The document discusses the physiology of hearing. It covers topics like acoustics, properties of sound, the external ear, middle ear, inner ear, bone conduction, and the auditory pathway. The middle ear functions to transmit sounds from the air to the inner ear fluid through a transducer mechanism involving the vibrations of the tympanic membrane, ossicles, and oval window. Sound waves in the inner ear cause motion of hair cells and generation of nerve impulses that travel through the auditory pathway to the brain.

1. PHYSIOLOGY OF HEARING Crisbert I. Cualteros, M.D.

3. acoustics Sound – a change in pressure (particle displacement)within an elastic medium - wave form - emanates from a vibrating system - travels faster in denser medium Intensity – refers to the strength of the sound; the - psychoacoustic equivalent of intensity is loudness

4. acoustics Decibel - the unit used to measure the intensity of a sound - named after Alexander Graham Bell - One – tenth of a Bel - range of human hearing 0 – 120 dB dB = 10 logIm / I ref where: Im = measure intensity I ref = reference intensity

5. acoustics The decibel scale has the following characteristics: 1. It is logarithmic and incorporates a ratio 2. It is not linear (i.e., an increase from 1 to 3 dB is not equal to an increase from 5 to 7 dB) 3. It is a relative measure (i.e., 0 dB does not indicate the absence of sound) 4. It is expressed with different reference levels

6. acoustics Frequency – refers to the number of cycles (complete oscillations) of a vibrating body per unit of time; the psychoacoustic equivalent of frequency is pitch.

7. acoustics Hertz (Hz) – unit used to measure frequency - formerly called cycles per second or cps - the human ear is capable of hearing from 20 – 20,000 Hz

8. acoustics Pure tone – a single frequency sound; rarely occur in nature Complex sound – has more than one frequency Noise – an aperiodic complex sound

9. acoustics Resonant frequency – the frequency at which a mass vibrates with the least amount of external force External auditory canal - 3000 Hz Middle ear - 800 Hz Tympanic membrane - 800 – 1600 Hz Ossicular chain - 500 – 2000 Hz

10. PROPERTIES OF SOUND Sound External auditory canal

11. EXTERNAL EAR - funnel-shaped “collector” of sounds - localization pinna

12. EXTERNAL EAR - directs sound waves - 2 to 4 kHz - 10 to 15 dB - total occlusion hearing loss not more than 40 dB - earplugs / earmuffs attenuate sound <30 dB External auditory canal

13. EXTERNAL EAR - monaural hearing - binaural hearing cannot localize sounds localize sounds

14. sound EAC middle ear

15. MIDDLE EAR air-filled - 1 to 2 cc

16. MIDDLE EAR - handle  embedded in the TM - neck  tensor tympani inserts - head  articulates with ant. surface of incus body in the epitympanum malleus

17. MIDDLE EAR - short process projects backward - long incus passes downward articulated with head of stapes incus

18. MIDDLE EAR - oval window  inner ear stapes

19. - axis of rotation:  line from short process of incus to neck of malleus MIDDLE EAR

20. MIDDLE EAR CONDUCTION A. Problem – transmission of sound between 2 99.9% reflected 0.1 % transmitted IMPEDANCE MATCHING 20-35 dB loss    different media (air  liquid)

21. MIDDLE EAR CONDUCTION B: TRANSDUCER MECHANISM 1. ratio of drum/ footplate area vibrating TM 55 sq. mm. foot plate 3.5 sq. mm. = 17 (25 to 30 dB) ______________ ______________ =

22. MIDDLE EAR CONDUCTION B. TRANSDUCER MECHANISM 2. Lever mechanism Length long process malleus Length long process incus ______________________________ =1.3 (2.5 dB)

23. MIDDLE EAR CONDUCTION B. TRANSDUCER MECHANISM 3. Phase difference/ Protection of the round window release valve sound waves striking both windows simultaneously  waves cancel each other different location / position ( phase difference )  ~ 4 dB change

24. MIDDLE EAR CONDUCTION B. TRANSDUCER MECHANISM 4. Natural resonance and efficiency of the outer and middle ear (500 to 3000 Hz)

25. MIDDLE EAR CONDUCTION B. TRANSDUCER MECHANISM 5. shape of the tympanic membrane Curved cone

26. Middle ear conduction

27. INTRINSIC MUSCLES OF THE MIDDLE EAR Tensor Tympani Stapedius

28. Sound  EAC  Middle Ear  Inner Ear (Footplate)

29. INNER EAR CONDUCTION The Stapes moves back and forth at the oval window  fluid displacement within the cochlea These waves cause up and down displacement of the basilar membrane which then pivots around the osseous lamina

30. INNER EAR CONDUCTION C. This results in side to side motion of the hair cells in relation to the tectorial membrane in which they are embedded . D. This shearing action initiates a local electrical potential, COCHLEAR MICROPHONIC, which trigger a nerve impulse.

31. INNER EAR CONDUCTION E. Portion of cochlear stimulated High Frequency  Lower hair cells Low Frequency  Apical cells F. Loudness - number of elements firing

32. BONE CONDUCTION Conductive mechanism of ME may be by-passed; natural structures stimulated directly through bone conduction Principle behind tuning fork tests

33. Conscious auditory pathway

34. Auditory pathway Sound EAC Middle ear Inner ear eighth nerve lateral lemniscus (medulla) Dorsal & ventral nuclei Inferior colliculus Medial geniculate body (thalamus) Auditory cortex

35. auditory pathway

36. salamat! Pls visit: http://crisbertcualteros.page.tl