1. S 319 < Auditory system >
Focus:
- Anatomy and function of the ear
- Sound transfer function of ear:
When sound is conducted into the ear, how sound is affected by that process
CD, Schedule)
Today what we are going to do is we will look at the anatomy and the function of each section of ear. Since this class is not anatomy class, we don’t go that detailed anatomy of ear. Actually if we need to see auditory anatomy in detail, we need one whole semester. But that is not our focus in this class. The reason why we need to go over the anatomy of auditory system is that we want to know When sound passes through our auditory system which are outer ear middle ear inner ear and central nervous system, how sound is changed by that process. When sound passes each subsystem of ear, the sound is gonna changed in a different way. Diane will lecture about the sound transfer function in detail next week.
So today I am going to try to give you more about basic background about our auditory system to understand their sound transfer functions for next week.
Q) How many you guys took anatomy class before?
If you did not take anatomy class before, then today might be a good introduction to anatomy of ear before you took it. But If you are taking now or you did take the anatomy class before, today might be too easy or just brief summary class.
So today you do NOT need to memorize detailed physiology. I have some interesting animations and pictures, so I hope you guys are more happy with them. Hope you enjoy various pictures of our ear today.

2. Outer ear Middle ear Inner ear
This Monday, Diane covered what the transfer function of system means. I will briefly review what the transfer function means.
Let’s say here we wanna measure sound transfer function.
And here we know what the input is and what the output is, then we can make an infer about what is happening between input and output, which is system. Here system can be outer ear, or middle ear, or inner ear. Whatever it was, it changed sound after the sound passes that part. Since sound is changed by that function of system we call it sound transfer function. So as long as we know the input and output, we can infer the transfer function. We will see how it works this week and next week.
Cf) Especially when I knew the output and the input measured with dB. Then how do I figure out the outer ear’s transfer function? For that we can subtract two transfer functions.But if we knew the output in linear unit like amplitude and input was also in linear value, then we need to divide them.
Outer ear
Middle ear
Inner ear

3. Four major divisions of auditory system - Anatomy
The outer ear
- pinna
- ear canal
- eardrum
2. The middle ear
- three ossicle bones;
(malleus, incus, stapes)
- two major muscles
(stapedial muscle, tensor
tympani)
- Eustachian tube
3. The inner ear
- cochlea (hearing)
- vestibular system (balance)
The central auditory system
Here is simplified picture of our whole auditory system.
Our auditory system has four separate systems: outer ear, middle ear, inner ear, and central auditory nervous system. Each system has some different subsystems. Let’s see the structures from outside to inside.
The outer ear has pinna, ear canal and ear drum.
And the middle ear has three ossicle bones; The biggest one among three is malleus (hammer), and the next one called the incus (anvil) and the smallest one called the stapes. These bones have mechanical movement to deliver the sound. Also there are two major muscles, which are stapedial muscle and tensor tympani. They work for protection of our ears. And we have eustachian tube working for equallizing pressure.
And then we have the inner ear. The inner ear has two parts. Here we have semicircular canals called vestibular system for balance and the snail-shaped cochlea for hearing.
And then we have central auditory system.
So We will see them in detail in a minute.

4. Four major divisions of auditory system – Function (CD, Figure 3.4.1)
Here we have a big picture of our whole auditory system. This picture is from our CD text.
As we can see, in the outer ear the sound is transmitted by air vibration. The major function of outer ear is protection, amplification, and sound localization (which means how we know where the sound is coming from).
In the middle ear the sound is transmitted by mechanical vibration. And they work for impedance matching, stimulating oval window, equalizing pressure between inside and outside.
In the inner ear the sound transmission gets more complicated. Mechanical vibration is changed to hydrodynamic and electrochemical work.
And then in the central auditory system, sound is transmitted by electrochemical work and delivered to nerve system. So our brain can process information of that sound.
This is real quick summary of each part. Then let’s take a look at each in more detail.
Any questions, so far?

5. Three parts of outer ear 1) Pinna 2) Ear canal 3) Ear drum
I. Outer ear (CD, Figure 3.4.2)
Three parts of outer ear
1) Pinna
2) Ear canal
3) Ear drum
Major function of outer ear 1) protection
2) amplification
3) sound localization
1) Here we have three major part of our outer ear which has pinna, the ear canal, and the eardrum.
(1) Pinna is the visual portion that comes to our mind when we talk about "the ear".
Take your partner who is next to you and see your friend’s pinna. Do you like the shape?
The pinna is the most outer part of the ear as you can see. It is actually cartilage covered with skin. 
How does the pinna look like?
Pinna has a funnel shape as a sound collector. Basically pinna collects sound into the ear canal through the air as a medium. So this horn shape of pinna works for amplification function because the sound pressure in the larger area goes into smaller area.
Then here I have a question. What else can be the important function of pinna?
How does it help our hearing? If someone does not have pinna, then how does it affect?

