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Hearing Physiology
Subject : Zoology
Lesson : Hearing Physiology
Lesson Developer: Dr. Mahtab Zarin and
Dr. Zubeda Khan
Department: Department of Zoology,
Delhi University
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Hearing Physiology
Table of Contents
INTRODUCTION
SOUND AND THEIR PROPERTIES
MECHANISM OF SOUND TRANSMISSION IN EAR
(1) Role of external ear in the hearing
(2) Role of middle ear in the hearing
(3) Role of inner ear in the hearing
(4) Transduction of mechanical vibrations into electrical signals
NEURAL AUDITORY PATHWAYS
CLINICAL ASPECT HEARING PHYSIOLOGY
PHYSIOLOGY OF EQUILIBRIUM
NEURAL PATHWAYS OF EQUILIBRIUM
USE OF VESTIBULAR INFORMATION
Summary
Exercises
Glossary
References
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Learning objectives

Explain the physiological anatomy of the ear;

Essential understanding of the sound for hearing;

Understanding of the functioning of each part of the ear;

List the various events in the physiology of hearing

To discuss various types of hearing loss and how it is measured.

Describe function of the receptor organs for equilibrium
INTRODUCTION
Hearing or audition is the ability to perceive sound from surrounding medium
through the ear. Hearing is imperative for animals to interact for various purposes.
Two systems are located in the ear: auditory system responsible in the hearing and
vestibular system resposible for maintaining equilibrium.
SOUND AND THEIR PROPERTIES
In terms of physics, sound is a vibration that proliferates as an audible mechanical
wave of pressure and displacement, across a medium for example air and water. In
physiological term, sound is the reception of such waves and their perception by
the brain. Anything capable of disturbing molecules- e.g., vibrating objects can serve
as a sound source. When there are no molecules, as in vacuum, there can be no
resonance. Followings are the characteristics of sound which we hear from a
medium:

Sound waves are vibrating between high and low pressure regions moving in
the same direction through some medium (for example air), it is so often termed
as pressure wave.

A sound wave measured over time consists of rapidly alternating pressures that
vary continuously from a
high during
compression of
molecules , to a low
during rarefaction , and back again.
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
The difference between the pressure of molecules in regions of compression and
rarefaction constitute the wave’s amplitude, that is associated with the loudness
of the sound; the greater the amplitue, the louder the sound.

The frequency of vibration of sound source is the number of zones of
compression or rarefaction in a given time.

The frequency of vibration is directly proportional the pitch we hear. The
sounds perceived most acutely by the human ear at frequencies ranging from
200 to 8000 hertz (Hz; 1 Hz = 1 cycle per second) (Fig 1a).

Any group of sound waves which are in irragular sizes leads to noise. Tone is a
sound with a particular pitch that is a group of sound waves are in regular sizes
and all exactly have same distances from each other (Fig 1b).

