L39 - Audition

Question 1

Function of external ear?

Answer 1

collection/ localization/ modification of sound

Question 2

2 functions of the middle ear?

Answer 2

Impedance matching

Sound attentuation via middle ear reflex

Question 3

Describe the physiology of impedance matching at the middle ear.

Answer 3

1. Air-borne vibration stops at eardrum
2. Fluid (perilymph, endolymph) vibration starts at oval window

2 media match via:

a) Area-ratio difference (exam) between the large eardrum and the small oval window
b) Leverage of ossicles bones (malleus, incus, stapes) (small contribution)

Question 4

Sequence of structures that transduce sound from external auditory canal to inner ear?

Answer 4

Air vibration in external auditory canal, vibrate ear drum at middle ear
» ossicle bones leverage (malleus, incus, stapes) at oval window of inner ear
» cause perilymph vibration&raquo_space; endolymph vibration
» movement of basilar membrane causing inner, outer hair cells’ cilia to bend and move against tectorial membrane
» Perilymph vibration
» Round window

Question 5

Describe the physiology and function of Middle ear muscle reflex?

Answer 5

For sound attenuation:

During exposure to loud sound / prior to vocalization, stapedius contracts to orient ossicles = less movement of stapes bone = less leverage at oval window:

Function:
• attenuate low frequency
• protect the inner ear
• improve speech discrimination in noise

Question 6

Describe how hair cells transduce sound signal to CN VIII?

Answer 6

Sound causes vibration of endolymph:

1) Shearing force displaces basillar membrane and Inner hair cells on it
2) cilia bend against the tectorial membrane
3) Pressure-sensitive K+ channels at tips of cilia increase membrane permeability of hair cells
4) High [K+] in endolymph rush into hair cells
5) Depolarization opens V-gated Ca2+ channels
6) Release Glutamate into synapse, excite CN VIII

Auditory nerve: generator potential, followed by action potential

Question 7

Ratio of inner and outer hair cells and compare their afferent innervation?

Answer 7

Ratio of outer to inner hair cells = 3:1

Outer hair cells:
10-100:1 afferent connection Innervated by 5% afferent

Inner hair cells:
1:1 afferent connection
Innervated by 95% afferent

Question 8

Describe how sound frequency is processed in the inner ear?

Answer 8

Need 2 mechanisms:

1) Longitudinal variation in the physical properties of basilar membrane
2) Tonotopic representation in the cochlea (Place-coding)

Question 9

Explain how the basillar membrane in inner ear partakes in frequency processing?

Answer 9

a) Basal portion (stiff, narrow, short stereocilia) responds best to high frequencies
b) Apical part (floppy, wide, long stereocilia) responds best to low frequencies

Question 10

Explain how place coding works to resolve frequency of sound?

Answer 10

Linear array of inner hair cells along the length of the cochlea corresponds to a linear profile of frequencies

Frequency of incoming tone determines the peak position of the travelling wave along the basilar membrane: Sharp cut-off beyond the maximal displacement

Question 11

Describe how the inner ear can process sound intensity? (think cocktail- party phenomenon)

Answer 11

i.e. Loud background noise processed in CNS

> > Efferent innervation of descending auditory pathway (Superior Olive)**

> > Organ of Corti amplifies very small vibration:
Outer hair cells shorten the length of their cell bodies

> > amplify the small mechanical vibration of basilar membrane (less contact between cilia and tectorial membrane)

> > improves the detection of weak auditory signal against background noise (selective filtering / attention)

Question 12

Describe the receptive field of auditory nerve cells.

Answer 12

Every cell has its characteristic receptive field = distinct V-shaped tuning curve:

Tip of curve = characteristic frequency (CF) = locus of hair cell on basilar membrane with which the auditory nerve innervates

Nerves are sharply tuned to exclude frequencies above CF

Question 13

Explain why the receptive of auditory nerve fibers is V-shaped?

Answer 13

Progressive sharpening of V-shaped tuning curve enhances frequency discrimination

i.e. the lower intensity the sound, the better frequency discrimination

> > discrimination of very soft (near threshold) tones with slightly different frequencies

Question 14

Describe the anatomical characteristics/ features of central auditory pathway?

Answer 14

• tonotopic projections
• bilateral networks, with strong crossed connections
• expansion in cell population of inferior colliculus and cortex
• hierarchical organization

Question 15

Function of the bilateral networks in the central auditory pathway?