6. I. Outer ear: (1) Pinna (Binaural cue to sound source location)
t Right
t Left
Left
ear
Right
ear
The important function of pinna is a cue for sound localilzation.
When we have no pinna, it gives a cue to sound localization when the sound source depending on the sound source location. For example, let’s say that we have a sound source at the left side. Then what is difference between right ear and left ear? Compared to right ear, the sound arrives earlier to the left ear a little bit more.
And also the distance to the sound source is closer, so the sound level to the left ear is greater than to the right ear because to the right ear our head blocked the sound transmission a little bit.
So different distance to each ear makes different arrival time and different intensities.
Then our ear and our brain are using these cues to determine the location of sound source.
* binaural cues:
Interaural time difference (ITD): sound takes longer to reach far ear, esp low freq
Interaural level difference (ILD): head shadows (blocks) sound going to far ear, less level, high freq(>2KHz) (higher freq part is more reduced by head because high freq has short wavelength)
And we can see here, some reflected sound to pinna interfere some direct sound. That means, the pinna will filter the sound to a certain degree. So it will also give a difference in the spectrum of a sound. (at either ear due to how the soundwaves interfere with obstructing objects). This is called interaural spectral difference (ISD).
* Different distances from source to each ear
=> different arrival times (Interaural time-difference)
and different sound level (interaural level-difference)

7. I. Outer ear: (1) Pinna (Spectral cue to sound source location)
+15° 0°
-15°
Outer ear gain
As you can see, the spectral feature of sound is changed depending on the sound elevation.
So spectral information is gonna be different depending on where the sound is coming from.
Here we have three different sound depending on the elevation of sound, high, medium and low elevation. And these sounds are filtered in a different way.
Cf) (spectral cues: head related transfer function (HRTF)). But that is different on the frequency of incoming input sound.
If the object is smaller than the wavelength of a sound, no change in the wave, so wave passes.
If the object is about equal in size to wavelength, the wave is refracted. A small shadow is created.
If the object is larger than the wavelength of a sound, the wave is reflected. Then a sound shadow is created (means Bigger head, more blocked.. Or shorter wavelength from high freq is more blocked.. -> higher freq has more head shadow)
The spectral feature of sound is changed depending on the sound elevation
=> Head Related Transfer Function (HRTF)

8. I. Outer ear (1) Pinna: Cases of abnormal pinna
Anotia
Microtia (Grade I)
Microtia (Grade II)
Microtia (Grade III)
Here I just put some pictures of abnormal pinna.
Don’t worry. You do not need to memorize how they are different.
I just thought you guys did not see these cases before except some au.d student.
So I thought it might give you guys more ideas about various hearing disorders.
They are the case for abnormal pinna. There are lots of abonormal cases.
But for outer ear, they have no pinna or they show abnormal size or shape of pinna.
1) The first case is Anotia technically means no ear.
2) We have three microtia cases here.
Microtia is divided into 3 separate classifications
Grade I: In this case, the ear is smaller than normal. Sometimes they have abnormal ear canal inside as well.
Grade II and Grade III show abnormal shape of pinna.
In these cases, their pinna cannot work in the normal way. The worse thing is if they have abnormal ear canal or ear drum, then it is impossible to get plastic surgery for that. In that case, we need to try hearing assistive device through bone, not air. I saw the one case when I was working in clinic. The girl has anotia, so she should wear bone conduction hearing aid. The shape is hairband, but it is made by heavy wire. So it really looks ugly.
So her mother had a hard time to make her use it. So I wrapped that ugly wire using pink ribbon. After that it looks more like hair band. Anyway in these cases they really have big trouble. Because they have hearing loss and also their looking is not normal. So it made them hurt emotionally.

