1. Physiology of
Hearing & Balance
Dr. Archana Sudhir

2. The Nature of Sound
 Sound is any audible
vibration of molecules
 Vibrating object
pushes air molecules
into zones of
compression
separated by zones of
rarefaction

3. Properties of Sound
 Frequency – the number of waves that
pass a given point in a given time
 Pitch – perception of different frequencies
(we hear from 20–20,000 Hz)
 Intensity – The power transmitted by a
wave through an unit area.
 Loudness – The perception of intensity.

4. Main Components of the Hearing
Mechanism
Divided into 4 parts
(by function):
 Outer Ear
 Middle Ear
 Inner Ear
 Central Auditory
Nervous System

5. Functions of the Outer Ear
 Gathers sound waves
 Increases Pressure in a frequency
sensitive way.
 Aids in localization

6. Functions of the Middle Ear
 Couple sound energy
to the cochlea
 Impedance matching
 Protects Cochlea
 Preferential
application of sound
to one window.

7. Impedance Transformer
 Large area of TM in
comparison to small area
of foot plate (pressure
increases inversely to the
ratio of these areas)
 Ossicular lever ratio
(Malleus is 1.3 times
longer than incus)
 Buckling action of TM
 Ligaments suspending
ossicles.

8. Impedance Efficiency
 Middle ear converts low pressure high
displacement movements of the ear drum
into high pressure low displacement
movements needed for the cochlear fluid
movement.
 50% of sound energy from TM gets
transmitted and absorbed in the cochlea.
 Without middle ear only 1% of sound
energy will be absorbed by the cochlea.

9. Role of Middle Ear Muscles
 Tensor tympani attaches to the neck of
malleus. It pulls the drum medially.
 Stapedius muscle attaches to the posterior
aspect of neck of stapes.
 Contraction of these muscles increase the
stiffness of ossicular chain thus blunting
low frequencies.
 Stapedius contracts in response to loud
sounds and acts as an in built ear plug.

10. Bone Conduction
 Bone vibration conducted through ext
canal
 Skull vibration – ossicles lag behind.
 Differential distortion of bony cochlea
 Direct vibration of osseous spiral lamina
 Skull vibration via CSF to endolymph

11. Structures of the Inner Ear
 Bony Labyrinth
 Bony Cochlea
 Vestibule
 Semi Circular Canals
 Membranous
Labyrinth
 Cochlea Duct
 Utricle & Saccule
 Semi Circular Canals

13. Organ of Corti
 16,000 hair cells have 30-100 stereocilia(microvilli )
 Microvilli make contact with tectorial membrane (gelatinous membrane
that overlaps the spiral organ of Corti)
 Basal sides of inner hair cells synapse with 1st order sensory neurons
whose cell body is in spiral ganglion

14. Movement of pressure waves through the cochlea

15. MOVEMENTS OF THE BASILAR MEMBRANE
AND THE DEFLECTION OF THE STEREOCILIA.

16. Potassium Gates of Cochlear Hair Cells
 Stereocilia bathed in high K+ concentration creating
electrochemical gradient from tip to base
 Stereocilia of OHCs have tip embedded
in tectorial membrane which is anchored
 Movement of basilar membrane bends
stereocilia
 Bending pulls on tip links
and opens ion channels
 K+ flows in -- depolarizing
it & causing release of
neurotransmitter stimulating
sensory dendrites at its base

17. Theories Of Hearing
 Place theory of Helm holtz
 Telephone theory of Rutherford
 Volley theory of Wever
 Traveling wave theory of Bekesy

18. CENTRAL AUDITORY PATHWAYS

19. Auditory Cortex

20. APPLIED PHYSIOLOGY
 EAC BLOCK - 30db HL
 TM PERFORATION - 26db HL
 TM PERFORATION WITH OSSICULAR INTERRUPTION -
26.5 +7.3+ 5=38.3dbHL
 TOTAL LOSS OF TM WITH OSSICULAR INTERRUPTION -
26.5 +7.3+ 16.2=50dbHL
 OSSICULAR INTERRUPTION WITH INTACT TM -
38+15=54dbHL
 OSSICULAR INTERRUPTION WITH INTACT TM WITH
CLOSED OVAL WINDOW - 60dbHL

21. Vestibular Apparatus
 Vestibule
 Utricle
 Saccule
 Semicircular canals
- lateral, superior, posterior
 Vestibular nerve

22. Equilibrium
 Static equilibrium is perception of head
orientation
 perceived by macula
 Dynamic equilibrium is perception of motion
or acceleration
 linear acceleration perceived by macula
 angular acceleration perceived by crista

