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| So here you have a diagram showing the basic organization anatomy of the ear and the pinna, this is the pinna. Then you have the auditory canal. And at the end, you have the tympanic membrane. After the tympanic membrane, you have three ossicles, three small bones that will transfer vibrations to the cochlea. |
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Now here you have a bigger figure of those three ossicles, the malleus, incus, and the stapes, that are going to transfer the mechanical displacement of waves traveling in the air in a fluid. Now it's going to go from air to fluid.
In air, sound waves travel at around 340 something meters per second, that depends on the pressure and on the temperature, of course. One little thing to keep in mind is that the tympanic membrane is around, in a normal human, between 50 and 60, somewhere, 55, square millimeters, and everything is now going to transfer from there, that is vibrating via the ossicles, into a much smaller membrane going to the oval window. And that oval window, the membrane at that point, is only around 3, 3.2 square millimeters. So there is a tremendous amount of amplification that is going to take place. |
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• The scala vestibuli is continuous with the scala tympani.
• Vibrations come in via the oval window and pressure gets released via the round window.
If we uncoil the cochlea, then we understand better things that might be happening. Now what you have in this diagram is, of course, the oval window, but going into the scala vestibuli. But keep in mind that the scala vestibuli and scala tympani are continuous. And they are continuous at the apex of the cochlea via the helicotrema. They contain perilymph, perilymph.
The scala media, which is the chamber in the middle, contains endolymph. So the name indicates endo-- inside, in the scala media, perilymph in the periphery. So those are scala vestibuli and scala tympani. Now the scala vestibuli is continues with this scala tympani. And therefore, the vibrations that are traveling now in the fluid are going to go all the way to the apex and travel down and continue into the scala tympani. At the end of the scala tympani, you have a round window, which is there to be able to dampen the vibrations, so that's the end of the pathway. |
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• Changes in pressure at oval window set up a traveling wave along the basilar membrane due to the movement of the endolymph in the scala media and perilymph in the scala vestibuli and scala tympani. The amplitude of the wave— its pressure—determines loudness.
• The traveling wave maximally deflects the base at high frequencies and the apex at low frequencies.
In terms of frequency, keep in mind that there is a tonotopic map, so frequencies are organized from higher frequencies to lower frequencies. One way to remember this is if you take a rope and you whip like that, you go like that, you have some higher frequencies at the beginning and then lower frequencies as you go far away. And here is basically the same thing. At the very beginning, at the base, you have the higher frequencies. And further down the cochlea, you have the lower frequencies. That's how it is tonotopic.
We are able to hear, we humans are able to hear, between about 20 Hertz and 20 kilohertz. Other animals have different capacities for hearing. Dogs have a very different capacity. Bats have different capacities, and so on. So hearing is something that is very peculiar to the species. And hearing, for humans, is considered to be one of those things that allows us to enjoy things such as, for instance, music and to be creative, and so on. We are species primates, in particular, that very tuned to using our ears for many reasons, including not only survival, but enjoyment.
Changes in pressure in the oval window set this traveling wave along the basilar membrane. And we are going to see in a second, is movement of the basilar membrane, where we are going to be moving hair cells that have cilia, and the tectorial membrane is going to rub on those and that's how we are going to transfer now all of these vibrations frame from air, into liquid, now into action potentials |
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A cross section through the cochlea cuts 2.5 turns.
• Three distinct chambers
1. Scala tympani
2. Scala media
3. Scala vestibuli
• Four important membranes 1. Basilar
2. Tectorial 3. Reissner’s 4. Reticular
And here, of course, in some of these schematics, one thing that is always present is also the vestibular system. The hearing system and the vestibular system, the auditory system and the vestibular, are intricate with each other. They're very, very close, and it's convenient to present them together. So now that we cut, we made a cut to the cochlea, you can see the scala tympani, the scala media, and the scala vestibuli. And what makes it important again, is that they have endolymph and they have perilymph.
And the ionic composition is different in terms of the concentration. In particular to us, what makes it interesting is that the scala media, that is the endolymph, is very rich in potassium. You have a very high amount of potassium in there. The perilymph, you have a smaller amount of potassium, much lower concentration of potassium.
