Physiology of hearing

DR NILESH KATE
MBBS,MD
ASSOCIATE PROF
DEPT. OF PHYSIOLOGY
PHYSIOLOGY
OF
HEARING

OBJECTIVES
 Stimuli or sound waves.
 Conduction of sound
waves.
 Transduction of sound
waves.
 Neural transmission of
signals.
 Encoding of signals.
 Applied.
Monday, March 28, 2016

AUDITION
 For social communication.
 Melophobia – fear of music.
 The scientific study of sound waves is known
as Acoustics.

SOUND
 perception of pressure waves
generated by vibrating air molecules

SOUND WAVES:
 Sound travels in
Waves through media
 Alternating Compression
(dense molecules) &
Rarefaction (loose
molecules) waves
 The simple sound is
the sinusoidal wave or
Pure Tone

PROPERTIES OF SOUND THE EAR
DETECTS
1- Pressure level (dB)
2- Frequency (cycle/sec = Hz)
3- Complexity

WAVE PATTERNS
 A is the record of a pure tone
 B has a greater amplitude
and is louder than A.
 C has the same
amplitude as A but a greater
frequency, and its pitch is
higher.
 D is a complex wave form
that is regularly repeated.
 E, which have no regular
pattern, are perceived as
NOISE

 The Human ear is sensitive to sounds over
a wide range of:
- Frequencies: 20 – 20,000 Hz
- Amplitudes: 0.0002 – 200 dyne/cm2
 The human ear can detect the difference
between two sounds occurring 10 μsec
apart in time
HOW SENSITIVE THE EAR TO
SOUNDS?

SOUND • Amplitude determines the loudness of sound
• Amplitude is measured in decibels

 Pitch discrimination is best in the 1000- to
3000-Hz range
 Poor at high and low pitches.
 Average individuals distinguish 2000 pitchs
 Musicians- Cortical Plasticity
 The pitch of the average male voice in
conversation is about 120 Hz and
 Average female voice about 250 Hz.
PITCH OF SOUND

CONDUCTION OF SOUND WAVES.
 Role of External Ear
Sound waves are collected by the Pinna and focused into the
External Auditory Canal
The vibration pass down the EAC and strike the TM

EXTERNAL EAR FUNCTIONS:
 Collecting sound waves
 Amplification of frequencies
2000
- 4000 Hz (Resonant
Frequency of EAC)
 Providing cues about the
vertical localization of a
sound source (by the Degree of
sound waves reflection over
the Pinna)

CONDUCTION OF SOUND
WAVES.
 Conduction from
tympanic membrane to
ear ossicles.
 Tympanic membrane
 Pressure Receiver –
Sensitive to Pressure
Change
 Resonator – Vibrate
with Pressure Change.
 Critically Dampens as
sound ends.

CONDUCTION OF SOUND WAVES
MECHANICALLY FROM MIDDLE EAR
TO INNER EAR
 Impedance matching.
 Phase differential
between oval and round
window.
 Natural resonance of
External Ear and Middle
Ear.
 Attenuation Reflex.

IMPEDANCE MATCHING.
 A person in water can not hear sound
produced out of it.
 As 99.9% sound get reflected from surface
of water due to Impedance.
 So as Air filled Middle ear conduct sound
to fluid filled Inner ear most of sound get
Reflected – Impedance Mismatching.
 Compensated by Inner Ear by
IMPEDANCE MATCHING.

IMPEDANCE MATCHING BY EAR
OSSICLES BY 3 MECHANISMS
 HYDROLIC ACTION OF
TYMPANIC MEMBRANE –
 Effective vibratory area of
tympanic membrane (55mm2)
is more than stapes oval
window surface area(3.2mm2)
 So force produced by sound
concentrated over small area
 Amplifying Pressure on Oval
Window

 LEVER ACTION OF
VESICLES.
 Handle of Malleus 1.3
times longer than Long
process of Incus,
providing Mechanical
Leverage Advantage.
 So Ossicles increases
force of movement by
1.3 times.

 CURVED MEMBRANE
EFFECT.
 Movement of
Tympanic membrane
more at Periphery
than at Center where
Malleus is attached.
 So provide some
leverage.

 So all these together
Increase sound
pressure 22 folds
 Impedance
Mismatching is mostly
compensated.
 If remove ossicles loud
sound hear as whisper.

MINIMUM AUDIBILITY CURVE
 Amplification of sound –
greatest between 1000-
3000 Hz.
 below 16 & above 20000 Hz
not amplified.
 Human ear can perceive
pitch between 16-20000 Hz
 Maximum sensitivity 1000-
3000 Hz

PHASE DIFFERENTIAL BETWEEN
OVAL AND ROUND WINDOW.
 Sound don’t reach both
windows simultaneously.
 When oval window receive
compression, round
window receive rarefaction.
 If sound reaches
simultaneously no
movement of Perilymph
& no hearing.

