Slide 1SoundAnimals Hear Different FrequenciesFourier AnalysisSpectrogramsIntensity of SoundPeripheral Auditory SystemMiddle EarCochleaBasilar Membrane Analyzes FrequencyBasilar MembraneOrgan of CortiInnervation of the CochleaTransduction of Sound into Neural SignalsSlide 15These Are Your StereociliaVolley TheoryAscending Auditory PathwaysBinaural ProjectionsSound Localization in Superior OliveStructure of the LSO and MSODescending pathway (efferents)Tonotopic MapThalamusAuditory Areas in NeocortexSlide 26Exciting the Cochlea Without SoundHearingFebruary 1 & 3, 2016Sound•Sound is the vibration of air molecules.•A tuning fork produces a sinusoidal vibration•The number of cycles each second is the frequency (Hertz)•We hear frequencies between 20 Hz and 20,000 HzAmplitude and Frequency of Sound WavesAnimals Hear Different FrequenciesFourier AnalysisFourier Baron Jean Baptiste Joseph FourierA square wave is the sum of odd harmonics: f, 3f, 5f, 7f, etc.SpectrogramsFrequency (Hz)Time (msec) Time (msec)/da/ /ta/• Spectrograms display how the frequency contents of a sound change over time.• Ear separates sound into it different frequency componentsIntensity of SoundDecibels of sound pressure (dB SPL) aredB SPL = 10 log10(P2/Pr2)= 20 log10(P/Pr)Where Pr is the reference pressure of 0.0002 dynes/cm2, the threshold of hearingPressure (dyne/cm2) dB SPL Experience0.0002 0 Threshold of hearing0.0020 20 Faint whisper0.0200 40 Quiet office0.2000 60 Conversation2.0000 80 City bus20.0000 100 Subway train200.0000 120 Loud thunder2000.0000 140 Pain and damagePeripheral Auditory System•Pinna (external ear) amplifies sound•Cochlea is where neurons transduce sound into neural signals. It is filled with fluid.•Middle ear compensates for the loss of intensity as vibrations in air are changed to vibrations in fluid.•Auditory nerve carries the signal to brain stem (medulla)Middle Ear•Middle ear amplifies sound by gathering energy over a large area and focusing it on a small area (similar to a thumb tack)•Muscles in middle ear can dampen sound, but act slowlyCochlea•The cochlea is actually two tubes coiled together•The top tube is divided into two parts by Reissner’s membrane•The tubes are filled with fluid•The two tubes are separated by the basilar membrane•The organ of Corti sits on basilar membrane and is the structure that changes sound vibrations into neural signalsBasilar Membrane Analyzes Frequency•Unroll the cochlea•High frequencies produce the largest vibrations of the basilar membrane near the base of the cochlea•Low frequencies produce the largest vibrations near the apex of the cochleaBasilar Membrane•The basilar membrane is short and stiff at the base of the cochlea (responds to high frequency)•It is wide an floppy at the apex of the cochlea (responds to low frequency)•Place theory of pitch perception: pitch is the location of the largest vibrationsBase of cochlea (short & stiff)Apex of cochlea (wide & floppy)High frequencyLow frequencytonotopic mapOrgan of Corti•In the organ of Corti, there are three rows of outer hair cells and one row of inner hair cells•There are about 9000-12000 outer hair cells and 3500 inner hair cells in each cochlea•On top of each hair cell is a band of stereociliaInnervation of the Cochlea•There are 30,000 to 50,000 auditory nerve fibers•95% of the auditory nerve fibers innervate the inner hair cells, only 5% go to the outer hair cells•There are also descending inputs onto the hair cellsTransduction of Sound into Neural Signals•Vibrations of the basilar membrane wiggle the stereocilia •The back and forth motion of the stereocilia opens and closes channels at their tips.•When the channels open positive ions enter and depolarize the hair cell, which releases neurotransmitter onto auditory nerve fibers.•Scala media contains a very high concentration of K+•K+ enters the hair cell when the channels at the tips of the stereocilia open and the hair cell becomes depolarized•K diffuses into perilymph at base of cells when tip channels are closed. No need for Na/K pumps in hair cellsEndolymph • very high K+Perilymph • low K+Basilar MembraneScala TympaniScala MediaReissner’s MembraneScala VestibuliThese Are Your StereociliaThese Are Your StereociliaOn Loud SoundsVolley TheoryBasilar membrane vibration in response to a 500 Hz toneMotion of stereocilia on hair cellsExcitatory neurotransmitter release Action potentials in auditory nerve fiber2 msec• In response to a 500 Hz tone, there are 500 action potentials spaced 2 msec apart• The volley theory proposes that the number of action potentials equals the frequency of the sound, and the interval between them equals the period of the stimulus frequency.Ascending Auditory Pathways•Auditory nerve projects to the cochlear nucleus on the side of the brain stem•From the cochlear nucleus, parallel pathways convey information to the inferior colliculus in the midbrainBinaural ProjectionsSound Localization in Superior Olive•Location of a sound must be computed by auditory system•Two cues –Difference in the arrival time of the sound at each ear–Intensity difference between the two ears caused by the head•Medial superior olive computes time difference•Lateral superior olive computes intensity differenceStructure of the LSO and MSO•LSO neurons respond to intensity differences between the ears•MSO neurons respond to the timing differences between the earsDescending pathway (efferents)•Two pathways project from the SOC to the cochlea–Medial system (around MSO) – large, myelinated fibers–Lateral system (around LSO) – thin, unmyelinated fibersTonotopic Map•Information from several brainstem auditory structures is combined in the inferior colliculus•Imaging studies have shown that there is a good tonotopic map in the inferior colliculus.•The goal is to combine the correct frequencies toThalamusAuditory Areas in Neocortex•Primary auditory cortex is in the temporal lobe on the bank of the Sylvian fissure•Wernicke’s area involved in speech analysis (Broca’s area is for speech production).•All components of speech identified and combined before the information reaches this level.Exciting the Cochlea Without Sound•Auditory nerve fibers send message to brain following depolarization produced by neurotransmitter.•For deaf people, the nerve can be depolarized by stimulating with electrical current from a cochlear