Auditory System

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Auditory System

• Consists of outer, middle, and inner ear
  sections
• Outer captures sound
• Middle ear amplifies and impedance matches
• Inner ear transduces and encodes

Outer Ear
Inner Ear
Middle Ear
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Sound waves
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The middle ear...

• Ossicles act as levers to
• transform air borne vibrations
• into fluid borne vibrations.
• Cochlea is a closed system, with
  incompressible fluid.
• What goes in, must come out.
• Perilymph (high Na, low K - like CSF) in scala
  vestibuli and scala tympani.
• Endolymph (low Na, high K - like intracellular
  fluid) in scala media.

• Conductive hearing loss
• Otitis media middle ear infection often due to
  eustachian tube restriction
• Otosclerosis tissue overgrowth restricting
  ossicle (stapes) movement

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Cochlear mechanics

• Basilar membrane
• In-out motion of stapes
• produces up-down motion of basilar membrane

• Resonant frequency varies
• along length
• Sharp frequency tuning

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Frequency specificity on basilar membrane

• Resonant frequency varies
• along length
• Most sounds have complex waveforms

• In mammals, cochlea is not straight, but
  coiled

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The organ of Corti...

• has two groups of receptors (hair cells)
• Inner hair cells (single row) receive 95 of all
  afferents
• Outer hair cells (3 rows) receive few afferents,
  but all efferents

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Hair cells are mechanoreceptors

• Stereocilia gated channels that open/close as
  stereocilia displace

Tip link
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Hair cells are mechanoreceptors that are
spatially tuned
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Hair cells are frequency selective

• Each cell responds with lowest threshold to a
  characteristic or best frequency (CF)
• Frequency tuning uses twomechanisms
• Basilar membrane vibration
• Resonant frequency of hair cell (membrane
  channels)

Base
Apex
Hair cell tuning curves Threshold vs. frequency
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Outer hair cells function to amplify/protect

• Cochlear amplification/protection
• Mechanical response of hair cell to
• Membrane voltage change
• Increase sensitivity of inner hair cells
• Sharpen tuning
• Regulated by efferents
• Produces otoacoustic emissions

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Hair cells are easily damaged

• Loud sounds (gt110db) explosions, gun shot
• Continuous exposure (gt95 db) lawn mower,
  musical amplifier
• Ototoxic antibiotics (aminoglycosides)
• Cancer treatment drugs (Cisplatnin)

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Auditory afferents are frequency tuned

• Each afferent responds with lowest threshold to
  a characteristic or best frequency (CF)
• Place code array of CFs along tonotopic map
• Rate code fibers rate proportional to sound
  frequency

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Cochlear implants

• Implanted through mastoid into scala tympani
• Only covers basal 40 50 of cochlea
• Generally destroys basilar membrane, how then
  does it work?
• Hint stimulating contacts pointing toward the
  spiral ganglion cells

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Central auditory processing
Main goals first where, then what
Sound Localization (where it is) Based on
comparisons between ears Differences in timing,
intensity
Sound Identification (what it is) Emphasizes
species-specific vocalizations Poorly understood
for humans, monkeys, Best understood for bats,
songbirds
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Central auditory pathway
Sound Localization Timing differences (low
frequencies, lt 1 kHz), good discrimination lt1
deg Intensity differences (high frequencies,
gt 3 kHz), good discrimination lt1 degBetween 1
3 kHz, both ITD and IID, poorer discrimination gt3
deg,
Auditory Cortex A1
Medial geniculate
Inferior colliculus
Cochlear nuclei
Dorsal CN
Ventral CN
Lateral lemniscus
Superior olivary complex
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Brainstem auditory evoked response (BAER)

• Clinical neurophysiology diagnostic
• Evoked potentials elicited by repetitive clicks
• Peaks at regular latencies are correlated with
  structures of auditory pathway
• Shifts in latency or changes in amplitude
  indicate lesion

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Cochlear nuclei

• CN is tonotopically organized
• Bushy cells (ventral or ACN) project bilaterally
  to SOC sound localization
• Multipolar cells project only to contralateral
  IC sound intensity
• Octopus cells project to contralateral IC speech
  frequency
• Pyramidal cells project monaurally to
  contralateral IC ?

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Central auditory pathway - II

• Monaural path
• Binaural path

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Auditory cortex

• Location mainly in Sylvian fissure superior
  temporal gyrus
• AI area 41, AII area 42 Tonotopy arranged in
  isofrequency columns
• Area 22 includes speech processing, damage
  results in Wernickes aphasia
• Areas 44 45 (Brocas aphasia) processes
  expressive speech and language

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Auditory cortex

• Wernickes area includes speech comprehension
  processing
• Brocas area processes expressive (production)
  speech and language