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Lecture 14

Lecture 14 - Hearing

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University of Toronto St. George
Michelle French

PSL300H1F L14; Oct. 14, 2011 endolymph-filled chamber sitting on top of basal membrane dividing cochlea into upper and lower where pressure pulses exterted oval window in upper chamber must have output for compressed fluid pressure flexible wall of round window w ossicles pressure pulse thru upper and lower (output at bottom) chambers filled w perilymph directs pressure across membrane distortion in basal membrane deflection of hair cells depolarization mechanical properties natural resonance of basal membrane different depending on where close to windows, stiff membrane so high natural freq to oscillate at; ~20,000hz; goes to window instead of fluid, max vibration at window Mechanoreceptor system broad at other end, very flexible, much lower Hair receptors problem is mechanical - stimulus = natural freq, as high; 30-50hz; goes thru all fluid seq of pressure pulses in air collected in auditory for max vibration at apex of basal membrane canal so membrane tuned to dif freqs Problem = must amplify pressure exterted on cochlea to maintain in fluid which is denser than Traveling wave (5 kHz) air maximally displace basal membrane at Middle ear mechanics particular pt Tympanum vibrates with sequence of sound pressure relative to window pulses (in air) area lever system of ossicles amplifies pressure on rest of membrane tympanum 22-fold by apex amplified pressure exerted on oval window of cochlea unaffected, (filled with fluid) 200 Hz Tone max distortion towards apex all membrane, most is affected since pressure pulses travel all the way thru cochlea to this pt note theres a connection at middle, air chamber connected to back of pharynx to allow for equalization of pressures across tympanum ex. Swallow to equalize pressure very sensitive hair cells sitting on top of basal membrane, extends shell curled up dif chambers throughout length of cochlea preferred freq at dif locations overlayed by flap of tissue tectorial membrane, cilia embedded axons of afferents collect in bundles, ganglion in wall of cochlea spiral ganglia cranial nerve bending affects opening of ion channels Hair cell amplification hair cells divided in two cells outer most numerous inner when outer hair cells depolarized contract pull separated by A-frame scaffold frame work, rod support on framework basal membrane bulges downward extension into reticular lamina, cilia stick thru to exaggerates vibration; good to amplify weak signals embed in tectorial membrane dont always have weak sounds, need mechanism for protecting hair cells from loud sounds efferents originate from within brainstem signals sent to cochlea adjusting sensitivity of hair cells inhibitory: so hyperpolarizing outer hair cells so dont contract as much so not amplifying the signal Olivocochlear Efferents 2 types
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