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Chapter 11

Chapter 11 The Auditory and Vestibular Systems

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University of Toronto Scarborough
Matthias Niemeier

Chapter 11The Auditory and Vestibular SystemsAudition sense of hearing and Vestibular system sense of balance have very different functions but similar structure and mechanism audition is an important communication and balance is for our personal internalized processSounds in our environment are translated into meaningful neural signalsMovements of our head are translated into a sense of where we areTHE NATURE OF SOUND y Sounds are audible variations in air pressure eg object moving towards a patch of air increases the density of molecules y Many sources of sounds produce PERIODIC variations in air pressure y Frequency number of compressed or rarefied patches of air that pass by our ears each second DETERMINES PITCH y Hertz Hznumber of cycles per second used to express sound frequency y Sound waves all propagate at the same speed they differ in frequency and intensity amplitudeHigher frequency is perceived as higher pitch and higher intensity is perceived as louder volumey Our auditory system can respond to pressure waves over a range of 20Hz to 20000 Hz BUT w age and exposure to noise y Intensitydifference in pressure between compressed and rarefied patches of air DETERMINES LOUDNESS y Unique tonal qualities come from simultaneous combination of different frequency eaves at different intensitiesTHE STRUCTURE OF THE AUDITORY SYSTEM y Pinnahelps collect sounds from a wide areaShape makes us more sensitive to sounds coming from ahead than behindConvolutions play a role in localizing soundsy Auditory canalprovides entrance to the internal ear extends 1 inch inside the skully Tympanic membraneeardrum y Ossiclesa series of bones connected to the medial surface of the eardrum which transfer movements of the eardrum into movements of a second membrane covering a hole in the bone of the skulloval window y Cochleafluidfilled contains the apparatus for transforming the physical motion of the oval window membrane into a neuronal responseFirst stages of the auditory pathway 1 Sound wave moves the tympanic membrane eardrum2 Tympanic membrane moves ossicles 3 Ossicles move the membrane at the oval window 4 Motion at the oval window moves the fluid in the cochlea5 Movement of the fluid in the cochlea causes a response in sensory neurons y Outer Earstructures from the pinna to the tympanic membrane y Middle Earmade up of the tympanic membrane and the ossiclesy Inner Earapparatus medial to the oval window y Medial geniculate nucleus MGNa relay in the thalamus which receives output from nuclei from the brainy Primary auditory cortex A1the area which the MGN projects to on the temporal lobeTHE MIDDLE EAR Variations in air pressure are converted into movements of the ossicles in the middle earComponents of the Middle Eary Three ossiclesmalleus hammer forms RIGID connection with incus anvil which forms FLEXIBLE connection with strapes stirrup footplate of the stapes moves in and out at the oval window to transmit sound vibrations to the fluids of the cochlea in the inner eary Eustachian tubeallows the air in the middle ear to be continuous with the air in the nasal cavities closed by valve
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