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cochlear biophysics .pptx

  2. INTRODUCTION three functional units within the cochlea (1) mechanoelectrical transduction The organ of Corti represents the "sensor" of the cochlea converting and amplifying mechanical sound stimuli into electrical signals. (2) endocochlear potential The stria vascularis is the cochlea's "battery," generating the energy mechanoelectrical transduction necessary for mechanoelectrical transduction and influencing cochlear fluid homeostasis. (3) electrical wires The spiral ganglion contains the neurons featuring axons that transport the electrical signals from the cochlea to the central nervous system
  3. • stapes footplate vibrate • compressional wave travels • pressure in the scala vestibuli is higher • pressure defference creates • cochlear partition to vibrate • traveling wave on the basilar membrane
  4. • shearing force between the stereocilia of the hair cells and the tectorial membrane • deflection of the hair cell stereocilia • If deflected in the direction of the tallest row • tip links to stretch
  5. • opening of gated potassium channels • hair cell depolarization • degranulation of neurotransmitter vesicles into the synaptic terminal • calcium influx through channels • propagates an action potential along the auditory nerve.
  6. PASSIVE COCHLEAR MECHANICS • Fundamental nature of cochlear mechanics is evident even in cadavers • Not requiring adenosine triphosphate (ATP) elements underlying passive mechanical transduction: 1)Basilar membrane 2)scala vestibuli and scala tympani 3)scala media 4)tectorial membrane Reissner's membrane : important homeostatic role No role as an element of passive cochlear mechanics.
  7. sound pressure waves are delivered to the scala vestibuli through oval window the higher pressure (or lower, depending on the direction of motion of the stapedial footplate) in scala vestibuli relative to scala tympani produces a pressure differential across the cochlear partition that set the partition into motion
  8. THE BASILAR MEMBRANE AND TONOTOPY 1) the width of the basilar membrane progressively increases toward apex. 2) the number of cells lining the basilar membrane increases along the basoapical length 3) the length of OHCs and stereocilia increase toward apex. 4) the thickness of the basilar membrane and the density of filaments decrease from base to apex
  9. The basilar membrane and organ of Corti complex is stiffer and less massive at the base than at the apex As a result, high-frequency acoustic events are preferentially transduced in the base, because it is stiff and less massive. The inverse is true in the case of low- frequency acoustic events
  10. ACTIVE COCHLEAR MECHANICS • The peak of the traveling wave in living animals is very sharply tuned at low levels of stimulation and exhibits nonlinear growth as sound levels increase • Basilar membrane vibration relative to stapes displacement is as much as two to three orders of magnitude greater under low stimulus level conditions than at higher levels
  11. So, cochlear mechanics are nonlinear Active mechanics is a metabolically labile energy-consuming process the active mechanical event underlying amplification is highly localized
  12. Role of Outer Hair Cells in Active Mechanics when OHCs were lost, with IHCs appearing unaffected, some cochlear functions were affected : 1)cochlear sensitivity reduced 2)frequency selectivity reduced 3)input-output curves acquired a more linear property in the region of OHC damage.
  13. OHCs contract or elongate • OHCs amplify the displacement of the basilar membrane • the voltage-sensitive motor protein underlying the process was identified and given the name prestin intracellular Cl- ions act as extrinsic charged voltage sensors.
  14. Conversion of Basilar Membrane Displacement to Radial Shearing Forces • displacement of the basilar membrane results in a radial shearing motion between the reticular lamina and the tectorial membrane, a motion that serves as the mechanical trigger of transduction currents
  15. • The tallest row of stereocilia protrudes from the apical surface of outer hair cell and embeds within the tectorial membranethe shearing motion between the reticular lamina and the tectorial membrane causes stereocilia to bend in the direction of the modiolus or the spiral limbus, depending on whether the basilar membrane is displaced toward the scala tympani or scala vestibuli, respectively
  16. • In contrast to OHCs, the stereocilia of IHCs do not appear to firmly contact the tectorial membrane Therefore, during basilar membrane vibration, the mechanical stimulus to IHC stereocilia is the flow of endolymph within the subtectorial space.
