The mechanism of hearing

PHYSIOLOGY OF HEARING

1. Dr Ajay Manickam JR – DEPT OF ENT AND HNS RG KAR MEDICAL COLLEGE The Mechanism of Hearing

2. The Mechanism of hearing

3. Broad classification Mechanical conduction of sound Transduction of mechanical energy to electrical impulses Conduction of electrical impulses to brain

4. 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 Catenary lever Ossicular lever Hydraulic lever

5. Mechanical conduction (Acoustic Transformer) Middle ear transformer mechanism Catenary lever – ear drum – 2times gain in sound pressure Ossicular lever – ossicles – handle of malleus 1.3 times longer than long process of incus – mechanical advantage of 1.3 Hydraulic lever – diff in surface between TM (55 sq mm) & Stapes (3.2 sq mm) – areal ratio – 14: 1

6. 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.

7. Conduction of electrical impulses to brain

8. 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)

9. The PLACE theory (1883) High frequency will excite BASAL region Low frequency will excite APICAL region

10. The FREQUENCY theory (1886) All frequencies activate entire length of basilar membrane All vibrations are portrayed to the brain without complex vibrations of cochlea

11. 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

12. 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

13. Thank you