Intensity of sound is measured in units termed as decibels (dB). Increase of
one decibel corresponds tenfold amplify in sound intensity (See Table 1).
Fig 1a. Formation of sound waves and their characteristics.
Source:
http://jdy-ramble-on.blogspot.in/2014/04/what-does-sound-
waves-look-like.html
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Fig 1b. The sound which have no regular pattern of waves perceived as Noise.
Sound which have same size of waves at same distances perceived as normal sound
or tone.
Source:
http://cnx.org/contents/07970e19-2e42-4b8e-9a7d-2749bf5d8529@15 CC
Image Credit: CC BY-SA 4.0
Table 1. Decibel levels of common sounds and their effects.
SOUND SOURCE
DECIBEL
EFFECTS
LEVEL
Breathing
10
Just easy to hear
Murmur
30
Very silence
Quiet office discussion
50-60
Easy hearing level below 60 dB
Vacuum cleaner, hair
70
Invasive; hinders with telephone talk
80
Annoying; constant exposure could
dryer
Metropolis traffic,
Garbage disposal
Rock concert
damage hearing
110-140
Threshold of pain begins at around
125 dB
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Shotgun blast, jet take-
130
Some permanent hearing loss likely
150
Tympanic membrane rupture,
off (200 foot distance )
Jet take- off ( 75 foot
distance )
permanent damage
Value addition: Did you Know
Hearing threshold and Auditory Masking
Hearing threshold
It is a level of sound intensity at which normal young adult can immediately
distinguish sound from quietness is defined as 0 dB at 1000 Hz. Sound turns out to
be uncomfortable to a normal ear at about 120 dB, and painful above 140 Db.
Auditory Masking
It happens when individual’s ability to hear one sound is decreased through the
presence of another sound. It occurs as the previous neural activity caused by the
first signal is reduced through the neural activity of the other sound. The degree to
which a given sound masks other sound is related to its pitch.
Source: Principles of Anatomy & Physiology- Tortora, G.J. & Derrickson, B.
And Text book of Medical Physiology by Guyton and Hall
MECHANISM OF SOUND TRANSMISSION IN THE EAR
The ear translates sound waves in the external environment into action potentials in
the auditory (VIII) nerves. This process involves following events:
Role of external ear in the hearing
The initial step in hearing is the entry of sound waves through auricle into the
external auditory canal. The appearance of the pinna, or auricle and the outer
auditory canal assist to amplify and direct the sound (See step 1 in Fig. 2).The
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sound waves reverberate through the sides and end of the outer auditory canal,
filling it with the nonstop vibrations of pressure waves.
Tympanic membrane
When sound waves hit the tympanic membrane, the alternating waves of high and
low pressure in the air make the tympanic membrane to vibrate ‘to and fro’ (See
step 2 in Fig. 2). The tympanic membrane vibrates gradually in reaction to low
frequency sounds and quickly in reaction to high frequency sounds. Therefore the
work of tympanic membrane in process of hearing categorized as:
1. It acts as a pressure receiver. It is extremely sensitive to pressure changes
produced by sound wave on its external surface.
2. It acts as a resonator. It starts vibrating freely when the sound waves
strikes.
3. It critically dampens the sound waves. As soon as the sound waves will
stop stretching the tympanic membrane, its vibrations are stopped almost
immediately.
Role of middle ear in the hearing
In the second steps, the middle area of tympanic membrane attaches to the malleus
that vibrates together with the tympanic membrane. This vibration is pass across the
air from the malleus to the incus and then to the stapes (See 3 in Fig. 2). Therefore
role of ear (auditory) ossicles in the hearing categorized as:
1. The ear ossicles (malleus, incus, stapes) performs as a lever system that
changes the resonant vibrations of the tympanic membrane into movement of
the stapes against the perilymph filled scala vestibuli of the cochlea.
2. The middle ear contains the air, inner ear contains fluid. Therefore, sound is
transmitted from air to the fluid. As fluid has got inertia, therefore, sound is
not transmitted so easily into the inner ear, it is transmitted by increasing the
pressure in the middle ear at oval window. This pressure is transferred to the
perilymph in the scala vestibuli (one of the chambers of cochlea in internal
ear).
As the stapes shifts ‘to and fro’, its oval shaped footplate,which is connected via a
ligament to the perimeter of the oval window, vibrates in the oval window.The
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vibrations at the oval window are about 20 times more forceful than the tympanic
membrane since the auditory ossicles proficiently transmit little vibrations scattered
over a big surface area into large vibrations at a lesser area.
Because liquid is more complicated to shift than air, the sound pressure pass on to
the internal ear have to be amplified. This is accomplished by a movable chain of
three small bones, the malleus, incus, stapes. These bones work as a piston and
couple the vibration of tympanic membrane to the oval window, a membrane
enclosed aperture separating middle and inner ears.
Tympanic reflex
Contraction of the middle ear muscles, i.e. tensor tympani and stapedius pulls the
foot plate of the stapes outward. Eventually, it reduces sound transmission.
Therefore, the loud sounds begin a reflex contraction of these muscles usually known
as the Tympanic reflex or attenuation reflex. Function of Tympanic reflex is most
likely twofold:
1. It protects cochlea and prevents strong sound waves from causing excessive
stimulation of the auditory receptors. Although, the responsing time for the
reflex is 40-160ms, it can not protect from shortly intense stimulation e.g.
sounds of gunshots.
2. It
masks low-frequency sounds in loud
environments. This generally
eliminates a major share background noise and permit an individual to
concentrate on sounds above 1000 hertz frequency, where most of the
relevant information in voice communication is transmitted.
Impedance Matching
As fluid of the cochlea in the inner ear has far greater inertia than air, it needs
inceased amount of pressure to cause vibration in fluid. In this way tympanic
membrane and ossicular system provide impedance matching between the sound
waves in air and sound vibrations in the fluid of the cochlea.
Bone and air conduction
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
Conduction of sound waves to the fluid of inner ear through the tympanic
membrane and the auditory ossicles, the main pathway for normal hearing is
called ossicular conduction.

Sound waves also initiate vibrations of the secondary tympanic membrane that
covers the round window. This process is known as air conduction, although it
has no role in normal hearing.

