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CHAPTER 8

CHAPTER 8. Peripheral Auditory Nervous System and Haircells. IHC Synapses. OHC Synapses. Efferents have greater surface area than afferents . Transduction-. Inner Hair Cells are the true sensory transducers, converting motion of stereocilia into neurotransmitter release.

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CHAPTER 8

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  1. CHAPTER 8 Peripheral Auditory Nervous System and Haircells

  2. IHC Synapses

  3. OHC Synapses • Efferents have greater surface area than afferents

  4. 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

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

  6. 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

  7. Two Descriptors for Neurons • Afferent (sensory)-- carrying signals toward the brain • Efferent (motor) -- carrying signals from brain to periphery

  8. Afferent & Efferent Neurons

  9. 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

  10. IHC Innervation Pattern

  11. OHC Innervation Pattern

  12. Inner hair cells • Synapse at the base with up to 20 afferent neurons • “Divergence” • Efferents synapse on afferent dendrites under IHCs

  13. IHC activation alters firing rate

  14. Neural Activity • Post-synaptic Potentials-- Local, Variable changes in voltage near synapse • Action Potentials-- Conducted through axon, “all or none,” “spike” • For image of AP’s traveling down an axon: http://bio.winona.msus.edu/berg/ANIMTNS/actpot.htm

  15. An Action Potential (or Spike)

  16. 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

  17. Spike Rate Increases Thru a 30 dB Range

  18. Signs of Peripheral Activation • Otoacoustic Emissions (OAEs) • Cochlear Potentials

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

  20. Recording OAEs

  21. Spectrum of Sound in Ear Canal

  22. OAEs: Measures of Cochlear Health

  23. 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 We’ll talk about 3 of these from the cochlea

  24. 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

  25. Summating Potential (SP) • Is a direct current or DC potential • Lasts for duration of stimulus. SP

  26. Compound Action Potential (CAP) • Summation of APs in large number of VIIIth nerve neurons • following onset (and offset) of stimulus CAP

  27. IHC activation alters firing rate

  28. Afferent neurons have their cell bodies in the Spiral Ganglion (4)

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