In this cross-section of the cochlea, there are about 2 and 1/2 turns. So in here, we go 1 turn, 2, and then 2 and 1/2 turns. And here you have the different components. So remember, this chamber is going to be continuous with these other chambers. All of this area here, is what we call the organ of Corti. And there, you have also four important membranes-- basilar, tectorial, Reissner, and reticular. And we will look into some of them in a little bit as we look more into the details. |
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Now when we move to histology, to real histology, how these things look like, here you have a real section showing you the different components. This is again, scala tympani. This is the scala vestibuli. So everything came in via the scala vestibuli, is going to go all the way to helicotrema. And then it's going to go into the scala tympani. Here you have Reissner's membrane separating scala vestibuli from scala media.
Now you can imagine how devastating it would be to have a hole, or a leaky membrane, Reissner's membrane, because I already told you, here you have a very high amount of potassium, here it is low. If you equalize these two chambers, then you are going to lose the capacity for sound transduction. Now this small apparatus here, the organ of Corti, is now depicted in this diagram. And what you have here is hair cells, depiction of the hair cells. And notice that there are two flavors to the hair cells. You have these three rows, in here, and then you have one row here which is a separate, if you would.
Now on top-- and I will tell you more about that in one second-- on top here, you have the tectorial membrane. Tectorial, the tectum. Remember, tectum is on the top. And this movement of this apparatus here will be such that the very tips of the cilia of the hair cells is rubbing against the tectorial membrane. And therefore, I will show you how it will have an effect for sound transduction.
The axons of the cells in the spiral ganglion, bipolar cells collecting information from the hair cells. Now I put this in here because I think it's absolutely beautiful to look at how these hair cells look like in real life |
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-Size of the cilia establishes the polarity of the cell
-Inner hair cells: 1 row ~3,500—~12 microns diameter; each inner hair cell is innervated by multiple ganglion cells
-Outer hair cells: 3–4 row 15,000–20,000—~8 microns diameter; multiple outer hair cells converge onto single ganglion cells
I think it's absolutely beautiful to look at how these hair cells look like in real life. Of course, doing an electron micrograph. In this case, a scanned electron micrograph. And what you see is these samples of cells. And here you have inner cells, which is one single row. And then the outer cells, which might be between three and four rows, usually three rows. The organization is extremely beautiful, in my opinion. |
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Have:
• Multiple stereocilia, but only one kinocilium (the tallest) which is lost during development in the cochlea
• Supporting cells are non-sensory and typically have microvilli
In this diagram, you have again, stereocilia. The tallest cilium, by the way, is called the kinocilium in these. It is usually lost in development. So you are left with multiple stereocilia, and the one kinocilium that is usually lost, and then supporting cells that are non-sensory cells that are supporting the whole apparatus here. Now when we move into the details remember, I said that they are organized from the tallest to the shortest. Is, in my opinion, amazing. |
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• Push cilia one way and the K+ channels will open
• Push cilia the opposite way and the K+ channels will close
• K comes in and activates Ca++ channels
• Ca++ comes in and initiates cascade of events for release of neurotransmitter
• Afferent fibers of the spiral ganglion cells fire APs
You have a connection between the tall cilium, and then the next one, and then a connection with the next one, to the next one, and so on. If you think, in those terms, like if my fingers were organized like that, when things are moved one way, you pull on the top of the cilia and you open-- it's like opening-- imagine that you have a lid on a pot. And when you pull, you pull the lid out, so the tall cilium pulls the lid on the next one. And that one pulls the lid on the next one, and so on. You are opening potassium channels.
If you push the cilia the opposite way, then you close them. It's a mechanical system. And it's extremely effective. So push of the cilia one way will open the potassium channels. And if you push it the other way, then it will close them. When calcium-- calcium is always involved, because remember, calcium is absolutely necessary for the release of transmitter-- but what happens here is that the potassium is the important ion for us. Potassium comes in and activates calcium channels and calcium then initiates a cascade of events that will result in the release of neurotransmitter.
Notice that the neurotransmitter is directly released onto the cell, which is going to receive that information. There is no action from these specialized receptor. There is no action going to someplace else. Developmentally and evolutionarily, an action is not necessary because everything is happening locally right there. So these are afferent fibers of the spiral ganglion cells are the ones that fire action potentials, not the receptor themselves. And here you have the reticular membrane keeping the cilia out into the compartment that has endolymph, that is high in potassium and separate from the rest of the cell, of the receptor. |
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Tip links connecting the taller cilium to the mechanical channel of the adjacent cilium mediate the opening and closing of the K+ channels.
