The mechanism of hearing

Dr Ajay Manickam

JR – DEPT OF ENT AND HNS

RG KAR MEDICAL COLLEGE

The Mechanism of Hearing

The Mechanism of hearing

Broad classification

Mechanical conduction of sound

Transduction of mechanical energy to electrical impulses

Conduction of electrical impulses to brain

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

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

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.

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

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)

The PLACE theory (1883)

High frequency will

excite BASAL region

Low frequency will

excite APICAL

region

The FREQUENCY theory (1886)

All frequencies activate

entire length of basilar

membrane

All vibrations are

portrayed to the brain

without complex

vibrations of cochlea

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

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

Thank you