L39 - Audition Flashcards
Function of external ear?
collection/ localization/ modification of sound
2 functions of the middle ear?
Impedance matching
Sound attentuation via middle ear reflex
Describe the physiology of impedance matching at the middle ear.
- Air-borne vibration stops at eardrum
- 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)
Sequence of structures that transduce sound from external auditory canal to inner ear?
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»_space; endolymph vibration
» movement of basilar membrane causing inner, outer hair cells’ cilia to bend and move against tectorial membrane
» Perilymph vibration
» Round window
Describe the physiology and function of Middle ear muscle reflex?
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
Describe how hair cells transduce sound signal to CN VIII?
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
Ratio of inner and outer hair cells and compare their afferent innervation?
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
Describe how sound frequency is processed in the inner ear?
Need 2 mechanisms:
1) Longitudinal variation in the physical properties of basilar membrane
2) Tonotopic representation in the cochlea (Place-coding)
Explain how the basillar membrane in inner ear partakes in frequency processing?
a) Basal portion (stiff, narrow, short stereocilia) responds best to high frequencies
b) Apical part (floppy, wide, long stereocilia) responds best to low frequencies
Explain how place coding works to resolve frequency of sound?
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
Describe how the inner ear can process sound intensity? (think cocktail- party phenomenon)
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)
Describe the receptive field of auditory nerve cells.
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
Explain why the receptive of auditory nerve fibers is V-shaped?
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
Describe the anatomical characteristics/ features of central auditory pathway?
- tonotopic projections
- bilateral networks, with strong crossed connections
- expansion in cell population of inferior colliculus and cortex
- hierarchical organization
Function of the bilateral networks in the central auditory pathway?
Crossed connections = decussate 2 times
to compare right / left ear in ascending pathways and allow LOCALISATION of sound
List the tract of the descending auditory efferent pathway?
Auditory cortex > medial geniculate body > inferior colliculus > Lateral lemiscus tract > superior olive > cochlear nucleus > cochlear nerve
List the lower brainstem centers in the central auditory pathway and explain their functions.
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
List the structures in the auditory pathway that resolves frequency, temporal pattern and localization of sound?
Frequency = basillar membrane + cochlear nucleus
Temporal pattern = cochlear nucleus
Localization = superior olivary complex (gross) + Higher centre in cerebral cortex (refined)
Describe how basic binaural sound processing works at the superior olivary complex?
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
List the midbrain centers in the central auditory pathway and explain their functions.
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
List the thalamic centers in the central auditory pathway and explain their functions.
medial geniculate body
1) Ascending relay to auditory cortex
2) Receives descending projection from auditory cortex to modulate ascending signals
Which structures in the auditory pathway receives descending projections from auditory cortex to modulate ascending signals?
Thalamus: Medial geniculate body
Inferior colliculus in midbrain
Which higher cortical areas are involved in auditory processing?
Primary auditory cortex
Wernicke’s area and Broca’s area for complex sounds i.e. language
Describe the organization of the primary auditory cortex? Major function?
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
Describe the location of Wernicke’s and Broca’s area and their connection and function?
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
Which brain area is affected in Receptive aphasia?
Wernicke’s area
Difficulty understanding written or spoken language
Which brain area is affected in expressive aphasia?
Broca’s area
Can express by writing but difficulty in turning thoughts into words
List 2 auditory tests and compare their functions?
- Pure tone audiometry = Measure hearing threshold in terms of intensity and frequency
- Brainstem auditory evoked response (BAER) = Assess hearing ability by Click-evoked neural activity of the auditory pathway
(use in infants)
2 categories of hearing disabilities?
Conductive or Sensorineural hearing loss
Explain the pathogenesis of presbycusis?
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
List 5 common causes disorders of hearing?
Tinnitus (= ringing sound in ear from extreme noise exposure/infection) Hereditary dysfunction Ototoxicity (e.g. Vancomycin) Tumor Temporal bone fracture (head trauma)
