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PSYCH 3A03 (56)
Paul Faure (56)

Week6 outline

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Paul Faure

This outline summarizes major points covered in lecture. It is not intended to replace your own lecture notes. Inner ear anatomy  Three temporal bone divisions:  1. Semicircular canals – part of vestibular system, but may also influence hearing because it shares fluid with inner ear  2. Vestibule  3. Cochlea  Semicircular canals open into vestibule, utricle, saccule, all involved in the sense of balance  Cochlea – coil structure within temporal bone, is the primary auditory organ  Central spiral axis of cochlea is modiolus  Osseous spiral lamina – spiral bony shelf that spirals along length of cochlea  Important to have system encased in bone for separation from other bodily sounds, and to keep entire system (from motion) so that it is sensitive to externally generations motion (sound) Inner ear anatomy  Helicotrema – apical tip of cochlear duct  Consists of three chambers: scala vestibuli, scala media, scala tympani  Scala vestibuli and scala tympani are contiguous at helicotrema  Scala media (Coclear Duct) contains Organ of Corti  Oval and round windows located near cochlear base  Relative motion of fluids in scala vestibuli and scala tympani cause movement of basilar membrane and shearing motion along hair cells Cochlear anatomy  Cochlear duct (scala media) – contains endolymph fluid and is sandwiched between scala vestibuli and scala tympani  Scala vestibuli and scala tympani – contain perilymph  Scala media: bound by Reissner’s membrane, basilar membrane, stria vascularis, spiral lamina  Basilar membrane  Tectorial membrane  Spiral ganglion – collection of nerve cell bodies that spirals along cochlear duct  Auditory nerve fibers – individual nerve cell axons that come from base of hair cell receptors Organ of Corti anatomy  Organ of Corti inside cochlear duct  Organ of Corti contains: inner hair cells (IHC; globular), outer hair cells (OHC; elongated), tectorial membrane, basilar membrane, stereocilia, pillars of corti, tunnel of corti  Supporting cells: Dieter (directly cup outer hair cells), Hensen’s cells, Claudius cells, pillars  Phalangeal processes of Dieter’s cells create membrane to support and encase the outer hair cells, one Dieter cell for each OHC Hair cells  Stereocilia point up into scala media  IHCs in one row on medial side of tunnel of Corti  OHCs in three rows on lateral side of tunnel of Corti  Hair cells slant toward each other  Tallest stereocilia contact tectorial membrane  ~ 40 stereocilia per IHC  ~ 150 stereocilia per OHC Cochlear dimensions  Scalae are larger (greater volume) at base than at apex  Basilar membrane is wider at apex than at base  Georg von Bekesy: studied ear/cochlear mechanics; won Nobel Prize for showing ear that the ear is a frequency analyzer (like a series of mechanical bandpass filters) Psych 3A03 15 October 2012 Week 6 Dr. Paul A. Faure Basilar membrane traveling wave  Stapes motion creates traveling wave in basilar membrane  Traveling wave results in B.M. deformation from base to apex  More displacement of membrane with greater sound pressure level (SPL)  Membrane motion mirrors stimulus in terms of compressions and rarefactions Basilar membrane is tonotopic  Mechanical structure; wider & under less tension at apex; stiffest at base  Vibrational mechanics; can vibrate more freely near at apex  B.M. traveling wave start at base and travels toward apex  Traveling wave of differential motion travels along basilar membrane  Peak amplitude of vibration is at a particular area  Resonate frequency varies systematically with spatial location Basilar membrane displacement  Maximum displacement of traveling wave varies with frequency  Different frequencies maximally excite different regions of basilar membrane  Asymmetry in envelope of traveling wave  Low frequency stimulation: base and middle of membrane also set into motion, but peak mechanical displacement occurs toward apex of B.M.  Mid frequency stimulation: base of membrane also set into motion, but peak mechanical displacement occurs near middle of B.M.  High frequency stimulation: stimulates base of B.M. only,  Compare phase of stapes motion to B.M.: larger phase shift with higher frequency e.g. @ 27 mm from base a 200Hz tone is 180° out of phase compared to stapes foot plate motion, resulting in a 2.5 ms travel time delay at this frequency ( = 0.005 s at 200 Hz) Basilar membrane motion summary  Basal end vibrates best at high frequencies, but is also set in motion by low frequency stimulation  Apical end vibrates only to low frequency stimulation  Phase delay between stapes motion and peak in b.m. traveling wave vibration  Greater motion to higher SPL sounds  Asymmetrical traveling wave envelope: falls off gradually at basal side but is steep at apical side. Basilar Membrane acts as a series of mechanical bandpass filters  Certain regions require less pressure to be maximally stimulated  B.M. motion of B.M. tuned, vibrating best at characteristic frequency (CF) for that region (position)  Because of asymmetry in envelope of B.M. traveling wave, shape of isodisplacement function (SPL to vibrate at certain amplitude) has shallow slope below B.M. CF, and a steeper slope above CF.  Traveling wave stimulates larger region of B.M. towards base than apex  Frequencies above CF for a given B.M. position, will cause little vibration, whereas frequencies below CF willl result in more vibrations for a single position along cochlea.  Any point along basilar membrane acts as a mechanical bandpass filter Motion of Basilar Membrane is Nonlinear  Motion of B.M. is not linear with respect to SPL at characteristic frequency (CF)  Input-output function of membrane velocity shows a compressive nonlinearity at CF.  Change is compressive because steepness of function becomes shallower at some SPLs.  Relationship is more linear at SPL extremes within CF  Relationship also more linear at frequencies above and below CF for that position  Compressive nonlinearity results in audible distortion products (e.g. harmonics and difference tones). Inner (IHC) and Outer (OHC) Haircells  Stereocilia geometrically arranged in rows; rows have different lengths of stereocilia.  Inner hair cells have afferent neurons connected to them and efferent neurons connected directly to the afferent neuron. Psych 3A03 15 October 2012 Week 6 Dr. Paul A. Faure  Outer hair cells have afferent neurons connected to them and efferent neurons connected to the hair cell, not the afferent neuron as with inner hair cells.  Stereocillia between and within rows are connected.  Tallest stereocilia contact tectorial membrane sitting above it.  Stereocilia within & between rows are joined by lateral cross-bridges & tip link bridges.  Links strengthen rows and keep stereocilia moving together.  Tip links play a crucial role in transduction process of mechanical (vibrational) to electrochemical energy. When one moves, it brings others around it with it. Motion of Haircell Stereocilia  Basilar membrane anchored to spiral ligament by osseous spiral lamina.  Haircells stimulated by bending (shearing) forces acting on stereocilia.  Tectorial membrane anchored by spiral limbus (free end)  Stereocilia must be strong, and tip link cross bridges give strength for shearing forces.  When tip links stretch, ion channels in haircell membrane open, causing permeability change that then depolarizes haircell; resulting in release of transmitter to afferent neuron below.  Hair cells are receptors, not neurons. Stereocilia Cross Bridges and Tip Links  Depolarization based on mechanical opening of cation (K+) channels.  Tip links synchronize K+ channel opening.  K+
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