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