Bone conduction is the transmission of vibrations of the bones of the skull to
the fluid in thw cochlea of the inner ear. Noticeably, It occurs when tuning forks
or other vibrating bodies are applied directly to the skull. This pathway also take
part in transmission of enormously loud sounds.
Role of inner ear in the hearing
The third event in hearing is the transmission of sound waves from tympanic
membrane via middle ear to the internal ear. This is accomplished in following
steps:
Sound transmission in cochlea
1. The vibrations of the stapes at the oval window put up waves of fluid pressure
in the perilymph of cochlea. Since the oval window bulge inward, it moves
forward the perilymph of the scala vestibule (See 4 in Fig. 2).
2. Sound waves are transmitted from the scala vestibule to the scala tympani
and ultimately to the round window, make it to swell outward into the middle
ear.
3. The sound waves travel through the perilymph of scala vestibule, then the
vestibular membrane, and then move into endolymph inside the cochlear
duct. Vestibular membrane (or Reissner’s membrane) is so thin and so easily
moved that it does not block the passage of sound vibrations from the scala
vestibule into the cochlear duct (scala media). So, in reference to the sound
conduction, scala vestibule into the cochlear duct are considered to be a
single chamber.
4. The significance of Reissner’s membrane is to uphold a special fluid in the
scala media which is needed for normal function of sound recepting hair cells.
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Basilar Membrane and Sound frequency
The basilar membrane is a fibrous membrane which divides the scala media from
the scala tympani. It has 20-30 thousands basilar fibers which are stiff, elastic
and reedlike structures. The length of the basilar fibers increases progressively
from the base of the cochlea to its apex. Thus, the stiff and short fibers near the
oval window of the cochlea vibrate best at a high frequency, whereas the longer
and supple fibers near the tip of the cochlea vibrate best at a low frequency.
Pattern of Vibration of the Basilar Membrane
Basilar membrane exhibits different pattern of sound waves of different
frequencies. Each sound wave is quite weak at the beginning but becomes strong
when it reaches that part of the basilar membrane which bears natural resonant
frequency equal to the particular sound frequency. At this position, the basilar
membrane can vibrate back and forth with so effortlessness that energy in the
wave is degenerated. Evantually, the wave expires out at this position and unable
to travel the remaining distance along the basilar membrane. Therefore, sound
wave of high-frequency travels simply a short distance along the basilar
membrane before reaching its rasonant point. Sound wave of medium-frequency
travels around middle-way and then dies out whearas sound wave of lowfrequency travels whole length of the membane.
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Fig. 2. Pattern of vibration of the basilar membrane.
Source: Text book of Medical Physiology by Guyton and Hall
Amplitude Pattern of Vibration of the Basilar Membrane: The amplitude
pattern of vibration of the basilar membrane demonstrates that the maximum
amplitude for 8000 Hertz takes place near the base of the cochlea, whereas
that for frequencies less than 200 Hertz is all the way at the apex of the
basilar membrane near the helicotrema where the scala vestibuli opens into
the scala tympani.
Value addition: Did you Know
Otoacoustic emissions
In addition to perceiving sounds, cochlea possesses the noticeable capability to
generate sounds. These usually inaudible sounds, called otoacoustic emissions
which may be detected through keeping a perceptive microphone adjacent to the
eardrum.
Source: Principles of Anatomy & Physiology- Tortora, G.J. & Derrickson, B.
Sound transmission in organ of Corti:
The organ of corti is the receptor organ that generates nerve impulses in
response to vibration of the basilar membrane.
Excitation of the hair cells: The sound waves in the endolymph arise
vibration in the basilar membrane. Therefore, the basilar fiber, rods of Corti,
and reticular lamina all move as a stiff unit. The downward and outward
motion causes the hair cells to move back and forth against the tectorial
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membrane. In this way, the hair cells are excited whenever the basilar
membrane vibrates (See 5 in Fig. 2).
Movement of the hair cells lead to bending of the stereocilia and ultimately to
the generation of nerve impulses in first-order neurons in cochlear nerve
fibers.
Hearing signals are transmitted mostly through inner hair cell:
Although there are 3-4 times as many outer hair cells as inner hair cells,
about 90 percent of the auditory nerve fibers are stimulated by inner cells
rather than by the outer cells. However, if the outer cells are injured while the
inner cells remain entirely functional, a huge amount of hearing loss takes
place. Therefore, it may be the outer hair cells by some means control the
sensitivity of the inner hair cells for different sound frequencies.
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Fig. 3. Transmission of sound to the inner ear.
Source:http://cnx.org/contents/b375ea7d-22d5-4f47-b10a-41dd93637896@4 cc
Value addition: Did you Know
Video on working of ear in hearing
See in the following hyperlink to know about the working of ear. This process
involves converting sound waves into a neural signal through the movement of hair
cells of cochlear duct in cochlea.
http://www.youtube.com/watch?v=GGqfRvCkt-w
Source: You tube
Transduction of mechanical vibrations into electrical signals
The hair cells convert mechanical vibrations into electrical signals. When the basilar
membrane vibrates, the hair bundles at the tip of the hair cell turn ‘to and fro’ and
glide at each other.
The stereocilia (the hair bundles or hairs) protruding from the ends of the hair cells
are composed of rigid structural protein framework. Each hair cell has about 100
stereocilia on its apical border. These stereocilia become progressively longer on the
side away from the modiolus. Tips of the shorter stereocilia are attached through a
thin filament (link protein) to side of its adjacent longer stereocilium.
Therefore, whenever the cilia are bent in the direction of the longer ones, the tips of
the smaller stereocilia are tugged outward from the surface of the hair cell. This
causes a mechanical transduction that opens 200-300 cation-conducting channels,
allowing rapid movement of cation, mainly positively charged potassium ions
