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Flashcards in Auditory I-III Deck (33)
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General physical nature of sound

  • sound radiates from vibrating sources
  • emitted as a serioes of pressure vaces of alternating compression (increased air density) and rarefaction (decreased air density)
  • primary auditory qualities:
    • amplitude (intensity)
    • frequency


Characteristics of sound intensity

  • intesity relates to force w/which air is compressed
    • increased force (higher amplitude) ==> increased air density
  • perceived as loudness
  • expressed on log scale = decibels of sound pressure level (dB SPL)
    • dB SPL = 20 * log [P1/P2]
      • P2 = standard reference pressure
      • i.e. if measured P1 = 10*P2, then sound = 20 dB SPL


Auditory threshold definition

  • lower lmit of sound detection by a patient
  • depends on frequency
  • threshold is used to measure hearing loss
    • smallest dB SPL that subject can detect at different frequences
  • [lower limit] threshold of any human = 20 x 10-6 N/m2


Characteristics of sound frequency

  • = number of times/sec that a sound wave reaches peak of rarefaction/compression
  • measured in Hertz (Hz = cycles/sec)
  • perceived as pitch
  • human hearing = 20 Hz - 20,000 Hz


Divisions of the ear

  • external ear
    • pinna
    • external auditory meatus (ear canal)
    • bounded by tympanic membrane
  • middle ear
    • contains ossicular chain (middle ear bones):
    • malleus
    • incus
    • stapes
  • inner ear
    • cochlea
    • semicircular canals (vestibular system)


Impedance mismatch in sound transmission @ ear

  • air = low impedance, while fluid = high impedance
    • aka fluid is more resistant to movement that air
  • if TM was directly acting on cochlea ==> inefficient transfer of sound energy ==> due to impedance mistmatch


Middle ear as "impedence matcher"

  • structure of TM + 3 ossicles allows ear to overcome impedence mistmatch
  • P = F/A ==> increase pressure by increasing force or by decreasing area ==> middle ear does both:
    • Area of the TM = 20x area of stapes footplate
    • orientation of ossicles ==> levering action ==> increased force


Conductive vs. sensorineural hearing loss + test to distinguish

  • conductive = due to degredation of mechanal transmisson of sound energy
  • sensorineural = damage or loss of hair cells or nerve
  • PE test for conductive vs. sensorineural:
    • place tuning fork near ear and then pressed against skull
    • in conductive hearing loss ==> tuning fork against skull will overcome deficit


Common causes of conductive hearing loss

  • fluid-filled middle ear e.g. otitis media
  • otosclerosis = arthirtic bone growth impedes movement of ossicles
  • malformation of ear canal
  • perforation/rupture of TM
  • interruption of ossicular chain
  • static pressure in middle ear


Common causes of sensorineural hearing loss

  • excessively loud sounds
  • exposure to ototoxic drugs
    • diuretics, aminoglycocide antibiotics, aspirin, cancer therapy durgs
  • age (presbycusis)


Structure of the inner ear

  • inner ear = coiled tube (cochlea) w/3 fluid-filled compartments
    • scala vestibuli
    • scala media
    • scala tympani
  • media and tympani are separated by basilar membrane (BM)
  • w/in scala media: on top of BM = organ of Corti
    • OoC contains inner hair cells ==> transduce sound into electrical signals
    • also contains outer hair cells (outer: inner = 3:1)


Mechanism of sound transduction ==> movement of basal membrane

  • Inner hair cells (w/in scala media @ cochlea) are attached to BM
  • movement of inner hair cells ==> movement of BM
  1. stapes compresses oval window ==> bulges into scala vestibuli
  2. compression is relieved by downward movement of BM ==> compression @ scala tympani ==> bulging of round window into middle ear
    1. opposit during rarefaction
  3. Due to varying mechanical properties of BM along cochlea, BM will actually vary in its response to frequencies along its length


BM response to frequencies by location in cochlea

  • BM @ base of cochlea = thinner, narrower, more rigid
    • vibrates best to high frequencies 
  • BM @ apex of cochlea = wider, more flexible, thicker
    • vibrates best to low frequencies 


Consequences of frequency-based arrangement of mechanical properties of BM

  • ==> "tonotopic arrangement/map"
  • = a topographic arrangement of "tones" (frequencies) along the length of the BM
  • ==> ability to organize the frequency senstivity of inner hair cells
  • ==> primary stimulus attribute that is mapped along cochlea is sound frequency (and intensity)


Mechanism of Inner Hair Cell transduction

  • hair cells project bunches of stereocilia (of varying lengths)
  • movement of bundle of stereocilia ==> change in membrane potential of hair cell
    • bundle pushed in direction of longest stereocilia ==> depolarization
    • bundle pushed in direction of shortest stereocilia ==> hyperpolarization

  • @ scala media: hair cells bathed in endolymph (K+-rich fluid) ==> endocochlear potential = +80 mV

    • bending of stereocilia ==> altered gating of NSC channels

    • in depolarization: mechanical force ==> opening of NSC ==> influx of K+ ==> depolarization


Mechanism of mechanical stimulation ==> opening of channels @ stereocilia

  • channels at tips of stereocilia are connected by "tip links" (= ~ springs)
  • bending of stereocilia causes tip-links to "pull" tops of stereocilia ==> mechanical opening fo channels 


Consequence of loss of endocochlear potential (+ example)

