Presentation on theme: "Physiology of Hearing & Equilibrium"— Presentation transcript:
1Physiology of Hearing & Equilibrium Dr. Vishal Sharma
2Parts of hearing apparatus Conductive apparatus: external & middle earConducts mechanical sound impulse to inner earPerceptive apparatus: cochleaConverts mechanical sound impulse into electricalimpulse & transmits to higher centers
3Role of external earCollection of sound waves by pinna & conduction to tympanic membraneIncreases sound intensity by dBCupping of hand behind pinna also increases sound intensity by 15 dB especially at 1.5 kHz.
5Role 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)
6Impedance 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.
7Described impedance matching in 1868 Hermann von HelmholtzDescribed impedance matching in 1868
8T.M. Catenary lever (curved membrane effect): Sound waves focused on malleus. Magnifies 2 timesOssicular Lever ratio:Length of handle of malleus > long process of incus.Magnifies 1.3 timesSurface area ratio (Hydraulic lever):T.M. = 55 mm2 ; Stapes foot plate = 3.2 mm2Magnifies 17 timesTotal Mechanical advantage: 2 X 17 X 1.3 = 45times = 30 – 35 dB
14Preferential sound pressure application (phase difference) Sound pressure preferentially applied to oval window by ossicular coupling while round window is protected by tympanic membraneSound pressure travels to scala vestibuli helicotrema scala tympani round window membrane yields scala media moves up & down movement of hair cells in scala media
15Preferential sound pressure application (phase difference) Yielding of round window membrane (push-pull effect) is necessary as inner ear fluids are incompressibleLarge tympanic membrane perforation loss of this function (push-push effect) no movement of inner ear fluids
38Theories 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.
39Theories of hearing Volley Theory (Wever, 1949) > 5 KHz: Place theory; <400 Hz: Telephone theory400 – 5000 Hz: Volley theoryGroups of fibres fire asynchronously (volleymechanism). Required frequency signal ispresented to C.N.S. by sequential firing in groupsof fibers as each fiber has limitation of 1 Khz.
41Bekesy’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.
42Georg von BekesyWon Nobel prize for his traveling wave theory in 1961
46Theories of bone conduction Compression theory: skull vibration from sound stimulus vibration of bony labyrinth & inner ear fluidsInertia 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
47Theories 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 membraneTonndorf’s theory: sound stimulus skull vibration rotational vibration of ear ossicles movement of stapes footplate
68Vestibulo-ocular reflex (VOR) Movement of head to left left horizontal canal stimulated & right horizontal canal inhibitedTo keep eyes fixed on a stationary point, both eyes move to right side by stimulating right lateral rectus & left medial rectus muscles