9. I. Outer ear: (2) Ear canal (CD, Figure 3.4.4)
Three parts of outer ear
1) Pinna
2) Ear canal
3) Ear drum
Major function of outer ear 1) protection
2) amplification
3) sound localization
Now we know in the outer ear, the sound is collected by the pinna, and goes in the ear canal here and directs town to the ear drum through air. And then ear drum picks that vibration in the air.
Then what happens in the ear canal and ear drum?
First of all, we don’t wanna little insects flying inside our ear unless someone keeps putting something in the ear.
Because the shape of ear canal is bent, slightly s shaped actually, and the diameter gets smaller, so the shape is protecting itself. Also along the ear canal we have cerumen or ear wax It helps protect against insects or dirty things and also it has an anti-bacterial protection role.
But we have another important function here, which is resonant frequency in the outer ear.
Our ear canal has ear drum at the one side, so it can be considered as a closed tube resonator. Then how does it make some difference? Diane will give some detail next week. So I will give you basic idea about it.
Cf) External canal and TM covered with epidermis (skin)
Epidermis migrates outward, so removes wax and heal most small TM perforations spontaneously…
Ear wax is a mixture of secretions from ceruminous and pilosebaceous glands and squames of epithelium, dust, and other debris. Do cotton-buds (Q-tips) predispose to impacted cerumen? This is a debatable point.
As long as it does not go in that deep, it is okay. But it goes in that deep, it can make a hole on the ear drum. Then
Perforations of the tympanic membrane is a common serious ear injury that may result from a variety of causes including projectiles or probes (e.g. Q-tips, pencils, paper clips, etc.). It will make conductive hearing loss accompanying tympanic membrane perforation in nature. .

10. I. Outer ear: (2) Transfer function of ear canal
As we can see from this picture, we have a "broad" resonance between about 2000 and 5000 Hz. That is because our external ear is a tube, closed at one end. This feature of ear canal increases sound level by apx 20 dB between Hz. And we can calcualte mathematically where the peak will be depending on the ear canal length. I guess Diane will show how to do that next week. So you guys will see pretty soon. So now we know the resonant frequency of the structure depends on its length and the size. So if some one has different length or size of the ear canal, then the resonant frequency will be changed.
Ex) children, Then if some adults and children have the same hearing loss, then the children can use the adults hearing aids?
No, even thought the hearing aids fit into children’s ears, they have different length of ear canal. So it will give different gain depending on different resonant frequency.
From Gelfand (1998)

11. I. Outer ear: (3) ear drum (CD, Figure 3.4.5)
Major function of outer ear
: As a boundary between outer
and middle ear
: Vibrates in response to sound
Three-dimensional finite element method (FEM) analysis of the middle ear
(3) As we can see all the way down the bendy ear canal we have the tympanic membrane or eardrum.
Ear drum which is called tympanic membrane separates the outer ear from middle ear. This is a fibrous membrane and it is relatively transparent in nature. So we can see the arm of the malleus of middle ear which is attached in the center of eardrum. Here we can barely see it.
When the sound travels through air, sound hits the ear drum and that vibration makes ear drum vibrate as well.
So the rate and pattern of vibration of ear drum depend on the frequency and complexity of the sound wave.  The magnitude of vibration depends on the level of the sound. I am not gonna how they are different at this time, but
I will show how they are different. To model that we need so complicated mathematics because ear drum has three layers and it had different stiffness depending on the area.
We will see three-dimensional modeling of ear drum.
The first one is showing ear drum vibration to 500Hz frequency. And the next one is for 2000Hz frequency.
Can you tell the difference? Compared the first one, as the frequency of sound goes up, the bottom part is more vibrating. Depending on the kind of sound, ear drum vibrates in a different way. .

12. II. Middle ear Three main parts of middle ear (1) Three Ossicle bones:
- Malleus(1), Incus(3), Stapes(6)
Function) Impedance matching
(2) Two muscles
- Stapedial muscle(5)
- Tensor tympani(9)
Function) Protection
(3) Eustachian tube(8)
Function) Equalizer of air pressure
Moving on the middle ear! We have a middle ear. In the middle ear, we have three small bones and two muscles and eustachian tube.
Let’s see the picture here. We have (7) ear drum. And we have three ossicle bones, (1) Malleus ; (3) Incus ; (6) Stapes footplate. These 3 bones are the smallest bones in our body and they work together (b/c they are joined each other and they are forming an ossicular chain). These bones are increasing sound level for impedance matching. WE will see how it works in a minute.
(2) And then here question is we know that our middle ear cavity is air filled, then how these bones are suspended?
Here These bones are suspended by some ligaments and muscles. Middle ear has two major muscles. This one is called the stapedial muscles(5) because it attaches to the stapes. And this one is called tensor tympani and it attaches to the malleus. When there is very loud low frequency noise, these muscles contract very quickly, so they reduce sound level of loud noises (up to 15 dB, depending on frequency). So their main function is protecting the ear from noise damage.
(3) And Here we have (8) Eustachian tube; This tube connects middle ear and nasopharynax region.
So this tube has a function of pressure equalizer. So, if there is some difference between inside middle ear and outside air, then the tube is opened and pressure is equalized.
So it happens when we are flying. When the air plane goes up, then the pressure outside is changed compared to inside middle ear, so the tube opens and equalizes the pressure difference. Then if this tube does not work, what happens?

