Sound Transduction 2

1
Sound Transduction 2

• Or how my phase got all locked up
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2
Previous lecture

• Inner ear -gt Cochlea -gt Scala media
• Scala media
• Endolymph filled chamber
• Surrounded by perilymph in the scala vestibuli
  scala tympani

3
Scala Media where it all happens

• Organ of Corti
• Organ o Corti contains
• Basilar Membrane (base)
• Tectorial Membrane
• Inner Hair Cells
• Outer Hair Cells
• Hair cells embedded in Bas. Membrane
• Outer Hair Cells contact Tect. Membrane

4
Basilar Membrane

• Properties of the Basilar Membrane
• Apex thin and stiff, Base broad and flexible
• Standing Waves
• Upward spread of masking
• Why do higher Frequencies get masked by lower
  frequencies?

5
Why does it matter that the Basilar Membrane
moves?

• Hair cell magic
• Outer Hair Cells
• 12,000 in three rows
• Afferent and Efferent connections
• Muscle fiber passive motion
• Inner Hair Cells
• 3,500 in single row
• Afferent connection
• Passive Motion

6
Actual Transduction!

• Wave along Basilar Membrane
• Causes inner hair cell shearing
• Shearing opens channel
• Endolymph in Scala Media attracts perilymph in
  Scala Tympani
• Change in electrostatic gradient within nerve
  cell
• Mechanical Electrical energy

7
What are the outer hair cells doing?

• Outer Hair cells motile embedded in Tectorial
  Membrane
• Theory 1. Change motion of Basilar Membrane
• Theory 2. Change position of Tectorial Membrane
• Opposite predictions for behavioral response of
  outer hair cells

8
What does the ear do with the Electrical Energy

• Theory 1 Place coding of frequency
• Position along basilar membrane
• Tonotopic map cochlea -gt cortex
• Why is this a good theory?
• Basilar membrane as tuned resonator (Bekesy,
  1960)
• Why is this an insufficient theory?
• Missing Fundamental Demo

9
Coding Sound (cont)

• Frequency Theory
• Synchronous firing rate relative to acoustic
  vibration
• Why is this a good theory?
• Phase Locking (Wever, 1970 Seeback, 1941)
• What is the problem?
• Firing rate of a neuron 1 1.5 kHz max
• Hearing 0.2 20 kHz
• But Volley Principle?

10
The Rest of the Auditory Pathway

• Cochlea lt-gt Cochlear Nerve
• Cochlear Nerve has tonotopic organization
• Coch. Nerve -gt Olivocochlear bundle x2
• Ipsi/contralateral projection
• Olivocochlear bundle -gt lateral lemniscus
• Efferent Afferent projections
• Lemniscus -gtInferior Colliculus
• Lemniscus detects binaural activity, interaural
  processing
• Inferior Colliculus -gt Medial Geniculate Nucleus
• Inferior Collic. Tightens tonotopic map
• MGN -gt Auditory cortex
• MGN tonotopic/binaural/initial multisensory
  input
• Auditory Cortex -gt Everywhere
• Primary auditory processing, speech center, etc.

11
Big Picture

• Outer Ear Spectral filter for direction
• Middle Ear Attacks impedance problem
• Inner Ear Converts sound to nerve impulse
• Midbrain (olivocochlear bundle) Bilateral
  effects
• Cortex Higher-level auditory processor