Physiology of hearing
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PHYSIOLOGYOF HEARING
Dr Seema S
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Outline
Introduction to sound Ear
Function as a transducer Parts and how it functions
External ear Middle ear Internal ear
Electrical potential Auditory path way
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SOUND
A form of energy propagates in the form of waves The speed of sound depend on the medium
through which the wave pass air - 343m/s water - 1482m/sec
Audible frequencies t for humans 20 to 20,000 cycles per second (cps, Hz).
It can detect the difference between two sounds occurring 10micro seconds apart in time
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EAR AS A TRANSDUCER
SOUNDENERGY
MECHANICAL ENERGY
ELECTRICAL ENERGY
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Technical jargons
•Strength of the sound•Loudness denotes the appreciation of sound intensity•Expressed in decibel (dB)
Amplitude/loudness
•Number of cycles per second•Pitch /Tone denotes the appreciation of frequency•Expressed in Hertz(Hz)
Frequency/Pitch/Tone
•Resistance offered by a medium to sound waves
Impedence
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RESONANCE•Resonance is the tendency of a system to oscillate with larger amplitude at some frequencies than at others
ATTENUATION•Attenuation is a general term that refers to any reduction in the strength of a signal
Values of hearing:
15-25dB —Whisper 35dB —Background noise 40-60dB —Background noise
( home ) 65—70dB –- normal speaking voice 130dB —painful noise 140-180dB —jet air craft engine noise
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Natural resonant frequencyEXTERNAL AUDITORY CANAL---------------
3000Hz
TYMPANIC MEMBRANE----------------------- 800-1600Hz
MIDDLE EAR--------------------------------------- 800Hz
OSSICULAR CHAIN--------------------------- 500-2000Hz
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External ear
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Functions of EXTERNAL EAR
Sound collection
Increasing pressure on the tympanic membrane in a frequency sensitive way
Sound localization
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Sound collection
Pinna- concha system catches sound over large area and concentrate it to smaller area of ext. auditory meatus.
This increases the total energy available to the tympanic membrane
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Pressure increase by EAC
If a tube which is closed at one end and open at other is placed in a sound field then pressure is low at open end and high at closed end.
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Sound localization
Because of its shape, the pinna shield the sound from rear end,change timbre,and helps to localize sound from infront or back
Cues for sound localization from right/left Sound wave reaches the ear closer to sound source before
it arise in farthest ear Sound is less intense as it reaches the farthest ear
because head act as barrier
Auditory cortex integrates these cues to determine location.
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Middle ear
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FUNCTIONS OF MIDDLE EAR
IMPEDENCE MATCHING
ATTENUATION
PHASE DIFFERENCIAL EFFECT
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Impedence mismatch
IF THERE WAS NO MIDDLE EAR SYSTEM ,99% OF SOUND WAVES WOULD HAVE REFLECTED BACK FROM OVAL WINDOW
MIDDLE EAR BY ITS IMPEDENCE MATCHING PROPERTY ALLOWS 60% OF SOUND ENERGY TO DISSIPATE IN INNER EAR
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“Impedance Matching” by the middle ear System
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(a) HYDRAULIC ACTION OFTYMPANIC MEMBRANE
Total effective area of tympanic membrane 45mm2
Area of stapes footplate is 3.2mm2
Effective areal ratio is 14:1
Thus by focusing sound pressure from large area of tympanic membrane to small area of oval window the effectiveness of energy transfer between air to fluid of cochlea is increased
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(b) Lever action of ossicles Handle of malleus is 1.3
times longer than long process of incus
Overall this produces a lever action that converts low pressure with along lever action at malleus handle to high pressure with a short lever action at tip of long process of incus
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(c) Action of tympanic membrane
Eustachian tube equilibrates the air pressure in middle ear with that of atmospheric pressure, thus permitting tympanic membrane to stay in its most neutral position.
A buckling motion of tympanic membrane result in an increased force and decreased velocity to produce a fourfold increase in effectiveness of energy transfer
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Total gain
Total transformer ratio=14x1.3x4=73:1
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Attenuation reflex
When loud sounds are transmitted through the ossicular system and from there into the central nervous system, a reflex occurs after a latent period of only 40 to 80 ms to cause contraction of the stapedius muscle and the tensor tympani muscle
The tensor tympani muscle pulls the handle of the malleus inward while the stapedius muscle pulls the stapes outward. These two forces oppose each other and thereby cause the entire ossicular system to develop increased rigidity, thus greatly reducing the ossicular conduction of low frequency sound
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Function of attenuation reflex To protect the cochlea from
damaging vibrations caused by excessively loud sound.
To mask low-frequency sounds in loud environments. This usually removes a major share of the background noise
To decrease a person’s hearing sensitivity to his or her own speech
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PHASE DIFFERENTIAL EFFECT
Sound waves striking the tympanic membrane do not reach the oval and round window simultaneously.
There is preferential pathway to oval window due to ossicular chain.
This acoustic separation of windows is achieved by intact tympanic membrane and a cushion of air around round window
This contributes 4dB when tympanic membrane is intact
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INNER EAR
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COCHLEA ---TWO FUNCTIONS…. A TRANSDUCER that translates
sound energy into a form suitable for stimulating the dendrites of auditory nerve.
AN ENCODER that programs the features of an acoustic stimulus so that the brain can process the information contained instimulating sound.
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Electrical potential of cochlea and CN VIII
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Endocochlear potential
An electrical potential of about +80 millivolts exists all the time between endolymph and perilymph, with positivity inside the scala media and negativity outside.
This is called the endocochlear potential, and it is generated by continual secretion of positive potassium ions into the scala media by the stria vascularis
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Cochlear microphonic
When basilar membrane move in response to sound stimulus electrical resistance at the tip of hair cells change allowing flow of K+ through hair cells and produce voltage fluctuations called cochlear micro phonic.
This is AC potential
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Summating potential
Produced by hair cells DC potential superimposed on VIII
nerve action potential
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Compound action potential
All or none response of auditory nerve fibres
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Central auditory pathway
• nerve fibers from the spiral ganglion of Corti enter the dorsal and ventral cochlear nuclei
• second-order neurons pass mainly to the opposite side of the brain stem to terminate in the superior olivary nucleus
• the superior olivary nucleus,the auditory pathway passes upward through the lateral lemniscus.
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Some of the fibers terminate in thenucleus 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, where all the fibers do synapse
Finally, the pathway proceeds by way of the auditory radiation to the auditory cortex, located mainly in the superior gyrus of the temporal lobe.
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Pecularities of auditory pathway
First,signals from both ears are transmitted through the pathways of both sides of the brain, with a preponderance of transmission in the contralateral pathway
Second, many collateral fibers from the auditory tracts pass directly into the reticular activating system of the brain stem
Third, 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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Function of auditory cortex
Perception of sound
Judging the intensity of the sound
Analysis of different property of sound
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