Auditory system

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Auditory system

  1. 1. Miguel B. Sebastian Jr. M.D., D.P.P.S. Department of Physiology OLFU
  2. 2.  The Human Ear
  3. 3.  Function of the muscles and ossicles Functions ◦ Protects the cochlea from damaging vibrations by excessively loud sounds ◦ Mask low frequency sounds in loud environments ◦ Decrease persons sensitivity to his or her own speech ◦ Latency – 40 to 80 milliseconds ◦ Contraction of tensor tympani and stapedius dampens the movement of ossicles and decreases the sensitivity of acoustic apparatus ◦ Reduces intensity of sound transmission by 30 – 40 dB
  4. 4.  Impedance Matching Device (tympanic membrane and ossicles) Acoustic impedance higher in fluid-filled cochlea than in air Depends on the following ◦ Ratio of the surface area of large tympanic membrane to that of the smaller oval window ◦ Mechanical advantage of the lever system formed by the ossicle chain
  5. 5.  Eustachian tube – equalizes pressure differences between external ear and middle ear Infection: fluid collects in middle ear  E.T. blocked  pressure differences  pain, displacement of the Tympanic Membrane  rupture
  6. 6.  Sound ◦ Produced by compression and decompression waves that are transmitted in air or in other elastic media such as water
  7. 7.  Sound Frequency ◦ Measured by cycles per second – hertz (Hz) ◦ Travels more slowly than light  Light – 300,000 km/sec (186,000 miles/sec)  Sound – 0.331 – 0.334 km/sec (0.2 miles/sec) – at 20 oC at sea level (↑ with temperature and altitude) - 335 m/sec in air (Berne & Levy)  Speed of sound: Solid > Liquid > Air
  8. 8.  Sound wave ◦ Expressed as Sound Pressure Level (SPL) ◦ Decibel (dB) ◦ SPL = 20 log P/Pr P= sound pressure Pr = reference pressure (0.0002 dyne/cm2) - the absolute threshold for human hearing at 1000 Hz
  9. 9.  Normal Human Ear ◦ Sensitive to pure tone with frequency of 20 to 20,000 Hz ◦ >100 dB – damage the auditory apparatus ◦ >120 dB – pain and permanent damage * As people age, their threshold at high frequency rises thereby reduces the ability to hear such tone (presbycusis)
  10. 10.  Sound waves  cause T.M. to oscillate  Oscillations transmitted to the scala vestibuli  creates a pressure difference between the S.V. and S.T.  displaces the basilar membrane  stereocilia of the hair cells bend. Upward displacement bends the stereocilia toward the tallest cilium (away from the modiolus) depolarizes the haircells. Downward deflection – bends the stereocilia away - hyperpolarized
  11. 11. • apex is wider than the base• tension is higher at the base than at the apex• base vibrate at higher frequency than the apex (frequency analyzer)
  12. 12. • length of the fibers is greater at the apex than at the base• fiber diameter is greater at the base than at the apex• base -- shorter and wider• apex – taller and slender• high –frequency resonance (base), low frequency resonance (apex)
  13. 13.  Ossicular Conduction ◦ Main pathway for normal hearing Air Conduction ◦ Unimportant for normal hearing ◦ Mediated by vibration of round window Bone Conduction ◦ Involves skull bone ◦ Plays a role in transmission of extremely loud sounds
  14. 14.  Presence of one sound decreases an individual’s ability to hear other sounds Due to the relative and absolute refractoriness of previously stimulated auditory receptors and nerve fibers to other stimuli
  15. 15.  Brodmann’s area 22 ◦ Concerned with the processing of auditory signals related to speech ◦ Right side – melody, pitch and sound intensity Planum Temporale ◦ Portion of posterior superior temporal gyrus ◦ Involved in language-related auditory processing
  16. 16.  Conductive Deafness ◦ Due to impaired sound transmission in external and middle ear ◦ Impacts all sound frequencies ◦ Causes:  Plugging of the EAC with cerumen or foreign body  Otitis externa and otitis media  Perforation of eardrum  osteosclerosis
  17. 17.  Sensorineural Deafness ◦ Due to loss of cochlear hair cells ◦ Problems with CN VIII ◦ Lesions within the Central Auditory Pathway ◦ Impairs the ability to hear certain pitches (permanent) ◦ Causes:  Aminoglycosides  Prolonged exposure to noise  Tumors and vascular damage
  18. 18.  Audiometer Tuning Fork
  19. 19. Thanks!!!

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