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hearing
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Middle ear Ear ossicles
Malleus, incus, stapes Reinforce sound
waves Impedance
matching High amplitude-low
pressure sound waves converted to low amplitude-high pressure waves
Sensitive: 300-3000 cps
Middle ear Acoustic reflex
Attenuation reflex tension dampens
vibrations by 30-40db sensitivity to own
speech Protects the cochlea
from loud tones Loud low tones Masks low tones
Middle ear Impedance matching
Tympanic membrane, oval window, ossicles
the force and pressure of waves going to the oval window: 22x
the sensitivity of hearing
Internal ear Cochlea
Scala vestibuli Oval window
Scala media Scala tympani
Round window
Endolymph Perilymph
Reisner’s membrane Basilar membrane
Internal ear Endocochlear
potential Endolymph – K, Na Perilymph – K, Na Positive scala media
80 mv electrical potential
Bodies of hair cells: perilymph, -70 mv
Apex of hair cells: endolymph, -150 mv
Internal ear Organ of Corti
Basilar membrane, basilar fibers
Rods of Corti, reticular lamina
Hair cells Inner hair cells Outer hair cells
Tectorial membrane
CN VIII
Vibration of bodies which can evoke an auditory sensation
Molecular motion in the direction of energy transmission
Rarefaction in pressure
Propagated with specifiable velocity according to the characteristics of the medium and independently of the intensity
density of the medium, velocity
Air 1000 ft/sec
Water 4700 ft/sec
Wood 13000 ft/sec
Steel 16500 ft/sec
Distance travelled by sound in one period
W=V/F W – wavelength V – velocity of sound F – frequency
500 cycle tone in air has a wavelength of 2 ft
Quality of the sensation that permits a sound to be classified in the scale from high to low
Determined by the frequency of the sound waves
Hertz
Amplitude modifies pitch 1000-3000 cps: frequency, pitch
Pitch does not change perceptively with changes in intensity
amplitude of high frequency waves, pitch >3000 cps: intensity, pitch
amplitude of low frequency waves, pitch <1000 cps: intensity, pitch
Telephone theory Pitch is a function of
the auditory center Basilar membrane
moves more as a unit
Place theory Pitch is a function of
the cochlea Pitch is determined by
the vibrating portion of the basilar membrane
Helmholtz’s resonance theory
Bekesy’s travelling wave theory
Helmholtz’s Resonance Theory Basilar membrane has fibers of different lengths
and diameter Basilar fibers
Length from oval window to helicotrema Diameter from oval window to helicotrema
Fibers vibrate selectively to tones of different frequencies
Short fibers near the oval window vibrate to higher frequencies
Long fibers near the apex vibrate to lower frequencies
Bekesy’s Travelling Wave Theory The cochlea is a tuned structure
Width of the basilar membrane increases from base to helicotrema
The basilar membrane exhibits graded stiffness Mass of organ of Corti increases towards the apex
Resonant frequency of cochlear partition is highest at the stapes, and decreases along its length
high frequency: displacement at the stapes low frequency: displacement at the apex Short wavelengths will die out more quickly
Bekesy’s Travelling Wave Theory
Length and diameter of hair cells Decreasing elasticity coefficient of basilar fibers
from the oval window to the apex Place principle
Frequencies are detected based on the area of the basilar membrane that is most stimulated
Loudness Psychologic reaction
to the intensity of the sound wave
Intensity Force or strength of
sound Depends on
amplitude
amplitude, intensity Inner and outer hair
cells are stimulated More nerve impulses
are produced Spatial summation
Stimulation of hair cells on the fringes of the resonating portion of the basilar membrane
Amplitude modifies pitch amplitude of high frequency waves, pitch
>3000 cps: intensity, pitch amplitude of low frequency waves, pitch
<1000 cps: intensity, pitch
1000-3000 cps: pitch does not change perceptively with changes in intensity
Loud sounds cause the width of the resonating membranes to increase in oscillations
Outer hair cells are stimulated with louder sounds
I = log E1/E2
I – intensity E1 – intensity of
observed sound E2 – intensity of
reference sound If intensity of sound A
is 10x that of sound B Ratio is 10:1 Log of ratio is 1 The intensities differ
by 1 bel
0.1 bel = 1 decibel 1 decibel = 1.26x
1 bel = 10 decibel 10 decibel = 12.6x
Least intensity that can be heard by the average person
Varies for each frequency within the pitch range
500 cps: threshold is highest2048 cps: threshold is lowest
<1000 cps: pitch , threshold
Low frequency: vibratory pressure
>3000 cps: pitch , threshold
High frequency: pain
1000-3000 cps: minimal thresholdThreshold increase for tones higher and lower than this
frequency
120 decibels Highest intensity of
sound that can be heard without pain
1000000 times the lowest auditory threshold
db hrs
60 Normal conversation
90 8 Shouting at 2 ft
100 2 LRT train passing
120 <0.25
Jet take-off, sandblasting
Louder sounds: Evoke vibrations of
greater amplitude in the basilar membrane and hair cells, thus nerve endings are stimulated at faster rates
Some hair cells are stimulated only when a certain intensity is reached
Spatial summation
Timbre Quality of sound Relative amplitudes of the various harmonics
yield a unique wave form for each sound source
Property of complex sounds Enables us to distinguish musical
instruments, voices, etc
Spatial pattern of neuronal stimulation Medial superior olivary nucleus
Difference in time arrival of the sound waves to each ear
Difference in the phase of the sound waves to each ear: For low pitch sound
Lateral superior olivary nucleus: Difference in intensities of the sound waves to each ear:
Localization is difficult for continuous sounds, pure tones, and noises
Cochlea CN VIII
Medial geniculate
body
Temporal lobeBA 41, 42:
6 tonotopic maps, isofrequency
columnsBA 22: Wernickie’s
area
Auditory radiation
Cochlear nuclei of restiform bodyDorsal nucleiVentral nuclei
Superior olivary nucleus
Superior olivary
complex
Lateral lemniscus
Inferior colliculus
Crossing-over in the brain stem Trapezoid body Lateral lemniscus
Commissure of Probst Inferior colliculi
Reticular activating system of the brain stem
Vermis
Retrograde pathways Cortex to cochlea
Superior olivary nucleus to organ of Corti
Final pathway Inhibitory: 15-20 db
Tuning of the receptor system
Brain stem to hair cells Shortening of outer hair
cells Change in stiffness of
outer hair cells
Whisper test Maximum distance
at which sound can be heard is determined and compared with that which can be heard by the normal ear
Expressed as a fraction of the normal
Whisper: 30ft
Tuning fork test Test fork is held close
to the ear Time from the moment
the fork is struck until the sound becomes inaudible is determined
It is compared with the normal time
Degree of hearing loss is determined by the difference
Audiometry Precise testing for auditory function Tones of varied intensity and pitch are generated Tests air and bone conduction
Conduction deafness Otosclerosis Otitis media Perforated tympanic
membrane Trauma
Central deafness Psychologic CVA
Sensory-neural deafness Ototoxic drugs Presbycusis Tumors Trauma
Webber test Tuning fork at
midline Check for laterality
Rinne test Tuning fork at ear
with lateralized sound Tuning fork at
mastoid process then at pinna
Air conduction > bone conduction