Hearing September 8 10 2014 Sound Amplitude and Frequency of Sound Waves 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 Hz Other Animals Hear Lower or Higher Frequencies Fourier Analysis Fourier Baron Jean Baptiste Joseph Fourier A square wave is the sum of odd harmonics f 3f 5f 7f etc Spectrograms ta Frequency Hz da Time msec Time msec Spectrograms display how the frequency contents of a sound change over time Ear separates sound into it different frequency components Intensity of Sound Decibels of sound pressure dB SPL are dB SPL 10 log10 P2 Pr2 20 log10 P Pr Where Pr is the reference pressure of 0 0002 dynes cm2 the threshold of hearing Pressure dyne cm2 dB SPL Experience 0 0002 0 Threshold of hearing 0 0020 20 Faint whisper 0 0200 40 Quiet office 0 2000 60 Conversation 2 0000 80 City bus 20 0000 100 Subway train 200 0000 120 Loud thunder 2000 0000 140 Pain and damage Peripheral 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 slowly Cochlea 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 signals Basilar 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 cochlea Basilar Membrane High frequency Base of cochlea short stiff tonotopic map Low frequency Apex of cochlea wide floppy 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 vibrations Organ 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 stereocilia Innervation of the Cochlea There are about 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 cells Transduction 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 Reissner s Membrane Scala Vestibuli Endolymph very high K Perilymph low K Scala Media Basilar Membrane Scala Tympani 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 cells These Are Your Stereocilia These Are Your Stereocilia On Loud Sounds Volley Theory 2 msec Basilar membrane vibration in response to a 500 Hz tone Motion of stereocilia on hair cells Excitatory neurotransmitter release Action potentials in auditory nerve fiber 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 midbrain Binaural Projections Sound 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 difference Structure of the LSO and MSO LSO neurons respond to intensity differences between the ears MSO neurons respond to the timing differences between the ears Descending pathway efferents Two pathways project from the SOC to the cochlea Medial system around MSO large myelinated fibers Lateral system around LSO thin unmyelinated fibers Tonotopic 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 to Thalamus Auditory 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 implant