Transcript
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Physiology of hearing
by
uma
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HEARINGNEEDS Adequate stimulus-sound
Mechanical Conduction of sound
transduction of mechanical energy to
electrical impulses
central auditory processing of the impulses in
the auditory pathway
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Sound
It is of successive waves of compression andrarefaction generated by vibrating object
Velocity of sound- 340m/s at the sea level
Propertiesof sound:
Physical dimension Perceptual dimension
Intensity Loudness
Frequency PitchComplexity timbre
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Intensity: pressure exerted by a sound stimulus
Strength of sound ,units-decebels
1dB=1/10 ofBell, normal range= 0 dB-120 dB
Frequency :no.of waves/sec;units-Hz
Frequency range measured in octave scales i.e. each F.is double the previous one eg;250,500,1000
Frequency range=20-20000Hz
Complexsound : sound more than one frequency eg;human voice
Puretone: a single frequency sound eg:250Hz
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Mechanical conduction
Pinna andE.A.C:collection of sound waves
Amplification of frequency 2000Hz-4000Hz
Tympanic membrane:partion wall between external
and middle ear.
As it moves to and fro, it is buckled in the regions
between the manubrium of malleus and the anterior
&posterior edges
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Ossicles: malleus ,incus and stapes-
Axis of rotation of ossicles and the axis ofsuspension by their ligaments coincide with their
centre of rotational inertia At low frequencies, mass effect is small, so the
ligaments play imp role in maintaining theposition of ossicles
At higher frequencies vibrations become morecomplex with rotatory motion along the long andshort axis of the footplate
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Internal ear
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Impedence matching Middle ear serves as acoustic transformer to match the
large, low impedence TM to much smaller, high
impedence oval window It is done by two principles
1)hydraulic action ofTM: the area ofTM is much largerthan that of footplate, the avg ratio is 18.75:1 so theforces collected over the TM or concentrated on a smallerarea to increase the pressure at the Oval window
2)lever action of the ossicles: geometrical lenth of themalleus is approx 2.1 times more than that of incus so itmultiplies the force 2.1 times and the velocity isd
ecreased
2.1 times Thus Lever action increases the impedence ratio 4.4
times(2.1 x2.1)
Vibrations are coupled preferentially to only OWproducing differential pressure b/w two windows which is
required for movement of cochlear fluids
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Transduction
Cochlea: it includes scala tympani,media andvestibuli.organ of corti is located in the scala media
Endolymph: formed by stria vascularis ,is like ICFcontains high K+ ion concentration(144Meq/l) and lowNa + ion concentration(16Meq/l)
Perilymph:it is not a simple ultra filtrate ofplasma.perilymph of scala vestibuli originates primarilyfrom the plasma and that of scala tympani from theplasma and the CSF
K+ion concentration in the scala vestibuli is some what
higher than in the scala tympani. The chemical andelectrical boarder b/w endo and peri lymph is thereticular lamina which include the transducing surfacesof the hair cells
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Haircells :are of two types inner and outer
Outer hair cells:amplify the travelling wave to increase thecochleas sensitivity and frequency selectivity
The movements of footplate are transmitted to the
cochlear fluids and basement memb, setting of the
shearing force
Thus the stimulus is coupled to the steriocilia by means of
shear or motion between the tectoral memb and the
reticular lamina
Stereocilia on the top of hair cells are rigid move as a stiff
bundle.When cilia are deflected in the direction of
kinocilia,ion channels in the membrane are opened
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Inner hair cells: single row, each has terminals
from 20 afferent nerve fibers
Potential grad
ient causes K+
entry into thehair cell from the endolymph there by
generation of action potentials occurs at the
bases of inner hair cells
Then the impulses are conducted to the brain
via ascending auditory pathway
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Phase Locking ofInner Hair Cells
Auditory nerve connected to inner hair cell tends
to fire at the same phase of the stimulating
waveform.