  17. Radial Displacement Patterns of the Basilar Membrane • The foot of the inner pillar cell lies near the bony spiral lamina • The foot of the outer pillar cell lies over the basilar membrane and is not supported by bone. • When the basilar membrane is displaced in a passive cochlea, movement occurs maximally near the foot of the outer pillar cell When OHCs contract, the reticular lamina pivots at the apex of the tunnel of Corti, and the basilar membrane and reticular lamina are pulled together, enhancing overall basilar membrane displacement.Greatest motion in active cochlea is in the domain of the OHC
  18. Hair Bundle Deflections and Receptor Potentials •Magnitude of the receptor potential is proportional to the degree of stereociliary deflection in the most sensitive region of its operating range •When the bundle is deflected toward the tallest stereocilia, the hair cell depolarizes •Deflections of the bundle in the direction opposite the tallest stereocilia hyperpolarize the cell
  19. Gating of Transduction Channels • transduction channels are located at each end of the tip link •When the bundle is deflected toward the taller stereocilia, tip links are stretched and physically open transduction channels, causing excitation •When the bundle is deflected away from the taller stereocilia, tip links relax, causing the channel to close.
  20. Hair cell transduction channels are nonselectively permeable • K+ carries most of the receptor current • When transduction channels open, K+ in the endolymph is flushed down a large electrochemical gradient • This inward transducer current flows across the basolateral membrane, producing the receptor potential and voltage- gated Ca2+ channels are activated, resulting in an influx of Ca2+ and release of neurotransmitter from the base of the hair cell.
  21. Hair Cell–ANF Synapses • a large presynaptic dense body commonly referred to as a ribbon. Each afferent dendrite receives input from a single “ribbon synapse” • When depolarized, Ca channels open and allow calcium to rapidly enter the cell, and intracellular Ca+ activates the protein otoferlin, which, in turn, triggers the fusion of synaptic vesicles with the presynaptic membrane and subsequent vesicular exocytosis at the hair cell-afferent bouton synapse
  22. • Each inner hair cell is contacted by several type I spiral ganglion cells (SGCs), each peripheral terminal ending in a single synaptic bouton, whereas type II SGCs innervate multiple outer hair cells. • Type I SGCs constitute 95% of the afferent innervation of the cochlea and both peripheral and central processes are myelinated, except peripheral to the habenula perforata, whereas type II SGCs are unmyelinated.
  23. ENDOLYMPH HOMEOSTASIS • extrusion and recycling of K+ after the depolarization of sensory cells are necessary components of mechanoelectrical transduction
  25. Central auditory system • It is the elaborate neural network that governs sound representation. • Most of auditory system is afferent • auditory receptors are selectively excited by sound frequency. • All other perceptual dimensions must be computed by integrating coincident activity across neural populations.
  26. Pathway • Eight (auditory ) nerve • Cochlear nucleus • Olivary nucleus(superior) • Lateral lemniscus • Inferior colliculus • Medial geniculate body • Auditory cortex
  27. Cochlear nucleus •First relay station for all ascending auditory information that originates in the ear, and it is located in the pontomedullary junction of the dorsolateral brainstem •The cochlear nucleus contains a number of cell types, each of these has unique somatic and dendritic characteristics. •These cell types display varying response properties to auditory stimulation and project to different targets
  28. • The temporal and spectral features of sound that originate in the ear are processed in the cochlear nucleus • These pathways project to the auditory brainstem, midbrain, and cortex and integrate information from the ear to determine the identity, intensity, and location of the sound source
  29. Superior Olivary Complex • relay station for auditory information from both ears • information from both cochlear nuclei is integrated in the SOC • important role in sound localization by analyzing interaural time and amplitude differences • allows for sound localization but also helps enhance auditory perception
  30. Lateral Lemniscus •formed by the three fiber tracts from the cochlear nucleus •lateral lemniscus sends out multiple branches; some terminate in the SOC
  31. Inferior Colliculus • processes frequency-specific information • receives auditory inputs from the lateral lemniscus, the cochlear nucleus, and the SOC, in addition, it receives projections from the somatosensory, visual, and vestibular systems • integrates information from both auditory and nonauditory source • sends fibers to the medial geniculate body of the thalamus
  32. Medial Geniculate Body •the auditory processing influenced by an abundance of inputs from the auditory cortex •important role in sound localization and processing of complex vocal communications, such as human speech.
  33. Auditory Cortex • primary auditory cortex is tonotopically tuned, with high frequencies being represented more medially and low frequencies more laterally. • Involved with integrating and processing complex auditory signals, which includes language comprehension