NROC64 : Lec 4 : Auditory and Vestibular Systems (nearly word-for-word what was said in lecture)

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4 Feb 2012
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NROC64 : Lec 4 : Auditory and Vestibular Systems
Slide 6:
Sound:
oAudible variations in air pressure, water pressure, other mediums
oTypically have some volume w/ mocs and there are certain regions in the volume where the
mocs are just packed denser together and areas where they are less dense this travels thru the
volume
oQuantify air pressure
Diagram:
oLongitudinal waves = waves don’t move much, they move back and forth but stay roughly on
same spot it’s just the density that actually travels (it’s just energy itself travelling in a
medium)
oAlso possible that these mocs do travel = wind
oCycle = from max pressure to min pressure and then to max pressure again; it doesn’t matter
which point you start, cycle would be the end of the same region that you started at
oFrequency = cycles/ second expressed in Hz
Higher freq = more cycles w/in same space; it doesn’t mean it travels more quickly (it
travels at same speed of sound in air medium (around 300 m/sec), but there’s just more
cycles)
oEnergy of sound wave depends on the amplitude of wave
Slide 7:
pitch and loudness are perceptual aspects of frequency and sound NRG
pitch = how we perceive frequencies (20 Hz – 20,000 Hz)
past age 15, can’t perceive frequencies greater than 20,000 Hz
Diagram:
oFrequencies that we can hear; we hear frequencies according to all sorts of processes that aren’t
based on physics (ex: 2 tones that are separated by an octave sound similar; higher tone octave
has the double frequency why this is is not based directly on physics)
oIt has to do with certain ways that we process frequencies and cultural aspects
oPitch is more complicated than just frequencies, it has PERCEPTUAL aspects to it
oLoudness = perceptual aspect of intensity
oGraph x-axis = diff frequencies and y-axis = NRG of the tones (dB)
oLog scales; curves represent where perception is roughly equal
oRed curve = atypical absolute threshold = point at which you start to perceive sound; if you go
below this point, tone becomes inaudible
oMeans that at low freq (like 20 Hz) has to have quite a bit of intensity (sound pressure NRG) in
order to be audible; frequencies of 3000 Hz for ex can have much lower sound NRG level
oSubjectively if you ask ppl, it appears to be at the same loudness even though it’s not; and it
continues at sounds that are louder than the absolute threshold
oIf louder, can still ask ppl to equate diff tones of frequency and ask them to match them in
loudness subjectively adjust them so that they appear to be the same get curves of diagram
For example, can have a curve where every single tone is adjusted to a sound level of 60
out of 100 (arbitrary #); get regular patterns, curvy and reflects absolute threshold
(suggests it has something to do w/ physiology and anatomy of inner ear, which happens
to hear best at this freq)
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Loudness in this way is more complicated than INTENSITY ITSELF, the more energy, the louder it
seems to you BUT IT REALLY DEPENDS ON THE FREQUENCY
Slide 10:
Auditory system:
oOuter ear,
oauditory canal = amplifies certain frequencies and protects other parts of the ear
otympanic membrane (boundary btn inner and outer ear) followed by 3 ossicles and inner ear
ofollowed by vestibular system
Slide 11:
Early auditory pathway stages:
oSound waves that hit tympanic membrane; ossicles amplify sound NRG which is focused onto
oval window which is important to transduce sound from medium that is lighter to a medium that
is more dense (endolymph / perilymph) this requires a lot of NRG and ossicles help to avoid
that
oMotion into cochlear fluid will induce shearing forces between certain membranes (basilar and
tectorial membranes) by which sensory neurons will (hair cells) will be stimulated
Slide 12:
Components of middle ear:
oMalleus, incus and stapes
oMuscles in between which have to do with modifying flexibility of the ossicles
oSound waves hit tympanic membrane and put it in motion = vibrations = and thereby moving
ossicles, movement is transferred to oval window at the end of the stapes
oEustachian tube = connection to your mouth; when plane ascends/descends, your ears may
experience pain (can swallow to get rid of pain)
oOssicles: one function and 2 mechanism to achieve it
Work as a lever mechanism: lever = small weight can lift larger weight if fulcrum is
closer to larger weight
2nd principle that’s implemented: pressure = force / area; if you have same force but make
area smaller, pressure increases
Slide 13:
needle, not much force, where you push is larger area, but tip of needle = small area = greater pressure
ossicles are forcing the pressure onto oval window; oval window is smaller than tympanic membrane
with the purpose being to amplify sound; it’s particularly necessary b/c air has low density and to get
sound to go thru, need ossicles to amplify sound
Slide 14:
Attenuation Reflex:
oThere are muscles attached to ossicles; the purpose of these muscles as they contract is to stiffen
ossicles thereby reducing leverage mechanism (this may be good sometimes)
oTo reduce the amount of NRG transduction / reduce increase of sound (make it not as large) b/c
there may be sounds that are simply too loud for your ears
oIn this way, you avoid saturation (where everything sounds equally loud and can’t tell the
difference anymore); and frequency seems similar when sound is very loud
oAnd want to avoid damage to ear
oIf gunshot sound, impossible to have attenuation reflex take place b/c that’s too short and the
reflex takes a bit of time
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oAttenuation reflex also helps to filter out lower frequencies; speech is at higher freq and can be
heard
oWhen you hear yourself speak, the frequencies travel thru the tissues of your own body (low freq
ones) and don’t need to listen to what you’re saying that much (might be confusing when you’re
trying to hear something else at same time) reduce sound of own voice
That’s what sensory systems tend to do: filter out info from own body that is not particularly interesting
Slide 15:
Anatomy of cochlea:
oOval window
oCochlea = one tube curled up
oTube segmented into 3 chambers
oOutside = scala vesibuli and at bottom = scala tympani and wedged in btn = scala media
oReissner’s membrane separates sc. vestibuli and sc. media
oBasilar membrane separates sc. media and sc. tympani
o3 chambers contain fluids that are somewhat diff
oupper and lower chamber contain same type of fluid = perilymph (like CSF); sc. media contains
endolymph
ostria vascularis cells (green) secrete endolymph which has high [K+] and low [Na+] which is
extracellularly unusual for a neural mechanism
know this from olfactory system that we have high concentration of K+ OUTSIDE the
cell
oat the very apex of the cochlea = helicotrema = where sound NRG travels and comes back thru
the other side
oendolymph potential is 80mV more positive than perilymph
Slide 16:
Basilar membrane in an uncoiled cochlea
oSeparates sc. media and sc. tympani
oBasilar membrane at the beginning (near oval window) = narrow and thick; at apex = wide and
floppy (like flippers)
oDifferently responsive to different frequencies; freq get separated on diff parts of basilar mem
oCertain parts of the membrane will move depending on which freq the sound is travelling with
achieve tonotopy (sounds separated by freq)
Slide 18:
physiology of the cochlea:
ostapes puts pressure at oval window, pushes perilymph into sc. vestibuli, goes around
helicotrema and towards round window that bulges out
higher freq waves = closer to oval window; lower freq near apex of basilar mem
endolymph movement bends basilar mem at the bottom of sc. tympani
Slide 19:
floppy and wide = low freq
another term related to tonotopy = place code
place code = frequencies are coded in terms of diff parts of the basilar mem; have displacements and diff
cells at each place
Slide 20:
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