from the surrounding endolymph in scala media into the stereocilia, that leads
depolarization of the hair cell membrane.
Depolarization rapidly spreads along the plasma membrane and opens voltage
gated Ca2+ channels in the bottom of the hair cell (Fig 4.). The resulting inflow of
the Ca2+ causes exocytosis of synaptic vesicles having neurotransmitter, which is
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probably glutamate. When more neurotransmitter is relayed, the frequency of nerve
impulses in the first-order sensory neurons which synapse with the base of the hair
cell enhanced.
Twisting of the stereocilia in the opposite direction shuts the transduction
channels, allow hyperpolarisation to occur, and decreases neurotransmitter
release from the hair cells. This reduces the frequency of nerve impulses in the
sensory neurons.
Fig 4. Transductions of sound signal in the ear. A: Sound waves pass through the
ear in which sound vibrations are transformed into electrical nerve impulses
through cochlea. Lastly, hearing information passes to brain through auditory
nerve system. B: a transverse section of sensory part of cochlea, depicting
structure of the sound-perceiving organ at cellular level. C: Tip link in
stereocilia is an entrance for the mechano-electrical transduction channel and
constitutes two distinctive anchor tips along stereocilia: the upper tip-link
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density (UTLD) and the lower tip-link density (LTLD). D: Sound induced
displacements of stereocilia that release pressure on tip links that open the
transduction channel. Then ions, K+ and Ca2+, enter through the opened
transduction channels and depolarize the hair cell.
Source: http://physiologyonline.physiology.org/content/nips/27/1/25/F1.large.jpg
Value addition: Did you Know
Endocochlear Potential
Electrical potential of around +80 millivolts exists all the time between endolymph
and perilymph, with positivity within the cochlear duct and negativity outside. This is
known as endocochlear potential. This potential is created through repeated
release of K+ ions into the cochlear duct through stria vascularis.
Significance of endocochlear potential: Tips of the hair cells protrude through
the reticular membrane and are bathed through endolymph of the cochlear duct,
while perilymph bathes the lower parts of the hair cells. Additionally, the hair cells
possess a negative intracellular potential of −70 millivolts corresponding the
perilymph but −150 millivolts corresponding the endolymph at their upper parts
where hairs project through reticular membrane and into the endolymph. Therefore,
this high electrical potential at tips of the stereocilia sensitizes the cell to great
extent. In this way, it increases its ability to react to smallest pitch of sound.
Source: Textbook of Medical Physiology- Guyton, A. C. & Hall, J. E.
NEURAL AUDITORY PATHWAYS
Bending of stereocilia of the haircells of the spiral organs causes the release of a
neurotransmitter which produces nerve impulses in the sensory neurons that is
supplied in the hair cells. The cells of the sensory neurons are positioned in the spiral
ganglia.
Nerve fibers from the spiral ganglion of Corti enter the dorsal and ventral
cochlear nuclei of the vestibulocochlear (VIII) nerve located in the upper part of
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the medulla oblongata. At this point, all the fibers synapse, and second-order
neurons pass mainly to the opposite side of the brain stem to terminate in the
superior olivary nucleus. A few second-order fibers also pass to the superior
olivary nucleus on the same side.
From the superior olivary nucleus, the auditory pathway passes upward through the
lateral lemniscus. Some of the fibers terminate in the nucleus of the lateral
lemniscus, but many bypass this nucleus and travel on to the inferior colliculus,
where all or almost all the auditory fibers synapse. From there, the pathway passes
to the medial geniculate nucleus in the thalamus, where all the fibers do synapse.
Finally, the pathway proceeds by way of the auditory radiation to the primary
auditory region of the cerebral cortex, positioned mainly in the superior gyrus
of the temporal lobe (Fig 5).
Because numerous auditory axons decussate in the medulla while others remain on
the same surface, both the right and left primary auditory regions collect nerve
impulses from both the right and left ears. The neurons responding to different
pitches are mapped along auditory cortex in a manner that corresponds to the region
along the basilar membrane.
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Fig. 5. Auditory Nerve Pathway
Source: http://mikeclaffey.com/psyc170/notes/notes-other-senses.html
Image Credit: This image is in the public domain because its copyright has expired.
This applies worldwide.
Sound Frequency Perception in the Primary Auditory Cortex
There are at least six tonotopic maps have been recognized in the primary auditory
cortex and auditory association areas. In each of these maps, high-frequency sounds
excite neurons at one end of the map, whereas low-frequency sounds excite neurons
at the opposite end. In most, the low-frequency sounds are located anteriorly, and
the high-frequency sounds are located posteriorly (Fig. 6).
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Fig 6. Perception regions of low to high Sound Frequencies in the Primary Auditory
Cortex
Source:
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localiza
tion/localization.html
Image Credit: This image is in the public domain because its copyright has expired.
This applies worldwide.
Sound localization (Fig 7)
There are two mechanisms through which can determine the direction from which
the sound is coming:
(1) Through the time interval between the entry of sound into one ear and into
the opposite ear. For example, a sound is louder in the right ear or reaches
sooner at the right ear than at left, you guess that the sound is coming from
the right direction. This mechanism works best at frequencies below 3000 Hz.
(2) Through the difference between intensities of the sounds in the two ears.
This intensity mechanism functions best at higher frequencies (above 3000
Hz) as the head operates as a sound barrier at higher frequencies.
Neural mechanisms: Cochlear nerves enter the brainstem and synapse with interneurons. Nerves from both right and left ears often converge on identical neuron.
Many of these inter-neurons are influenced by varied entrance times and intensities
of input from two ears. The possible neural mechanisms for detecting sound direction
are followed as:

