Physiology of the Ear


To understand the physiology of the Hearing sense, we must first understand some specific things about sound waves.

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The only information an originator of sound puts out is something called a sound wave. A sound wave is made of compressions and rarefactions formed by the original vibration of an object which makes sound.
Imagine, a vibrating gong; pushing and pulling on the air around it as it moves in and out. A push on the air makes air particles run into each-other and creates a pocket of denser air, while a pull on the air creates a pocket of less dense air. As a gong vibrates back and forth, the compressions and rarefactions which occur create fluctuations or waves in the air currency which will travel through the air and into someone's ear.
Imagine that this grid is the normal air pressure surrounding a resonating originator of sound. When the sound begins, it pushes lines of the grid closer together in a ring around the origin of the sound and interchangeably, pushes out a ring of grid lines farther apart after that.
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If we look at a cross section of a sound wave we see frequency and amplitude and these two things determine pitch and volume of a sound.
<-Amplitude changing
<-Frequency changing
High pitch associates with high frequency and low with low. Intense amplitude associates with loud sounds and diminutive amplitude associates with quiet sounds.

...........................................................................High frequency=High Pitch.................... Low Frequency= Low Pitch
..............................................................Intense Amplitude= Loud Sound ........................... Diminutive Amplitude=Quiet Sound







Simply Hearing...


Hearing is an interesting sense because to conduct sound, no chemical changes take place within the hearing structure, only mechanical. This is different from the senses of taste, smell, and vision.

Our ear does three mechanical things to take a sound and turn it into a decipherable piece of information.
1. Direct sound from the atmosphere into the body
2. Sense the sound waves
3. Process the sound ultimately through nerve receptors

We will deal with Outer, Middle, and Inner ear.


OUTER

The shape and position of the pinna is beneficial to this sense. The pinna is the outermost part of the ear and situated on either side of the head. Its large and fleshy conical shape captures sounds and narrows them into the ear canal. You will notice that the pinna is ridged. As sound waves enter through the pinna, they bounce off its ridges, and depending from which direction the sound originates from, above, below, in front of, or behind, it hits the pinna differently and as it is processed, the brain recognizes these combinations of patterns to tell from where a sound originates. Horizontally, a sound will always enter the ear it is closest to first, left of right ear, and by comparing the relay of sound to either ear, the brain can also tell if the sound comes from the right or left.
wikimedia commons:The Ear Canal
wikimedia commons:The Ear Canal
wikimedia commons: The Pinna
wikimedia commons: The Pinna


From the pinna, the waves travel through a simple skin covered canal called the ear canal, which, outside of the epidermal covering, is surrounded by the cranium. At the end of the ear canal rest a structure like the head of a tight drum, the eardrum, also called the tympanic membrane. This membrane is kept taught by the tensor tympani muscle and is incredibly sensitive to changes in air pressure. For this reason, it waves with the atmospheric sound waves, in and out. The ear drum is actually the other sensory element of the entire hearing structure; the elements from here on only work to conduct the eardrum's initial message of vibration to the brain.
Past the ear drum is the middle ear.



MIDDLE


The middle ear is a small cave-like chamber filled with air. It holds the three ear bones, the ossicles, which transfer and amplify the ear drum's movement to the inner ear. It also holds the opening of a second canal called the Eustachian, which runs from middle ear chamber to the throat. More about the Eustachian canal at the bottom o
Wordle.net
Wordle.net
f this page.
The three ear bones conduct and amplify the ear drum's movements like this:
The malleus, commonly called "hammer", is attached to the inner face of the ear drum and receives all of the ear drum's energy, then conducts it along its lever-like body to the incus, commonly called "anvil", which in turn moves against the stapes, commonly called "stirrup", whose "faceplate" reaches to the inner ear through an opening called the oval window.

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wikimedia commons: The stapes in comparison to a 10 cent Euro piece
wikimedia commons: The stapes in comparison to a 10 cent Euro piece

The movements of the ear drum must be amplified because the next conductor of the sound wave is in the inner ear which is completely filled with fluid. Because of fluid's inertia, it does not move as easily as air and so more force will be needed to conduct a wave in the inner ear than it takes to move the eardrum which is surrounded on both sides by air. Good thing that the transfer of a sound wave from ear drum to inner ear amplifies the wave's energy nearly 22 times over! How? Two simple ways.
1. The lever action of the first ear bone, the malleus, amplifies the energy.
2. But the greatest benefactor to the amplification is the difference is size between eardrum and faceplate of the stapes bone. The eardrum is about eighteen times larger in surface area. It is simple hydraulics which magnifies the wave's energy because the stapes takes the transferred pressure it receives and expends over a much smaller space, giving a shorter, but stronger push, for each vibration transferred into the inner ear.


