be
equal in both ears. If I don’t hear it in one ear,
then I have either a conductive or sensorineural loss. It doesn’t tell me which ear, and it doesn’t tell
me which loss. So I have to do the second test,
which is called the Rinne test. For that you have to
put the tuning fork on the mastoid process,
which I’m going to do now. And ask the patient to tell you
when they no longer hear it. And when they no longer
hear it, then you take it and you put it just
in front of their ear, and normal person should
continue to hear it for about twice as long as
they heard it through the bone. And that is normal. With a sensorineural loss,
it will be shortened. And with a conductive loss,
there will be no air conduction because the vibration of the
tympanic membrane is not getting through to the oval window to
set up a sound wave travelling in the cochlear fluid. So let’s look at a neurologist
doing this on a patient who has a hearing loss and see if you can tell whether the
boy has a sensorineural loss or a conductive loss.>>Can you hear that there? Hear it over here? Harder to hear over there? Okay. All right. Can you feel it in the
middle or to one side? Feel it over there? Okay. All right. Which, can you feel
it back here? Okay. Tell me when it stops.>>It stopped.>>Can you still hear it? Okay. Tell me when
it stops over here.>>It stopped.>>Can you hear it? No.>>And so from that you can see that this young boy has a
conductive hearing loss. So now I want to show
you an axial radiograph through the pons. So let’s just orient ourselves. Here is our pons, and here
we have the fourth ventricle with a bit of the
vermis of the cerebellum. And here we have
cerebellum on either side. And now what I want to
show you is the seventh and eighth nerve along with
the semicircular canals and the cochlea. So let’s look here. Here we have the
cerebellum with the vermis, the fourth ventricle, the pons. And then here you
see both the cochlea and the semicircular canals,
cochlea and semicircular canals on each side, along
with the bright CSF in the subarachnoid space. And the basilar artery. And notice you can see two dark
lines representing both the facial and vestibulocochlear
nerves coming into the junction between the medulla
and the pons. So this is normal. The cochlea is in front of
the semicircular canals. Here is the cochlea. You can almost imagine
two and a half turns. Now the next axial section
I’m going to show you is with a tumor in the
cerebellopontine angle. Again, we have the
same relationship. We have the pons,
the fourth ventricle. This is different. The CSF is not bright
in this one. And you can see the
tumor right here in the cerebellopontine angle. This is the angle. And it’s actually invading into
that internal auditory meatus. So this patient would have had
not only a sensorineural hearing loss, but would have
also had signs and symptoms of facial
paralysis. And most likely problems with
disequilibrium and balance and gait because of the
vestibular part of the nerve. As a matter of fact, these
acoustic Schwannomas, as they are called, often
occur more frequently on the vestibular part of the
vestibular acoustic nerves, or the cochlear vestibular
nerve. However you want to call it. Now we’ll look next at this
same tumor in a coronal section. Now to orient you, this
is a coronal section. So we have our lateral
ventricles, our third ventricle. This would be the thalamus. Here is the temporal lobe
on one side and the other. And here would be the midbrain and the pons bulging
here, and the medulla. And here is our tumor in
the cerebellopontine angle. And here would be the
part that is growing into the internal
auditory meatus. And so this is a
left acoustic neuroma with a sensory neural
hearing loss on the left side. Recall in radiographs this
side is always the left side, as if you were standing
and facing the patient. And so this is the
take home message that the auditory
system is fascinating. Most of the problems are
peripheral or have to do with the cerebellopontine angle
because after that the crossings of the auditory system
all the way to the cortex, make it a difficult system to
use for neurologic localization.

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