Primarily, the superior olivary nucleus is partitioned into two segments: (i)
the medial superior olivary nucleus and (ii) the lateral superior olivary
nucleus.

The lateral nucleus is involved in detecting the direction from which the sound
is emanating through the difference in intensities of the sound arriving the
two ears.

The medial superior olivary nucleus has specific mechanism for detecting time
interval between sound signals reaching the two ears.
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Fig 7. Mechanisms of Sound Localization.
Source: http://cnx.org/contents/29cade27-ba23-4f4a-8cbd-128e72420f31@5 CC
CLINICAL ASPECT OF HEARING PHYSIOLOGY
A. DEAFNESS
It is an inability of an individual to hear either wholly or partly. Deafness is of two
main types: conductive and nerve deafness.
1. Conductive Deafness
It is due to sound transmission defect either in the external or middle ear. Therefore,
it is characterized by partial loss of hearing. The hearing loss is fairly uniform
throughout the frequency range but it is never complete. It is because the skull
bones themselves conduct sound to the cochlea (bone conduction) and the basilar
membrane can be set into vibration.
Causes of conductive deafness are as follows:
(i) Wax or foreign bodies in the external ear.
(iI) Thickening of the tympanic membrane due to repeated infections, therefore, its
vibrations decrease.
(iii) Otitis media i.e.middle ear inflammatory disorders which damage the tympanic
membrane and / or the ear ossicles.
(iv) Otosclerosis (ear ossicles sclerosis) i.e. pathological fixation of the foot plate
of stapes in the oval window.
(v) Blockage of the eustachian tube.
2. Nerve Deafness
It is due to either defects of the internal ear (hair cells) or damage of neural
pathways. Therefore, it is characterized by complete loss of hearing.
Causes of nerve deafness are as follows:
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(i) Aging: Hearing gradually decline with age, called Presbycusis.
It is probably
due to gradual cumulative loss of hair cells and cortical neurons.
(ii) Hereditory.
(iii) Injury to VIII nerve (acoustic trauma).
(iv) Hazards of industrial noises (prolonged exposure to noises damages the hair
cells, initially it manifests as loss of sensitivity of hearing in 300-3000 Hz range
resulting in the impairment of the subject’s understanding of conversation ).
(v) Toxic degeneration of VIII nerve such as due to streptomycin injection,
quinine, measles, meningitis, etc.
(vi) Tumors of VIII nerve (acoustic neuroma)
(vii) Vascular damage in medulla which leads to destruction of the auditory
pathways.
B. TINNITUS
It is a ringing sensation in the ears by irritative stimulation of eiyher the internal ear
or the auditory (VIII) nerve.
HEARING TESTS
It is done by either of following methods:
A. Use of the human voice: A conversational voice (60 dB) which should be
heard at 3.5 meters in each ear separately; if extends to 6 meters, it can be
presumed
that
the
subject
has
normal
hearing.
Lists
of
spondee
(phonetically balanced) words are used for this test, which should be repeated
using the whispered voice.
B. Turning fork tests: The most widely used for these tests to distinguish
between conductive and nerve deafness are forks vibrating at 256 or 512 Hz.
C. Audiometry: Auditory acuity (sharpness of hearing) can be measured with
the help of an audiometer. This device presents the subject with pure tones
of various frequencies through earphones. At each frequency, the threshold
intensity is determined and plotted on a graph as a percentage of normal
hearing. This provides an objective measurement of the degree of deafness
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and a picture of the sound range most affected (Fig. 8.). The device consists
of the following parts:
1.
Electronic oscillator: it can generate pure tones that range from low
to high frequencies.
2.
Intensity dial: it helps to adjust the threshold intensity of hearing for
each tone.
3.
A headphone.
Fig. 8. An audiogram is a form the audiologist uses to graph the results of a
hearing test.
The vertical lines represent the test frequencies, arranged from low pitched on the
left to high pitched on the right. The horizontal lines represent loudness, from very
soft at the top to very loud at the bottom.
Circles represent scores for the right ear, and Xs are used for the left ear. The
scores are plotted on the audiogram and compared to results obtained from persons
with normal hearing (the unshaded area). Speech sounds at an average speech level
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Value addition: Did you Know
Electronic devices
may also be plotted, to give some information about which sounds are audible to the
listener.
Source: http://www.hawkinshearing.com/how-to-read-an-audiogram.html
Image Credit: This image is in the public domain because its copyright has expired.
This applies worldwide.
After of audiogram, range of hearing loss lies in followings:





Normal = less than 25 db HL
Mild = 25-40 db HL
Moderate = 41-65 dB HL
Severe = 66-90 db HL
Profound = more than 90 db HL
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It can help compensate for damage to the intricate middle ear, cochlea, or neural
structures.
Hearing aids amplify incoming sounds which than pass via the ear canal to the
same cochlear mechanism used for normal sounds.
When substantial damage has occurred. However, hearing aids cannot correct the
deafness, the electronic devices known as cochlear implants may in some cases
partially restore the functional hearing.
A cochlear implant is a machine that decodes sounds into electric signals which
could be perceived by the brain. Such a device is helpful for people with deafness
which is arise from the injure to hair cells in the cochlea. The external parts of a
cochlea implant consist of (1) a microphone worn around the ear that picks up
sound waves, (2) a sound processor, which may be placed in a shirt pocket, that
converts sound waves into electrical signals, and (3) a transmittor, worn behind the
ear, which receives signals from the sound processor and passes them to an internal
receiver.
See the video how the cochlear implant works in following link
http://www.youtube.com/watch?v=u8LpjkfvaSU
Source: Vander’s Human Physiology and youtube
EQUILIBRIUM PHYSIOLOGY
Two types of equilibrium or balance are found i.e. static equilibrium and dynamic
equilibrium.
(i) Static equilibrium is meant to the balance of the position of body (primarily
the head) in relation to gravitational force. Movements of body which triggers
the receptors for static equilibrium contain tilting the head and speeding up or
down, such as when the body is being moved in lift or in a car that is
speeding up or slowing down.
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(ii) Dynamic equilibrium is meant to maintain the position of body (primarily
the head) in relation to sudden movements for example rotational clockwise
or anticlockwise movements.
(1)
Role of Utricle and Saccule:
The utricle and saccule convey sensory massage related to speed of positioning up
and down, back and forth and alterations in head position in response to the forces
of gravity. This is important to maintain the static equilibrium.
Responses of the Hair Cells to Directional changes
Each hair cell composed of 50-70 small cilia, the stereocilia and one large and true
cilium, the kinocilium. The kinocilium is always placed to one side, and the
stereocilia become gradually shorter toward the other side of the cell. The tip of each
stereocilium to the subsequent longer stereocilium and lastly, to the kinocilium are
attached through thin filaments.
Due to these attachments, when the stereocilia and kinocilium bend in the direction
of the kinocilium, the filamentous attachments pull in series on the stereocilia,
draging them outside from the hair cell. Eventaully, this opens many hundred fluid
channels in the cell membrane of neurone around the bases of the stereocilia. These
channels are then conducts huge numbers of positive ions. Therefore, positive ions
discharge into the cell from the nearby endolymphatic fluid, leads to depolarization
of receptor membrane. On the other hand, bending the pile of stereocilia in the
opposite direction (back to the kinocilium) decreases the tension on the attachments
and eventually this closes the ion channels, therefore leads to hyperpolarization of
receptor.
At normal and resting positions, the neurones leading from the hair cells transmit
continuous nerve impulses at the rate of 100 impulses/second. If the stereocilia are
bent in the direction of the kinocilium and the nerve impulses increases (i.e. so many
hundred nerve impulses per second). On the other hand, bending the stereocilia
away from the kinocilium decreases the nerve impulse, subsequently it gets turned
off completely. As the orientation of the head in space changes and the weight of the
statoconia bends the cilia, relevent signals are transmitted to the brain to control
equilibrium (Fig 9).
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Fig 9. Stimulation of hair cells occurs through depolarization and Inhibition of hair
cells occurs through hyperpolarization.
Source:https://humanphysiology2011.wikispaces.com/10.+Sense+Organs
cc
Image Credit: CC BY-SA 3.0
Maintenance of Static Equilibrium
It is particularly vital that the different hair cells are positioned in different directions
in the maculae of the utricles and saccules, with the purpose that at different
positions of the head, different hair cells become excited. The patterns of stimulation
of the different hair cells conveyed to the the neural pathway specialized for the head
position corresponding to the gravitational pull. Eventually, the vestibular, cerebellar
and reticular motor systems stimulate the proper postural muscles to maintain
apposite equilibrium.
Detection of Linear Acceleration
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In response to alteration in position of head, the gelatinous otoliths (statoconia)
move corresponding to the gravitational and drag against hair cells. Consequently,
the stereocilia bend along with the otoliths and nerve cells are stimulated (Fig. 10).
When our body is suddenly push forward which means the body accelerates, otoliths
(statoconia) which have greater inertia than the surrounding fluid, fall backward on
stereocilia and the information of disequilibrium conveyed through the neural
pathway. Subsequently, it leads an individual to bend forward so that the resulting
forward movement of the statoconia becomes equivalents to the affinity for the
statoconia to fall backward due to the acceleration. At this stage, the nervous system
senses a state of appropriate equilibrium beyond which the body does not need to
bend forward. In this way, the maculae function to maintain equilibrium during linear
acceleration.
Fig 9. Static equilibrium: Gravity bends the hair cells when the head tilts forward.
Source: http://cnx.org/contents/b375ea7d-22d5-4f47-b10a-41dd93637896@4 CC
(2) Semicircular Canals
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Hearing Physiology
The three semicircular canals function in dynamic equilibrium. The semicircular
canals perceive information about angular accelerations in rotational movements and
position of head in three perpendicular axes.
Detection of Head Rotation (Angular Acceleration)
If an individual start to rotate his/her head in any direction, the inertia of the fluid in
one or more of the semicircular canals leads the fluid to remain stationary whereas
the semicircular canal rotates with the head. This leads fluid to move from the canal
and through the ampulla so that the cupula bend to one side. Rotation of the head in
the opposite direction leads the cupula to bend to the opposite side.
Hundreds of stereocilia plus kinocilium from hair cells placed on the top ampullary
protrude into the cupula. The kinocilia of these hair cells are all oriented in the same
direction in the cupula, and bending the cupula in that direction results in
depolarization of the hair cells, whereas bending it in the opposite direction
hyperpolarizes the cells. After that, relevent electrical signals from the hair cells
are conveyed through route of the vestibular nerve to the neural pathway
specialized for sensing and proceesing the information of rotational change of the
head and the rate of rotational change in each of the three planes of space (Fig.
11).
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Hearing Physiology
Fig 11. In dynamic equilibrium, hair cells in semi-circular canals move with the
motionless endolymph, in turn hair bundles bend when head rotates.
Source: http://cnx.org/contents/b375ea7d-22d5-4f47-b10a-41dd93637896@4 CC
Value addition: Did you Know
Video on vesibular system
See in the following hyperlink to know about the vesibular system. This involves
Semicircular Canals, Utricle and Saccule.
http://www.youtube.com/watch?v=UKaBZprL3t4
Source: You tube
NEURAL PATHWAYS OF EQUILIBRIUM
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When hair bundles of the hair cells bend in the semicircular ducts, utricle, or
saccule results in the release of a neurotransmitter (probably glutamate), which
generates nerve impulses in the sensory neurons that are supplied in the hair cells.
The soma or cell bodies of sensory neurons are situated in the vestibular ganglia.
Nerve impulses move along the axons of these neurons, which constitute the
vestibular branch of the vestibulocochlear (VIII) nerve. The majority of these
axons synapse with sensory neurons in vestibular nuclei, the main integrating
centers responsible for maintaining equilibrium, in the medulla oblongata and spinal
cord. The vestibular nuclei also receives input from the eyes and proprioceptors in
the neck and limb muscles that indicate the position of the head and limbs. The rest
of the axons go into the cerebellum through the inferior cerebellar peduncles.
Bidirectional pathways connect the cerebellum and the vestibular nuclei (Fig 12.).
Next, signals are sent into the reticular nuclei of the brain stem, as well as down the
spinal cord by way of the vestibulospinal and reticulospinal tracts. The signals to the
cord control the interplay between facilitation and inhibition of the many antigravity
muscles, thus automatically controlling equilibrium.
The flocculonodular lobes of the cerebellum are especially concerned with dynamic
equilibrium signals from the semicircular ducts. In fact, destruction of these lobes
results in almost exactly the same clinical symptoms as destruction of the
semicircular ducts themselves. It is believed that the uvula of the cerebellum plays a
similar important role in static equilibrium.
Signals transmitted upward in the brain stem from both the vestibular nuclei and
the cerebellum by way of the medial longitudinal fasciculus cause corrective
movements of the eyes every time the head rotates, so the eyes remain fixed on a
specific visual object. Signals also pass upward (either through this same tract or
through reticular tracts) to the cerebral cortex, terminating in a primary cortical
center for equilibrium located in the parietal lobe deep in the sylvian fissure on the
opposite side of the fissure from the auditory area of the superior temporal gyrus.
These signals apprise the psyche of the equilibrium status of the body.
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Hearing Physiology
Fig 12. The neural pathways of equilibrium.
Source:
http://classconnection.s3.amazonaws.com/704/flashcards/586704/png/equilibriu
m_pathway1310130521369.png
USE OF VESTIBULAR INFORMATION
Vestibular information (Fig. 13.) is used in three ways:
1. First use is to control the movement of eye muscles so that, in spite of
changes in head position, the eyes can remain fixed on the same point.
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2. The second use of vestibular information is in reflex systems (that is
vestibular postural reflexes) to maintain upright posture and balance.
3. The third use of vestibular information is to provide consciousness of the
body position and speed of the movement, sensitivity of the area neighboring
the body, and memory of spatial information.
Fig. 13.Vestibular information and pathways.
Source: https://humanphysiology2011.wikispaces.com/10.+Sense+Organs
Image Credit: CC BY-SA 3.0
Value addition: Did you Know
Disorders related to the Vestibular System
1. Nystagmus
Nystagmus is a large, jerky, back-and-forth movement of the eyes that can occur in
response to unusual vestibular input in healthy people; it can also be a pathological
sign.
2. Vertigo
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Unexpected inputs from the vestibular system and other sensory systems can induce
these sicknesses. Vertigo, defined as an illusion of movement generally rotating
followed with conditions of nausea and dizziness. Such disruptions can result from
strokes, irritation caused in the labyrinths by infection, movable particles of calcium
carbonate in the semicircular canals, or excessive consumption of alcohol.
3. Motion sickness
Motion sickness is a condition that occurs when you experience unfamiliar patterns
of linear and rotational acceleration. Symptoms of motion sickness include palenes,
restlessness, excess salivation, nausea, dizziness, cold sweats, headache, and
malaise that may progress to vomiting. Medications for motion sickness are usually
taken in advance of travel and include scopolamine in time- release patches or
tablets , dimenhydrinate and meclizine.
Source: Vander’s Human Physiology
Summary