NATURAL RESONANCE OF
EXTERNAL EAR AND MIDDLE EAR.
 Natural Resonance –
allow some frequency to
pass more easily to inner
ear.
 External auditory canal –
3000 Hz
 Tympanic membrane –
800-1600 Hz.
 Middle ear – 800 Hz.
 Ossicular chain – 500-2000
Hz.

ATTENUATION REFLEX.
 Tympanic
reflex/Acoustic reflex
 Protective reflex.
 Reduces sound pressure
amplitude by Changing
mobility &
Transmission
properties of Ear
ossicles.

ATTENUATION REFLEX.
 Stimulus - loud sound.
 Latent period – 40-
80ms.(sudden loud
sound; bomb explosion –
Deafness)
 Reflex Activity –
contraction of 2 muscles
 Tensor tympani
 Stapedius.

ATTENUATION REFLEX.
 Tensor Tympani – pull
Malleus inwards
 Stapedius – pulls
stapes outwards.
 Both makes Ossicular
system rigid & no
vibrations.
 Sound intensity
Decreased by 30-40 db.

ADVANTAGES OF
ATTENUATION REFLEX.
 Prevents Damage to cochlea
from loud sound.
 Attenuates & Mask all low
frequency environmental
sounds & allow person to
concentrate on sounds above
1000 Hz.
 Reduces sound produced
during vocalization &
chewing.

TRANSDUCTION OF SOUND
WAVES
 Transduction of sound from
Mechanical to Electrical
occur in ORGAN OF CORTI
in inner ear.
 Vibration of Basilar
membrane.
 Stimulation of hair cells
 Membrane potential change in
hair cells
 Neural transmission of signals.

VIBRATION OF BASILAR
MEMBRANE.
 Sound waves from middle
ear pass to inner ear
through Oval window by
in & out movement of
stapes.
 Wave spread along Scala
Vestibuli to Scala tympani
as a travelling wave.
 As it passes it Vibrate
basilar membrane.

STIMULATION OF HAIR CELLS
 Movement of basilar
membrane causes organ of
corti to move up & down.
 Hair of the outer hair cells are
embedded in Tectorial
Membrane.
 As both Tectorial membrane
& basilar membrane moves,
they slide each other with
movement.

STIMULATION OF HAIR CELLS
 As organ of Corti Moves
up, tectorial membrane
slide foreward moving
stereocilia Away from
limbus.
 As organ of Corti Moves
Down, tectorial
membrane slide backward
moving stereocilia
towards limbus.

STIMULATION OF
HAIR CELLS
 Bending of stereocilia
stimulate hair cells
 Depolarization – as
stereocilia bend away
from limbus.
 Hyperpolarization –
as stereocilia bends
towards limbus.

MEMBRANE POTENTIAL
CHANGE IN HAIR CELLS
 Change in membrane
potential is directly
proportional to degree
of displaement.
 Describe under 2
conditions
 At rest
 During stimulation.

RESTING MEMBRANE
POTENTIAL FROM HAIR CELLS
 At rest 2 potentials are
recorded
 Endocochlear potential.
 Resting potential of hair
cells.

ENDOCOCHLEAR POTENTIAL.
 Endolymph in scala media secreted
by stria vascularis has
 High conc of Na-K-ATPase & unique
electrogenic K pump
 So it has high K conc & electrically
positive to perilymph.
 So potential developed between
Endolymph & Perilymph is
Endolymphatic potential or
Endocochlear potential
 + 80 mv.

RESTING POTENTIAL OF HAIR
CELLS.
 Each hair cell has negative
RMP with -70 mv.
 At the upper end of hair cell
potential difference
between ICF & endolymph
is -150 mv
 Large negative potential &
lack of K conc difference
make hair cells highly
sensitive.

ACTION POTENTIAL IN HAIR
CELLS
 Cochlear Microphonic
potential
 Gating of K channels is
controlled by movement of
stereocilia.
 As stereocilia bend away from
Limbus – K channels open –
Depolarization.
 As stereocilia bend towards
Limbus – K channels close–
Hyperpolarization.

COCHLEAR MICROPHONIC
POTENTIAL
 Similar to generator potential as
 No latency or refractory period.
 Do not obey all or none law.
 Resistance to ischemia & anesthesia.
 Base of cochlea respond to all frequency, apex
respond to low frequency of sound.
 When organ of corti damaged due to prolonged
exposure to loud sound, potential produced by this
band of sound is abolished.

ACTION POTENTIAL IN
AUDITORY NERVE.
 As hair cells Depolarize –
Ca channels open – Ca
enters – release synaptic
transmitter – activates
receptor sites on afferent
neurons – Action
Potential.
 Loudness of sound
determine Frequency of
Action Potential.

