physiology of different parts of the ear and auditory cortex

1. Physiology of Hearing
Dr. Dina Merzeban
lecturer of Medical Physiology
Faculty of Medicine - Fayoum University

2. 1- Wavelength.
Velocity of sound:
In Air ≈ 340 m/s
Waves of compression and
rarefaction
Sound

3. 2- Frequency (Pitch)
Our ear can hear
frequencies between
20 – 20’000 Hz.
Average speech
frequency: 2000 – 5000
Hz

4. 3- Amplitude: Intensity or Loudness
change in pressure as
the sound wave passes
by a point
Measured by Decibel
(dB)
Intensity of sound (dB) = 20 Log
Psound
Pspl
Sound
Pressure
level

5. Sound Intensity
(dB)
Index
0 Threshold of hearing
40 Whisper
60 Normal conversation
80 Traffic noise
100 Pneumatic drill
120 Jet plane
Above 140 Pain and damage to auditory
receptors

8. Function of the External ear
1- Collection of sound stimuli.
2- Increase sound pressure at the
tympanic membrane.
3- Sound localization.
4- Protective function:
– Wax.
– Keep air moist & warm.

9. Function of the tympanic
membrane
• It vibrates in response to sound.
• Its vibration is transmitted to the middle ear
• It has the following characteristics:
• 1- elastic
• 2- tense
• 3- aperiodic (resonator)
• 4- damped.

10. Middle Ear
• Air-filled
• Connected to nasopharynx via Eustachian
tube.
• Three ossicles:
– Malleus
– Incus
– Stapes
• Two muscles:
– Tensor tympani.
– Stapedeus

11. Function of the middle Ear
1- Couples sound energy to the cochlea.

12. 2- Application of sound preferentially to the
oval window.

13. 3- Impedance Matching:
tympanic membrane: 55 mm2
Oval window: 3.2 mm2
Ratio: 17:1
Lever action: 1.3 time
The impedance of the fluid in the cochlea is about 30
times greater than that of air
Amplification of sound
Surface area of: 17 times
Amplification = 17 x 1.3 = 22 times

14. Function of the Middle Ear
4- Middle ear muscles:
 Tensor tympani: keeps the tympanic membrane
tense.
 Stapedius: limits the movement of the stapes.
** Acoustic reflex. Latency is 40-160 msec.
5- Physical protection of the cochlea.

15. Function of the Middle Ear
• Function of the Eustachian Tube:
It connects the middle ear with the
nasopharynx
Normally Closed, but it opens during
chewing, swallowing & yawning
1- Equalizes pressure on tympanic
membrane.
2- Lined by ciliated epithelium:
drainage of middle ear.

16. Sound Transmission to Inner
Ear
1- Ossicular route:
Via tympanic membrane and ossicles.
2- Air route:
Through secondary tympanic membrane.
Only when tympanic membrane and ossicles are damaged.
3- Bone route:
Via skull bones

17. Inner Ear (Labyrinth)

18. Reissner
membrane
Basilar
membrane
Scala vestibuli
Scala tympani
Scala media

19. Cochlear Organization

26. Mechanism of stimulation of
Auditory Nerve

34. Auditory cortex is concerned with:
1- Perception of sound and giving the
psychic sensation of:
-Pitch
-Loudness
-Direction
-Pattern
2- Centrifugal (olivo-cochlear) fibers.

35. Bilateral lesion of the auditory cortex:
-Marked reduction of hearing, but NOT complete
deafness.
-Abolishes recognition of tonal pattern and sound
localization
Unilateral lesion of the auditory cortex:
Slight reduction of hearing in the opposite ear
Lesion in auditory association areas:
Inability to interpret meaning of the sound heard

36. Auditory Encoding
The ability of the auditory system to
determine:
1- Sound frequency (pitch).
2- Sound intensity (loudness).
3- Sound localization.
4- Tonal and sequential sound pattern

37. Determination of Sound Frequency
Place principle

39. Tonotopic Organisation:

41. Humans can discern a difference in frequency of
just 0.1 percent.
This means that humans can tell the difference
between sounds at frequencies of 1,000 Hz and
1,001 Hz.

42. Determination of Sound Intensity:

43. The auditory system determines loudness in three
ways.
 First, the amplitude of vibration of the basilar
membrane increases so that hair cells excite nerve
endings at more rapid rates.
 Second, more and more hair cells on the fringes
of the resonating portion of the basilar membrane
become stimulated.
 Third, outer hair cells become recruited at a
significant rate.

44. Localization of Sound:

46. Determination of sound pattern:
Cortical function
Detect combination or sequence of tones

47. Hearing Impairments
Tinnitus:
Irritative stimulation of
Inner ear
VIII cranial nerve
Deafness:
Conduction deafness Diseases of external or
middle ear
Nerve deafness Diseases of inner ear
or VIII cranial nerve

48. Testing of hearing

50. Hearing Test: Rinne Test

52. Hearing test: Weber Test
Conductive
Deafness
Nerve
Deafness

53. Hears better in
diseased ear
Hears better in
healthy ear

54. Audiogram

55. Audiometry

56. Normal Hearing

57. Conductive Hearing Loss

58. Sensorineural Hearing Loss
presbyacusis

59. Mixed Hearing Loss

60. Olfaction
The Sense of Smell
• Selection of food.
• Flavor of food.
• GIT function regulation.
• Sexual behavior.
• Long-term memories.

62. Olfactory Mucosa
Supporting cell
Receptor cell
Basal cells
Bowman’s gland

63. Properties of Olfactory Receptors
• Sensitivity: very high
• Discrimination: 10’000 odors.
• Intensity: limited (30% change).
• Adaptation: very rapid.
• Stimulation: by adsorption of odorant
molecules causing receptor potential.
• Single receptor responds to many odors.
• EOG

65. Olfactory Stimulus: Odorant substance
Volatile Water soluble Lipid soluble
Odorant
Binding
Protein
+ G
protein
Adenylcyclase
Phospholipase C
Open
Na+
channels
Depolarization of
receptor cell
Olfactory
receptor +
+

66. PG cell
Granule
cell

67. Anterior
commissure
Medial
olfactory area
Primitive responses
Orbito-
frontal
cortex
Pyriform & prepyriform
Amygdaloid
Likes & dislikes
Conscious
analysis of
odors

69. Types of Papillae
Circumvallate
papillaeFoliate
papillae
Fungiform
papillae

71. Taste Buds

73. Mechanism of stimulation of Taste receptors:
Sweet substances: Open Na+ channels
+ adenylate cyclase
increasing cAMP
closing K+ channels
Salty substances: Open Na+ channels
Bitter substances: Increase IP3 increasing
intracellular Ca++
Sour substances: Increase H+ closing
K+ channels

74. Innervation of Tongue
facial
glossopharyngeal
vagus

75. From Tongue to Brain

76. Gustatory Pathway
Anterior 2/3 innervated by
branch of VII (Facial): Chorda
tympani
Geniculate ganglion
Posterior 1/3 innervated by
branch of IX (Glossopharyngeal)
Petrosal ganglion
Epiglottis and Pharynx innervated by
branch of X (Vagus)
Nodose ganglion
All terminate in
gustatory area
of nucleus of
the solitary
tract
thalamus
Gustatory cortex