Sound waves are vibrating between high and low pressure regions moving in
the same direction through some medium e.g. air. The sounds perceived most
intensely by the human ear at frequencies ranging from 200 to 8000 hertz.

When sound waves, coming through outer ear, hit the tympanic membrane.
Eventually, the tympanic membrane vibrates gradually in response to low
frequency sounds and quickly in reaction to high frequency sounds. It acts as a
pressure receiver and resonator.
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Hearing Physiology

The ear ossicles (malleus, incus, stapes) performs as a lever system that
changes the resonant vibrations of the tympanic membrane into movement of
the stapes.

Because fluid in the inner ear is more complicated to shift than air, the sound
pressure pass on to the internal ear have to be amplified. This is accomplished
by a movable chain of three small bones, the malleus, incus, stapes along
with tympanic membrane.

The sound waves are changed greatly by the tympanic membrane and ear
ossicles into vibrations of the foot plate of the stapes. These vibrations set up
sound waves in fluid present in cochlea. This further cause the vibration of
basilar membrane.

The vibration of basilar membrane cause movements in the basilar fiber, rods
of Corti, and reticular lamina eventually leads to the excitation of hair cells.

Movement of the hair cells lead to bending of the stereocilia and that opens
transduction channels i.e. positively charged potassium ions move from the
surrounding endolymph in scala media into the stereocilia, that leads
depolarization of the hair cell membrane. Ultimately it results in the generation
of nerve impulses in first-order neurons in cochlear nerve fibers.

Movement of the stereocilia in the opposite direction shuts the transduction
channels, allow hyperpolarisation to occur, and decreases neurotransmitter
release from the hair cells.

Nerve fibers from the spiral ganglion of Corti enter the dorsal and ventral
cochlear nuclei of the vestibulocochlear (VIII) nerve. The hearing pathway
further proceeds in the primary auditory region of the cerebral cortex.

There are two mechanisms through which can determine the direction from
which the sound is coming: One through the time interval between the entry of
sound into one ear and into the opposite ear; through the difference between
intensities of the sounds in the two ears.

In response to alteration in position of head, the otoliths in macula of utricle
and saccule move corresponding to the gravitational and drag against hair cells.
Consequently, the stereocilia bend along with the otoliths and nerve cells are
stimulated to maintain the static equilibrium and detect the linear acceleration.
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Hearing Physiology

In response to rotation of the head, fluid to move from the semicircular canals
and through the ampulla so that the cupula bend to one side. Rotation of the
head in the opposite direction leads the cupula to bend to the opposite side. In
this way, it maintain the dynamic equilibrium and detect the angular
acceleration.

