Auditory I-III

Question 1

General physical nature of sound

Answer 1

• 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

Question 2

Characteristics of sound intensity

Answer 2

• 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

Question 3

Auditory threshold definition

Answer 3

• 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

Question 4

Characteristics of sound frequency

Answer 4

• = 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

Question 5

Divisions of the ear

Answer 5

• 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)

Question 6

Impedance mismatch in sound transmission @ ear

Answer 6

• 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

Question 7

Middle ear as “impedence matcher”

Answer 7

• 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

Question 8

Conductive vs. sensorineural hearing loss + test to distinguish

Answer 8

• 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

Question 9

Common causes of conductive hearing loss

Answer 9

• 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

Question 10

Common causes of sensorineural hearing loss

Answer 10

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

Question 11

Structure of the inner ear

Answer 11

• 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)
    *

Question 12

Mechanism of sound transduction ==> movement of basal membrane

Answer 12

• 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

Question 13

BM response to frequencies by location in cochlea

Answer 13

• 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 *

Question 14

Consequences of frequency-based arrangement of mechanical properties of BM

Answer 14

• ==> “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)

Question 15

Mechanism of Inner Hair Cell transduction

Answer 15

• 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

Question 16

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

Answer 16

• 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

Question 17

Consequence of loss of endocochlear potential (+ example)

Answer 17

• ==> 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

Question 18

Overall steps from airborne sound ==> electrical nervous signals

Answer 18

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.

Question 19

Afferent sensory nerves supplying the ear

Answer 19

• 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

Question 20

Mechanism of cochlear amplification

Answer 20

• 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

Question 21

Clinical important of OHCs/cochlear amplification

Answer 21

• 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

Question 22

Response of ANFs to sound transduced by IHCs

Answer 22

• 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

Question 23

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

Answer 23

• 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”)

Question 24

Anatomical pathway: ANFs ==> brainstem

Answer 24

• 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

Question 25

Clinical significance of anatomical pathway of ANFs

Answer 25

• 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

Question 26

Auditory pathway: Inferior colliculus ==> cerebral cortex

Answer 26

• 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

Question 27

Primary fxns of the auditory system

Answer 27

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

Question 28

Location in nervous system of sound localization

Answer 28

• 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

Question 29

Acoustical cues for sound localization

Answer 29

• **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

Question 30

Anatomic locations of encoding of acoustical cues of soudn localization

Answer 30

• 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

Question 31

Auditory portion of the thalamus =

Answer 31

medial geniculate body (MGB)

Question 32

Location of auditory cortex

Answer 32

• auditory areas of cortex = superior temporal gyrus
• primary auditory cortex (A1) = Broadmann’s area 41
• A1 is surrounded by secondary auditory cortex (A2) = Broadmann’s area 42

Question 33

Mapping of sound @ cortex

Answer 33

• tonotopic map = spatially organized by response to given frequency
• anterior neurons @ A1 = respond to low frequencies
• posterior neurons @ A1 = respond to high frequencies