Audition

Question 1

Nature of sound

Answer 1

• audible variation in air pressure
• cycle = distance between successive compressed patches of air
• frequency = number of cycles per second = pitch
  
  • expressed in Hz or KHz
  • human range = 20-20000Hz
• intensity = amplitude

Question 2

Ultrasound vs Infrasound

Answer 2

• unltrasound = greater then 20000Hz
  
  • dogs, bats, fish
• infrasound = lower than 20Hz
  
  • whales, elephants

-predator and prey usually have similar ability to detect sound

Question 3

Movement of sound

Answer 3

1. Sound waves move tympanic membrane
2. Tympanic membrane moves ossicles
3. Ossicles move membrane at oval window
4. Motion at oval window moves fluid in cochlea
5. Movement of fluid in cochlea causes response in sensory neurons

Question 4

Auditory pathway

Answer 4

1. Auditory receptors in cochlea
2. Brain stem neurons (where processing occurs)
3. Medial geniculate nucleus (MGN)
4. Auditory cortex

Question 5

Attenuation reflex

Answer 5

• tympani muscle = anchored to malleus and skull
• stapedius muscle = anchored to stapes and skull
• when muscles contract the ossicles become more rigid and diminish sound
• sound attenuation greater at low frequency than high

Question 6

Structure of inner ear

Answer 6

• reisners membrane separates scala vestibuli and scala media
• basilar membrane separates scala media and scala tympani
• organ of corti sits on top of basilar membrane
  
  • contains auditory receptor neurons
• tectorial membrane hangs over
• scala vestibuli + tympani contain perilymph
• scala media contains endolymph

Question 7

Cochlea

Answer 7

• base
  
  • closest to round window
  • narrow and stiff
  • high frequencies
• apex
  
  • furthest from round window
  • wide and floppy
  • low frequencies

Question 8

Tonotopy

Answer 8

-systematic organization of sound frequency within an auditory system

Question 9

Hair cells

Answer 9

• auditory receptors that have stereocilia
• sit between basilar membrane and reticular lamina
• rods of corti span both membranes for support
• cells outside rods of corti = outer hair cells
• cells inside rods of corti = inner hair cells
• stereocilia extend above reticular lamina into endolymph
  
  • connected to both membranes
• hair cells form synapses on neurons whose cell bodies are in spinal ganglion
  
  • axons from spinal ganglion enter auditory nerve to project to cochlea nuclei in medulla)

Question 10

Transduction

Answer 10

• sound waves cause stereocilia to bend back and forth, so hair cells generate receptor potential that alternately hyperpolarizes and depolarizers
• stereocilia can move from 0.3nm—> 20nm
• inward ionic flow generates hair cell receptor potential
• K+ influx causes depolarization, activating VGCC to trigger release of glutamate to activate spinal ganglion
• majority of auditory info from inner hair cells

Question 11

Cochlear amplifier

Answer 11

-outer hair cells act like tiny motors that amplify movement of basilar membrane

Question 12

Audition pathways

Answer 12

• afferent from spinal ganglion enter brainstem in auditory-vestibular nerve
• at medulla, axons branches to innervate DORSAL COCHLEAR NUCLEUS + VENTRAL COCHLEAR NUCLEUS ipsilaterally
• axons from ventral cochlear nucleus project to BOTH SUPERIOR OLIVE (bilateral)
• axons then ascend in lateral lemniscus to INFERIOR COLICULUS of midbrain (bilateral)
  
  • ALL ascending pathways CONVERGE in inferior colliculus
• neurons in inferior colliculus project to MGN, then A1

Question 13

Characteristic frequency

Answer 13

• frequency at which neuron is most responsive to

- in MGN some cells respond to complex sounds like vocalization and some response to simple selective frequency

Question 14

Intensity coding

Answer 14

-encoded by firing rather + number of activated neurons

Question 15

Tonotopy

Answer 15

• map of basilar membrane in cochlear nuclei
• not enough to deduce/encode frequency because we need intensity AND frequency

-tonotopy alone above 5khz

Question 16

Phase locking

Answer 16

• consistent firing of cell at same phase of sound wave
• frequency of AP = frequency of sound wave
• mainly for low frequencies
• volley principle= at higher frequencies neurons surrounding all collaborate to fire an AP to cover each cycle of sound wave
  
  • pooled activity creates phase locking manner

-humans use tonotopy alone above 5khz

Question 17

Sound localization

Answer 17

• horizontal localization requires comparison of both ears, vertical doesnt
• interaural time delay: time difference between sound reaching either ear (no delay if straight ahead)
• continuous sounds harder to localize
• interaural intensity difference: relationship between the direction the sound comes from and the extent to which your head shadows the sound to one ear
  
  • only high frequencies

Question 18

Duplex theory

Answer 18

=time delay (low freq) + intensity difference (high freq)

Question 19

Cochlear nuclei

Answer 19

• receives input ipsilaterally ONLY
  
  • monaural
• superior olive received input bilaterally
  
  • binaural

Question 20

To the cortex

Answer 20

• axons leaving MGN project to A1 via internal capsule
  
  • ACOUSTIC RADIATION
• A1 tonotopy:
  
  • low frequencies: low, anterior
  • high freq: posterior, medial

Question 21

Distinguishing speech

Answer 21

-manipulate sound do that it is unintelligible but has same frequency and intensities —> ROTATED SPEECH

Question 22

Deafness

Answer 22

• A1 Lesions:
  
  • unilateral = almost normal audition (due to bilateral projection) - sound localization deficits
  • bilateral = deaf
• Conduction deafness = problems with conduction from outer to inner ear
  
  • sometimes rescued by hearing aid
• nerve deafness = damage to hair cells/auditory nerve
  
  • cant be rescued