The flocculonodular lobes of the cerebellum are especially concerned with
dynamic equilibrium signals from the semicircular ducts. the uvula of the
cerebellum plays a similar important role in static equilibrium.
Exercises
Multiple choice questions
1. Under resting conditions the cilic of the inner ear are connected by what
structures?
A. Ossicles
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B. Tip links
C. Bipolar cells
D. Stapes
Answer: B
2. What is the first structure in the brainstem which receives auditory input from the
ear?
A.
Inferior colliculi
B.
Nucleus raphe magnus
C.
Solitory nucleus
D.
Cochlea nucleus
Answer: D
Which structure vibrates when struck by sound waves tunnelling down from external
ear canal?
A. Cochlea
B. Eustachean tube
C. Pinna
D. Tympanic membrane
Answer: D
Which structures are responsible for the transfer of vibrations from the external ear
to the inner ear?
A. Auditory ossicles
B. Cochlea
C. Eustachean tube
D. Round window
Answer: A
Malleus, incus and stapes together are known as
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Hearing Physiology
A. Cochlea
B. Auditory ossicles
C. Eustachean tube
D. Round window
Answer: B
Which area is important in equilibrium?
A. Vestibular apparatus
B. Cochlea
C. Auditory ossicles
D. Eustachean tube
Answer: A
Which cranial nerve carries impulses from semi-circular canals for the sense of
hearing?
A. Tenth
B. Fourth
C. Sixth
D. Eighth
Answer: D
In sound, pitch is measured in _____, and intensity is measured in _____.
A.
B.
C.
D.
nanometers (nm); decibels (dB)
hertz (Hz); decibels (dB)
decibels (dB); nanometers (nm)
decibels (dB); hertz (Hz)
Answer: B
Which of the following are found both in the auditory system and the vestibular
system?
A. basilar membrane
B. semicircular canals
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C. ossicles
D. hair cells
Answer: D
Define following:

Sound waves

Tympanic reflex
Write short notes on:

Audiometry

Role of tympanic membrane and three ossicles in hearing

Organ of the corti
Long answer type questions:
2. How are sound waves transmitted from the auricle to the spiral organ of corti.
3. How do hair cells in the cochlea and vestibular apparatus transduce
mechanically vibrations into electrical signals?
4. What are the auditory pathways to detect hearing signals? Explain with the
diagram.
5. Explain the difference between static equillibrium and dynamic equilibrium.
6. What is the use of vestibular information?
7. Describe the equilibrium pathways.
8. Explain the role of middle ear in the hearing.
9. Discuss various types of hearing losses.
Glossary
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Hearing Physiology
Sound: It is a vibration which transmitted as a characteristically perceptible
mechanical wave of pressure and deflections occurs through a medium like air or
water which is registered as hearing phenomena.
Sound Frequency: It is the number of cycles per unit of the time. Frequency is
measured in cycles per second (cps) or Hertz (Hz).
Resonance: It is ability of a system to oscillate with greater amplitude at some
frequencies. Frequencies at which the response amplitude is a relative maximum are
known as the system's resonant frequencies, or resonance frequencies.
Transduction: This process is to convert the sound waves into hearing signals. It
invoves the depolarization of stereocillia to increase nerve impulses in order to carry
hearing signals and hyperpolarization of stereocillia to decrease nerve impulses.
Tympanic reflex: : The loud sounds start a reflex contraction of these muscles i.e.
tensor tympani and stapedius is known as the tympanic reflex or attenuation reflex.
It protects cochlea and prevents strong sound waves from causing excessive
stimulation of the auditory receptors.
Hearing aids: It amplify incoming sounds which than pass via the ear canal to the
same cochlear mechanism used for normal sounds.
Static equilibrium: It is meant to maintain the position of body (primarily the head)
in response to the gravitational force through the action of utricle and saccule.
Dynamic equilibrium: It is meant to the maintain body position (primarily the
head) in relation to rotational movement and speed of the movement through the
action of semicircular canals.
References
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Hearing Physiology
1. Tortora, G.J. & Grabowski, S. Principles of Anatomy & Physiology. 13th
Edition, p.642.
2. Moyes, C. D. and Schulte, P. M. (2006). Principles of Animal Physiology, p.
248.
3. Hill, R. W., Wyse, G. A. and Anderson, M. (2006). Animal Physiology. p.355.
4. Randall, D., Burggren W. and French, Kathleen (2001). Eckert Animal
Physiology.
5. Widmaier, E.P., Raff, H. and Strang, K.T. (2008). Vander’s Human Physiology,
XI Edition, McGraw Hill.
6. Guyton, A.C. and Hall, J.E. (2011). Textbook of Medical Physiology, XII
Edition,
Harcourt Asia Pvt. Ltd./W.B. Saunders Company.
7. Ganong, William F. Review of Medical Physiology. XXI Edition. Mc Graw Hill
8. Textbook of Physiology by Prof. A.K. Jain.
9. Anatomy And Physiology: In health and illness. Ross and Wilson (Tenth
Edition)
Web links
1. http://jdy-ramble-on.blogspot.in/2014/04/what-does-sound-waveslook-like.html
2. http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/l
ocalization/localization.html
3. http://www.hawkinshearing.com/how-to-read-an-audiogram.html
4. http://michaeldmann.net/mann8.html
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