The Peripheral Auditory System

1. The Peripheral Auditory System George Pollak Section of Neurobiology

2. 1 • Hair cells, the transducers of the auditory system, and how they work. stereocillia of inner hair cells stereocillia of outer hair cells Organ of Corti Basilar membrane

4. stereocilia on one hair cell

5. 9

9. Potential difference between Endolymphand cell interior Potential difference between Perilymph and cell interior Kout Kout endolymph = 0mV = ~-70mV Ek= 58 log Ek= 58 log Kin Kin Hi K+ Lo Na+ Hi K+ Lo Na+ Hi Na+ Lo K+ perilymph

10. -45 mV

11. Hi K+ low Na+ Hi K+ low Na+ Hi K+ low Na+ Hi Na+ low K+ Hi Na+ low K+

12. small leakage of K+ into cell No K+ into cell K+ into cell -45 mV -70 mV

13. Next, we are going to build a cochlea

14. Stapes Basilar membrane

15. Sound is changed from a pressure wave in the air into mechanical movements on the basilar membrane

19. round window

20. Traveling waves on basilar membrane oval window round window

21. The structure of the basilar membrane causes it to perform a frequency to place transformation

22. Apex responds maximally to low frequencies flexible wide and thin Stiff Narrow and thick Base responds maximally to high frequencies Basilar Membrane has continuously changing dimensions along its length

24. Basilar membrane converts frequency to a place of maximal response

25. Frequency-to-Place Transformation in the Cochlea

26. The motion on the basilar membrane causes shearing of the cilia on hair cells and thereby causes the hair cells to depolarize and hyperpolarize in response to sound

27. Organ of Corti Basilar membrane

28. shearing of stereocillia Organ of Corti basilar membrane basilar membrane

31. Why are there two types of hair cells?

33. 98% of the fibers that project into the central auditory system are innervated by inner hair cells!! 98%

34. What are the outer hairs doing? Answer: they act as amplifiers of the mechanical motion of the basilar membrane generated by sound

35. release of transmitter Hi K+ K+ ----- depolarization hyperpolarization

36. Evoked mechanical responses of isolated cochlear outer hair cells. _ + _ + _ + Electromotility: OHC can change length in response to voltage change Direct evidence of an active mechanical process in the organ of Corti depolarized hyperpolarized

37. Dancing hair cell 42

38. Outer hair cells are the only cells in the body that express prestin. Even inner hair cells do NOT have prestin. + + + + + + + +

39. Show movie of how Prestin works

41. Positive feedback loop Basilar membrane motion Sound stimuli Hair bundle deflection Change in length of hair cells Membrane potential change OHC IHC Sensory signal transmission

42. Normal response with cochlear amplifier Apex base Basilar membrane response without cochlear amplifier Apex base

43. How motion of basilar membrane generates tuning curves in auditory nerve fibers and thereby imparts frequency selectivity to auditory nerve fibers

45. 5 50 dB SPL 6 7 8 10 11 9 Frequency ( kHz) base apex 6 kHz 60 7 kHz 50 40 Intensity (dB SPL) 8 kHz 30 20 9 kHz 10 10 kHz

46. 30 dB SPL 5 best frequency 6 7 8 10 11 9 Frequency ( kHz) Tuning Curve The most basic feature of an auditory neuron base apex 6 kHz 60 7 kHz 50 40 Intensity (dB SPL) 8 kHz 30 20 9 kHz 10 10 kHz

48. tuning curves in animals with no outer hair cells or in animals without prestin gene tuning curves in normal animals high Sound intensity low low high frequency

49. How is the tonotopic organization that was first established on the basilar membrane preserved in in the central auditory system?

50. Auditory cortex Medial geniculate Medial geniculate Inferior colliculus Inferior colliculus Cochlear nucleus Cochlear nucleus Superior olive Superior olive Auditory nerve Auditory nerve Cochlea Cochlea Flow of Information Along the Central Auditory Pathway