Audi%on The Physical S%mulus Auditory Anatomy & Physiology •  Light energy normally is described in terms of wavelength (700 nanometers to < 400 nanometers). •  Sound energy normally is described in terms of frequency (rate of vibra%on) ranging from ~16 or 20 Hz to 20,000 Hz. •  The frequency range of visible light extends from 400,000,000,000,000 Hz (400 trillion Hz) (for red) to 7,500,000,000,000,000 Hz (for violet) Wavelength & Frequency •  There is an inverse rela%onship between wavelength (λ) and frequency. •  Wavelength = velocity/frequency •  For light, velocity = 186,000 miles/second or 260,400,000 meters/second •  For sound, the velocity is about 370 meters per second. •  The wavelengths of light are very short. •  The wavelengths of sounds are very long The Auditory S%mulus Physical Proper%es of Sound •  For simple sine waves –  Amplitude: Sound Pressure Level (dB) [roughly perceived as loudness] –  Frequency: cycles per second (Hz) [roughly perceived as pitch] –  Phase [speaker wires, noise-­‐canceling headphones] •  Complex waves (Fourier analysis) Sound Transmission •  Sound can travel through any elas%c medium that has iner%a (but not through a vacuum) [sci-­‐fi movies] •  Air molecules in random mo%on that becomes non-­‐random with sound s%mulus •  Molecules push against neighbors, so molecule at sound source doesn’t go to your ear •  Impedance mismatch: difference in elas%city of media for transmibng sound Overview •  hdp://www.youtube.com/watch?
v=46aNGGNPm7s&feature=related Outer Ear •  Pinna: car%laginous outer flap helps to localize high frequency sounds as well as front from back •  Concha (cave) is the deep center of pinna •  External auditory meatus (opening) •  External auditory canal (2-­‐3 cm long) •  Eardrum (tympanic membrane) marks end of outer ear, seals off middle ear •  Effect of outer ear is to cause an increase in amplifica%on of ~10-­‐15dB in a frequency range of 1.5-­‐7kHz…mainly due to concha and external auditory canal •  Ear wax: from secre%ons of glands, repels water, traps dust, insect repellant? •  hdp://www.stupid.com/stat/WAXX.html ”Don’t s%ck anything in your ear that's smaller than your elbow!" Middle Ear •  Cavity is about 2 cm3 •  Connected to nasal cavity by eustachian tube [equalizes pressures] •  Tympanic membrane is .1mm thick and 55-­‐90 mm2; composed of two sets of fibers, one set radiates from the center to the outside and one is formed into rings •  Ossicles are the smallest bones in our bodies: Malleus (hammer) Incus (anvil) Stapes (s%rrup), footplate implanted in oval window Func%on of Middle Ear •  Conduc%on –  Conduct sound from the outer ear to the inner ear •  Protec%on –  Creates a barrier that protects the middle and inner areas from foreign objects –  Middle ear muscles may provide protec%on from loud sounds •  Transducer –  Converts acous%c energy to mechanical energy –  Converts mechanical energy to hydraulic energy •  Amplifier –  Transformer ac%on of the middle ear –  only about 1/1000 of the acous%c energy in air would be transmided to the inner-­‐ear fluids (about 30 dB hearing loss) Tympanic Membrane •  The eardrum separates the outer ear from the middle ear •  Creates a barrier that protects the middle and inner areas from foreign objects •  Cone-­‐shaped in appearance –  about 17.5 mm in diameter •  The eardrum vibrates in response to sound pressure waves. •  The membrane movement is incredibly small –  as lidle as one-­‐billionth of a cen%meter Ossicles •  Malleus (hammer) •  Incus (anvil) •  Stapes (s%rrup) smallest bone of the body Transformer Ac%on #1 Area ra%o of TM to Stapes TM ~ 55 mm2 (part that moves) Area ra%o about 17:1 Stapes ~ 3.2 mm2 Transformer Ac%on #2 Lever Due to Malleus/Incus Length Incus 7 mm Malleus 9 mm Normal balance beam has equal lengths on both sides of the fulcrum. Ossicles are ≠ Middle Ear Muscles •  Tensor tympani –  Adached to malleus –  Innervated by V, trigeminal nerve •  Stapedius –  Adached to stapes –  Innervated by VII, facial nerve •  Middle Ear Muscle Func%on: –  Help maintain ossicles in proper posi%on –  Protect inner ear from excessive sound levels •  When ear exposed to sound levels above 70 dB, the muscles contract, decreasing amount of energy transferred to inner ear –  This protec%ve reflex termed "acous%c reflex" Inner Ear Balance •  Linear mo%on •  Rotary mo%on Inner Ear Transduc%on •  hdp://www.youtube.com/watch?
v=1JE8WduJKV4&feature=related Cochlea •  The cochlea resembles a snail shell and spirals for about 2 3/4 turns around a bony column •  Within the cochlea are three canals: –  Scala Ves%buli –  Scala Tympani –  Scala Media Anatomy of the adult mammalian cochlea
Barald, K. F. et al. Development 2004;131:4119-4130
Inner and Outer Hair Cells •  About 3,500 inner hair cells arranged in a single row •  About 12,000 outer hair cells arranged in 3 or 4 rows •  Auditory nerve contains about 30,000 afferent fibers (to the brain) and 500 efferent fibers (from the brain) •  IHC get most of the afferent fibers and OHC get most of the efferent fibers OHC vs. IHC Func%on Electron micrograph of a hair bundle as seen from above. Note: (1) cilia are arranged according to height, (2) cilia are arranged in a very dis?nc?ve pa@ern, variously described as a ‘W’ or some?mes a ‘V’ shape. modiolus spiral ligament IHCs modiolus OHCs spiral ligament Outer Hair Cell Mo%lity •  hdp://www.youtube.com/watch?
v=Xo9bwQuYrRo •  Few afferent connec%ons (to the brain) •  Elonga%on and contrac%on of cells serves as a cochlear amplifier •  Efferent input to the outer hair cells modifies their response •  Contain pres%n in their walls, which drives mo%lity Stereocilia •  About .05 micrometers in width •  Displacement of .3 nm (3/10 of a billionth of a meter) can give rise to sound percep%on—
equivalent to the top of the Sears tower moving about 5 cm