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
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
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
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
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.
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
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