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Hearing
SOUND
- longitudinal pressure wave (the direction of
oscillation and propagation are parallel, energy is
transported through the change of density of the
medium)
- elastic medium is required for propagation (e.g.
air)
- frequency range: <20 Hz: infrasound, 20 Hz – 20
kHz: audible sound, >20 kHz: ultrasound
Physical properties of the sound
-
frequency, f („pitch”): temporal periodicity
period time, T
wavelength, : spatial periodicity
amplitude, A („loudness”): maximal energy of sound
intensity, I: sound power (p) per unit area (A)
propagation velocity, v: depends on the elastic properties of the medium
I
P
A
v f
Absolute intensity:
- hearing threshold (lowest audible intensity): ~10-12 W/m2 (at 1000 Hz),
- pain threshold (maximal intensity): ~10 W/m2
Relative intensity
(sound intensity level, n): a logarithmic measure of the sound intensity of
a sound relative to a reference value (I0 = 10–12 W/m2), (dB, decibel)
Perception of sound
Phon-scale:
Human ear has different sensitivity for different frequencies.
Phon-value of a given sound means the sound pressure level
in dB of a sound at a frequency of 1000 Hz, that sounds just as
loud as the given sound.
Sounds at different frequncies and relative intensities, but
with the same loudness sensation defines the equal loudness
contours, the so-called isophon-curves (Fletcher curves)
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n  10 lg
I
I0
THE HUMAN EAR
Outer ear
- Parts: auricle, external
membrane (eardrum)
auditory
canal,
tympanic
- Function: drives the sound to the tympanic membrane
- Two ears: helps the sound localization
Middle ear
- Between tympanic membrane – oval window
- Auditory ossicles: hammer – incus – stapes
- Function: amplification of sound pressure (~22x)
o
Auditory ossicles: lever-like function (~1.3x)
o
Eardrum – oval window size difference: pressure
amplification (~17x)
Inner ear
- In the petrosal portion of temporal bone: semicircular
canals (balance), cochlea (hearing)
- Cochlea: 3 parallel liquid filled compartments,
- Cochlear duct: the middle canal, filled with endolymph:
- Basilar membrane: basal part of cochlear duct
- Organ of Corti: on the basilar membrane, contains hair
cells
- Tectorial membrane: bends over the hair cells
- Hair cells: organized stereocilia on their apical surface,
connected with a protein fiber (tip link). 3 row outer
hair cells: amplification, 1 row inner hair cells:
sensation
THE FUNCTION OF THE ORGAN OF CORTI
Sound perception: mechano-electic transduction
The mechanical stimulus (sound) transform to electric signal (action potential)
1. Sound vibrates the basilar membrane
2. Hair cells are pressed to the tectorial membrane.
3. Stereocilia bend
4. Tip-link are streched
5. Ion channels open mechanically, K+ influx
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6. Depolarization
7. Voltage gated Ca2+-channels open, Ca2+ influx
8. Neurotransmitter release, stimulus transmitted to the afferent neuron: depolarization, action
potential
Frequency recognition: tonotopic localization (Békésy’s place theory):
Sound wave can cause vibration on the basilar membrane only at a location specific for its
frequency
Oval window: stiff basilar membrane – high pitch tone
Apex of the cochlea: loose, wide basilar membrane – low pitch tone
1. The sound stimulus cause a surface
wave, which propagates from the oval
window to the cochlear apex.
2. The propagation speed decreases (45
m/s – 2 m/s) as the basilar membrane
become wider and looser.
3. The rearward waves catch up with the
precedings: the amplitude suddenly
increase, than decrease
4. The position of the maximum of the
resulting wave depends on the
frequency, while its amplitude on the
intensity.
5. The vibration of the basilar membrane stimulate the hair cells, causing electrical potential
change.
6. The stimulus originated from a specific location of the organ of Corti is transmitted through the
acoustic nerve to the corresponding location of the temporal lobe, causing the frequency
sensation.
Outer hair cells as mechanical amplifiers
Prestin: voltage sensitive motor proteins located in the membrane of the
outer hair cells
1. When the outer hair cells detect the displacement of the basilar
membrane, K+ channels open causing depolarization
2. prestin change conformation causing longitudinal change in the cells:
electro – mechanical transduction
3. Amplifies the vibration of the basilar membrane
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THE PROCESS OF HEARING
1.
Sound wave vibrates the eardrum
2.
Auditory ossicles amplify the wave
3.
Through the oval window the vibration is transmitted to the liquid compartment of the inner
ear
4.
The basilar membrane get vibrated
5.
Hair cells are pressed to the tectorial membrane
6.
The bending of the stereocilia opens the ion channels
7.
Hair cells depolarize
8.
Stimulus transmitted through the acoustic nerve to the brain
9.
Temporal lobe: primary auditory cortex
10. Assotiation area: language recognition (Wernicke area)
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