Peripheral Auditory Nervous System and Haircells

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CHAPTER 8

• Peripheral Auditory Nervous System and Haircells

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IHC Synapses
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OHC Synapses

• Efferents have greater surface area than
  afferents

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Transduction-

• Inner Hair Cells are the true sensory
  transducers, converting motion of stereocilia
  into neurotransmitter release.
• Mechanical? Electro-chemical
• Outer Hair Cells have both forward and reverse
  transduction--
• Mechanical ? Electro-chemical
• Mechanical? Electro-chemical

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Sensory Physiology

• The basic unit of the nervous system is the
  Neuron or nerve cell
• Neurons undergo voltage changes-
• SPONTANEOUSLY
• IN RESPONSE TO STIMULATION

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The Neuron

• Dendrites receive synaptic stimulation
  (neurotrans.)
• Action Potential generated in soma near axon
• AP conducted along axon from Node to Node
  (saltatory conduction)
• AP produces release of neurotransmitter at
  terminal boutons

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Two Descriptors for Neurons

• Afferent (sensory)-- carrying signals toward the
  brain
• Efferent (motor) -- carrying signals from brain
  to periphery

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Afferent Efferent Neurons
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4 Types of Cochlear Neurons

• INNER HAIR CELLS
• Multiple (10 to 20) Afferent synapses
• (Efferents synapse on afferent dendrites)
• OUTER HAIR CELLS
• Large Efferent synapses engulf base of cell
• Small ( not very active) Afferent synapses

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IHC Innervation Pattern
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OHC Innervation Pattern
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Inner hair cells

• Synapse at the base with up to 20 afferent
  neurons
• Divergence
• Efferents synapse on afferent dendrites under IHCs

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IHC activation alters firing rate
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Neural Activity

• Post-synaptic Potentials-- Local, Variable
  changes in voltage near synapse
• Action Potentials-- Conducted through axon, all
  or none, spike
• For image of APs traveling down an axon
• http//bio.winona.msus.edu/berg/ANIMTNS/actpot.htm

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An Action Potential (or Spike)
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Action Potentials

• Are generated spontaneously
• At a slow rate by some neurons
• At a faster rate by some neurons
• And occur more frequently with STIMULATION
• Spike rate increases through a range of about 30
  dB

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Spike Rate Increases Thru a 30 dB Range
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Signs of Peripheral Activation

• Otoacoustic Emissions (OAEs)
• Cochlear Potentials

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Otoacoustic Emissions

• Low-level sounds produced by the cochlea and
  recordable in the external ear canal.
• Spontaneous
• Click-evoked
• Distortion Product
• Stimulus Frequency

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Recording OAEs
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Spectrum of Sound in Ear Canal
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OAEs Measures of Cochlear Health
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Cochlear Potentials

• Resting Potentials voltages which exist without
  external stimulation
• e.g., Endolymphatic Potential,
• Cell Membrane Potential
• Stimulus-Related Potentials voltages occurring
  in response to sounds
• Well talk about 3 of these from the cochlea

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Cochlear Microphonic

• Least valuable from a clinical standpoint.
• Is an alternating current (AC) response that
  mirrors the waveform of low to moderately intense
  sound stimuli
• Appears to arise from outer hair cells in the
  basal-most turn of the cochlea

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Summating Potential (SP)

• Is a direct current or DC potential
• Lasts for duration
• of stimulus.

SP
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Compound Action Potential (CAP)

• Summation of APs in large number of VIIIth nerve
  neurons
• following onset (and offset) of stimulus

CAP
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IHC activation alters firing rate
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Afferent neurons have their cell bodies in the
Spiral Ganglion (4)