Displacement of stereocilia by as little as 0.3 nm can result in a receptor potential.
Here you have a depiction again, of how the system works mechanically. And although very, very light-- I don't know if you can really see the details again, at the level of an electron micrograph-- it shows a protein chain, basically, which is the tip link and it's again, exactly as I depicted. It's like opening the lid of a pot and then being able-- as if you have a rope, and you pull it open, or you put it back and close it.
I think it's a very beautiful mechanical system. Extremely, extremely effective. Displacement of stereocilia by as little as 0.3 nanometers can result in the receptor potential being initiated. A nanometer remember, it's 1 times 10 to the minus 9. So atomic scale is extremely, extremely sensitive. |
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Now we move into the pathways, how the information is going to travel. So information is being collected by the cochlear nerve. Cochlear, remember, cochlear then [? audiation, ?] and is going to be going to the dorsal cochlear nuclei or to the ventral cochlear nuclei. And then from the dorsal cochlear nuclei, it goes contralateral inferior colliculus. And then from there, to the medial geniculate nucleus. Medial geniculate is in the thalamus. All sensory modalities, except for olfaction, have to go through the thalamus before they move into the cortex, before the information moves through the cortex.
From the ventral cochlear nucleus, is ipsilateral to the superior olivary nucleus. And then contralateral to superior olivary via the trapezoid body, and the inferior colliculus. And eventually, all the information will get also to the medial geniculate. Medial geniculate, I tell you now, medial, remember with M for music, medial geniculate nucleus or the thalamus, M, music, medial, is involved with sound. If you have lateral geniculate, lateral geniculate with L, light, that's for vision, that we will do soon.
Now because of the multiple decussations that you see here at different levels, it's almost impossible to get unilateral hearing loss. Because of the central nervous system, it just doesn't happen because many things are crossing. You can have unilateral loss if, for instance, you have an obstruction in the ear. So if I just put my finger in one of my ears, then I obstruct that side and then I have unilateral loss. But once it's in the central nervous system, it's really not possible because there are too many things which are crossing. |
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On the next slide, I have exactly the same information in a more anatomical way. So you can do it yourselves as an exercise. Just follow the actions come in here from the spiral ganglion cells, collecting information from the cochlea. And then follow them into the dorsal and the ventral cochlear nuclei. Pay attention to where information is being crossing and where it is going.
And then finally, to the media geniculate nucleus. And from there, to the primary auditory cortex, which is located here in the insula, is the Heschl's transverse gyrus, Brodmann area 41, next to the superior temporal gyrus, and also involving some areas of the superior temporal gyrus. With this, we come to conclude our part number 1 that has to do with the auditory and the vestibular systems. |
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Here you have the vestibular system and together with the auditory. So you still have the cochlear. And you see what we call the semicircular canals. So you have three of them because we have three planes of movement, basically. We have space is in three dimensions. So these guys are important, of course, one for vertical, one horizontal, and then they cover all of the possibilities.
And we cannot go into a humongous amount of detail in here, but this whole system is embedded in bone. And here you have, more or less, the location of the system. Here you have the different parts, the different ganglia that are involved and so on, and everything. Just pay attention to how closely connected it is to the cochlea.
In fact, the cranial nerve eight is vestibulocochlear. So many times, for instance, infections that have to do-- that are affecting the cochlear part have also an impact on the vestibular system. You might be very well aware of the fact that the middle ear might be affecting audition, but it also affects the vestibular system. And that's because of these very close proximity.
The endolymph that is produced for the scala media of the cochlea is the same endolymph which is produced and which is used by the vestibular system. So there are many similarities between both vestibular and auditory systems. |
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The cochlea and the vestibular system are in the petrous temporal bone
Stria vascularis: common source of endolymph for both
Hair cell transduction in both depends on high K+ in the endolymph (~150 mM) and normal levels
in the perilymph (~7 mM)
They are embedded in the petrous temporal bone. And the source, again, is the stria vascularis that is produced in endolymph for both systems. Remember, the important ion in here is potassium. In the endolymph, potassium is around 150 millimolar. And the perilymph is around about only 7 millimolar. And so very, very different amounts of potassium. |
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Remember, potassium is the ion that moves into the receptor, and potassium opens-- potassium is positive, so it depolarizes those cells in either system. And then calcium comes in, after depolarization, and calcium is the one that initiates the cascade of events that will result in neurotransmitter release.
