Download The Inner Ear - Lectures For UG-5

Auditory Transduction
The Inner Ear
5.3.13
Outer Ear
• Pinna collects the sound and directs it to ear
canal
• Because of the length of the ear canal, it is
capable of amplifying sounds with
frequencies in range 2000-3000 Hz. Ear canal
acts as a resonator for this fundamental
frequency
Basic parts of human ear
a. Outer ear
b. Middle ear

The Eardrum (tympanic membrane)
Auditory Ossicles
The Tympanic Cavity

The Eustachian Tube


c. Inner ear
Middle Ear
Ossicles
Ear Drum
Eustachian Tube
Ossicles
Auditory Ossicles
• The function of the auditory
ossicles is to transmit sound
from the air striking the
eardrum to a fluid-filled
labyrinth inside the inner ear
(Cochlea).
• The bones are connected by
small ligaments and transmit the
vibratory motions of the
eardrum to the inner ear.
Transmission of sound wave by
ossicles to inner ear
• Being connected to the hammer, the movements of
the eardrum will set the hammer, anvil, and stirrup
into motion at the same frequency of the sound
wave.
• The stirrup is connected to the inner ear; and thus
the vibrations of the stirrup are transmitted to the
fluid of the inner ear and create a compression
wave within the fluid
Importance of Middle Ear
• One may wonder why the incident sound
wave collected by outer ear is not incident
directly on the fluid of inner ear
• The primary reason is that of a very poor
matching of the impedance of the air and
the cochlear fluid
• Middle ear acts as an impedance matching
device
Importance of Middle Ear
• Acoustic impedance is a measure of the resistance
of a medium to being disturbed by a change in the
external pressure
• When a sound wave is traveling in one medium and
is incident upon an interface with a second medium,
a certain fraction of sound energy will be reflected
and a certain fraction will be transmitted
• If the impedances of two materials are very different,
sound will not easily pass from one to the other
• If two stones are tapped together in air and the ear is
in air, the sound made is clearly audible. Sound
conducts well through air.
• If two stones are tapped together underwater and the
ear is underwater, the sound made is, again, clearly
audible. Sound conducts well through water.
• On the other hand, if two stones are tapped
together in air and the ear is underwater (or the
other way round), the sound made is almost
imperceptible.
• Sound does not conduct well from air to water or
from water to air. This is because the impedances
of water and air do not match, and most of the
sound is reflected off the interface between the
two media, remaining in the medium in which it
was generated
• The impedance of the fluid in the cochlea is about 30
times greater than that of air, and if the sound were
applied directly to the cochlear fluid, most of it
(~97%) would be reflected, leaving only 3%
transmission.
• It is necessary to somehow compensate for this
difference, to match the characteristics of one
material to that of the other
• Ossicles chain works as impedance matching device
Sound amplification by middle
ear
Middle ear amplifies sound by a
combination of three mechanisms
• The area ratio advantage of the ear drum to
the oval window
• The lever action of ossicles
Sound amplification by middle
ear
• The largest contribution comes from
area advantage
• The force that is exerted over the
large area of the tympanic membrane
is transmitted to the smaller area of
oval window
• The area of the eardrum is about 22
times larger than the oval window.
Therefore, the pressure on the oval
window is increased by the same
factor
• This feature enhances our ability of
hear the faintest of sounds
Sound amplification by middle
ear
• Ossicles amplify the sound reaching eardrum by lever action
With a long enough lever,
you can lift a big rock with a
small applied force on the
other end of the lever. The
amplification of force can be
changed by shifting the pivot
point
Sound amplification by middle
ear
• The three tiny bones of the middle ear act as levers to
amplify the vibrations (pressure) of the sound wave.
• The pivot point or fulcrum is located farther from the
tympanic membrane than from the stapes.
• The force at the oval window is amplified. The
mechanical advantage is 2
• The resulting vibrations would be much smaller
without the levering action provided by the bones
Amplification of sound with
frequency in range 2000-3000Hz
• In the frequency range around 3000Hz, there is an increase in
the pressure at the eardrum due to the resonance of the ear
canal. This amplifies the sound pressure by a factor of 2
• Lever action amplifies by another factor of 2
• Smaller area of oval window amplifies the sound by a factor of
22
• amplification = 2 x 2 x 22 =88
• This accounts for the high sensitivity of ear to this frequency
range
The Tympanic Cavity and the
Eustachian Tube
• The tympanic cavity is an air chamber
surrounding the ossicles within the middle ear
• The Eustachian tube is a membrane lined tube
(approximately 35 mm long) that connects the
middle ear space to the back of the nose (the
Pharynx)
• The Eustachian tube does not directly relate to
the mechanical process of hearing
Functions of the Eustachian tube
• Pressure equalization:
Air seeps in through this tube to maintain the middle ear at
atmospheric pressure
A rapid change in the external air pressure such as may occur
during an airplane flight causes a pressure imbalance on the
two sides of the eardrum.
The resulting force on the eardrum produces a painful
sensation that lasts until the pressure in the middle ear is
adjusted to the external pressure
Volume control by muscles of
middle ear
• The ossicles are connected to the walls of the
middle ear by muscles that also act as a volume
control
• If the sound is excessively loud, these muscles as
well as the muscles around eardrum stiffen and
