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Physiology of hearing
Physiology of hearing
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Physiology Of Hearing



Physiology Of Hearing

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Physiology Of Hearing

  1. 1. PHYSIOLOGY OF HEARING Crisbert I. Cualteros, M.D.
  2. 3. acoustics <ul><li>Sound – a change in pressure (particle displacement)within an elastic medium </li></ul><ul><li>- wave form </li></ul><ul><li>- emanates from a vibrating system </li></ul><ul><li>- travels faster in denser medium </li></ul><ul><li>Intensity – refers to the strength of the sound; the - psychoacoustic equivalent of intensity is loudness </li></ul>
  3. 4. acoustics <ul><li>Decibel - the unit used to measure the intensity of a sound </li></ul><ul><li>- named after Alexander Graham Bell </li></ul><ul><li>- One – tenth of a Bel </li></ul><ul><li>- range of human hearing 0 – 120 dB </li></ul><ul><li>dB = 10 logIm / I ref </li></ul><ul><li>where: </li></ul><ul><li>Im = measure intensity </li></ul><ul><li>I ref = reference intensity </li></ul>
  4. 5. acoustics <ul><li>The decibel scale has the following characteristics: </li></ul><ul><li>1. It is logarithmic and incorporates a ratio </li></ul><ul><li>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) </li></ul><ul><li>3. It is a relative measure (i.e., 0 dB does not indicate the absence of sound) </li></ul><ul><li>4. It is expressed with different reference levels </li></ul>
  5. 6. acoustics <ul><li>Frequency – refers to the number of cycles (complete oscillations) of a vibrating body per unit of time; the psychoacoustic equivalent of frequency is pitch. </li></ul>
  6. 7. acoustics <ul><li>Hertz (Hz) – unit used to measure frequency </li></ul><ul><li>- formerly called cycles per second or cps </li></ul><ul><li>- the human ear is capable of hearing from 20 – 20,000 Hz </li></ul>
  7. 8. acoustics <ul><li>Pure tone – a single frequency sound; rarely occur in nature </li></ul><ul><li>Complex sound – has more than one frequency </li></ul><ul><li>Noise – an aperiodic complex sound </li></ul>
  8. 9. acoustics <ul><li>Resonant frequency – the frequency at which a mass vibrates with the least amount of external force </li></ul><ul><ul><ul><li>External auditory canal - 3000 Hz </li></ul></ul></ul><ul><ul><ul><li>Middle ear - 800 Hz </li></ul></ul></ul><ul><ul><ul><li>Tympanic membrane - 800 – 1600 Hz </li></ul></ul></ul><ul><ul><ul><li>Ossicular chain - 500 – 2000 Hz </li></ul></ul></ul>
  9. 10. PROPERTIES OF SOUND <ul><li>Sound </li></ul><ul><li>External auditory canal </li></ul>
  10. 11. EXTERNAL EAR <ul><li>- funnel-shaped “collector” </li></ul><ul><li>of sounds </li></ul><ul><li>- localization </li></ul><ul><li>pinna </li></ul>
  11. 12. EXTERNAL EAR <ul><li>- directs sound waves </li></ul><ul><li>- 2 to 4 kHz </li></ul><ul><li>- 10 to 15 dB </li></ul><ul><li>- total occlusion hearing loss </li></ul><ul><li>not more than 40 dB </li></ul><ul><li>- earplugs / earmuffs attenuate </li></ul><ul><li>sound <30 dB </li></ul><ul><li>External auditory canal </li></ul>
  12. 13. EXTERNAL EAR <ul><li>- monaural hearing </li></ul><ul><li>- binaural hearing </li></ul><ul><li>cannot localize sounds </li></ul><ul><li>localize sounds </li></ul>
