Download 017-018 Special Senses lecture 3-4 Physiology of Hearing

Nervous System
Physiology
Special Senses
By
Dr. SHAHAB SHAIKH
PhD MD MBBS
Lecture: Physiology of Hearing
••••••••••••••••••••••••••••••••••
Faculty of Medicine
Al Maarefa Colleges of Science & Technology
Introduction
• Hearing, Auditory Perception, or Audition is the
ability to perceive sound by detecting vibrations or
changes in the pressure of the surrounding medium
through the ear.
• Hearing is the neural perception of sound energy.
• Hearing mainly involves two aspects:
– the identification of the sounds (what) and
– their localization (where).
• Hearing is performed primarily by the auditory
system: mechanical waves, known as vibrations are
detected by the ear and transduced into nerve
impulses that are perceived by the brain.
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Sound
• Sounds are produced when vibrating objects, such
as the plucked tuning fork or guitar strings, produce
pressure pulses of vibrating air molecules, better
known as sound waves.
• Sound waves are often simplified as plane waves,
which are characterized by these generic properties:
– Frequency,
– Amplitude
– Direction
• Sound that is perceptible by humans has
frequencies from about 20 Hz to 20,000 Hz.
• Humans can detect the difference between two
sounds occurring 10micro seconds apart in time 3
Audible Sound Range
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Sound
• Generally loudness of sound is correlated with the amplitude
of sound wave (subjective interpretation of the intensity of a
sound)
• Decibel:
–
–
–
–
logarithmic scale to measure the intensity of sound waves
human amplitude range - 0 dB - 120 dB
avg sound intensity of human speech is 65 dB
Greater than 100 dB can cause damage to auditory apparatus
• The pitch is correlated with the frequency ( no of waves or
cycles per second / Hz)
– The sound frequencies audible in humans - 20 Hz to 20000 Hz ,with
greatest sensitivity from 1000 to 4000 Hz
– Normal pitch:
Male – 120 Hz
Female - 250 HZ
• Pure tone sound is sound wave with single
frequency
• Most of the sound are mixture of pure tones
Sound Terminologies
Amplitude/loudness
• Strength of the sound
• Loudness denotes the appreciation of sound intensity
• Expressed in decibel (dB)
Frequency/Pitch/Tone
• Number of cycles per second
• Pitch /Tone denotes the appreciation of frequency
• Expressed in Hertz(Hz)
Impedence
• Resistance offered by a medium to sound waves
Resonance
• Resonance is a phenomenon that occurs when a vibrating
system or external force drives another system to oscillate with
greater amplitude at a specific preferential frequency.
Attenuation
• Attenuation is a general term that refers to any reduction in the
strength of a signal.
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Functional Anatomy
External (Outer) ear:
•
•
•
•
Auricle
External acoustic meatus
Tympanic membrane
Ceruminous glands
Middle ear.:
• Tympanic cavity
• Osicles
– Malleus (hammer)
– Incus (anvil)
– Staples (stirrup).
• Eustachian tube.
Inner ear
• Bony and membranous
labyrinth
– Cochlea: hearing
– Vestible: equilibrium
– 3 perpendicular semicircular
canals: equilibrium
Sound Transmission
•
In the human ear, a sound energy is transmitted through four separate mediums
along the auditory system before a sound is perceived: in the outer ear—air, in the
middle ear— mechanical, in the inner ear liquid and to the brain—Electrical.
SOUNDENERGY
MECHANICAL
ENERGY
ELECTRICAL
ENERGY
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Sound Transmission
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Sound Transmission
External Ear:
• Air transmitted sound waves
are directed toward the
Tympanic Membrane
• Functions of External Air:
– Sound collection
– Increasing pressure on the
tympanic membrane in a
frequency sensitive way.
– Sound localization
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Sound Transmission
Middle Ear:
• When sound wave strikes the
tympanic membrane (also called EAR
DRUM),
the tympanic membrane
moves.
• Motion of the eardrum sets the
ossicular chain i.e. Malleus, Incus &
Stapes into motion.
• Functions:
– Maintains resting air pressure on both sides of TM equal
– Transfers movements of the tympanic membrane to the
inner ear
– Impedance matching
– Attenuation
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Impedance Matching
• The Impedance (resistance) for the
sound waves to travel in air and fluid is
different.
• The middle ear plays role in matching this
impedance as follows:
– The amplitude of movement of the stapes footplate
with sound vibration is only three fourth as much as
amplitude of the handle of the malleus
– This increases the force of movement by 1.3 times
– The surface area of tympanic membrane is about 55
square mm.& that of stapes is 3.2 sq mm
– This 17 fold difference times the 1.3 fold ratio of the
lever system causes about 22 times as much total
force exerted on the fluid of the cochlea
Attenuation Reflex
• When loud sounds are transmitted through the ossicular
system to the central nervous system, a reflex occurs to
cause contraction of the stapedius muscle and the tensor
tympani muscle.
