Special Senses-Sense of Hearing...complete lecture
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Transcript of Special Senses-Sense of Hearing...complete lecture
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SENSE OF HEARING
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Sense of Hearing
Mechanism by which EAR
Receive sound waves
Discriminate their frequencies
Transmit auditory information to central nervoussystem
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ANATOMY OF EAR1. The outer ear
- pinna
- ear canal
- eardrum
2. The middle ear
- three ossicle bones;
(malleus, incus,stapes)
- two major muscles
(stapedial muscle,tensor
tympani)
- Eustachian tube
3. The inner ear
- cochlea (hearing)
- vestibular system(balance)
4. The central auditorysystem
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OUTER EAR
Three parts ofouter ear
1) Pinna
2) Ear canal
3) Ear drum
Major functionof outer ear
1)amplification
2) soundlocalization
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MIDDLE EAR
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Three main parts ofmiddle ear
(1)Three Ossicle
bones:- Malleus(1), Incus(3),
Stapes(6)
Function)Impedancematching
(2)Two muscles
- Stapedial muscle(5)
- Tensor tympani(9)
Function)Protection(attenuation reflex)
(3)Eustachian tube(8)
Function)Equalizer ofair pressure
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TRANSMISSION OF SOUND
WAVES THROUGH MIDDLE EAR The tip end of handle of
malleus is attached tocenter of tympanicmembrane and at thisattachment is constantlypulled by tensor tympanimuscle which keep tympanicmembrane tensed
Sound vibrations on anyportion of tense tympanicmembrane is transmitted tomalleus and then to incusand then to stapes foot plate
on oval window window, setsthe fluid of inner ear intomotioneventually excitinghearing receptors.
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IMPEDANCE MATCHING
Sound transmission in fluid (inner ear) requiresmuch higher pressure than sound transmission in air(outer ear)
Therefore, the tympanic membrane and ossicular
system provide impedance matching between thesound waves in air and the sound vibrations in the
fluid of the cochlea.
The amplitude of movement of the stapes faceplate
with each sound vibration is as much as the
amplitude of the handle of the malleus
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This increases theforce of movement about 1.3
times.
The surface area of the tympanic membraneis about 55 square millimeters, whereas the
surface area of the stapes averages 3.2 square
millimeters. This 17-fold difference times the 1.3-fold ratio
of the lever system causes about 22 times as
much total force to be exerted on the fluid ofthe cochlea as is exerted by the sound waves
against the tympanic membrane.
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ATTENUATION REFLEX
When loud sounds aretransmitted through the
ossicular system and from
there into the central
nervous system, a reflexoccurs after a latent period
of only 40 to 80
milliseconds to cause
contraction of the
stapedius muscle and, to a
lesser extent, the tensor
tympani muscle
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The tensor tympani musclepulls the handle
of the malleus inward while the stapedius
muscle pulls the stapes outward. These two
forces oppose each other and thereby causethe entire ossicular system to develop
increased rigidity
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IMPORTANCE
1. Toprotect the cochlea from damaging vibrationscaused by excessively loud sound
2. To mask low-frequency sounds in loud
environments. allows a person to concentrate onsounds above 1000 cycles per second
3. Decrease a persons hearing sensitivity to his or
her own speech.
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INNER EAR
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FUNCTIONAL ANATOMY OF
COCHLEA
The cochlea is a systemof 3 coiled tubes
(1) scala vestibuli(contains perilymph)
(2) scala media (contains
endolymph)(3) scala tympani.
(contains perilymph)
Reissners membrane
Basilar membrane
Organ of Corti
Helicotrema
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BASILAR MEMBRANE Fibrous membrane Separates the scala
media from the scalatympani
20,000 to 30,000 basilar
fibers Projects from modiolus,
toward the outer wall
Stiff, elastic, reed likestructures
Fixed at basal end butfree at distal end
Vibrates at free end
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The lengths of the basilar fibers increaseprogressively
beginning at the oval window (base) of the cochleato the helicotrema (apex)
The diameters of the fibers, however, decrease frombase
to apex
The stiff, short fibers near the oval window of the
cochlea vibrate best at a very high frequency
Long, limber fibers near the tip of the cochlea vibrate
best at a low frequency.
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the
Cochlea
When stapes footplatemoves inwards againstthe oval window, roundwindow must bulgeoutwards
Sound waves causebasilar membrane to bendin direction of roundwindow
As a result basilar fibersdevelop elastic tension,
initiates a fluid wave thattravels along the basilarmembrane towards theapex
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Different sound frequencies on
basilar membrane
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is for discrimination of sound waves
of different frequencies
High frequency at the base Low frequency at the apex
Intermediate frequency at the intermediatedistance between two extremes
Spatial organization of nerve fibers in cochlearpathway from cochlea to cerebral cortex
Specific brain neurons are activated by specificsound frequencies
So nervous system detects different soundfrequencies according to position of stimulatedpart of basilar membrane
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VOLLEY OR FREQUENCY
PRINCIPLE
Low frequencysounds 20 to 2000cycles per secondssend volleys of nerve
impulsessynchronized at samefrequency throughcochlear nerve tocochlear nuclei, which
distinguish thedifferent frequenciesof volleys.
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Sound frequencies are discriminated from oneanother on the basis of
Place of maximum stimulation of nerve fibers from
the organ of corti lying on the basilar membrane.