Answer 15

Crossed connections = decussate 2 times

to compare right / left ear in ascending pathways and allow LOCALISATION of sound

Question 16

List the tract of the descending auditory efferent pathway?

Answer 16

Auditory cortex 
> medial geniculate body 
> inferior colliculus 
> Lateral lemiscus tract
> superior olive 
> cochlear nucleus 
> cochlear nerve

Question 17

List the lower brainstem centers in the central auditory pathway and explain their functions.

Answer 17

1) Cochlear nucleus
= analysis of frequency and temporal pattern

2) Superior olivary complex
= binaural processing (time and intensity cues from 2 ears) for gross** differentiation of sound direction.
Accurate localization at higher cortical areas

Question 18

List the structures in the auditory pathway that resolves frequency, temporal pattern and localization of sound?

Answer 18

Frequency = basillar membrane + cochlear nucleus

Temporal pattern = cochlear nucleus

Localization = superior olivary complex (gross) + Higher centre in cerebral cortex (refined)

Question 19

Describe how basic binaural sound processing works at the superior olivary complex?

Answer 19

2 ears receive sound signal
i.e. Sound reaches left ear first. Right ear receive signal later due to longer path taken by sound wave

Outer hair cells of left ear = first compute sound source

Comparison of time and intensity cues from both ears = gross spatial info. about sound source

Question 20

List the midbrain centers in the central auditory pathway and explain their functions.

Answer 20

Inferior colliculus:

1) Detects frequency modulation (FM), amplitude modulation (AM) in speech

2) Reflex center (by sensorimotor pathway):
a) Novel sound = head-orienting response
b) Loud sound = startle response

3) Receives descending projection from auditory cortex to modulate ascending signals

Question 21

List the thalamic centers in the central auditory pathway and explain their functions.

Answer 21

medial geniculate body

1) Ascending relay to auditory cortex
2) Receives descending projection from auditory cortex to modulate ascending signals

Question 22

Which structures in the auditory pathway receives descending projections from auditory cortex to modulate ascending signals?

Answer 22

Thalamus: Medial geniculate body

Inferior colliculus in midbrain

Question 23

Which higher cortical areas are involved in auditory processing?

Answer 23

Primary auditory cortex

Wernicke’s area and Broca’s area for complex sounds i.e. language

Question 24

Describe the organization of the primary auditory cortex? Major function?

Answer 24

Functional columns (areas 41, 42) 
Organized into tonotopic patterns (similar to visual cortex) by iso-frequency bands and binaural bands 

1) Specific spatial patterns are fired for discrimination of sounds
2) Accurate localization of sound

Question 25

Describe the location of Wernicke’s and Broca’s area and their connection and function?

Answer 25

1) Wernicke’s area in the temporal lobe (area 22), mostly in the dominant hemisphere – For language recognition
2) Broca’s area in the frontal lobe (areas 44, 45), mostly in the dominant hemisphere – For language perception.

Connected by huge bundle of fibers: Arcuate fasciculus

Question 26

Which brain area is affected in Receptive aphasia?

Answer 26

Wernicke’s area

Difficulty understanding written or spoken language

Question 27

Which brain area is affected in expressive aphasia?

Answer 27

Broca’s area

Can express by writing but difficulty in turning thoughts into words

Question 28

List 2 auditory tests and compare their functions?

Answer 28

1. Pure tone audiometry = Measure hearing threshold in terms of intensity and frequency
2. Brainstem auditory evoked response (BAER) = Assess hearing ability by Click-evoked neural activity of the auditory pathway
   (use in infants)

Question 29

2 categories of hearing disabilities?

Answer 29

Conductive or Sensorineural hearing loss

Question 30

Explain the pathogenesis of presbycusis?

Answer 30

presbycusis = deterioration of hearing ability of high-frequency sounds with ageing

Sensory: loss of high-frequency hair cell (at base of inner ear: prone to damage)

Neural: loss of cochlear neuron

Question 31

List 5 common causes disorders of hearing?

Answer 31

 Tinnitus (= ringing sound in ear from extreme noise exposure/infection) 
 Hereditary dysfunction
 Ototoxicity (e.g. Vancomycin)
 Tumor 
 Temporal bone fracture (head trauma)