2. physiology of hearing and balance
TRANSCRIPT
Physiology of Hearing and
Balance
Dr. Krishna Koirala
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 and transmits to higher centers
Role of external ear
• Collection of sound waves by pinna and
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• 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 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 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 high pressure, low amplitude vibration suitable for driving cochlear fluids
Hermann von Helmholtz
Described impedance matching in 1868
• 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 = 45 times 30 – 35 dB
• 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
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 down movement of hair cells in scala media
• Contd
• 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 inner ear fluids
Ossicular break + intact T.M. : 55-60 dB lossOssicular break + T.M. perforation : 45-50 dB loss
– Movement of basilar membrane
– Shear force between tectorial membrane &
hair cells
– Cochlear microphonics
– Nerve impulses
Transduction of mechanical energy to electrical impulses
Organ of Corti
Auditory pathway
• Eighth (Auditory) nerve
• Cochlear nucleus
• Olivary nucleus (superior)
• Lateral lemniscus
• Inferior colliculus
• Medial geniculate body
• Auditory cortex
Theories of Hearing1. Place / Resonance Theory (Helmholtz, 1857)
– Perception of pitch depends on selective vibration
of specific place on basilar membrane
2. Telephonic Theory (Rutherford, 1886)
– Entire basilar membrane vibrates but pitch is related
to rate of firing of individual nerve fibers
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 of 2 - 5 fibres
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
•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
George von Bekesy Won Nobel prize for his traveling wave theory in 1961
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 footplate vibration of inner ear fluids contd.
• 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 position is maintained by 4 organs
– Vestibular apparatus (inner ear)
– Eye
– Posterior column of spinal cord
– Cerebellum
Vestibular apparatus• Semicircular canals
– Angular acceleration and deceleration
• Utricle
– Horizontal linear acceleration and deceleration
• Saccule
– Vertical linear acceleration and deceleration
Physiology of head movement
Head Movement Semicircular canal stimulated
Yaw Lateral
Pitch Posterior + Superior
Roll Superior + Posterior
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 and right horizontal canal inhibited
• To keep eyes fixed on a stationary point,
both eyes move to right side by
stimulating right lateral rectus and left
medial rectus muscles