Physiology of Hearing &

EquilibriumDr. 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

• 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)

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

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

– Movement of basilar membrane

– Shear force between tectorial membrane & hair cells

– Cochlear microphonics

– Nerve impulses

Transduction of mechanical energy to electrical 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 hearingPlace / 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 hearingVolley 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.

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.

Bekesy’s travelling wave theory

Georg von Bekesy

Won Nobel

prize for

his

traveling

wave

theory in

1961

Bekesy’s travelling wave theory

Theories of bone conductionCompression 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 movementHead 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

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