reduce the transmission of sound to the inner ear
Basic parts of Human Ear
I.
II.
III.
IV.
Ear anatomy
Outer ear
Middle ear
Inner ear
Semicircular canals
Cochlea (Latin for snail.)
Inner Ear
Semicircular Canals
(Balance)
Cochlea
(Transducer/
Microphone)
The Inner Ear
• The inner ear can be thought of as two organs: the
semicircular canals which serve as the body's
balance organ and the cochlea which serves as the
body's microphone, converting sound pressure
impulses from the outer ear into electrical
impulses which are passed on to the brain via the
auditory nerve
The Inner Ear
• The cochlea is a snaillike structure divided
into three fluid-filled
compartments/ducts
• The scala vestibuli and
scala tympani are
filled with fluid called
perilymph while scala
media is filled with
endolymph
The Cochlea
Transmission of sound into organ of
corti
• The small bone called the stirrup, one of the ossicles, exerts force
on the thin membrane called the oval window by piston action,
transmitting sound pressure information into the perilymph of the
scala vestibuli
• Then through Reissner's membrane and the basilar membrane to
the scala tympani. In the scala tympani, the vibrations pass again
through perilymph to the round window at the base of the
cochlea.
The displacement in the cochlea caused by
movement of the stapes is almost all
across the basilar membrane. The energy
dissipation at the round window is
necessary to prevent pressure-wave
reflections within the cochlea
Organ of Corti: The body’s
Microphone
• On the basilar membrane sits the sensory organ of the ear,
the organ of Corti which acts as a transducer (converting
sound energy into electrical energy)
• It is composed of a complex of supporting cells and
sensory or hair cells atop the thin basilar membrane
• There are some 16,000 -20,000 of the hair cells distributed
along the basilar membrane which follows the spiral of the
cochlea. There are 3500 inner hair cells and 12,000 outer
hair cells in each ear
• Each hair cell has up to 80 tiny hairs projecting out of it
into the endolymph
Organ of Corti
Function of hair cells
• Research of the past decades has shown that outer hair
cells do not send neural signals to the brain, but that they
mechanically amplify low-level sound that enters the
cochlea.
• The inner hair cells transform the sound vibrations in the
fluids of the cochlea into electrical signals that are then
relayed via the auditory nerve to the auditory brainstem
and to the auditory cortex
Generation of Receptor Potentials by
Inner Hair Cells (Sensory receptors)
• The upper ends of the hair cells are held rigid by
the reticular lamina and the hairs are embedded in
the tactorial membrane
• Due to the movement of the stapes both the
membranes move in the same direction and they
are hinged on different axes so there is a shearing
motion which bends the hairs in one direction
Hair cell shearing
Tectoral membrane
Hair cells
Basilar membrane
Sheared hairs
Generation of Receptor Potentials
by Inner Hair Cells
• Endolymph is rich in K+ ions while perilymph in
Na+ ions
• The deflection of the hair-cell stereocilia opens
mechanically gated ion channels that allow K+ ions
to enter and depolarize the cell.
• The influx of K+ from endolymph in Scala media
depolarizes the hair cells producing receptor
potentials across the hair cell membrane.
Resonance Place Theory of Pitch
Perception by Helmholtz
• Pitch can be distinguished through differences
in sound wave frequencies
• Different areas of the basilar membrane
resonate/ respond to different pitches due to
different levels of flexibility along the
membrane
Resonance Place Theory of Pitch
Perception by Helmholtz
• Higher
frequencies
stimulate the membrane
closest to the oval
window,
lower
frequencies
stimulate
areas further along (apex)
• These
regions
then
stimulate neurons to send
signals to specific areas
of the brain and thus leads
to certain perception of
pitch
Loudness of sound and frequency of
action potentials
• The louder the sound is, the greater height or
amplitude of the vibrations in the sound waves,
the more movement of hairs/stereocilia of hair
cells and thus more action potentials
• Greater the frequency of action potentials,
louder the sound is
• If you could hear someone talking, that means
the voice is loud enough to generate action
potentials in the sensory neurons of your ear.
Loudness of sound and frequency of
action potentials
• If they raise their voice, that causes an increase
in the APs to your brain. If they lower their
voice into a whisper, the frequency decreases.
• If they lower their voice to the point where you
can’t hear them, then that means you’re not
even generating ONE action potential. So if
you can’t hear a sound, it doesn’t mean there’s
no sound in the room, it means the sound is too
soft for you to hear.
Why do our own voices sound different to us
when we hear them on a recording vs. when
we hear them as we speak
• This is because there are two different ways in
which we hear sounds. One is through air
conduction, and the other is bone conduction.
• Everyday sounds we hear are primarily hear
through air conduction, which is basically sound
waves traveling through our ear canal and
impacting our eardrum, and eventually to the
cochlea of the inner ear.
• When we speak, however, we hear our voice
through both air conduction and bone conduction.
• Bone conduction is the conduction of sound to the inner ear
through the bones of the skull. The vibrating of our bones and
body tissue transmits sounds directly to the cochlea.
• The skull conducts lower frequencies better than air, people
perceive their own voices to be lower and fuller (heavier) than
others do.
• When we hear our voice on a recording, that's how it sounds to
everyone else, as we are then hearing it through air conduction
only
• You can note the difference in your voice by talking with the
ears plugged