Hearing

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Hearing

  1. 1. Ears & Hearing 10-34
  2. 2. <ul><li>Sound waves funneled by pinna ( auricle ) into external auditory meatus </li></ul><ul><li>External auditory meatus channels sound waves to tympanic membrane </li></ul>Ears & Hearing - Outer Ear Fig 10.17 10-47
  3. 3. <ul><li>Malleus (hammer) is attached to tympanic membrane </li></ul><ul><ul><li>Carries vibrations to incus (anvil) </li></ul></ul><ul><ul><li>Stapes (stirrup) receives vibrations from incus, transmits to oval window </li></ul></ul>Ears & Hearing - Middle Ear continued Fig 10.18 10-49
  4. 4. <ul><li>Stapedius muscle , attached to stapes, provides protection from loud noises </li></ul><ul><ul><li>Can contract & dampen large vibrations </li></ul></ul><ul><ul><li>Prevents nerve damage in cochlea </li></ul></ul>Ears & Hearing - Middle Ear continued 10-50 Fig 10.18
  5. 5. Ears & Hearing - Cochlea <ul><li>Consists of a tube wound 3 turns & tapered so looks like snail shell </li></ul>Fig 10.19 10-51
  6. 6. Ears & Hearing - Cochlea continued <ul><li>Tube is divided into 3 fluid-filled chambers </li></ul><ul><ul><li>Scala vestibuli , cochlear duct , scala tympani </li></ul></ul>Fig 10.19 10-52
  7. 7. Ears & Hearing - Cochlea continued <ul><li>Oval window attached to scala vestibuli (at base of cochlea) </li></ul><ul><li>Vibrations at oval window induce pressure waves in perilymph fluid of scala vestibuli </li></ul><ul><li>Scalas vestibuli & tympani are continuous at apex </li></ul><ul><ul><li>So waves in vestibuli pass to tympani & displace round window (at base of cochlea) </li></ul></ul><ul><ul><ul><li>Necessary because fluids are incompressible & waves would not be possible without round window </li></ul></ul></ul>10-53
  8. 8. Ears & Hearing - Cochlea continued <ul><li>Low frequencies can travel all way thru vestibuli & back in tympani </li></ul><ul><li>As frequencies increase they travel less before passing directly thru vestibular & basilar membranes to tympani </li></ul>Fig 10.20 10-54
  9. 9. Ears & Hearing - Cochlea continued <ul><li>High frequencies produce maximum stimulation of Spiral Organ closer to base of cochlea & lower frequencies stimulate closer to apex </li></ul>Fig 10.20 10-55
  10. 10. Spiral Organ (Organ of Corti) <ul><li>Is where sound is transduced </li></ul><ul><li>Sensory hair cells located on the basilar membrane </li></ul><ul><ul><li>1 row of inner cells extend length of basilar membrane </li></ul></ul><ul><ul><li>Multiple rows of outer hair cells are embedded in tectorial membrane </li></ul></ul>Fig 10.22 10-56
  11. 11. Spiral Organ (Organ of Corti) <ul><li>Pressure waves moving thru cochlear duct create shearing forces between basilar & tectorial membranes, moving & bending stereocilia </li></ul><ul><ul><li>Causing ion channels to open, depolarizing hair cells </li></ul></ul><ul><ul><li>The greater the displacement, the greater the amount of NT released & APs produced </li></ul></ul>10-57
  12. 12. Neural Pathway for Hearing <ul><li>Info from 8th nerve goes to medulla, then to inferior colliculus, then to thalamus, & on to auditory cortex </li></ul>Fig 10.23 10-58
  13. 13. Neural Pathways for Hearing <ul><li>Neurons in different regions of cochlea stimulate neurons in corresponding areas of auditory cortex </li></ul><ul><ul><li>Each area of cortex represents different part of cochlea & thus a different pitch </li></ul></ul>Fig 10.24 10-59
