7. Cochlear Functions Transduction- Converting acoustical- mechanical energy into electro-chemical energy. Frequency Analysis-Breaking sound up into its component frequencies

8. 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

9. Hair Cell Activation Involves Ion Flow into cell Through channels in the stereocilia Bending stereocilia causes # of open channels to change. Toward Modiolus = Fewer channels open Away from Modiolus = More open

10. Ion Channels are opened by “TIP LINKS” Tip Links connect tip of shorter stereocilia to the side of a stereocilium in the next taller row Bending toward taller rows pulls tip links Bending toward shorter rows relaxes tip links

11. Tip Links

12. Resting (or Membrane) Potentials Inner Hair Cell = - 45 mV Outer Hair Cell = - 70 mV

13. Stereocilia bent toward tallest row Potassium flows into cell Calcium flows into cell Voltage shifts to a less negative value More neurotransmitter is released

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

15. The Neuron

16. 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

17. An Action Potential (or Spike)

18. 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

19. Spike Rate Increases Thru a 30 dB Range

20. Cochlear Mechanics Passive: Bekesy’s Traveling Wave Theory Active: Outer Hair Cell Motion enhances stimulation of inner hair cells

21. The Traveling Wave BASE APEX

22. Bekesy’s Theory describes Passive Mechanics Based on work in “dead” cochleae Highly damped -- not sharply tuned Active Undamping occurs in live and healthy cochleae Like pumping on a swing--adds amplitude

23. The Active Component Adds to Bekesy’s Traveling Wave

24. The Active Component Improves Sensitivity for soft sounds Improves frequency resolution

25. Frequency Tuning Curves Show these Effects = plots of response threshold as a function of frequency They have a characteristic shape sharp tip (shows best sensitivity at one freq) steep high frequency tail shallow low frequency tail

26. Tuning Curves Passive Only Active + Passive

27. More on Tuning & Tuning Curves: Frequency of “tip” is called the CHARACTERISTIC FREQUENCY Can be seen for:  basilar membrane,  hair cells,  nerve cells

28. Tonotopic Organization Mapping of Characteristic Frequency by place in the auditory nervous system In the Auditory portion of cranial nerve VIII the map is spiral-shaped with highest CF neurons on the outside, lowest CF neurons at the center.

29. Head-Related Effects Head-Baffle--the mere presence of your head alters the sound field. Head Shadow - loss of energy at far ear for frequencies above approx 2000 Hz

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

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

32. Recording OAEs

33. Spectrum of Sound in Ear Canal

34. OAEs: Measures of Cochlear Health

35. 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

36. 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

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

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