Here you have an anatomical placement, again, a diagram showing the placement of the system. So you have both hemispheres with respect to the skull. They are complementary. You have a set on one side and then the other side on the other side of the head. |
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There are certain-- besides the many similarities, of course, there are differences, and some of these differences are shown in the following diagrams, in more diagrams which are coming in.
Now, of interest, I would like you to notice these otoliths, which are sitting on top of something which is gelatinous. It's a gelatinous layer. Now, this is different from what you have in the auditory.
Now, you have hair cells in here, but you have these gelatinous layer on top of which you have some crystals. These are calcium carbonate crystals. And I'll talk about it a little bit more. Those are otoliths.
You have something called the ampulla in which is going to be affected by the movement of the endolymph, the endolymph flow. And so a couple of different compartments here, the utricle and the saccule, are in charge of detecting particular types of movement. |
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• The saccule is oriented to detect linear vertical acceleration.
• The utricle is oriented for linear horizontal acceleration.
• The semicircular canals detect angular/rotational acceleration.
The saccule is oriented to detect linear vertical acceleration. The utricle is oriented for linear horizontal acceleration. The semicircular canals for angular rotational acceleration.
Now, one easy way to remember this-- if you connect the end points of the letter S, you end up with something like that. That is to detect, then, vertical acceleration. And if you connect the ending points of the letter U, you end up with something which is horizontal. So utricle is for detecting horizontal-oriented acceleration. And then the semicircular canals, of course, for angular or rotational acceleration. And here you have some of the details of the hair cells and the placement in the utricle and so on. |
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• The hair cells of the utricle and saccule are activated when the mass of the otoliths shift, causing a deflection of stereocilia.
• Otoliths are small particles composed of a combination of a gelatinous matrix and calcium carbonate.
So in this diagram, you have hair cells of the utricle and saccule that are activated when the mass of the otoliths is shifting, causing a deflection of the stereocilia. So the principle is the same. Basically, you have these little calcium carbonate crystals so that they don't fall on top of the-- although sometimes, under some conditions, they may fall on top of the hair cells-- that's not a good condition. They are embedded on top of the gelatinous layer.
But it's the tilting of the system that still allows the cilia to move. And they can move one way or move the other way. That's what happens when you are moving your head in this direction or when you are going like that with your head. And so the system is at work detecting changes in the position of the head. The otoliths are very small, but heavy enough to produce deflections in the stereocilia. |
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| Here you have, again, using electron micrographs, stereomicroscopy at this level, showing you kinocilium and the stereocilia and otoconia, the crystals, right on top of calcium carbonate. These guys are very small. They are about 1 to 5 micrometers in diameter. So these are tiny crystals, but, never the less, extremely important. |
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• Shifting of the otoconia and the underlying otolithic membrane result in the deflection of the hair cell cilia.
Now, in terms of what is happening, here you have the idea of the tilting. Now, what I want you to notice is how these hair cells are organized and the fact that you have something called the striola that is sort of a midline separating hair cells that are oriented as if one is looking one way and the other one is looking the other way.
So in this part here, you have the high cilium towards the midline, and on the other one you have the high cilium again to the midline. What happens, then, is that if you tilt your head this way, then these guys are going to close the potassium channels that should be open. And these are the ones are the opposite, so they are opening. And all of these together constitutes the otoconia. And this is the otolithic membrane.
So shifting of the otoconia and the underlying otolithic membrane results in the deflection of the hair cells. And that deflection of the hair cells with either close or open the channels, and then potassium will be able to get in or not. And if it gets in, then it will initiate a cascade of events after the entrance of calcium. |
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The flow of endolymph in the semicircular canals can occur bi-directionally and can persist after the movement of the head stops.
Now, in the semicircular canals, you have a similar situation in which you have endolymph rotating. And so that flow of endolymph in the canals can occur bidirectionally. So the fluid can go one way or can go the other way. It use hydraulics. So if it goes like that, the fluid goes in this direction, and then the opposite if you change depending on the forces.
At the bottom here, you have a couple of structures called the cupula and the ampulla. and there is that you will have interesting events happening in terms of what is going on with the hair cells. |
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• Angularacceleration shifts the chamber of the superior semicircular canal, but the mass of the endolymph lags. The lag results in the displacement of the cupula and the stereocilia embedded within.