  13. 14. <ul><li>sound </li></ul><ul><li>EAC </li></ul><ul><li>middle ear </li></ul>
  14. 15. MIDDLE EAR <ul><li>air-filled </li></ul><ul><li>- 1 to 2 cc </li></ul>
  15. 16. MIDDLE EAR <ul><li>- handle </li></ul><ul><li> embedded in the TM </li></ul><ul><li>- neck </li></ul><ul><li> tensor tympani inserts </li></ul><ul><li>- head </li></ul><ul><li> articulates with ant. surface of </li></ul><ul><li>incus body in the epitympanum </li></ul><ul><li>malleus </li></ul>
  16. 17. MIDDLE EAR <ul><li>- short process projects backward </li></ul><ul><li>- long incus passes downward </li></ul><ul><li>articulated with head of </li></ul><ul><li>stapes </li></ul><ul><li>incus </li></ul>
  17. 18. MIDDLE EAR <ul><li>- oval window </li></ul><ul><li> </li></ul><ul><li>inner ear </li></ul><ul><li>stapes </li></ul>
  18. 19. - axis of rotation:  line from short process of incus to neck of malleus MIDDLE EAR
  19. 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)
  20. 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) ______________ ______________ =
  21. 22. MIDDLE EAR CONDUCTION B. TRANSDUCER MECHANISM 2. Lever mechanism Length long process malleus Length long process incus ______________________________ =1.3 (2.5 dB)
  22. 23. MIDDLE EAR CONDUCTION <ul><li>B. TRANSDUCER MECHANISM </li></ul><ul><ul><li>3. Phase difference/ Protection of the round window </li></ul></ul><ul><ul><ul><li>release valve </li></ul></ul></ul><ul><ul><ul><li>sound waves striking both windows simultaneously  waves cancel each other </li></ul></ul></ul><ul><ul><ul><li>different location / position ( phase difference ) </li></ul></ul></ul><ul><ul><ul><li> ~ 4 dB change </li></ul></ul></ul>
  23. 24. MIDDLE EAR CONDUCTION <ul><li>B. TRANSDUCER MECHANISM </li></ul><ul><ul><li>4. Natural resonance and efficiency of the outer and middle ear (500 to 3000 Hz) </li></ul></ul>
  24. 25. MIDDLE EAR CONDUCTION <ul><li>B. TRANSDUCER MECHANISM </li></ul><ul><ul><li>5. shape of the tympanic membrane </li></ul></ul><ul><ul><ul><li>Curved cone </li></ul></ul></ul>
  25. 26. Middle ear conduction
  26. 27. INTRINSIC MUSCLES OF THE MIDDLE EAR Tensor Tympani Stapedius
  27. 28. Sound  EAC  Middle Ear  Inner Ear (Footplate)  
  28. 29. INNER EAR CONDUCTION <ul><li>The Stapes moves back and forth at the oval </li></ul><ul><li>window  fluid displacement within the cochlea </li></ul><ul><li>These waves cause up and down displacement of </li></ul><ul><li>the basilar membrane which then pivots around the osseous lamina </li></ul>
  29. 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.
  30. 31. INNER EAR CONDUCTION E. Portion of cochlear stimulated High Frequency  Lower hair cells Low Frequency  Apical cells F. Loudness - number of elements firing
  31. 32. BONE CONDUCTION <ul><li>Conductive mechanism of ME may be by-passed; natural structures stimulated directly through bone conduction </li></ul><ul><li>Principle behind tuning fork tests </li></ul>
  32. 33. Conscious auditory pathway
  33. 34. Auditory pathway <ul><li>Sound EAC Middle ear </li></ul><ul><li>Inner ear eighth nerve lateral lemniscus </li></ul><ul><li> (medulla) </li></ul><ul><li>Dorsal & ventral nuclei Inferior colliculus </li></ul><ul><li>Medial geniculate body (thalamus) Auditory cortex </li></ul>
  34. 35. auditory pathway
  35. 36. <ul><li>salamat! </li></ul><ul><li>Pls visit: </li></ul>