This document discusses the anatomy and physiology of hearing and balance. It describes the parts of the hearing apparatus including the external, middle, and inner ear. Sound is conducted through the ear canal and vibrations are amplified by the ossicles in the middle ear before being transduced into nerve impulses in the cochlea. Hair cells in the organ of Corti detect sound vibrations and transmit signals to the brain. Several theories attempt to explain the mechanisms of hearing such as place theory and traveling wave theory. The vestibular system in the inner ear, along with visual and proprioceptive cues, helps maintain balance and orientation.

1. Physiology of
Hearing and
Balance
Dr. Krishna
Koirala

2. Parts of Hearing Apparatus
• Conductive apparatus: external and
middle ear
–Conducts mechanical sound
impulse to inner ear
• Perceptive apparatus: cochlea
–Converts mechanical sound
impulse into electrical impulse

3. Role of external ear
• Collection of sound waves by pinna
and conduction to tympanic
membrane
• Increases sound intensity by 15-20
dB

5. Role of middle ear
• Impedance matching mechanism (Middle
ear transformer or amplifier function)
• Preferential sound pressure application
to oval window (phase difference by
ossicular coupling)
• Equalization of pressure on either sides

6. Impedance matching
mechanism
• When sound travels from air in middle ear
to fluid in inner ear, its amplitude is
decreased by fluid impedance
• Only 0.1 % sound energy goes inside inner
ear
• Middle ear amplifies sound intensity to
compensate for this loss by converting
sound of low pressure, high amplitude to

7. Hermann von Helmholtz
Described impedance matching in 1868

8. • 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
• T.M. Catenary lever (curved membrane effect):
– Sound waves focused on malleus. Magnifies 2
times
• Total Mechanical advantage: 1.3x17x2 =

11. • Property to allow certain sound
frequencies to pass more readily
– External auditory canal : 2500 – 3000
Hz
– Tympanic membrane : 800 - 1600 Hz
– Ossicular chain : 500 – 2000 Hz
– Range : 500 – 3000 Hz (speech
frequency)
Natural Resonance

13. 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 to helicotrema → scala
tympani → round window membrane
yields → scala media moves up and

14. • Yielding of round window membrane
(push-pull effect) is necessary as
inner ear fluids are incompressible
• Large tympanic membrane
perforation leads to loss of push-
pull effect with no movement of

16. Ossicular break + intact T.M. : 55-60 dB
loss
Ossicular break + T.M. perforation : 45-

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

20. Organ of
Corti

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

27. Theories of Hearing
1. Place / Resonance Theory
(Helmholtz, 1857)
– Perception of pitch depends on selective
vibration of specific place on basilar
membrane
2. Telephonic Theory (Rutherford,

28. 3. Volley Theory (Wever, 1949)
– > 5000 Hz: 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

29. 4. 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 and dies off
• Sound frequency is determined by point of
maximum amplitude

30. • 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

31. George von Bekesy Won Nobel prize for
his traveling wave theory in 1961

33. Theories of bone conduction
• Compression theory
– Skull vibration from sound stimulus →
vibration of bony labyrinth and inner ear
fluids
• Inertia theory
– Sound stimulus → skull vibration but ear
ossicles lag behind due to inertia
– Out of phase movement of skull and ear
ossicles → movement of stapes

34. • 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

36. Physiology of equilibrium
• Balance of body during static or
dynamic position is maintained by 4
organs
–Vestibular apparatus (inner ear)
–Eye
–Posterior column of spinal cord

37. Vestibular apparatus
• Semicircular canals
– Angular acceleration and deceleration
• Utricle
– Horizontal linear acceleration and
deceleration
• Saccule
– Vertical linear acceleration and

39. Physiology of head
movement
Head Movement Semicircular canal
stimulated
Yaw Lateral
Pitch Posterior +
Superior
Roll Superior +
Posterior

40. Nystagmus (fast
component)

41. 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

53. Vestibulo -ocular reflex (VOR)
• Movement of head to left → left
horizontal canal stimulated and
right horizontal canal inhibited
• To keep eyes fixed on a stationary
point, both eyes move to right side
by stimulating right lateral rectus