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2. physiology of hearing and balance
- 1. Physiology of Hearing and Balance Dr. Krishna Koirala
- 2. 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
- 3. 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
- 5. 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)
- 6. 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
- 7. Hermann von Helmholtz Described impedance matching in 1868
- 8. • Ossicular Lever ratio: – Length of handle of malleus > long process of incus – Magnifies 1.3 times • Effective surface area ratio of TM to stapes footplate: – 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
- 11. • 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
- 13. 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
- 14. • 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
- 16. Ossicular break + intact T.M. : 55-60 dB loss Ossicular break + T.M. perforation : 45-50 dB loss
- 19. – Movement of basilar membrane – Shear force between tectorial membrane & hair cells – Cochlear microphonics – Nerve impulses Transduction of mechanical energy to electrical impulses
- 20. Organ of Corti
- 24. Auditory pathway • Eighth (Auditory) nerve • Cochlear nucleus • Olivary nucleus (superior) • Lateral lemniscus • Inferior colliculus • Medial geniculate body • Auditory cortex
- 27. Theories of Hearing 1. 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
- 28. 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
- 29. 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
- 30. • 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
- 31. George von Bekesy Won Nobel prize for his traveling wave theory in 1961
- 33. 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.
- 34. • 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
- 36. 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
- 37. Vestibular apparatus • Semicircular canals – Angular acceleration and deceleration • Utricle – Horizontal linear acceleration and deceleration • Saccule – Vertical linear acceleration and deceleration
- 39. Physiology of head movement Head Movement Semicircular canal stimulated Yaw Lateral Pitch Posterior + Superior Roll Superior + Posterior
- 41. 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
- 53. 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