  • ==> sensorineural deafness due to loss of driving force for transduction
  • e.g. mutation @ subunit connexin 32 (important in active transport of K@ stria vascularis)  ==> collapse of endocochlear potential ==> major cause of congenital deafness
    • stria vascularis = epithelium @ scala medai


Overall steps from airborne sound ==> electrical nervous signals

  1. Airborne pressure waves in the external ear canal set up vibrations of the eardrum 
  2. Eardrum vibrations move the 3 ossicles 
  3. Vibrations of the stapes on oval window set up traveling waves in the cochlear fluids 
  4. These fluid waves cause a vertical displacements of the basilar and tectorial membranes 
  5.  The relative shearing force between membranes bends the ciliary bundles of the hair cells 
  6.  Ciliary bending leads to depolarization and hyperpolarization of the membrane potential, 
  7.  Which causes increased and decreased rates of transmitter release, respectively 
  8.  Transmitter (aspartate or glutamate) causes depolarization of the afferent auditory nerve fiber 
  9.  This results in action potentials that are sent to second order neurons in the brainstem. 


Afferent sensory nerves supplying the ear

  • signals generated by inner hair cells ==> "auditory nerve" = CN VIII
    • aka the spiral ganglion
  • Type I ANFs (auditory nerve fibers) innervate inner hair cells
    • 10-30 ANFs/IHC
  • Type II ANFs innervate outer hair cells
    • 1 ANF/10 OHC


Mechanism of cochlear amplification

  • OHC (outer hair cells) are poorly innervated by afferent nerves, but play an important role as "cochlear amplifier"
  • OHCs respond to changes in voltage with a change in length ==> pulling of the BM towards or away from the tectorial membrane ==> change in mechanical frequency selectivity of BM
  • This response at given frequencies leads to amplification via larger and sharper responses of the BM


Clinical important of OHCs/cochlear amplification

  • damage @ OHCs ==> sensorineural deafness
  • OHCs are more sensitive than IHCs to:
    • ototoxic antibiotics: streptomycin or gentomycin
    • prolonged exposure to loud sounds
  • OHCs can create sounds = "otoacoustic emissions"
    • method of testing sensorineural hearing in infants


Response of ANFs to sound transduced by IHCs

  • sounds response is characterized by a frequency tuning curve
    • = number of APs/sec corresonds to the sound frequency
    • max APs fired @ "characteristic frequency" to which fiber is sensitive
  • frequency tuning arises from mechanical frequency selectivity of BM


Coding of frequency/pitch: high frequency vs. low frequency

  • frequency is partly encoded by place along cochlea where afferent fibers innervate an IHC
  • high frequency = frequency tuning curve
  • low frequency (<1000 Hz)= using temporal pattern of action potentials ("phase lock")


Anatomical pathway: ANFs ==> brainstem

  • ANF = cell bodies @ spiral ganglion ==> travel w/ CN VIII ==> bifurcate @ brainstem to ventral cochlear nucleus and dorsal cochlear nucleus (both @ inferior cerebellar peduncle)
  • VCN axons = "trapezoid body" ==> cross midline
  • DCN axons = "dorsal acoustic stria" ==> cross midline
  • axons rejoin ==> lateral lemniscus ==> inferior colliculus @ midbrain
  • along the way some axons terminate at various nuclei @ pons:
    • superior olivary complex
  • some axons from cochlear nucleus join the ipsilateral lateral lemniscus


Clinical significance of anatomical pathway of ANFs

  • axons from cells @ cochlear nucleus ==> some ipsilateral lateral lemniscus and some contralateral lateral lemniscus
  • lesions rostral to cochlear nuclei DO NOT lead to unilateral deafness
  • lesions caudal to cochlear nuclei DO lead to unilateral deafness


Auditory pathway: Inferior colliculus ==> cerebral cortex

  • cochlear nuclei + superior olivary complex ==> inferior colliculus
  • inferior colliculus ==> 
    • ispilateral medial geniculate @ thalamus OR
    • contralateral inferior colliculus/medial geniculate
  • medial geniculate ==> primary auditory cortex (A1) @ superior temporal gyrus


Primary fxns of the auditory system

  1. identify what in the environment produced the sound
  2. identify where in space that sound came from


Location in nervous system of sound localization

  • ANFs code frequency, intensity, and temporal patterns of sound, but not location ==>
  • sound location must be computed centrally based on neural representations of spectral and temporal characteristics


Acoustical cues for sound localization

  • **Each cue is encoded in a separate pathway through the brainstem
  • Interaural time delays = differences in time of arrival between the ears
  • Interaural level differences = head creates an "acoustic shadow" for high frequency sounds to the far ear
    • small ILDs for low frequency
    • larger ILDs for higher frequency <== primary use = distinguish high frequency sounds
  • Monoaural spectral shape = arise from direction-based sound wave modification by interaction w/pinna


Anatomic locations of encoding of acoustical cues of soudn localization

  • ITDs = encoded @ medial superior olive
    • afferent inputs cary timing info via phase locked neural responses 
    • most sensitive to lower frequencies
  • ILDs = encoded @ lateral superior olive
    • mostly sensitive to high frequency sounds
    • cues from ipsilateral ear to LSO = excitatory
    • cues from contralateral ear to LSO = inhibitory
  • Spectral cues = ecnoded @ dorsal cochlear nucleus