Auditory System Barb Rohrer (SEI614 2-5086)
Sounds arise from mechanical vibration (creating zones of compression and rarefaction; which ripple outwards) Transmitted through gaseous, aqueous or solid medium (vacuum?) Any object capable of creating disturbance can create sound 2 properties perceived by hearing - Frequency - perceived as pitch - Amplitude - perceived as loudness Nature of sound
Measures of sound intensity Sounds measured in decibels (db) (typically measured by microphones) 0 db = sound pressure at faintest sound heard Increase in 20 db is 10-fold increase in sound pressure and a 20-fold increase in power (power goes with the square of the pressure) Most sensitive hearing occurs at 1-4 KHz Frequencies below & above must be louder to be heard (tuning curve) Above 120 db is painful
Measures of sound intensity sound is white noise (a mix of all audible frequencies) second sound uses ½ power as first sound: 10 log (P 2 /P 1 ) = 10 log 2 = 3 db 0 db = sound pressure at faintest sound heard One decibel is close to the Just Noticeable Difference (JND) for sound level.
Mechanical conduction components: Outer ear Pinna - direction sensitivity (limited) Ear canal - natural resonance at 1-4 KHz; important for speech fricatives (amplification) Middle ear - overcomes 30 db loss of energy going from air to water Tympanic membrane to ossicles (malleus, hammer; incus, anvil; stapes, stirrup) to oval window membrane Area ratio of two membranes (17:1) Structure of the ear Middle Outer ear ear - Lever action of the ossicles (1.3:1) Inner ear Inner ear - cochlea Transduction of sound to neural activity
Boney cavity in temporal bone Filled with fluid Coiled divided lengthwise by membranous tube basilar membrane cochlear duct (scala media) follows cochlear spiral Cochlea structure Scala vestibuli (above basilar membrane) Scala tympani (below basilar membrane) S. vestibuli & tympani filled with perilymph (like extracellular fluid; high in Na + ) S. media filled with endolymph (like intracellular fluid; high in K + ) Fluid movement: basilar membrane Stapes imposes vibration on scala vestibuli; transmitted across basilar membrane to scala tympani; pressure relief by round window
Fluid filled chamber vibrated by stapes movement Divided by basilar membrane Receptor cells rest on basilar membrane Model of cochlea Membrane stretched out to increase length Coiled to take up less space
Vibration causes a traveling wave to move down the basilar membrane Independent of method of stimulation Cochlear fluid movement Different frequencies cause maximal vibration at different places along the basilar membrane
Cochlea organization 3 tubes separated by basilar membrane (BM) Organ of corti rests on BM Pressure diff. across s. media cause vibration of BM Receptor cells (Hair cells) rest on basilar membrane 1 row of inner; 3 rows of outer Stereocilia of hair cells covered by stiff, gelatinous tectorial membrane Afferents come from inner hair cells (spiral nerve fibers) Outer hair cells regulate frequency sensitivity (input from superior olivary complex) Organ of corti
Hair cell stimulation Vibration of basilar membrane pushes stereocilia against tectorial membrane (TM) TM is stationary Bending of stereocilia results in depolarization of hair cells Hair cells release neurotransmitter onto afferent fiber terminals EMs of hair cells A = transmission EM B = scanning EM
Hair cell stimulation Vibration of basilar membrane pushes stereocilia against tectorial membrane Bending of stereocilia results in depolarization of hair cells Cation influx depolarizes cell (scala media contains high K + such that K + rather the Na + carry current; thus many drugs which affect kidney also affect cochlea) Depol. opens voltage-gated Ca 2+ channels Hair cells release neurotransmitter (glutamate) onto afferent fiber terminals (cell bodies in cochlear nucleus)
Mechanical properties of basilar membrane Base (near oval window) basilar membrane: narrow, rigid Apex (helicotrema) basilar membrane: wide, loose Tonotopic localization: Different frequencies cause maximal amplitude at different places along BM High freq. - max vibration at base Low freq. - max vibration at apex Each afferent fiber arises from small point on basilar membrane cochlear implants take advantage of this tonotopic arrangmenet Bone vs air conduction (test for middle ear damage)
Noise damage to hair cells Many stereocilia of the outer and inner hair cells damaged Some hair cells die (we only have 16,000 hair cells) A = normal B = high noise exposure Guinea pig cochlea - scanning EM Hair cells sensitive to high frequency more sensitive to noise damage
Tonotopic representation of frequency Cochlear afferent responses tuned to one frequency Each frequency causes max vibration at one part of cochlea Each frequency excites afferents from one area of cochlea Afferents responding to each frequency project to separate areas in ascending relay sites As intensity of sound is increased: a. Firing rate of active fibers is increased b. More fibers are recruited from adjacent parts of cochlea
Ascending auditory pathways Cochlear afferents (cochlear component of IIX cranial nerve) Cochlear nucleus (brainstem) Ipsilateral input Superior olivary nuclei Beginning of bilateral input Nucleus of lateral lemniscus Inferior colliculus Medial geniculate (thalamus) Auditory cortex - temporal lobe (Brodmann s area 41 and 42) Tonotopically organized binaural interaction: intensity and time differences help localize sound
Located on superior temporal lobe (Brodmann s area 41 and 42) Tonotopically organized (Cells in the same vertical column respond to same frequency) New cell properties Direction sensitivity (intensity and phase delays) (humans can discern a shift in sound source from midline corresponding to an interaural delay as small as 10 µsec) Tone pairs Frequency modulation (important for processing speech) Auditory cortex
QUESTIONS? Barb Rohrer (SEI614 2-5086) rohrer@musc.edu