Physiology of Special Sense Cochlear mechanism

1. Cochlear mechanism

2. In order for sounds to be
audible it has to be
transmitted to the inner ear

3. Inner ear is located
in petrous part of
temporal bone

4. Inner ear
has two
parts

5. Cochlea is
coiled tube

6. Cochlea is composed
of bony and
membranous part

8. Inner ear has receptors

9. Cochlea is
composed of 3
tubes

10. Scala Tympani

11. Scala Vestibuli

12. Scala Media

13. Inner ear is connected to middle ear via OW

14. We can understand the cochlea by uncoiling it

15. Reissner membrane separate SV to SM and Basilar membrane separates ST to SM

16. Perilymph in scala vestibuli and tympani and Endolymph in scala media

21. Sound wave transmission from middle to inner ear

22. • Soundwave
from O.W –
perilymph of
vestibuli

24. Basilar membrane
It is not uniform the whole length

25. Basilar membrane
fibers
• Length increases
• Diameter decreases

30. Organ of corti rests on basilar membrane

31. Organ of corti composition

32. Inner and
outer hair
cells in organ
of corti

36. • Inner hair cells are
arranged in single
row
• Outer hair cells in 3-5
rows

38. ~ 100
Stereocilia
are present on
hair cells

39. Note: the lenghth of stereocilia- largest stereocilia is away from modiolus

40. Tiplinks on Stereocilia

42. Tiplink connected to mechanically gated Potassium
channel

44. • Tips of
Stereocilia
embedded in
tectorial
membrane
• Reticular
lamina

46. Sound wave moves basilar membrane up and down

47. Reissner
membrane is so
thin as if it doesnot
exist

48. When basilar memebrane moves, whole organ of corti moves as single unit

55. Channels on
sterocilia

57. Basilar membrane
movement moves
hair cells

58. Movement of
stereocilia causes
depolarization or
hyperpolarization

62. • The hair cell is synapsed to nerve
fiber at its base

63. 90-95% synapse
occurs on inner hair
cells

64. Note: Number of
synapses on inner
hair cels

67. Fluid in scala vestibuli, tympani and media

69. • Endolymph is formed by
stria vascularis- in outer
wall of scala media-
Vascular layer
Capillaries

70. 8

71. Endocochlear
potential

73. 7
150

76. Potassium in scala
media will flow
into the hair cells
when potassium
channels open

78. Determination of frequency

79. Place principle
• Each Hair cell occurs at localized position in cochlea
• Each innervated by cochlear nerve fibers
• Specific fibers of VIII nerve sense specific frequency
• Fibers that innervate hair cell at base of basilar membrane carry higher
frequency
• So each location within nerve respond best to particular frequency
• This arrangement is tonotopic- the orderly spatial arrangement of neural
elements corresponding to the separation of different frequencies.

82. Volley principle
• For sounds below
2000 Hz

86. Firing rate hypothesis
• As the stimulus gets more
intense, the basilar
membrane vibrates at a
greater amplitude
• More amplitude  more
depolarization

87. Number of neurons hypothesis
• More hair cells
stimulate more
no. of neurons