1. Physiology of Hearing & Equilibrium
Dr. Vishal Sharma

2. Parts of hearing apparatus
Conductive apparatus: external & middle ear
Conducts mechanical sound impulse to inner ear
Perceptive apparatus: cochlea
Converts mechanical sound impulse into electrical
impulse & transmits to higher centers

3. Role of external ear
Collection of sound waves by pinna & conduction to tympanic membrane
Increases sound intensity by dB
Cupping of hand behind pinna also increases sound intensity by 15 dB especially at 1.5 kHz.

5. Role of middle ear in hearing
Impedance matching mechanism (step – up transformer or amplifier function)
Preferential sound pressure application to oval window (phase difference by ossicular coupling)
Equalization of pressure on either sides of tympanic membrane (via Eustachian tube)

6. Impedance matching mechanism
When sound travels from air in middle ear to fluid in inner ear, its amplitude is ed by fluid impedance.
Only 0.1 % sound energy goes inside inner ear.
Middle ear amplifies sound intensity to compensate for this loss. Converts sound of low pressure, high amplitude to high pressure, low amplitude vibration suitable for driving cochlear fluids.

7. Described impedance matching in 1868
Hermann von Helmholtz
Described impedance matching in 1868

8. T.M. Catenary lever (curved membrane effect):
Sound waves focused on malleus. Magnifies 2 times
Ossicular Lever ratio:
Length of handle of malleus > long process of incus.
Magnifies 1.3 times
Surface area ratio (Hydraulic lever):
T.M. = 55 mm2 ; Stapes foot plate = 3.2 mm2
Magnifies 17 times
Total Mechanical advantage: 2 X 17 X 1.3 = 45
times = 30 – 35 dB

12. Natural Resonance
Property to allow certain sound frequencies to pass more readily to inner ear.
External auditory canal = 2500 – 3000 Hz
Tympanic membrane = Hz
Ossicular chain = 500 – 2000 Hz
Range = 500 – 3000 Hz (speech frequency)

14. Preferential sound pressure application (phase difference)
Sound pressure preferentially applied to oval window by ossicular coupling while round window is protected by tympanic membrane
Sound pressure travels to scala vestibuli  helicotrema  scala tympani  round window membrane yields  scala media moves up & down  movement of hair cells in scala media

15. Preferential sound pressure application (phase difference)
Yielding of round window membrane (push-pull effect) is necessary as inner ear fluids are incompressible
Large tympanic membrane perforation  loss of this function (push-push effect)  no movement of inner ear fluids

17. Ossicular break + intact T.M. = 55-60 dB loss
Ossicular break + T.M. perforated = dB loss

20. Transduction of mechanical energy to electrical impulses
Movement of basilar membrane
Shear force between tectorial membrane & hair cells
Cochlear microphonics
Nerve impulses

23. Cochlear hair cells

28. Transducer Mechanism

30. Auditory pathway Eighth (Auditory) nerve Cochlear nucleus
Olivary nucleus (superior)
Lateral lemniscus
Inferior colliculus
Medial geniculate body
Auditory cortex

38. Theories of hearing Place / Resonance Theory (Helmholtz, 1857)
Perception of pitch depends on selective vibration of specific place on basilar membrane.
Telephone Theory (Rutherford, 1886)
Entire basilar membrane vibrates. Pitch related to rate of firing of individual auditory nerve fibers.

39. Theories of hearing Volley Theory (Wever, 1949)
> 5 KHz: Place theory; <400 Hz: Telephone theory
400 – 5000 Hz: Volley theory
Groups of fibres fire asynchronously (volley
mechanism). Required frequency signal is
presented to C.N.S. by sequential firing in groups
of fibers as each fiber has limitation of 1 Khz.

41. Bekesy’s travelling wave theory
Sound stimulus produces a wave-like vibration of basilar membrane starting from basal turn towards apex of cochlea . It increases in amplitude as it moves until it reaches a maximum & dies off. Sound frequency is determined by point of maximum amplitude. High frequency sounds cause wave with maximum amplitude near to basal turn of cochlea. Low frequency sound waves have their maximum amplitude near cochlear apex.

42. Georg von Bekesy
Won Nobel prize for his traveling wave theory in 1961

43. Bekesy’s travelling wave theory

46. Theories of bone conduction
Compression theory: skull vibration from sound stimulus  vibration of bony labyrinth & inner ear fluids
Inertia theory: sound stimulus  skull vibration but ear ossicles lag behind due to inertia. Out of phase movement of skull & ear ossicles  movement of stapes footplate  vibration of inner ear fluids

47. Theories of bone conduction
Osseo-tympanic theory: sound stimulus  skull vibration but mandible condyle lags behind due to inertia. Out of phase movement of skull & mandible  vibration of air in external auditory canal  vibration of tympanic membrane
Tonndorf’s theory: sound stimulus  skull vibration  rotational vibration of ear ossicles  movement of stapes footplate

49. Physiology of equilibrium
Balance of body during static or dynamic
positions is maintained by 4 organs:
1. Vestibular apparatus (inner ear)
2. Eye
3. Posterior column of spinal cord
4. Cerebellum

50. Vestibular apparatus Semicircular canals
Angular acceleration & deceleration
Utricle
Horizontal linear acceleration & deceleration
Saccule
Vertical linear acceleration & deceleration

51. Orientation of semicircular canals

53. Physiology of head movement
Semicircular canal stimulated
Yaw
Lateral
Pitch
Posterior + Superior
Roll
Superior + Posterior

54. Nystagmus (slow component)

55. Nystagmus (fast component)

56. Semicircular canal stimulated
Nystagmus Direction
Right Lateral
Right horizontal
Left Lateral
Left horizontal
Right Superior
Down beating, counter-clockwise
Left Superior
Down beating, clockwise
Right Posterior
Up beating, counter-clockwise
Left Posterior
Up beating, clockwise

68. Vestibulo-ocular reflex (VOR)
Movement of head to left  left horizontal canal stimulated & right horizontal canal inhibited
To keep eyes fixed on a stationary point, both eyes move to right side by stimulating right lateral rectus & left medial rectus muscles

72. Thank You