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Introduction to the Auditory Periphery. Basic Cochlear Mechanics Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Lecture Organization • Why bother with cochlear function and cochlear mechanics. • Basic concepts on how the ear encodes the incoming sounds. 4 Basic cochlear Macro-mechanics. 4 Basic cochlear Micro-mechanics • Concept: The healthy ear is NON linear Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists A few words about sound Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 1 Acoustic Waves or SOUNDS The Sound Transmission Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Acoustic Waves or SOUNDS High Frequency Low Frequency Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Example of a graph showing the human voice Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 2 Structures of the Auditory System Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists The AuditorySystem © Video Copyright Inserm 2002 Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists The periphery © Video copyright Oticon 2001 Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 3 The evolution of cochlear mechanics Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Before the 1800s • The first studies were primarily anatomical and they identified the major features of the auditory periphery (eardrum, middle-ear ossicles, cochlea) • The coiled BM was described by Du Verney (1683) • Reissner (1851), Corti (1851) and Deiters (1860) applied new technologies and discovered structures which were named after them. Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 4 Helmholtz- 1863 In 1863 Helmholtz suggests to the scientific community that the frequency selectivity of the cochlea can be explained by a bank of INDEPENDENT and cascaded resonators (filters), formed by the neural fibers. Although his hypothesis is appropriate for an introduction to cochlear mechanics, in essence he was wrong, because he neglected the fluid coupled BM segments. Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Georg von Békésy • In his pioneering work from 1924 to 1960 , Georg von Bék ésy used new micro-dissection techniques , a light microscope and stroboscopic illumination to observe the propagation of sound stimulation in cadaver cochleas. • He coined the sound propagation profile as the TRAVELLING wave. • He was the first to observe that different frequencies were mapped at different “lengths” of the Basilar membrane, coining the term tonotopicity . Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Georg von Békésy Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 5 Cochlear Tonotopicity © Copyright Inserm 2002 Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists The Travelling Wave mechanics a step by step tutorial Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists INSIDE the cochlea there is NO SOUND !!. The acoustical vibrations arriving at the stapes are being transformed into a pressure wave (TW) which moves from 10 -100 times MORE SLOWLY than sound !! Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 6 External + Internal Views Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Inside the Cochlea Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists The organ of Corti Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 7 The site of sound encoding 4,000 IHCs 12,000 OHCs Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists The Travelling Wave mechanism gives the ear the possibility to project onto its sensory epithelium a detailed spatial image of the incoming acoustical stimulation. Stavros Hatzopoulos Ph.D. The TravelingWave Lecture 1: Cochlear Mechanics for Audiologists Note: At the peak the BM displacement is a FRACTION of a nm !!!! © Video Copyright Oticon 2001 Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 8 Advantages of using a TW • The distribution of the stimulus energy in a tonotopical manner (by frequency) relieves the auditory nerve fibers of conveying themselves the sound vibration. To recall that the fibers transduce badly frequency changes above 500 Hz !! Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Advantages of using a TW • The TW is the means to achieve the sharp localization and focusing of the incoming sound vibrations. Such an action offers: 4 Enhanced detection sensitivity . immunity, therefore greater frequency selectivity. 4 Reduction of the bulk of information which are processed by the inner hair cells and the neural fibers. 4 Greater noise Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Background INFO 4The vibration of the cochlea is induced by pressure forces within the scalae , which give rise to the notion of a Travelling Wave (TW) . 4The first attempts to explain this behavior , by von Békésy, considered the travelling wave as a passive phenomenon. Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 9 Origin of the Place-Frequency map 4The cochlea is characterized by a grading of partition STIFFNESS from base to the apex. This fact generates the known cochlear TONOTOPICITY. Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists The Travelling Wave Nobili, AshMore and Mamano Neuroscience 1998 Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists TW Background INFO • • The TW grows in amplitude, comes to a maximum, and then declines sharply. This is the result of the difference in STIFFNESS, MASS and WIDTH along the basilar membrane. Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 10 Cochlear Mechanics step by step Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists The Travelling Wave Generation • The reason that the TW amplitude increases moving from the base to the Apex, is the reduction in the wave propagation speed. • WHY IS THIS HAPPENING ? Because the basilar membrane becomes more compliant (less stiff) and wider with the distance from the stapes. • As the TW slows down (its speed declines) the energy it carries is being concentrated in a smaller distance. Therefore the amplitude of the vibration increases . Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists As a consequenceof the increase in the amplitude of the vibration the velocities of the “up” and “down” motion increase. • • The increase in motion implies an increase in the viscous drag (friction) and the energy losses. • As we move towards the point of resonance the energy losses due to friction increase. The point where the energy losses equal the energy gain corresponds to the peak of the TW . Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 11 Basic Cochlear Macro--Mechanics Macro Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Cochlear Micromechanics • At the end of the 80s and the beginning of the 90s research was shifted to the understanding of the active components in the cochlear sound processing. • Brownell (1982, 1985), Flock (1988, 1990), Evans (1986) and later Ruggero and Rich (1992) have shown that the OHCs are the ACTIVE components responsible for the responsiveness of the BM vibration. Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Cochlear Micromechanics • The motile features of the OHCs are enhancing the motion of the Basilar Membrane at their characteristic frequency , which results in a “better perception ” of that frequency. • It was realized in the early 90s that OAEs can help us understand the functioning of the cochlear processing (or cochlear amplifier-a term coined by Davis in 1983) Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 12 Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Cochlear Mechanics: Summary of events 1. Vibration of the Basilar Membrane and bending of the OHC and IHC stereocilia. 2. Transduction into cellular receptor potentials. 3. Generation of length changes in the OHCs. 4. Mechanical feedback of these changes on the BM and TM. 5. Enhancement of the TW peak. Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Cochlear micro micro--mechanics: A synopsis 1. Polarization = OHCs shrink and BM moves upwards 2. Hyperpolarization = OHCs expand and BM moves downwards Cochlear Mechanics © Video Copyright Promenade ‘round the cochlea Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 13 The CLEAR advantage Active vs passive mechanics © Copyright Promenade ‘round the cochlea Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists OHC motility recordings in-vitro Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Motility of the OHCs Case A: ©Video Copyright Joe Santos-Sacchi ComplexStimulus (music) Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 14 Motility of the OHCs Case B: SimpleStimulus ( 1 kHz tone) © Copyright J. Ashmore 1999 Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Motility of the OHCs Free Field © Copyright Holley and Ashmore 1988 Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists From Mechanics to Hearing Science Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 15 Locating SNHL deficits Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Simulation of Noise Effects 1 2 3 © Copyright Inserm 2002 Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists Ototoxicity Effects Cisplatin Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 16 Stavros Hatzopoulos Ph.D. Lecture 1: Cochlear Mechanics for Audiologists 17