The mechanism of hearing
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Transcript of The mechanism of hearing
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Dr Ajay Manickam
JR – DEPT OF ENT AND HNS
RG KAR MEDICAL COLLEGE
The Mechanism of Hearing
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The Mechanism of hearing
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Broad classification
Mechanical conduction of sound
Transduction of mechanical energy to electrical impulses
Conduction of electrical impulses to brain
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Mechanical conduction Pinna - determine origin of
sound – concha acts like a megaphone to concentrate sound – 6dB increase in sound pressure
EAC - in concert with pinna increase sound pressure by 15 to 22 dB at 4000Hz
Middle ear transformer mechanism
1. Catenary lever
2. Ossicular lever
3. Hydraulic lever
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Mechanical conduction (Acoustic
Transformer)
Middle ear transformer mechanism
1. Catenary lever – ear drum –2times gain in sound pressure
2. Ossicular lever – ossicles –handle of malleus 1.3 times longer than long process of incus – mechanical advantage of 1.3
3. Hydraulic lever – diff in surface between TM (55 sqmm) & Stapes (3.2 sq mm) –areal ratio – 14: 1
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Acoustic transduction
By the lever effect
sound waves enter the
liquid medium from air
medium
Thus by transduction
sound waves are
converted to electrical
waves by hair cells,
basilar membrane.
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Conduction of electrical impulses to brain
1
• Hair cells peripheral / spiral ganglia central
2
• Cochlear nerve
3
• Ventral & dorsal Cochlear nucleus
4
• Superior olivary nucleus
5• Lateral lemniscus
6• Inferior colliculus
7• Medial geniculate body
8• Auditory cortex
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Theories of hearing
Helmhotz’s place theory (1883)
Rutherford’s frequency theory (1886)
Wever’s volley resonance theory (1949)
Von bekesy’s travelling wave theory (1960)
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The PLACE theory (1883)
High frequency will
excite BASAL region
Low frequency will
excite APICAL
region
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The FREQUENCY theory (1886)
All frequencies activate
entire length of basilar
membrane
All vibrations are
portrayed to the brain
without complex
vibrations of cochlea
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VOLLEY RESONANCE theory (1949)
High frequencies –
perceived in basal turn
Low frequency – frequency
stimulation
Intermediate frequencies –
asnchronous discharges
which then combine
actively to represent the
frequency of stimulus
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TRAVELLING WAVE theory (1960)
Begins from base move to
apex
Independent of frequencies
Region of maximum
displacement varies
according to frequency
High pitched – short
travelling wave
Low pitched – long wave
maximum displacement
near apex
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Thank you