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Lecture 4

Lecture 4.docx

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Matthias Niemeier

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Lecture 4  the auditory system and the vestibular system Short answer question  describe the three pathways of the visual system that are differently specialized for motion, color and shape o magnocellular pathway  motion o parvocellular pathway  fine detail o koniocellular pathway  non specific Introduction  sense of hearing o audition  detect sound  perceive and interpret nuances  sense of balance o vestibular system  head and body location  head and body movements Hearing and balance  why so close together? o lateral line organs  small pits or tubes o function  sense vibration or pressure changes The nature of sound  sound o audible variations in air pressure, water pressure etc o range  20 Hz to 20000 Hz o pitch  perpetual aspect of frequencies  amount of cycles in an amount of space  two aspects of sound may occur at the same intensity but at different frequencies o perceived as louder than the other o loudness  perpetual aspect of intensity  amplitude  two aspects of sound may occur at different intensities but be perceived as having the same loudness  sound frequency o number of cycles per second expressed in hertz  cycle o distance between successive compressed patches  high frequency o more cycles in same amount of space Overview  the structure of the auditory system  physiology of the cochlea  auditory pathways  how the brain encodes intensity and frequency  sound localization  the auditory cortex The structure of the auditory system  auditory system diagram o slide 9  pinna  auditory canal  tympanic membrane  ossicles  oval window  cochlea Early auditory pathway stages  sound waves  tympanic membrane  ossicles  oval window  cochlea fluid  movements of basilar and tectorial membranes o sheering forces  sensory neuron response The middle ear  components of the middle ear diagram o slide 11  ossicles  malleaus  incus  stapes o foot plate What is the function of the ossicles?  sound force amplification o pressure = force / area  20x greater pressure at oval window than tympanic membrane  moves fluids inside the cochlea The attenuation reflex  response where onset of loud sound causes tensor tympani and stapedius muscle contraction  functions o adapt ear to loud sounds  avoid saturation of sound  damage o filter out low frequency  understand speech better  which occurs at higher frequencies o reduce sound of our own voice  which occurs at lower frequencies (relative) The inner ear  anatomy of the cochlea diagram o slide 14  scala  vestibuli o reissner's membrane below  media o tectorial membrane in middle o basilar membrane below  tympani o basilar membrane above  basilar membrane  organ of corti sits on this membrane  analogous to flippers for scuba diving o thick bottom (more posterior)  higher frequencies o wide top (more anterior)  lower frequencies  perilymph o fluid in scala vestibuli and scala tympani  endolymph o fluid in scala media  high K+  low Na+  endocochlear potential o endolymph electric potential 80 mV more positive than perilymph Basilar membrane in an uncoiled cochlea  apex o 5x wider o 100x less stiff Physiology of the cochlea  pressure at oval window pushes perilymph into scala vestibuli o round window membrane bulges out  endolymph movement bends basilar membrane near base o wave moves towards apex  why is basilar membrane more stable at a certain point?  frequency of sound o lower frequencies of sound toward the front o higher frequencies of sound toward the back  results in more fluctuation of basilar membrane Basilar membrane; place code  slide 18 o different frequencies affect different locations of the basilar membrane The organ of corti and associated structures  diagram o slide 19  organ of corti  rods of corti  inner hair cells   outer hair cells  basilar membrane  as it moves up and down o organ of corti moves tectorial membrane up and down  imposes shearing forces  move left and right relative to one another Transduction by hair cells  sound o basilar membrane upward and towards modiolus   reticular lamina up and stereocilia bends outwards  TRPA1 channels o tip links (elastic filaments)   Ca2+ entry   neurotransmitter (glutamate?) o depolarization  stereocilia move towards the longer stereocilia o hyperpolarization  stereocilia move towards the shorter stereocilia  diagram o slide 21 The innervation of hair cells  one spiral ganglion fiber o one inner hair cell o numerous outer hair cells Amplification by outer hair cells  function o sound transduction  motor proteins (prestin?) o change length of outer hair cells  efferences Study: Mammano and Ashmore  introduction o outer hair cells  amplification o sense displacements and feed back forces to enhance basilar membrane motion o reverse transduction o are forces large enough to move basilar membrane?  methods o stimulus pipette  scala media o laser interferometer  reticular lamina and basilar membrane o acronyms  IHC  inner hair cell  TM  tectonic membrane  OHC  outer hair cell  TC  triangle of corti  RL  reticular lamina  SM  scala media  BM  basilar membrane  ST  scala tympani  results o stimulation near OHC  provides greatest response  depolarization o  pulls reticular lamina down  hyperpolarization o  pushes reticular lamina up o diagram  slide 26  depolarization  basilar membrane displaced towards scala media  reticular lamina displacement opposite  voltage dependent  BM displacements accompanied by dampened oscillations  analysis  the more stimulation o  the more depolarized  the less stimulation o  the more hyperpolarized o pharmacological control experiment  superfusion with 10 mM sodium salicylate  diminish electromotility in OHCs o 50% reduction in basilar membrane response  discussion o OHC  shorten with depolarization o opposite displacements of reticular lamina and basilar membrane consistent with a model with 2 degrees of freedom o displacements of ~1 nm near auditory threshold Auditory pathways  central auditory processes o auditory pathways  more complex than visual pathways  anatomy  auditory nerve fiber  dorsal cochlear nucleus  ventral cochlear nucleus  superior olive
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