NEURAL TRANSMISSION OF
SIGNALS.
 Spiral ganglion
 Cochlear nuclei
 Superior olivary nucleus
complex, trapezoid Nucleus
& N of lateral lemniscus.
 Inferior colliculus
 Medial geniculate body.
 Auditory cortex (41,42,
22,21,20)

SALIENT FEATURES OF
AUDITORY PATHWAY.
 Bilateral representation.
 Descending pathway.
 Role in brain stem & spiral
acoustic reflex.
 Role in general arousal.
 Spatial organization.
 Features of auditory cortex.
 Features of other cortical
areas with audition.

SALIENT FEATURES OF
AUDITORY PATHWAY.
 Bilateral
Representation – form
Medulla onwards ear is
Bilaterally represented
in Auditory pathway.
 Descending Pathway –
there is significant
Descending pathway
forming feed-forward &
feedback loop.

SALIENT FEATURES OF
AUDITORY PATHWAY.
 Role in Brain Stem & Spiral
Acoustic Reflex – Integration
of Visual & Auditory
information occurs due to
interconnection between
Superior & Inferior Colliculi.
 Role in General Arousal –
Due to Auditory Pathway
collateral to Reticular
Formation & Cerebellum

SALIENT FEATURES OF
AUDITORY PATHWAY.
 Spatial Organization – different parts of organ of
corti respond to different frequency.
 There is Tonotopic organization in cochlear nuclei
maintained in superior olivary nucleus, inferior colliculus,
MGB & auditory cortex.
 Same as Retinotopic organization & Somatotopic
organization.

FEATURES OF AUDITORY
CORTEX.
 Tonotopic organization.
 Column Organization –
 Isofrequency Columns – Neurons
have same characteristic frequency.
 Summation Columns – Neurons
responsive to binaural than
Monaural inputs.
 Suppression Columns – Neurons
less responsive to Binaural than
Monaural stimulation.

FEATURES OF OTHER CORTICAL
AREAS WITH AUDITION.
 Hemispheric Specialization.
 During language learning area
22 concerned with processing
of auditory signals related to
speech is more active on left
than right.
 Area 22 on right side is more
concerned with melody, pitch &
sound intensity.

FEATURES OF OTHER CORTICAL
AREAS WITH AUDITION.
 Plasticity of auditory
pathways.
 If person becomes deaf
before language skills
developed, Viewing sign
language activates auditory
association area.
 Musicians have larger
auditory area & larger
cerebellum than non-
musicians.

NEURAL PROCESSING OF
AUDITORY INFORMATION
 Encoding of
Frequency.
 Encoding of Intensity
(loudness)
 Feature Detection.
 Localization of Sound
in space

ENCODING OF FREQUENCY.
 Human ear can
discriminate sound
between 60-20,000 Hz
range.
 Encoding occurs in
cochlear nerve.
 Explained by Theories
of Hearing.

PLACE THEORY OR BEKESY
TRAVELLING WAVE THEORY
 Discriminate sound between 2000-
20000 Hz.
 Basilar Membrane is Mechanical
Analyzer of sound frequency.
 Pattern of movement of basilar
membrane is that of Travelling Wave.
 Basilar membrane Near oval window
vibrate in response to sound of Higher
frequency & Near Apex respond to
Lower Frequency.

PLACE THEORY OR BEKESY
TRAVELLING WAVE THEORY
 This different response to different frequency is due
to systematic variations in Mechanical Properties
in basilar membrane.
 So Higher frequencies are represented in Basal turn
& Lower Near Apex.
 So same response by Hair cells & Auditory Nerve
fibres.
 Thus there is Spatial Organization of Auditory
pathways from hair cells to Auditory Cortex.

FREQUENCY THEORY
 Discriminate low frequency
sound below 2000 Hz.
 For very low frequency
sound there is
synchronization between
frequency of sound & rate of
discharge through cochlear
nerve.
 So Frequency of Action
potential in auditory nerve
determine Loudness than
pitch.

PITCH OF SOUND
 PITCH – subjective
sensation of frequency
of sound.
 Higher frequency
greater is pitch.
 Discrimination of pitch
also depend on
 Loudness
 Duration.

ENCODING OF INTENSITY (LOUDNESS)
Occurs at level of cochlear nerve.

FEATURE DETECTION.
 Higher auditory
centers respond to
particular feature.
 CORTICAL Neurons
respond to shift of
Note from high to low
frequency.

LOCALIZATION OF SOUND IN
SPACE
 Can separate location by 1
degree.
 Center – Brain Stem relay
nuclei superior olivary
nucleus.
 Clues –
 Time lag between entry of
sound in 2 ears
 Difference in intensity that
reaches 2 ears.

Physiology of hearing

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