13. II. Middle ear (3) Eustachian Tube
Comparison of Eustachian tubes In adults and children
: shorter, smaller, less steep eustachian tube in children
=> Hard to be drained away from middle ear
When the Eustachian tube fails to open, a negative pressure is gonna be built up so it makes some low frequency hearing loss (b/c it increases the stiffness in the middle ear). And also it fails to open, middle ear cannot be ventilated. This might affect more children. Here as we can see, adults and children have different ET.
How they are different?
Especially in children, the Eustachian tube is rather more horizontal and smaller and shorter, so something bad like bacteria kind of thing cannot be drained away from middle ear well even though it opens. So it gives warm, moist space in the middle ear which is best place for bacteria. So that is why the children can get otitis media more often which is the common middle ear disorder in children.

14. II. Middle ear (CD, Figure 3.4.7)
Middle ear cavity
Function of ossicles
% sound is reflected due to high impedance of fluid in the cochlea (0.1% sound is only passed = - 30 dB sound loss from air - fluid impedance mismatch)
- Middle ear bones overcome the loss of sound by increasing sound pressure (+34dB)
=> Impedance matching
Here we have three middle bones again. The arm of the malleus is attached to the eardrum, and the footplate of the stapes is attached to the oval window of the cochlea (inner ear). What they are doing is they transmit vibrations of the eardrum into the cochlea. But here we have a problem. Because the impedance of air and impedance of liquid is really different. Which one is bigger? Impedance in the liquid is much bigger than that of air.
We can think of an example. Let’s say we are in a swimming pool. And we are under water in a swimming pool. And then we cannot hear well the speech of outside even though the voice is loud. Try it later at the gym. That is because the impedance of liquid is so high, most of sound is reflected when the sound hits the water. And 99.9%, most of sound is lost. In other words, only 0.1% of power is passed.
That sound loss gives us -30dB sound level loss just because of impedance mismatch between air and liquid.
But fortunately our middle ear bones overcome that sound loss. The process is called impedance matching because they are matching, making up that loss.
Then how does it happen?

15. II. Middle ear (CD, Figure 3.4.9)
Three mechanisms for impedance matching
1) Area ratio of the ear drum to the stapes footplate (20:1)
=> 20 log (20/1) = +26dB SPL
* Basic concept: p = f/a
2) Lever action of the ossicles (1.3:1)
=> 20 log(1.3/1) = +2 dB SPL
3) Buckling of ear drum
( x 2 pressure increase
=> 20 log(2/1) = +6dB SPL
What they are doing is they are amplifying sound level to overcome mismatched impedance.
It can work due to their physical structure.
1) First of all, we have a really big ear drum relative bones. Especially, ear drum is really big and stapes footplate is really small. Here as we can see, the area of ear drum is twenty times bigger than the area of stapes footplate. (Ear drum 60 mm2, stapes 3mm2).
Using equation of decibel, we calcualte how much gain it boosts.
Area ratio is 20, so area of ear drum to area of stapes footplate 20/1 = 20 log (20/1)=26dB gain is boosted by this area ratio between ear drum and stapes footplate.
(The same concept=> If we think about hitting a nail with a hammer, we put the force to the head of the nail. But the force is gonna be bigger at the point of nail. Why is it?
Because when the same force is applied, then the pressure is gonna be incrased from larger to smaller area. p=f/a)
2) They work as like lever. Because as we can see in this picture, the arm of malleus is longer than that of incus. So different distance makes lever ratio. So this lever action gives another increase about 1.3 times which is equal pressure increase by 2dB. (What that means is that the stapes is displaced much less than TM. TM is displaced up to 2mm, but stapes is displaced by 0.1mm.)
(3) buckling of the ear drum.
As we saw before ear drum changes its shape in a complicate way when the sound hits ear drum. Each part of ear drum response to different frequency in a different way. So ear drum itself can increase force when ear drum moves. This buckling effects increase pressure by 6 dB (by a factor of 2).
All together, these three factors provide 26dB+2dB+6dB = more 34dB gain
(Or linearly, 20*1.3*2=by a factor of 52)

