How the Ear Works
- With Pictures - Page 2
For page 1 introduction of How the Ear Works - Click here
Putting it all together
What the outer ear does
Sound waves from the external world travel
through the auditory canal to the tympanic
membrane (ear drum). The auditory canal can
resonate and amplify sounds within a frequency
range of about 60 Hz to 8000 Hz by up to a factor of 10.
The difference in pressure between the sound wave striking the outer surface of the eardrum and normal atmospheric pressure on the inside of the eardrum causes the eardrum to vibrate.
Successive compressions and decompressions of air waves reaching the eardrum result in changes in pressure between the outer ear and the middle ear. The
Eustachian tube helps to keep the middle ear at atmospheric pressure
so sounds are transmitted at true frequencies in
the middle ear..
What the Middle Ear Does
From the vibrating ear drum, and in
sequence, the malleus (hammer), the incus (anvil), and the stapes (stirrup), vibrate and transfer the vibrations to the oval window of the inner ear.
What the Inner Ear Does
The movement of the stapes, the third small
bone and the footplate, causes the oval window
membrane to vibrate which, in turn, causes
pressure waves to move in the fluids of the vestibular
canal (scala vestibuli). These pressure waves
also cause the fluids in the organ of Corti to
move causing the tectorial membrane to vibrate
laterally. After moving through the helicotrema,
the pressure waves then run through the tympanic canal
(scala tympani) causing the basilar membrane to move
in a "vertical" manner (90 degree angle to its
planar surface.
As the fluids of the canals move, still at the same frequency of the original air wave(s), the round window membrane at the end of the tympanic canal functions as a sound multiplier, moving outward or inward at a 20X multiple as the oval window membrane is pushed in or sucked out by the stapes.
As a pressure wave passes through the fluids of the two canals, there is
first movement of the tectorial membrane, then movement of the basilar membrane and, along with it, the
fluids in the organ of Corti
containing the hair cells. A wave form with a high frequency will only affect the initial part of the basilar membrane, which is
very thin than the final segment that responds
to low frequencies and is much stiffer.
At a mechanical level, measurements of the basilar membrane can be made similar to a wave frequency spectrum.
The higher frequency waves are picked up at the cochlear opening; the lower frequency waves are picked up towards the apex
at the top of the cochlea.
The basilar membrane will only distort at a
specific location when a frequency is detected
that matches the basilar frequency sensitivity attribute
at that location. Inner hairs at that location
distort (shear) and generate chemicals that cause an electrical current
to jump across to the auditory nerve endings and
thence to the brain. Outer hairs in the
immediate area of the basilar membrane stretch
and add amplitude to the inner hair shearing.
A pressure wave of
a particular frequency will pass through the entire
length of the scala vestibuli and scala tympani,
but the basilar membrane will only distort inner
hair cells at
the location where the basilar membrane attributes
match a specific frequency. This point is
actually a cancelled wave. In the cochlear
fluids, waves build up to a maximum for each
particular pitch and then rapidly fall away to
nothing. In other words, the brain only hears
what specific inner hairs transmit to it, based
on the distortion of the basilar membrane at a
specific location.
The image below shows a simulated basilar
membrane, rolled flat. The base end is the
opening of the cochlea and the apex is the top
of the cochlea. For a 2000Hz (2KHz)
wave and a 6000Hz (6KHz) wave, the location of
the peak of the wave varies at different
pitches: for high-pitched sounds, the wave peaks
vibrate (and are sensed) near the base of the
cochlea (6KHz example), whereas for low-pitched
sounds the peaks vibrate (and are sensed) near
its apex (narrow end) (2kHz example).
The above example is for only two
frequencies. During our daily activities, the
basilar membrane is continually distorting
thousands of times a second at various
frequencies, and the frequency-associated inner
hairs are continually firing electrical and
chemical messages each time they are "pushed" by
the basilar membrane. These actions provide the
hearing experience.
For Biochemists Only :) Electrochemical
Sequences of the Inner Ear
The following are the physical and chemical sequences of events that allow hearing to occur from the cochlea:
The insides of the inner hair cells are normally
at about a –40 millivolt resting potential relative
to the rest of the body. They achieve this by
pumping positive ions like sodium (Na+) out of the
cell, as do many cells in the body, leaving a net
negative charge. But the tops of the hair cells are
located in the middle channel, the scala media,
which contains a special fluid called endolymph that
is potassium (K+) rich and has a +80 millivolt
positive charge.
As a wave reaches its peak at a specific location
in the cochlea, the outer hair cells near this peak
give a small, physical "push" or amplification to
enhance the movement of the basilar membrane. Much
like a carpet on a floor being pushed into folds,
the basilar membrane's action of "folding" acts as
an amplifier. The positive terminal (K+) is
connected to the outer part of the upper surface of
the inner hair cell, and the negative terminal (CA
2+) is inside the hair cell. The buckling of the
basilar membrane causes the endolymph in the
tympanic canal (high in potassium (K+) with a
positive charge) to squirt towards the hairs of the
inner hair cells which have a negative charge
(calcium - CA). The hairs are deflected (sheared)
and very small channels open up near the bottom tips
of the hairs across from the auditory nerve endings.
The potassium atoms (K+) in the endolymph flow
through these small channels, being propelled by the
differential charges. This action is called
depolarization - a positive change in the voltage
across a cell's membrane. In this case, the negative
charge approaches neutral.
On the bottom and sides of the hair cells there
are inputs to the auditory nerve which are separated
from the hair cell by a small synapse.
When depolarization occurs, special voltage
sensitive calcium (Ca 2+) channels open in the hair
cell, triggered by the voltage rise and let calcium
into the cell. The calcium causes the hair cell to
release a quantity of a special chemical called a
neurotransmitter that stimulates the auditory nerve
to fire. (It does this by forcing the nerve to open
up its own ion channels at the synapse which raises
the voltage in the nerve fiber. This triggers still
other adjacent voltage sensitive channels in the
fiber to open which results in a domino effect that
causes a wave of depolarization to propagate along
the entire nerve.)
Translating from chemicals to electricity
When the the potassium (K+) comes in contact
with the hair cell, it alters the voltaic charge on
the membrane
of the hair cell. The chemical sensitivity of the
hair cell to potassium triggers an action at the bottom
of the hair. Small parcels of chemicals
(calcium influx via voltage-gated channels)
cause neurotransmitters in the form of
glutamate to be released from the base of
the hair cell over a synapse, causing the
nerves across the synapse to become active
and send an electrical impulse towards the
brain.
The feedback loop
In addition to sending information to the brain, the inner ear is subject to feedback regulation from the brain. Neurons in the brainstem send messages back to the cochlea
through the auditory nerve based on the "content" of the messages the brain is receiving from the inner hair cells i.e. what sounds it is hearing. This feedback is terminated at the outer hair cells
which have about 10% of the connections to
the auditory nerve. When instructed by the
brain,
ducts at the base of these hairs release the neurotransmitter acetylcholine to inhibit the action of inner hair cells.
It is thought that one of the functions of this
feedback/inhibitor mechanism is to protect the cochlea (and the sensitivities of the user) to loud and sudden noises.
It is also postulated that the brain's signals
also induce another response in the outer hairs
that assists with the cochlear protection
mechanism. The hypothesis states that the effect of the acetylcholine also
causes the outer hair cells
to stiffen or even to move in a
counter-cyclical motion to cause dampening or interference
patterns to reduce damaging sounds. However this
theory has not been proven to date.
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