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Lecture 14

PSL300H1 Lecture Notes - Lecture 14: Cholinergic, Dyslexia, Brainstem


Department
Physiology
Course Code
PSL300H1
Professor
Michelle French
Lecture
14

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PSL300H1F L14; Oct. 14, 2011
Mechanoreceptor system
Hair receptors – problem is mechanical - stimulus =
seq of pressure pulses in air – collected in auditory
canal
Problem = must amplify pressure exterted on
cochlea to maintain in fluid which is denser than
air
Middle ear mechanics
Tympanum vibrates with sequence of sound pressure
pulses (in air)
lever system of ossicles amplifies pressure on
tympanum 22-fold
amplified pressure exerted on oval window of cochlea
(filled with fluid)
note there’s a connection at middle, air chamber
connected to back of pharynx to allow for
equalization of pressures across tympanum – ex.
Swallow to equalize pressure – very sensitive
shell curled up – dif chambers
endolymph-filled chamber sitting on top of basal
membrane dividing cochlea into upper and lower
where pressure pulses exterted
oval window in upper chamber
must have output for compressed fluid pressure
flexible wall of round window w ossicles pressure
pulse thru upper and lower (output at bottom)
chambers filled w perilymph directs pressure
across membrane distortion in basal membrane
deflection of hair cells depolarization
mechanical properties – natural resonance of basal
membrane different depending on where
close to windows, stiff membrane so high natural
freq to oscillate at; ~20,000hz; goes to window
instead of fluid, max vibration at window
broad at other end, very flexible, much lower
natural freq, as high; 30-50hz; goes thru all fluid
for max vibration at apex of basal membrane
so membrane tuned to dif freqs
Traveling wave (5 kHz) maximally displace
basal membrane at
particular pt
relative to window
area
rest of membrane
by apex
unaffected,
200 Hz Tone max distortion
towards apex
all membrane, most
is affected since
pressure pulses
travel all the way
thru cochlea to this
pt
hair cells sitting on top of basal membrane, extends
throughout length of cochlea
preferred freq at dif locations
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overlayed by flap of tissue – tectorial membrane,
cilia embedded
axons of afferents collect in bundles, ganglion in
wall of cochlea – spiral ganglia cranial nerve
hair cells divided in two cells
outer – most numerous
inner
separated by A-frame scaffold – frame work, rod
support
extension into reticular lamina, cilia stick thru to
embed in tectorial membrane
when basal membrane bouncing up and down to
stimuli, movement causes mechanical deflection of
tectorial membrane back and forth across hair cells,
bends stereocilia
ensure cilia bent, much bend most to get
hyperpolarization
bundles polarized – long on one side, short on other,
connected by tip links
bend towards long depolarize, release transmitter
EPSPs & act pot in afferents
all afferents synapse w inner hair cells not outer hair
cells
since outer hair cells are efferents
inner ones are sensory
Outer: useful for amplifying stimulus when
depolarized contract; get flat when
hyperpolarized
Tip Links
bending affects opening of ion channels
Hair cell amplification
when outer hair cells depolarized contract pull
on framework basal membrane bulges downward
exaggerates vibration; good to amplify weak
signals
don’t always have weak sounds, need mechanism for
protecting hair cells from loud sounds
efferents originate from within brainstem
signals sent to cochlea adjusting sensitivity of
hair cells
inhibitory: so hyperpolarizing outer hair cells so
don’t contract as much so not amplifying the
signal
Olivocochlear Efferents – 2 types
Negative feedback from superior olivary feedback
complex in brainstem
Adjust sensitivity of outer hair cells, decrease
contraction
Outer hair cells have self-regenerating oscillations
problem – ringing in ears; unless have effective
damming mechanism like this, get ringing
Cholinergic efferents: inhibitory; reduce OHC
mechanical amplification
GABAergic efferents: long-term effects?
Depending position in cochlea, any given afferent
synapsing w hair cells will have a preferred freq
If at centre of cochlea – 500-600hz preferred
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