16. II. Middle ear - Impedance matching
In total, 20 x 1.3 x 2 increase in pressure by middle ear and ear drum ( + 34 dB SPL).
It works for mismatched impedance
(99.9% sound loss = apx - 30 dB)
Three-dimensional finite element method (FEM) analysis of the middle ear
So in total, we have 20 x 1.3 x 2 increase in pressure by middle ear bones and ear drum.
So they boost pressure that much, so we got 34 dB SPL back from three mechanisms.
As we know, theoretically 99.9% sound loss from impedance mismatch is about 30 dB sound loss.
So it is close, not exactly, it about overcomes that air/liquid impedance difference.
So That is we can say that the middle ear matches the acoustic impedance between the air and the fluid.
Basically so many things can be calculated using mathematics even this kind of physically complicated things.
But this is kind of over-simplifed calculation.
Actually middle ear is really complicated since each bone has different impedance. They respond to different frequency in a different way. I don’t go more detail for that, but I will show you how they move in a different way.
This is three-dimensional modeling of middle ear.
It is hard to tell how they are different exactly, but we are calculating that detailed difference and then modeling their movement. And then we can predict what kind of different disease or different structures give different hearing loss or type. (The middle ear acts as high-pass filter, as would be expected from a stiff structure. It will "pass" those frequencies that get a "boost" from the external ear resonance.)

17. III. Inner ear (CD, Figure 3.4.11)
Twp parts of inner ear
1) Cochlea (Hearing)
- Scala vestibuli
- Scala media
- Scala tympani
2) Vestibular system (balance)
Major function of inner ear
1) Hearing
(It transmits sound to
neural impulse and gives
resonant frequency)
2) Balance
Moving one. Now we are gonna talk about inner ear.
Main structures of inner ear is vestibular system and cochlea. Each one has different function. Vestibular system is for balance and cochlea is for hearing. For balance, 3 semi-circular canals are working. So if we have abnormal function of vestibular system, we cannot maintain balance. 
So in that case, we feel serious dizziness all the time like when we are drunken. That is really serious torture.
For hearing, the cochlea is the main structure. As we can see, cochlea is a "snail shaped" structure and it is divided into three fluid-filled parts;  scala vestibuli, scala media and scala tympani.  (Two membranes create these three compartments.  The first is Reissner’s membrane which separates the scala media from the scala vestibuli.  The second membrane is the basilar membrane separating the scala media from the scala tympani.) 
Then how is the sound transmitted to inner ear?

18. III. Inner ear – Cochlea endolymph perilymph perilymph
So this is cross section of ear.
When the sound is vibrating the stapes footplate, it gives vibration in the fluid of the cochlea. This movement in the fluid gives wave-like motion to
Basilar membrane. Here we have basilar membrane down here. Along the BM, organ of corti is here. And we have sort of detectors for that vibration in this organ of corti. These detectors allow us to determine the presence of the vibration.
Then let’s see how the basilar membrane shows wavelike motion first.
perilymph

19. III. Inner ear – Resonance of Basilar membrane
Here we have basilar membrane. How do we encode frequency information along the Basilar Membrane?
Let’s imagine we rolled out the basilar membrane. Then we have this shape of basilar membrane here. This part which is closer to stapes is base part and the other end is apex part.
As we can see, the membrane is narrow and stiff at the base which is closer to stapes.
As we go along the BM to the apex, the membrane is wider and more flexible (less stiff) at the helicotrema end. Our basilar membrane has different stiffness and mass along the BM, from base part to the apex part.
Physically, stiffness and mass can affect the resonant frequency. When we have more stiffness, we have resonant frequency at higher frequency. Here, That is why the high frequency has maximal vibration at the base part because base part has more stiffness.
In contrast, at the apex which has more mass, we have maximal vibration (resonance) at lower frequency.
So it seems like most of things in physiology can be explained by mathematics and physics.
(CF) Since this vibration looks like traveling along the BM, so we call it traveling wave.
According to the ’Place Theory’ the BM can be likened to a set of graded resonators or strings. Those near the stapes ’resonate’ at high frequencies and those at the helicotrema end, at low frequencies. The travelling wave will reach its maximum point of displacement at the position along the BM corrosponding to the frequency of the stimulus sound. So, the lower the frequency, the further down the length of the BM the point of maximum displacement will be.)
Figure 15.32

20. III. Inner ear – Cochlea
Then now we know different frequency makes different movement along the basilar membrane.
Now we need to look at the orgain of corti in more detail.
Figure 15.28