23. The Saccule and Utricle
 Saccule & utricle
chambers containing
macula
 patch of hair cells with their
stereocilia & one kinocilium
buried in a gelatinous
otolithic membrane
weighted with granules
called otoliths
 otoliths add to the density &
inertia and enhance the
sense of gravity and motion
Otoliths

24. Macula of Saccule and Utricle
 With the head erect, stimulation is minimal, but when the head is tilted,
weight of membrane bends the stereocilia (static equilibrium)
 Linear acceleration is detected since heavy otolith lags behind (one type of
dynamic equilibrium)

25. Crista Ampullaris of Semicircular Ducts
 Crista ampullaris consists of hair cells buried in a mound of
gelatinous membrane
 Orientation of ducts causes different ducts to be stimulated by
rotation in different planes

26. Crista Ampullaris & Head Rotation
 As head turns, the endolymph lags behind
pushing the cupula and stimulating its hair cells

27. Equilibrium Projection Pathways
 Unmyelinated plexus at the base of
sensory epithelium gives rise to primary
vestibular neuron
 Central processes of primary vestibular
neurons synapses with vestibular
nucleus of pons, cerebellum

28. Vestibular Nuclei
Cristae of SCC &
Cerebellum
Superior
vestibular nuclei
Bechterew
Medial
Longitudinal
Fasciculus
Cerebellum &
Utricular Macula
Lateral
vestibular nuclei
Dieter
Vestibulo Spinal
Tract, Reticulo
Spinal Tract
Cristae Cerebellum Medial vestibular
nuclei Schwalbe
Medial
Longitudinal
Fasciculus
Utricular &
Sacular Maculae
Descending
vestibular nuclei
Cerebellum &
Reticular
Formation

29. Ascending Vestibular Projections
Lateral &
Superior
vestibular nuclei
Thalamus
Sensori Motor
Cortex
Visual Projections
Proprioceptive
Projections

30. Vestibular Reflexes
 Vestibulo-spinal
 Helps maintain center of gravity
 Vestibulo-ocular
 Helps maintain stability of visual field
 Vestibulo-collic:
 Helps to maintain stability of the head during movement
of the torso.

31. Vestibulo Ocular Reflexes

33. CLINICAL RELEVENCE
GIDDINESS
1. NON CORRECTABLE VISUAL IMPAIRMENT.
2. NEUROPATHY.
3. VESTIBULAR DYSFUNCTION.
4. CERVICAL SPONDYLOSIS.
5. ORTHOPAEDIC DISTURBANCES.
6. CARDIAC DISORDERS.
7. NEUROLOGICAL DEFICITS.

34. ASSESMENT
 HISTORY
 IDENTIFICATION OF PRESENCE/
ABSENCE OF VESTIBULAR
COMPONENT.
1. VESTIBULO-SPINAL FUNCTION.
2. VESTIBULO – OCULAR FUNCTION.

35. VESTIBULO-SPINAL FUNCTION
 ROMBERGS TEST
 UNTERBERGERS TEST

36. VESTIBULO-OCULAR FUNCTION
 NYSTAGMUS
 INVOLUNTARY DEVIATION OF EYES AWAY
FROM DIRECTION OF GAZE FOLLOWED BY A
RETURN OF THE EYES TO THEIR ORIGINAL
POSITION.
 3 TYPES
1. CENTRAL
2. OCULAR
3. VESTIBULAR

37. VESTIBULAR NYSTAGMUS
 RHYTHMIC
 FAST AND SLOW PHASES
 NAMED AFTER FAST PHASE.
 3 TYPES
1. SPONTANEOUS
2. POSITIONAL
3. INDUCED.

38. VESTIBULAR NYSTAGMUS
 SPONTANEOUS NYSTAGMUS
 GRADE 1.
 GRADE 2.
 GRADE 3.
 POSITIONAL NYSTAGMUS
 HALLPIKE MANOEUVRE

39. INDUCED NYSTAGMUS
 ROTATIONAL TESTS
 Nystagmus Induced by accelerating and
decelerating rotating chair, tests both labyrinths
simultaneously
 CALORIC TESTS
 COWS- cold water opposite side, warm water
same side, direction of nystagmus
 Extent of caloric response indicates function of
labyrinth

40. Electronystagmograghy
 Positive potential between the cornea
and retina recorded as eyes move from
straight ahead gaze
 Test includes different head positions,
eyes open, closed and caloric tests