• This cause the entire ossicular system to develop increased
rigidity, thus greatly reducing the sound conduction
• This reflex occurs after a latent period of about 40 to 80 ms.
• Function:
– To protect the cochlea from
damaging vibrations caused by
excessive sound
– To mask background noise in
loud environments.
Sound Transmission
Inner Ear:
• The ossicular chain transfers energy
from air of external ear to the fluid
medium of the inner ear via the stapes
attached to the oval window.
• Movement of the oval window creates
motion in the cochlear fluid and along
the Basilar membrane. Motion along
the
basilar
membrane
excites
frequency specific areas of the Organ
of Corti, which in turn stimulates a
series of nerve endings.
• Nerve impulses are relayed through the
VIII C.N., through various nuclei along
the auditory pathway to areas to the
brain. It is the brain that interprets the
neural impulses.
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Inner Ear
Organ of Corti
Organ of Corti
Organ of Corti
• Organ of corti has specialized type of nerve cells
called hair cells
• Single row of inner hair cells numbering about
3500 and
• 3 to 4 rows of outer hair cells numbering about
12,000
• 90-95% of cochlear nerve ending synapse on inner
hair cells
Sound Transmission
• Basilar membrane motion causes depolarization of the hair
cells.
• While the hair cells do not produce action potentials
themselves, they release neurotransmitter at synapses with
the fibers of the auditory nerve, which does produce action
potentials.
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Sensory Transduction
• Kinocilia - Stereocilia are
Linked
• Displacement Opens K+
Channels
• Depolarization → release of
glutamate
• K+ flows through cell
• Glutamate → increase
spike rate in auditory nerve
Inner Ear Function
• Thus the inner ear has 2 main functions:
1. Sensory transduction:
• pressure waves are transformed into neural impulse.
2. Frequency analysis – Place Principle
Resonance of the basilar membrane
Stapes
Scala
Cochlear
vestibuli
nerve
Perilymph
Oval
window
Round
window
Scala
tympani
Basilar
membrane
Cochlear
duct
(a)
Base
Relative
lengths
of basilar
fibers
within
different
regions
of basilar
membrane
Hz
20,000
(b)
(High notes)
Apex
Basilar
membrane
500 Hz
4000 Hz
Hz
1500
Hz
500
Hz
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(Low notes) (c)
24,000 Hz
Place Principle
The method used by nervous system to detect different sound
frequencies is to determine the position along the basilar membrane
that are most stimulated. This is called the “place principle
Hair Cell Innervation
• Hair cells are the sensory
receptors of both the auditory
system and the vestibular
system
• They derive their name from
the tufts of stereocilia that
protrude from the apical
surface of the cell, largest
one
being
called
the
Kinocilium
• Nerve fiber innervation is much denser for inner hair cells
than for outer hair cells. A single inner hair cell is innervated
by numerous nerve fibers, whereas a single nerve fiber
innervates many outer hair cells.
Function of Inner & Outer Hair cells
Inner Hair Cells:
• They transform the mechanical forces of sound
into electrical impulses.
Outer Hair cell:
• Increases the sensitivity of inner hair cells for
different frequency and intensity.
Auditory Pathway
Auditory cortex
From there it passes to medial
geniculate nucleus
Pathways passes through lateral
lemniscus but many bypass this and
travel to inf. colliculus
Second order neuron from here
cross to opposite side of brain stem
and terminate in sup olivary nucleus
Fibers from the spiral ganglion
enters ventral cochlear nuclei
located in the upper part of medulla
Peculiarities of auditory pathway
• Signals from both ears are
transmitted through the pathways
of both sides of the brain, with a
preponderance of transmission in
the contralateral pathway
• Many collateral fibers from the
auditory tracts pass directly into the
reticular activating system of the
brain stem
• A high degree of spatial orientation
is maintained in the fiber tracts
from the cochlea all the way to the
cortex
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Sound Ananlysis
Perceiving Pitch (Frequency):
• location of vibration on the basilar membrane
Appreciation Of Loudness Of Sound:
• Intensity or loudness of sound correlates with two
factors:
1. Rate of discharge from the individual fibers of auditory
nerve
2. Total number of nerve fibers discharging.
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Sound Ananlysis
Localization Of Sound
• Sound localization is the ability to detect the source
from where sound is produced.
• Source of sound is Localized by comparing sounds
from both ear for:
– relative intensity - the amplitude of sound waves
hitting the different ears
– relative timing - the difference in timing in which a
sound reaches both ears
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Pathophysiology of hearing
• Conduction deafness
–
That caused by impairment of the physical
structures of the ear that conduct sound to
cochlea like outer ear and middle ear.
• Sensorineural deafness
–
It is the deafness caused from impairment of the
Hair cells, auditory nerve etc.
References
• Human physiology by Lauralee Sherwood,
9th edition
• Text Book Of Physiology by Guyton & Hall,
12th edition
THANK YOU
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