Download The Inner Ear (Cochlea)

The Incredible Ear
How do we perceive the crack of lightening, crunch of chips, buzz of a bee, and pop
of a soda can? Quite simply, we pick up these, and all other sounds, from the help of
a remarkable organ known as the “ear” through the process of “hearing”. Unlike
taste, vision, or smell, which all involve chemical reactions, the process of hearing is
entirely mechanical. This means that it is completely based on physical movement.
Beginning from the source, we will trace the path of a sound wave through the
outer, middle, and inner ear all the way to the brain.
Source: http://www.ahschicago.com/how-does-your-ear-work/
The Source
For an object to produce a sound, it has to vibrate matter. This matter could be solid
like the earth or liquid like water, but most of the time it is gaseous in the form of
air. When the item vibrates, it starts a chain reaction and moves the particles
directly surrounding it, which in turn cause the movement of the particles
surrounding those particles, which consequently cause the movement of the
particles surrounding those particles. This process continues on and on until the
particles reach their next destination – the outer ear.
If the object bows outward at first, it will cause “compression” in which the particles
move forward and collide with each other due to high pressure. However if the
object bends inward, it will cause “rarefaction” in which the particles are pulled in
and move backwards due to low pressure. Each object creates a different, specific
series of compressions and rarefactions that subsequently create individual sound
waves. Depending on the frequency of how fast the particles move forward and
backward, the sound we hear will either be higher pitched (rapid variations) or
lower pitched (fewer fluctuations). In addition, depending on how many of the
particles are moved, the sound will be loud (considerable amount of movement) or
soft (slight shifts).
The Outer Ear (Pinna)
The next step in the process of hearing is directing the sound waves into the ear. The
large surface area and multiple ripples in the outer ear, or pinna, act as a funnel to
catch sound waves and send them down the ear canal [1] to the eardrum [2]. Unlike
other animals, such as dogs and cats, humans are incapable of moving the outer part
of their ear to focus on a sound from a particular direction. This is because our
pinnae face forward, lay rather flat against our heads, and lack significant muscle.
However, we can still determine whether a sound is coming from behind, in front,
above, or below depending on the distinct sound wave pattern created by the
reflection off the pinna. Also, the horizontal position of the sound can be determined
by comparing information from both ears. If a noise happens on the right side of the
body, the sound wave will arrive at the right pinna sooner and louder than it does at
the left pinna.
The Middle Ear
The Eardrum
After going through the ear canal, the sound wave reaches the eardrum,
where the fluctuations in air pressure are sensed. Also known as the
tympanic membrane, the eardrum is a thin, extremely sensitive, cone-shaped
piece of skin about 10 millimeters wide. This flap of skin acts like a drum,
which is pulled and pushed, based on the compressions and rarefactions of
the sound wave. Louder sound waves move the drum a greater distance,
while higher pitched sound waves move the drum more rapidly. No matter
where the sound wave hits, the membrane will vibrate since it is kept taut by
the inward pull of the tensor tympani muscle.
The Eustachian Tube
Even the slightest variations in air pressure will cause the eardrum to move
back and forth a good distance. This is directly due to the equal air pressure
on either side of the eardrum, which is maintained by the eustachian tube
[7]. Since the eustachian tube connects the middle ear and the throat, air is
allowed to flow freely and provide the same pressure as the atmosphere.
The Ossicles
The ossicles is a group of three, tiny, connected bones: the malleus or
hammer[3], the incus or anvil [4], and the stapes or stirrup [5]. When the
eardrum vibrates, it moves the malleus from side to side like a lever since
they are in contact. The malleus then in turns hits the incus, which pushes on
the stapes, which presses on the faceplate of the inner ear [6].
Overall, the function of the ossicles is to amplify the force created by the
movement of the eardrum into enough pressure to move the liquid-filled
inner ear. This amplification is needed since fluid has a higher inertia than
air, meaning that it is a lot harder to move. These bones are extremely
effective, increasing the force that is made by the eardrum 22 times, for two
reasons. First, there is a huge size difference between the eardrum and the
stapes. The eardrum has 18 times the surface area of the stapes and when
force is concentrated on a smaller surface area, the pressure is greatly
increased. Secondly, the malleus is longer than the incus, which forms a basic
lever between the eardrum and the stapes. Because of basic energy
principles, when the malleus moves a greater distance, it causes the incus to
move with greater force.
The Inner Ear (Cochlea)
The stapes from the middle ear rests against the faceplate of the cochlea [8], a
liquid-filled system of tubes that resemble the circular shell of a snail. When air
pushes in on the eardrum, the ossicles move forward and push in on the cochlear
fluid. Conversely, when air pulls on the eardrum, the ossicles move backward and
pull the fluid with it. Hence, waves are created in the inner-ear fluid depending on
the air-pressure fluctuations of the sound wave. The cochlea is full of tiny hair cells,
which sense this motion. These hairs transform the liquid vibrations into electrical
impulses that travel along the auditory nerve [9] and finally reach the brain.