INNER

The inner ear consists of the cochlea, semicircular canals, the vestibular nerve, and the cochlear nerve, but actually, for the sense of hearing, only the cochlea and cochlear nerve are used.
When the stapes moves in and out, it does two things:

1. Pushes on the cochlear fluid of the inner ear, transferring waves of pressure into the fluid.
  • To account for the the bulges of pressure throughout the cochlea, a structure called the round window sits on the exterior of the cochlea just inferior to the oval window. The Round window is an opening covered with a flexible membrane which interchangeably sucks in and bulges out as the fluid in the cochlea is pumped by the action of the stapes.
2. Pulls and pushes on the base of the basilar membrane.

The second of these is especially important to the continued conduction of the sound wave. The basilar membrane accepts the sound wave signal from the movement of the stapes in a peculiar way do to its form. First, think of the cochlea a tube; though in reality it is a tube all rolled up like a snail's shell, it is easier to envision the basilar membrane inside if the cochlea is rolled out. Now, the basilar membrane reaches from end to end of the cochlea, the oval window being proximal and the opposite end being distal; however, its thousands of fibers to not run from proximal to distal end, they run perpendicular to. The fibers also change in form the more distal they become. Every fiber is distinctive, the proximal fibers being short and stiff and the distal being longer and more flexible. These fibers, varying in shape, allow the basilar membrane to conduct the sound wave traveling from one end of its body, the proximal end attached to the oval window, to its distal end in a distinctive way. As a wave travels down the membrane it does not lose much energy, but as every pitch of that wave within the human hearing range comes to a particular fiber in the membrane it stops and resonates with that fiber, releasing all of its energy onto that fiber and others around it with less intensity, causes these fibers all to vibrate. The proximal fibers resonate with high pitch sounds and the distal fibers with the low pitches.

.....................................................................Proximal fibers=High pitch.................................... Distal fibers=Low pitch

Now that the pitches of a sound are separated to resonate along different areas of a membrane, all that is needed left to determine where along the membrane fibers are resonating is a series of nerve receptors. These nerve receptors arrive in the form of an organ, the organ of corti, which houses thousands of things called hair cells. Nerve receptors run into each hair cell and are activated with that hair cell moves, and the connection: a hair cell moves when a fiber near it begins to vibrate. The organ of corti lies along the surface of the entire basilar membrane and so its hair cells can feel movement from any of the fibers and their movement is felt by the nerve receptors within them. The nerves bundle together and form the cochlear nerve which leaves the inner ear system and travels to the auditory cortex where the information is processed.














.........Want a virtual tour and summary
.........of the ear's workings for the sense
.........of hearing? Watch this quick video!















Everyday Hearing Phenomena...


The Eustachian Canal: Remember the middle ear chamber? It is an ear filled chamber where the bones of the ear and the inner side of the ear drum move around freely. This would not be possible if the pressure inside of this chamber was different than the pressure on the outer side of the eardrum. The outer side of the eardrum opens out to the outside atmospheric pressure so how does an air filled chamber inside of the body attain the exact same pressure? Through the Eustachian canal. If you remember, the Eustachian canal runs from the middle ear obliquely downward and medially to meet up at the throat. Whatever air pressure enters into the throat is then equalized with the middle ear chamber thru the pathway of the Eustachian canal. The usage of this canal usually means that we can always hear, but of course is not always the case.One common scenario where usage of the Eustachian canal is of no help to hearing is if a human's head is underwater and they are holding their breath.
The Tympanic Membrane: The tympanic membrane is a lot more miraculous than we would think. All background noise, the sound of one's own voice, both are blocked out by the work the tympanic membrane. When our brain receives a long drone of low pitched sounds or the sound of our own voice, it sends signals to the tensor timpani muscle to pulls the ear drum more taught. This is going on constantly so that you can forget about unimportant sounds and focus on what are, usually, higher pitched sounds. Lower pitched sounds become more and more undetectable as the ear drum is pulled more taught and after awhile, the sound will easily be ignored.






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