NRSC 2100 1st Edition Lecture 15 Auditory System Hearing Sound Properties Propagates through gases liquids and solids metals Cycle successive compression and rarefaction of medium Frequency pitch cycles per seconds Hertz Hz humans 20 20 000 Hz Intensity loudness logarithmic scale decibels dB very sensitive wide range 0 160 dB Complexity timber additivity of simple waves gives rise to complex waves most sounds are complex The Human Ear and Sound Propagation Sound wave processing Sound waves Tympanic membrane Ossicles Oval window Cochlear fluid Hair cells transducers Sensory neurons response Middle Ear Mechanisms These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Sound Amplification by the Ossicles The Attenuation Reflex Onset of loud sound causes tensor tympani and stapedius muscle contraction Function Adapt ear to loud sounds improves speech perception Cross Section of The Cochlea Perilymph Fluid in scala vestibuli and scala tympani Endolymph Fluid in scala media Endocochlear potential Endolymph electric potential 80 mV more positive than perilymph Inner Ear The Cochlea Uncoiling the pea size cochlea Perilymph is continuous between scala vestibuli and tympani Movement of stapes imparts movement to basilar membrane The Hearing Structure Within the Cochlea The Organ of Corti Outer and inner hair cells embedded in Deiter s cells and Rods of Corti within the Organ of Corti stereocilia contained at the top of each hair cell extending partially into tectorial membrane Inner hair cells are mostly responsible for providing auditory information eventually reaching the brain outer hair cells regulate tightness of the cochlea Transduction of Sound Pressure by Hair Cells Sound pressure perilymph endolymph movement basilar membrane movement hair cell stereocilia movement against tectorial membrane stereocilia movement or polarization of hair cells via tip link regulation of TRPA1 channels or of voltage gated calcium channels or of neurotransmitter release onto terminals of spiral ganglion neurites Stereocilia Approximately 100 at the top of each hair cell Movement toward long stereocilia depolarization Movement toward short stereocilia hyperpolarization Short to long stereocilia connected via tip link filaments Coding Of Sound Intensity Loudness Determined by both firing frequency and number of spiral ganglion neurons related to hair cell activity on basilar membrane ex soft sound fewer AP loud sound more AP from spiral bipolar neurons Coding Of Sound Frequency 1 Place coding a Different frequencies vibrate basilar membrane at different spots places b Works for moderate to high frequencies 200 to 20 000 Hz near base high frequencies 20 000 Hz near apex lower frequencies 200 Hz 2 Phase lock coding also called rate coding a Frequency of sound frequency of action potentials locked on specific wave phase b Multiple phase locked fibers together volleys c Works for low to medium frequency sounds 20 4000 Hz Coding Of Sound Localization Horizontal Plane 1 Interaural time delay Time taken for sound to reach from ear to ear 2 Interaural intensity difference Intensity differences between the two ears 3 Duplex theory of sound localization both mechanisms employed Interaural time delay 20 2000 Hz Interaural intensity difference 2000 20000 Hz Coding Of Sound Localization Vertical Plane Precise shape of pinna provides slightly different echo times to sounds coming from different vertical angles Frequency tonotopic organization maintained throughout auditory system all the way to primary auditory cortex Primary auditory cortex posterior superior temporal gyrus