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1) Physical properties of sound 2) Peripheral mechanisms in audition ear structure

Auditory System. 1) Physical properties of sound 2) Peripheral mechanisms in audition ear structure transduction 3) Coding. 1. Functions of the Auditory System. 1) Sound identification (What is it? ) Frequency Intensity 2) Sound localization (Where is it?)

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1) Physical properties of sound 2) Peripheral mechanisms in audition ear structure

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  1. Auditory System 1) Physical properties of sound 2) Peripheral mechanisms in audition ear structure transduction 3) Coding 1

  2. Functions of the Auditory System • 1) Sound identification (What is it? ) • Frequency • Intensity • 2) Sound localization (Where is it?) • Interaural timing differences • Interaural frequency differences • 3) Communication 2

  3. 10% of the population has hearing disorders 1 in 3 people over the age of 60 has hearing loss 40% of the cells that detect sound are destroyed by age 65 3

  4. Physical Properties of Sound Movement of a single pulse of sound Sound is energy produced by vibrating bodies, which produces a disturbance in air molecules. The disturbance travels as a longitudinal wave, which has a region of condensation (close together) and a region of rarefaction (far apart). Region of condensation Region of rarefaction 4

  5. Properties of Sound Speed: In air, the speed of sound is 344m/s. Frequency: The number of sound pulses that travel past a fixed point in a second. Species - Frequency Range Humans 20 - 20,000 Hz Bats 1000-100,000 Hz Pitch: Perception of frequency. High frequency sound is heard as a high pitch. Intensity: Amount of air pressure. Loudness Perception of intensity. Loudness proportional to log (Intensity) Normal breathing is about 10 dBs A typical conversation is 60 dB A jet taking off is 140 dB 5

  6. Sound Illusion: Doppler Shift The frequency of sound that reaches our ears changes when the sound source is moving. The change in perceived pitch due to movement is known as the Doppler shift. 6

  7. THE EAR Outer--- funnels sound; converts sound into physical vibration Middle---transmits vibrations to the inner ear Inner---converts fluid movements into neural firing 7

  8. The EAR OUTER MIDDLE INNER 8

  9. THE OUTER EAR (1) Deflects sound towards auditory canal (external auditory meatus) (2) Assists in sound localization (3) Converts sound to physical vibration External Auditory Meatus Tympanic Membrane 9

  10. The EAR MIDDLE 10

  11. Incus Malleus Stapes Tympanic Membrane Function of Middle Ear 1. Three bones in the middle ear transmit the signal malleus- hammer incus-anvil stapes-stirrup 2. The middle ear amplifies signal by increasing the pressure on the oval window 3.Inner ear muscles provide adjustable intensity control(dampen sounds) 11

  12. INNER EAR 12

  13. Inner Ear: Chambers of the Cochlea Scala vestibuli-contacts oval window Scala media-enclosed by other 2, contains endolymph Scala tympani-in contact with round window 13

  14. The Cochlea Unwound Scala Vestibuli Oval Window Helicotrema Scala Media Scala Tympani Round Window 14

  15. Fluid movement in “unwound” cochlea • Fluid flows from oval window to round window Fluid path taken by wavelength outside of audible frequency Fluid path taken by wavelength within audible frequency 15

  16. Cochlear Representation Of Sound - Theories Sound Frequency Cochlear Representation Place theory: Frequency is represented by the location of the mechanical response. Low frequency sounds are closer to the helicotrema. High frequency are closer to the oval window Frequency Theory: This theory postulates that the cochlea vibrates in synchrony with the sound 16

  17. Georg Von Bekesy 1961 Nobel prize Von Bekesy studied cochlear response by opening a window to the cochlea and observing the effect of frequency on movement of the basilar membrane. 17

  18. Discovery One: Basilar membrane has very tiny vibrations 18

  19. Discovery Two: Different sounds produce vibrations at different places Sound caused a traveling wave, with a maximum point of deflection that varied as a function of frequency. Tuning Curves for various frequency signals Low Frequency Deflection Distance from Stapes High Frequency 19

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  21. Resonance properties of basilar membrane tune it to different frequencies BasalApical Thick Thin Stiff Floppy High frequency Low frequency stapes Basilar membrane helicotrema A TONOTOPIC MAP!!! 21

  22. Active Response is caused by outer hair cells, which inject energy back into vibration of cochlear membranes Shape of Cochlear Response to Sound Actual Response Difference between actual response and response predicted by von Bekesy is the active response. Response seen by Von Bekesy 22

  23. Transduction of Sound by Hair Cells Structure of Organ of Corti Tectorial membrane Inner Hair Cells Outer Hair Cells Basilar Membrane 23

  24. The hair bundle of hair cells 24

  25. How do hair cells sense mechanical stimulation? 60 dB causes 10 nm deflection 25

  26. Tip Link Hypothesis of Transduction 1) The stereocilia are connected together through tip links. The tip links end on gating springs. 2) Fluid movement in one direction puts pressure on gating links, leading to ion channels opening. 3) Fluid movement in opposite direction reduces pressure on gating links, leading to ion channels closing. 26

  27. Tip Link Hypothesis of Adaptation • Channel connected to a • sliding motor • Motor moves channel, • relieves tension on tip link • Allows the channel to • rapidly adapt 27

  28. The channel is a potassium channel!!!! “normal” cell hair cell Na K extracellular 0 mV +80 mV K K -45 mV -45 mV 28

  29. Hearing aids or cochlear implants? What’s the difference? 29

  30. Beyond the Ear…How sound is encoded in the brain • Basics of the auditory pathways • Properties of higher order auditory cells 30

  31. Auditory Pathways- The highlights Parallel pathways, divergence, crosstalk 31

  32. Coding of Auditory Information Lower Levels (cochlea, auditory nerve, cochlear nuclei) Tonotopic Organization Superior Olive---Sound location receives input from both ears Interaural timing differences---sound reaches one ear first, determine sound location by the difference in time Interaural frequency differences--- sound frequency gets distorted by head, another way to determine location 32

  33. Processing of Location in Inferior Colliculus • Cells show varying kinds of binaural interactions. • Some show an excitatory response to input to both ears (EE cells) • Others show excitation to input to one ear, and inhibition • to input to the other ear. • Interaural phase difference cells fire maximally when the • input to the two ears is out of phase by a specified amount. • Cells sensitive to temporal order of input to two ears. 33

  34. Major concepts for hearing • Ear is specialized to detect sounds • Cochlea has a tonotopic map • Hair cells transduce sound information quickly • Direct gating of ion channels • Rapid adaptation • Brain retains maps of different frequencies • Brain computes sound location by differences in • frequency and timing between the two ears 34

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