  14. 14. Hearing Impairments <ul><li>Conduction deafness occurs when transmission of sound waves to oval window is impaired </li></ul><ul><ul><li>Impacts all frequencies </li></ul></ul><ul><ul><li>Helped by hearing aids </li></ul></ul><ul><li>Sensorineural ( perceptive ) deafness is impaired transmission of nerve impulses </li></ul><ul><ul><li>Often impacts some pitches more than others </li></ul></ul><ul><ul><li>Helped by cochlear implants </li></ul></ul><ul><ul><ul><li>Which stimulate fibers of 8th in response to sounds </li></ul></ul></ul>10-60
  15. 15. Vestibular Apparatus <ul><li>Provides sense of equilibrium </li></ul><ul><ul><li>=orientation to gravity </li></ul></ul><ul><li>Vestibular apparatus & cochlea form inner ear </li></ul><ul><li>V. apparatus consists of otolith organs ( utricle & saccule ) & semicircular canals </li></ul>Fig 10.11 10-35
  16. 16. Semicircular Canals <ul><li>Provide information about rotational acceleration </li></ul><ul><li>Project in 3 different planes </li></ul><ul><li>Each contains a semicircular duct </li></ul><ul><li>At base is crista ampullaris where sensory hair cells are located </li></ul>Fig 10.12 10-42
  17. 17. <ul><li>Utricle and saccule provide info about linear acceleration </li></ul><ul><li>Semicircular canals, oriented in 3 planes, give sense of angular acceleration </li></ul>Vestibular Apparatus continued Fig 10.12 10-37
  18. 18. <ul><li>Hair cells are receptors for equilibrium </li></ul><ul><ul><li>Each contains 20-50 hair-like extensions called stereocilia </li></ul></ul><ul><ul><ul><li>1 of these is a kinocilium </li></ul></ul></ul>Vestibular Apparatus continued Fig 10.13 10-38
  19. 19. <ul><li>When stereocilia are bent toward kinocilium, hair cell depolarizes & releases NT that stimulates 8th nerve </li></ul><ul><li>When bent away from kinocilium, hair cell hyperpolarizes </li></ul><ul><ul><li>In this way, frequency of APs in hair cells carries information about movement </li></ul></ul>Vestibular Apparatus continued Fig 10.13 10-39
  20. 20. Utricle & Saccule <ul><li>Have a macula containing hair cells </li></ul><ul><ul><li>Hair cells embedded in gelatinous otolithic membrane </li></ul></ul><ul><ul><ul><li>Which contains calcium carbonate crystals (= otoliths ) that resist change in movement </li></ul></ul></ul>Fig 10.14 10-40
  21. 21. Utricle & Saccule continued <ul><li>Utricle sensitive to horizontal acceleration </li></ul><ul><ul><li>Hairs pushed backward during forward acceleration </li></ul></ul><ul><li>Saccule sensitive to vertical acceleration </li></ul><ul><ul><ul><li>Hairs pushed upward when person descends </li></ul></ul></ul>Fig 10.14 10-41
  22. 22. Semicircular Canals <ul><li>Provide information about rotational acceleration </li></ul><ul><li>Project in 3 different planes </li></ul><ul><li>Each contains a semicircular duct </li></ul><ul><li>At base is crista ampullaris where sensory hair cells are located </li></ul>Fig 10.12 10-42
  23. 23. Semicircular Canals continued <ul><li>Hair cell processes are embedded in cupula of crista ampullaris </li></ul><ul><li>When endolymph moves cupula moves </li></ul><ul><ul><li>Sensory processes bend in opposite direction of angular acceleration </li></ul></ul>Fig 10.15 10-43
  24. 24. Neural Pathways for Equilibrium & Balance Fig 10.16 10-44
  25. 25. Nystagmus & Vertigo <ul><li>Vestibular nystagmus is involuntary oscillations of eyes that occurs when spinning person stops </li></ul><ul><ul><li>Eyes continue to move in direction opposite to spin, then jerk rapidly back to midline </li></ul></ul><ul><li>Vertigo is loss of equilibrium </li></ul><ul><ul><li>Natural response of vestibular apparatus </li></ul></ul><ul><ul><li>Pathologically, may be caused by anything that alters firing rate of 8th nerve </li></ul></ul><ul><ul><ul><li>Often caused by viral infection </li></ul></ul></ul>10-45

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