There you have the deflection at the level of the ampulla. And it is this mechanism of moving. So if you move-- if the forces are going one to the left, then deflection will be to the right. The angular acceleration is, therefore, detected according to shifts in the semicircular canals.
All of this is a complex mechanism. It's now going to send information in a manner very similar to how we already describe for how auditory system. So remember, important things in here is that potassium is the ion that is in charge of having-- that is involved in the conversion of these mechanical signals now converted into action potentials. And the reason it happens is because there is a differential amount of potassium in the endolymph as compared to perilymph, and that's how you transfer one type of signal into action potentials. |
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The information, then, travels to the vestibular nuclei. I hope you remember this schematic. Remember, this is the thalamus, and now the cerebellum has been removed. So here you have where it was called-- this is the superior cerebellar peduncle, the medial cerebellar peduncle, and inferior.
At this level, in color, is that you have the different vestibular nuclei in the gray matter, of course, of the brain stem. So these different nuclei, the cells on those nuclei, are receiving information from the vestibular system |
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So here you have, again, information being shared. And in reality, a lot of other things are going on, including, for instance, the information that has to go and keep the cerebellum informed because, remember, the cerebellum has to be able to make adjustments to find movement. The vestibular system is engaged all the time with even the slightest type of movement.
And then limb motor neurons and motor neurons in the neck and so on, motor neurons all over, have to be engaged to be able to react accordingly and so that when you move, when you move one part of your body and you change the center of gravity and you change your balance, the vestibular system is able to react, and then the reflexes are able to react accordingly so that you don't lose enough tone in the muscles and collapse, and everything has to be in tandem.
When we lecture and when we try to explain these very complex problems, we have to do it in little pieces because, of course, we cannot handle the whole thing in one shot. But you would be amazed at the humongous amount of computation that happens by just picking something up.
By picking it up, I am engaging the whole body. I'm engaging the vestibular system. I am engaging the muscles, the reflexes. If I happen to be talking to you and if you happen to be talking to me, the auditory system would be engaged and the visual system is engaged. So we are, indeed, very, very complex machines in terms of the computation that is happening for us to be able to just function normally. No wonder that changes in all of these systems could have devastating consequences. |
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| Hair cells from the labyrinth are innervated by |
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the bipolar cells of Scarpa’s ganglion (~20,000 cells).
The neurons of Scarpa’s ganglion project centrally via the VIII nerve to the four vestibular nuclei in the floor of the fourth ventricle. |
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| Cortical representation of vestibular input is not |
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| well defined. Intraoperative stimulation studies suggest regions of superior temporal gyrus, rostral to Brodmann’s area 41/42 (auditory). |
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| Secondary projections predominately from the medial and inferior vestibular nuclei also |
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innervate the cerebellum via the juxtarestiform body.
Vestibular computation is complex and seems to be distributed. |
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| alternating slow and rapid eye movements |
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| introduction of warm or cold water into the external ear will generate convection currents in the endolymph and the sensation of movement |
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includes recurring periods of vertigo. The pathology suggests elevated levels of endolymph are responsible.
• Sustained treatment with antibiotics, such as streptomycin, can cause vestibular and hearing loss because the antibiotics kill the hair cells; in the case of audition, damage appears to occur predominately in the outer hair cells. |
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| occurs when the data provided by the vestibular system (movement) is not consistent with the data provided by the visual or somatosensory systems. |
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cochlear implants
In terms of for the auditory, just to mention one improvement over the years has been the case with cochlear implants. This number is no longer up to date. This is much larger right now worldwide. It's many, many several million people already. And these systems are getting better and better with time.
Now, I would like to end by telling you a tiny story, one minute only, that has to do with autism with the auditory system. And I'm, unfortunately, unable to give you many more details due to time, but suffice it to say that many of the children that were thought to be autistic were not at all.
What happened was that there was a problem in the hair cells of the auditory system. And so when people talked to them, they were unable to hear exactly what was going on. So of course, if they just look around and they see somebody moving their mouths but the sounds don't make any sense, they go [MOANS], then they are not engaged, they are not socially-- they were not socially engaged.
And researchers were able to find this out. And they put implants that changed the frequency components-- in other words, cochlear implants-- that were able to make corrections to the frequencies that these children were hearing. And the results were amazing. Once the proper frequencies were able to be detected, the children were absolutely reactive to human speech, and that has made a tremendous impact on the social lives and the quality of life of all of these patients. |
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