9/2 Auditory System - Crockett Flashcards
conductive apparatus
function
parts
fx: transform sound waves into mechanical vibrations
- have to compensate for loss of energy that occurs when sound goes from gaseous medium to fluid medium
parts
- external auditory meatus (ear canal)
- tympanic membrane
- ossicles → amplify force at oval window, help compensate for diff in impedance between middle ear (air) and inner ear (fluid)
- malleus
- incus
- stapes
ossicle conditions that can cause hearing loss
- otosclerosis: footplate of stapes gets locked in place due to bone growth around annular ligament
test with Rinne’s test : if bone doncutance is more sensitive than air conducted stumuli → disease process with ossicles!
- 512 tuning fork
- use in conjunction with Weber’s test
- results…
- conduction deafness: low freq loss
- sensory deafness: high freq loss
how is sound conducted?
- tympanic membrane shakes → ossicles conduct sound → fluid moves (from scala vestibuli to/through scala tympani)
- cochlea transforms fluid waves into neural activity
Organ of Corti has 1 inner and 3 outer rows of hair cells (sensory receptors)
- kinocilium is lost during devpt, but basilar plate gives cell same kind of polarity
- as basilar membrane vibrates, stereocilia are bent
- outer hair cells are embedded in tectorial membrane, bend due to shearing action
- inner hair cells are NOT embedded, bend due to fluid movement
auditory hair cells
have clusters of stereocilia at apical ends ordered by size
kinocilium is lost during devpt; have a basal body instead
- bending of sterocilia towards basal body → depolarization
- bending away from basal body → hyperpolarization
mechanically sensitive ion channels
- hair cells act like ‘electrical valves’
- tips of stereocilia are linked by protein filaments
- when stereocilia bend, they open mechanosensitive ion channels in stereocilia → let in depolarizing K current
- upward deflection = depol
- downward deflection = hyperpol
functions of inner and outer hair cells
inner hair cells : detection of sound
outer hair cells : amplification and dampening of sound via motor function
- contract and elongate in synchrony with sound-sensitive receptor potentials due to action of motor proteins (prestins)
ototoxic agents
can affect both auditory and vestibular fx
common ototoxins:
- aminoglycoside antibiotics (streptomycin, gentamicin)
- salicylates
trophic factors may help protect hair cells
hair cell efferents
hair cells are innervated by efferents from superior olivary nuclear complex
- outer hair cells: mostly contralateral innervation
- medial olivocochlear bundle
- direct efferent connection, small afferent connection
- inner hair cells (type I): mostly ipsilateral innervation
- lateral olivocochlear bundle
- axoaxonic contacts with bipolar cells
perception of pitch
2 concepts
temporal coding
2 concepts explain how you can use multiple neurons to hear high freq sound
- phase locking: detection of waves in regular pattern, in a certain interval
-
volley principle: fire, skip a few waves, fire again
- works to approx 5000Hz
place coding
different regions of the cochlea are selectively responsive to different freqs of sound
- basilar membrane is stiff near oval window and flexible near apex
- individual hair cells may be tuned electrically and mechanically at diff points along basilar membrane
hair cells have stereocilia that vary in length
- stereocilia at base are short/stiff, at apex are long/flexible
- in lower vertebrates, hair cell membranes show spontaneous oscillations which systematically vary in frequency over teh length of the basilar membrane, matching its mechanical resonance
innervation of hair cells
90% of bipolar cells located in the spiral ganglion innervate inner hair cells
- inner hair cells are innervated by 10 afferents each
10% of spiral ganglion cells innervate outer hair cells
- a single primary sensory afferent can innervate multiple outer hair cells
- outer hair cells also directly innervated by efferent fibers
cell types in cochlear nucleus
ventral cochlear nucleus
- stellate cells (multipolar) : fire in regular trains
- chopper : have specific tuning cures, resp for frequency coding
- bushy cells : receive input from one or a few massive axonal terminals (end bulbs) and fire a single axon potential at onset of sound. role in sound localization, sharpening tuning
dorsal cochlear nucleus
- fusiform cells : role in locating sounds at diff elevations, respond to a broad range of freqs
ascending auditory pathway
characterized by bilateral representation at every level above cochlea
implication: cant get deafness in ONE ear except in cases of damage to CN VIII or cochlear nuclear complex
- CAN see problems with localization of sound in other injury!
localization of sound in space
time differences : medial superior olivary nucleus
- low freq is the most useful (large wavelengths)
- time diff in signals received from right and left ears (and lengths of axons of neurons transmitting them) allows for localization of sound
intensity differences : lateral superior olivary nucleus (and medial nucleus of trapezoid)
- high freq is most useful
auditory cortex
- organized in columns
- also has topographical organization (sections that correspond to apex and base of cochlea)