For Low frequency stimuli Nt will be released in
pockets and concentratedduring the
depolarising phase of the hair cells response
It is seen only at low frequency stimuli. By 3-5 Hzfrequency the AC response is so small that phase
locking of action potential is negligible
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Cental auditory pathway E
xtend
s from cochlear nuclei to the aud
itory area in thecerebral cortex
First order neurons: bipolar cells connecting the haircells of organ of corti to the dorsal and ventral cochlearnuclei. Cell bodies in the spiral ganglion are housed with
in the bony spiral lamina
These auditory division of fibers are arranged in atonotopic fashion which is maintained in the cochlearnuclei also
Low frequency fibers are represented ventrolaterally andhigh frequency fibers dorsomedially
Fibers near the base of the cochlea will respond best to ahigh frequency and fibers near the apex to a low
frequency
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Second order neurons: starts from cochlear nuclei and
project to inferior colliculus as dorsal pathway, as ventral
pathway to the ipsilateral and contra lateral SOC.
Superior olivary complex: it contains an S-shaped lateral
olivary nucleus, disk shaped medial olivary nucleus and
the medial nucleus of trapezoid body together withsmaller peri olivary nuclei
The neurons from the SOC pass through the lateral
lemniscus in the mid brain
Neurons that localize the sound with inter aural delay arelocated in MOC
Neurons that detect differences in the sound intensity are
located in LOC
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Inferior colliculus: contains central nucleus which
receives the major auditory input and outer dorsalcortex and external lateral cortex
It is involved in sound localization and auditory moto
responses eg:controling the middle ear muscles incase of noise
Third order neurons: connects inferior colliculus to
medial geniculate body which has dorasl,ventral and
medial divisions
Ventral division-transmits information on auditory
discrimination to the auditory cortex
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Dorsal division-sends axons to the auditory
association cortex for maintaining and
d
irectingauditory attention
Medial division- functions as a multi sensory
arousal system
Fourth order neurons: connect MGB to the cerebral
cortex
These neurons pass through the sub lenticular
portion of the internal capsule
Primary auditory cortex(areas 41,42 of broadmen):
it is the gyrus of heschl on the upper part of the
superior temporal gyrus in the sylvian fissure
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Theories of hearing Place theory:it states that perception of sound
depends upon the selective vibration s of a specific
place on the basilar membrane. frequency analysis
is carried out in the inner ear
Temporal/timing theory: perception of sounddepends on the temporal patterns with which
neurons respond to the sound in the cochlea
As the basilar membrane vibrates, each clump ofhair cells along its length is deflected in time
More intense the vibration is the more hair cells
ared
eflected
&more likely they are to cause nerve
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Telephone theory(Rutherford):Perception is
related to the rate of firing of individual nerve
fibers
Hair cells transform sound vibrations to the
nerve in similar frequency&litude.so there isno analysis of the complex vibrations in the
cochlea which occurs in the brain
Volley theory:higher frequencies are perceivedby the place mechanisams,low frequencies by
the telephone mechanism and the intermediate
frequencies by both
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Travelling wave theory: sound waves from the
OV produce a wave which travels along thebasilar memb to the apex of the cochlea, called
classical traveling wave
As it moves there is increase in the amplitude.the point of maximum amplitude on the BM
depends upon the frequency of the sound
high frequency prod
uced
travelling wave peaksnear the waves and low frequency wave
stretched further up to the apex and finally dies
away
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Oto acoustic emissionsLow intensity sounds produced by the movements of
OHC of the cochlea
Can be recorded & measured by placing a
microphone receiver in the deep EAC
OAC can be recor
ded
in all normal hearing personsirrespective of age& consciousness. OAC are of two
types;spontaneous&evoked
OAC in response to high frequency sound stimulus
originate from the basal turn of the cochlea& have a
shorter latency
Absence ofOAC indicates structurally damaged or
non functional OHC eg:acoustic trauma,ototoxic
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References
Scott brown 7th edition
Pl dhingra
Anirban Biswas
Internet
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