Auditory Transduction (2002) — Transcript

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1. 0:01(orchestra tuning)
2. 0:25(conductor taps podium)
3. 0:30The sense of hearing is accomplished by a process known as "auditory transduction."
4. 0:35The ear converts sound waves in the air into electrical impulses, which can be interpreted by the brain.
5. 0:42As sound enters the ear, it passes through the external auditory canal, where it meets the tympanic membrane.
6. 0:51The tympanic membrane then vibrates in response to the sound.
7. 0:56(orchestral music plays)
8. 1:00Sounds of a lower pitch, or "frequency", produce a slower rate of vibration.
9. 1:05(low-pitched sound plays)
10. 1:06And sounds of a lower volume, or "amplitude", produce a less dramatic vibration.
11. 1:11(quiet, low pitched sound plays)
12. 1:12Higher frequency sounds produce faster vibrations.
13. 1:16(high-pitched sound plays)
14. 1:18The tympanic membrane is cone-shaped, and articulates with a chain of three bones, called the "auditory ossicles".
15. 1:26They consist of the malleus, ...
16. 1:29...the incus, ...
17. 1:31...and the stapes.
18. 1:34The movements of the tympanic membrane vibrate the ossicles...
19. 1:37...passing on the information of frequency and amplitude.
20. 1:45The three bones pivot together on an axis, shown here in red.
21. 1:50(rhythmic, repetitive sound plays)
22. 1:56The pivotal axis is due to a series of ligaments which hold the bones in place within the middle ear cavity.
23. 2:03The anterior malleal ligament...
24. 2:07...and the posterior incudal ligament are of particular importance for the pivotal axis.
25. 2:13Two structures which normally obscure this view of the middle ear have been removed.
26. 2:17They are: the chordae tympani nerve...
27. 2:20...and the tendon of the tensor tympani muscle.
28. 2:24Through the ossicles, the vibrations of the tympanic membrane are transferred to the footplate of the stapes.
29. 2:31(music plays)
30. 2:39The stapes moves with a piston-like action, which sends vibrations into a structure called the "bony labyrinth".
31. 2:51The labyrinth is filled with a fluid called "perilymph".
32. 2:56If it were a completely closed and inflexible system...
33. 2:59...the movement of the stapes would be unable to displace the perilymph...
34. 3:03...and therefore, unable to send vibrations into the bony structure.
35. 3:08Due to the flexibility of a small membrane called the "round window",...
36. 3:11... the stapes' movement can displace the perilymph, allowing vibrations to enter the labyrinth.
37. 3:18(music plays)
38. 3:26The corridor leading to the round window is found in the spiral portion of the bony labyrinth...
39. 3:30...known as the "cochlea".
40. 3:34Vibrations produced by the stapes are drawn into the spiral system...
41. 3:39...and return to meet the round window.
42. 3:46The portion of the spiral passage in which vibrations ascend to the apex of the cochlea...
43. 3:50... is called the "scala vestibuli".
44. 3:55The descending portion of the passage is called the "scala tympani".
45. 4:01A third structure, called the "cochlear duct", is situated between the scala vestibuli and the scala tympani.
46. 4:09The cochlear duct is filled with a fluid called "endolymph"...
47. 4:13...and when viewed in cross-section, the membranes separating the two fluid filled systems are visible.
48. 4:19They are: "Reissner's membrane"...
49. 4:23...and the "basilar membrane".
50. 4:25The membranes are flexible, and move in response to the vibrations traveling up the scala vestibuli.
51. 4:32The movements of the membranes then send vibrations back down to the scala tympani.
52. 4:42A specialized structure, called the "organ of Corti"...
53. 4:45...is situated on the basilar membrane.
54. 4:49As the basilar membrane vibrates, ...
55. 4:51..., the organ of Corti is stimulated,...
56. 4:53which sends nerve impulses to the brain via the cochlear nerve.
57. 5:00The actual nerve impulses are generated by specialized cells within the organ of Corti...
58. 5:04...called "hair cells."
59. 5:07The hair cells are closely covered by a structure called the "tectorial membrane".
60. 5:14As the basilar membrane vibrates...
61. 5:16...the tiny clusters of hairs are bent against the tectorial membrane...
62. 5:20...triggering the hair cells to fire.
63. 5:25The entire basilar membrane does not vibrate simultaneously.
64. 5:30Instead, specific areas along the basilar membrane move variably...
65. 5:34... in response to different frequencies of sound.
66. 5:38Lower frequencies vibrate the basilar membrane closer to the apex of the cochlea...
67. 5:43...whereas higher frequencies produce vibrations closer to the base.
68. 5:50This arrangement is known as "tonotopic organization".
69. 5:53(music plays)
70. 6:02Together, this sequence of events is responsible for our acoustic perception of the world around us.
71. 6:09(music plays)