21. III. Inner ear – Inner hair cells (IHC) & Outer hair cells (OHC)
Here above BM, we have three rows of outer hair cells and one row of inner hair cell. And at the top of them we have tiny (hair-like) haircells which are called stereocilia (from the tops of the outer hair cells to the under tectorial membrane).
Here also we can see three rows of outer hair cells and one row of inner hair cell.
Then why do we need both inner hair cell and outer hair cells?
Because they have different roles for sound transmission.
1) IHC Only inner hair cell is the sensory hair cell so it sends messages about sound to the brain. They produce sensation of hearing
2) OHC: Since they have motility (contract to stimulation), so they can modify BM movement or shape (for more fine tuning). Also they are like amplifcation system because they improve sensitivity and frequency resolution.
but they work together for overall sensation of hearing, so If we lost one of them, it produces hearing loss.
Then how can our ear tell exactly which frequency is sounding?
When there is vibration of basilar membrane at different place, these hair cells are excited or fired.
What the fire or excite means... basically these hair cells send chemical or electrical signals to the brain through the auditory nerve. Let’s see how it works.
Inner hair cells: produce sensation of hearing
Outer hair cells: modify BM response and act as amplification system

22. III. Inner ear – Sound transduction
Then how the frequency-dependent feature can be delivered to the brain?
For that hair cells need to change basilar membrane motion into electrochemical current.
When the basilar membrane moves upward, haircells also have upward movement. Then it makes stereocilia to bend outward.  Then then as we can see tip links are stretched (tip links are attached to ion channels), so it releases neurotransmitters and action potential. These neurotransmitters produce a discharge in the connected auditory nerve fibers so it transmits neural impulses to the brain. 
In opposite direction, the basilar membrane moves downward during a vibration, the tectorial membrane makes the cilia to bend inward. Then the firing is gonna be reduced.
(because it opens mechanically gated ion channels and it makes a graded potential and the release of a neurotransmitter).

23. IV. Central auditory system (CD, Figure 3.4.14)
So actually, in the brain, the sound does not look like a pressure wave any more. Because our inner ear transmits the sound to neural impulse for neural fiber, At this point of our brain, it really does not know anything about the sound outside world. Our brain is just coding neural impulses from our ears because our brain is composed of a neuron.
This picture is showing general auditory Pathway to the Brain. When we have neural impulse from cochlea, they pass through spiral ganglion to the cochlear nuclei. From cochlear nuclei. , impulses are sent to the to the superior olivary complex, to the lateral lemniscus, to the inferior colliculus, and to the medial geniculate body, until it reaches the final place in the brain, the auditory cortex.  This is over over simplification. But this has been basic idea for auditory pathway.
(cf) There are two types of nerves at the base of the hair cells: "Afferent nerve fibers" carry sensory information away from the cells to the brain while "Efferent nerve fibers" bring information from the brain to the hair cells.  These afferent neural pulses are then collected and sent out the internal acoustic meatus via the auditory nerve thus translating mechanical information into neural information.  Once the auditory nerve has received the neural impulses, it continues the signal through various pathways in the brainstem. )

24. IV. Central auditory system - Auditory pathway to brain
Tonotopy!
Here the point is that.
Like BM, each auditory nucleus maintains the frequency-specific tuning introduced by the cochlea and, in addition, extracts other features of the sound stimulus carried by the neural code. So this feature can be maintained in the brainstem (cochlear nucleus, superior olivary complex, lateral lemniscus), the midbrain (inferior colliculus), the thalmus (medial geniculate), and the cortex.  Ultimately, these nerve centers allow the brain to extract information about the sound stimulus and the environment from which it comes. So tonotopy means different frequency information is maintained from cochlea to brain. so we can tell which frequency is sounding for stimuli.

25. Overall, how sound travels through the ear...
Outer ear:
Acoustic energy, in the form of sound waves, passes pinna, ear canal. Sound waves hit the ear drum, causing it to vibrate like a drum.
Middle ear:
It sets three ossicle bones (malleus, incus, stapes) into motion, changing acoustic energy to mechanical energy. These middle ear bones mechanically amplify sound and compensate mismatched impedance.
Inner ear and Central auditory nervous system:
When the stapes moves in and out of the oval window of the cochlea, it creates a fluid motion, hydrodynamic energy. It causes membranes in the Organ of Corti to shear against the hair cells.
This creates an electrochemical signal which is sent via the auditory nerve to the brain.

26. QUESTIONS?