Download Physiology of Hearing & Equilibrium

Physiology of
Hearing &
Equilibrium
Dr. Vishal Sharma
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
Role of external ear
• Collection of sound waves by pinna &
conduction to tympanic membrane
• Increases sound intensity by 15-20 dB
• Cupping of hand behind pinna also increases
sound intensity by 15 dB especially at 1.5 kHz.
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)
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.
Hermann von Helmholtz
Described impedance matching in 1868
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
Natural Resonance
• Property to allow certain sound frequencies to
pass more readily to inner ear.
• External auditory canal = 2500 – 3000 Hz
• Tympanic membrane = 800 - 1600 Hz
• Ossicular chain = 500 – 2000 Hz
• Range = 500 – 3000 Hz (speech frequency)
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
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
Ossicular break + intact T.M. = 55-60 dB loss
Ossicular break + T.M. perforated = 45-50 dB loss
Transduction of mechanical
energy to electrical impulses
– Movement of basilar membrane
– Shear force between tectorial membrane &
hair cells
– Cochlear microphonics
– Nerve impulses
Cochlear hair cells
Transducer Mechanism
Auditory pathway
• Eighth (Auditory) nerve
• Cochlear nucleus
• Olivary nucleus (superior)
• Lateral lemniscus
• Inferior colliculus
• Medial geniculate body
• Auditory cortex
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.
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 2 - 5 fibers as each fiber has limitation of 1 Khz.
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.
Georg von Bekesy
Won Nobel
prize for
his
traveling
wave
theory in
1961
Bekesy’s travelling wave theory
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
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
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
Vestibular apparatus
Semicircular canals
Angular acceleration & deceleration
Utricle
Horizontal linear acceleration & deceleration
Saccule
Vertical linear acceleration & deceleration
Orientation of semicircular canals
Physiology of head movement
Head Movement
Semicircular canal
stimulated
Yaw
Lateral
Pitch
Posterior + Superior
Roll
Superior + Posterior
Nystagmus (slow component)
Nystagmus (fast component)
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
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
Thank You