1. CHAPTER 8 Peripheral Auditory Nervous System and Haircells

3. IHC Synapses

4. OHC Synapses Efferents have greater surface area than afferents

5. 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

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

7. 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

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

9. Afferent Afferent & Efferent Neurons

10. 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

11. IHC Innervation Pattern

12. OHC Innervation Pattern

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

14. IHC activation alters firing rate

15. 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

16. An Action Potential (or Spike)

17. 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

18. Spike Rate Increases Thru a 30 dB Range

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

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

21. Recording OAEs

22. Spectrum of Sound in Ear Canal

23. OAEs: Measures of Cochlear Health

24. 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

25. 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

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

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

28. IHC activation alters firing rate

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