(depends upon positions along the basilar
membrane that are most stimulated)
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Loudness (amplitude/intensity)
Perception of loudness is related to
1- sound pressure level in db
2- duration of sound
POWER LAW:Psychophysical perception is log linear with
stimulus magnitude
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DETERMINATION OF LOUDNESS
1) Amplitude of vibration of basilar membrane andhair cells increases leading to excitation of
nerve endings at more rapid rates.
2) Spatial summation: more and more hair cells
stimulated leading to transmission throughmany nerve fibers rather than few.
3) Stimulation of outer hair cells
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Function of organ of corti
Receptor organ
Single row of internal
hair cells
3 or 4 rows of outer
hair cells
90-95% cochlear
nerve endings
terminate on innerhair cells (detection of
sound)
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Reticular
lamina
Stereocilia
Tectorialmembrane
Triangular
rods of
Corti
Basilar
fibers
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Hair Cell Receptor Potentials and Excitation
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Hair Cell Receptor Potentials and Excitation
of Auditory Nerve
Fibers
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Endocochlear Potential
An electrical potential of about +80 millivoltsexists all the time between endolymph and
perilymph, with positivity inside the scala media
and negativity outside
Generated by continual secretion of positivepotassium ions into the scala media by the stria
vascularis.
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Central auditory pathway
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AUDITORY CORTEX
Supratemporal portionof superior temporalgyrus
Primary auditory cortexexcited by projections
from medial geniculatebody
Secondary auditorycortex (auditoryassociation area)excited by impulsesfrom primary auditorycortex and thalamicassociation area
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Low frequency
anteriorly
High frequency
posteriorly
Discriminates sound
frequencies
Detection of direction Sudden onset of
sound
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Importance of auditory cortex
Discrimination of tonal and sequential sound patterns Destruction of both primary auditory cortices leads to
high reduction of ones sensitivity for hearing
Destruction of one side only slightly reduces hearing
in the opposite ear because of many crossoverconnections in auditory pathway
However, it does affect ones ability to localize thesource of a sound, because comparative signals inboth cortices are required for the localization
Lesions that affect the auditory association areas onlyleads to unable to interpret the meaning of the soundheard.
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Determination of sound direction
(1) The time lag between the entry of sound into one ear andits entry into the opposite ear
(2) The difference between the intensities of the sounds inthe two ears.
The first mechanism functions best at frequencies
below 3000 cycles per second The second mechanism operates best at higher
frequencies
The two mechanisms cannot tell whether the sound isemanating from in front of or behind the person or fromabove or below. This discrimination is achieved mainly by
thepinnae of the two ears. The shape of the pinnachanges the quality of the sound entering the ear,depending on the direction from which the sound comes.
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Centrifugal Signals from the Central Nervous
System to Lower Auditory Centers
Retrograde pathways (inhibitory) from cortex tocochlea which allows the person to direct his or
her attention to sounds of particular qualities
while rejecting sounds of other qualities
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Conduction deafness:
Anatomical site: Middle ear (ossicular chain), tympanic membrane or
external ear.
Causes: Ear wax, infection (otitis media), foreign body, tumor, tympanicmembrane perforation, genetic causes.
Weber test: Sound localizes to affected ear (ear with conductive loss).
Rinnes test: Negative, Bone conduction> air conduction (bone/air gap)
Treatment: In most of the cases hearing loss is reversible. Antibiotics,
antifungal, surgical repair, cochlear implants.
Perceptive deafness:
Anatomical site: Root cause lies in vestibulocochlear nerve, the inner
ear (damage to hair cells) or central processing centers of brain.
Causes: Noise, trauma, infections, congenital, aging.
Weber test: Sound localizes to normal ear.
Rinnes test: Positive, air conduction> bone conduction (both air and
bone conduction are decreased equally, but difference between them
unchanged).
Treatment: Loss permanent. Hearing aids
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Audiogram:
Is plotted on the basis of interrelationship ofsound intensity with frequency.
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The Vestibular System
VestibularLabyrinthOtolith organs
Utricle and
SacculeSemicircular
canals Anterior,
Posterior andLateral orhorizontal
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Function of vestibular apparatus
Is the sensory organ for detecting sensations ofequilibrium.
Accurate control requires accurate information
Sensory inputs:
1- vestibular system
2- visual system
3- proprioceptive system
4- cutaneous sensations
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Otolith organs
Both utricle and sacculecontain a sensory areainside known asMACULA
Macula of utricle lies in
horizontal plane anddetermine orientation ofhead when head isupright
Macula of saccule liesin vertical plane anddetermine orientation ofhead when person islying down
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Function of Otolith organs
Static equilibrium Linear acceleration
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Semicircular Ducts
ANTERIOR POSTERIOR
LATERAL
Three planes of space
Ampulla Endolymph
Crista ampullaris
Cupula
Hair cells Stereocilia
Kinocilia
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Crista ampullaris
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Crista ampullaris
RrRotationof head
Bending of cupu
Stereocilia bend Receptor cell fires
Synapse activated
Stimulation of nerve endings Dynamic equilibrium sense
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Function of semicircular ducts
Angular acceleration Rapid intricate
changing body
movements
Predictive role
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VESTIBULAR PATHWAY:
First order neurons in Scarpas ganglion 2ndorder neurons in vestibular nuclei at junction
of pons and medulla
Then pathway ascends to parietal cortex, area for
equilibrium
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Neural connections
Cerebellum Motor nuclei of CN 3,4 & 6
Reticular formation
Spinal cord