Download The Inner Ear

Auditory Transduction
The Inner Ear
11.6.12
Ossicles as levers
Levers increase force
To oval
window
Stapes footprint
Oval window
Ear drum
22 times amplification of sound pressure due to difference in surface area
of ear drum and oval window
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
• The impedance Z is the product of the mass density
ρ and speed of sound
• It determines 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 oval window, 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
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 snail-like 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
Uncoiled 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.
• Each hair cell has up to 80 tiny hairs projecting out of it
into the endolymph
Organ of Corti
Generation of Receptor Potentials
by Hair Cells
• 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 Hair Cells
• Endolymph is rich in K+ ions
• The bending of hairs depolarizes the hair cells
producing receptor potentials across the hair cell
membrane.
• K+ from endolymph enters into hair cells
Neurotransmitters are released. Nerve endings are at
the base of hair cells. These impulses travel to the
auditory areas of the brain for processing
Action Potential by Sensory
Neurons
• Sensory receptors like hair cells do not directly
generate action potentials
• Instead sensory receptors generate receptor
potentials which vary in intensity with stimulus
• These changes in membrane potential are passed
to adjacent sensory neurons which may generate
an action potential, if the incoming stimuli are
sufficient for the neurons to reach threshold
• Receptor potential is a graded potential for
sensory neurons
Pitch Perception and Resonance of Basilar
Membrane (Bekesey theory of pitch perception)
• Pitch can be distinguished through differences in sound
wave frequencies
• Different areas of the basilar membrane are sensitive to
different pitches due to different levels of flexibility
along the membrane
• 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
Sensitivity of Ear to Pressure
Variations
• The sensation of hearing is produced by the
response of the nerves in the ear to pressure
variations in the sound wave
• The nerves in the ear are not the only ones that
respond to pressure, as most of the skin contains
nerves that are pressure sensitive.
• However, the ear is much more sensitive to
pressure variations than any other part of the body