Download P312Ch11_Auditory II (EarDetails)

Topic 16 – Processing Sound
Auditory Apparatus
Outer ear
Earwig
Earwigs are fairly abundant and
found in many areas of the world.
There is no evidence that they
transmit diseases to humans or
other animals. Their pincers are
commonly believed to be
dangerous, but in reality even the
curved pincers of males cause
little harm to humans.[45] It is a
common myth that earwigs crawl
into the human ear and lay eggs
in the brain.[46][47] Finding earwigs
in the human ear is rare, as most
species do not fly and prefer dark
and damp areas (such as
basements) rather than typical
bedrooms.[4]
Pinna – Ear flap. Some amplification compared to a simple hole in head.
Auditory canal
1” long; About 6 cc in volume
Auditory canal resonates at about 3400 Hz
This means that sounds at 3400 Hz are more intense – about 5-10 db - than sounds of other
frequencies
Tympanic membrane - Ear drum
Vibrates in unison with the air pressure changes.
Transforms air pressure changes into movement.
Most likely vibrates in unison with the vibration of the device causing the sound.
Punctures leave danger of intrusion.
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Middle ear
A Rube Goldberg-seeming device to increase the force of the vibrations of the eardrum.
Example of a Rube Goldberg cartoon
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Middle ear continued . . .
Three smallest bones of body.
Eardrum connects to malleus
Malleus connects to incus.
Incus connect to stapes.
Stapes is attached to a membrane that is stretched across a hole in the cochlea called the Oval Window.
Stapes
Incus
Malleus
Bones move back and
forth in unison with
eardrum.
Functions of middle ear
1. To increase force of vibrations of eardrum. (G8 p. 270, Fig. 11.14)
Construction of the bones is analogous to a lever.
Low force
Eardrum - Malleus
High force
Stapes – Oval window
Eardrum
- Malleus with other factors results in a 22:1 increase in the force of movement from
This lever action
in conjunction
the eardrum movement to the movement of the oval window membrane.
Approximately a 30 db increase in sound level. That is, sounds would be 30 db lower than they are if we
didn’t have the middle ear.
Mike – demo this if you have the sound equipment in the room.
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Functions of middle ear continued . . .
2. To modulate intensity of sound reaching inner ear through the acoustic reflex
There are two sets of muscles attached to the bones of the middle ear . . .
Tensor tympani muscle connects to malleus
Stapedius muscle connect to stapes
When these muscles contract . . .
Malleus is pulled to one side – so doesn’t impart as much movement to incus
Stapes is forced to move from side-to-side rather than back and forth
The effect of contraction of these muscles is to reduce the sound pressure reaching the inner ear by several
db.
They contract in two types of situations.
A) In response to loud sounds, but the latency is about .150 seconds, too long to prevent damage.
.150 is about 1/6 of a second. In that time, 83 pressure changes of a 500 Hz tone will affect the inner
ear. 833 pressure changes of a 5,000 Hz tone of a 5000 Hz tone would get through. Here’s a combination
tone 500+5000 – in the first .002 seconds (2 milliseconds), 1 major and 11 minor pressure changes
occurred.
B) In response to self-produced noises in the mouth.
So there are two possible reasons for the acoustic reflex . . .
1) To reduce levels of loud sounds, although latency is too long to protect against high frequency sounds of
high intensity. It’s like having an anti-missile defense system that has to be taken out of storage before it’ll
work.
2) To reduce noise of self-produced sounds, the distraction of eating and talking,
This is probably the major function of these muscles.
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Inner ear (G8 p. 270)
Structure is cochlea
A cave in the skull.
Filled with fluid
Two windows covered with membranes – oval window and round window.
Semi-circular canals
Auditory Nerve
Stapes attached to oval
window
Round window
The whole auditory apparatus, so you
can see the relative sizes of the
structures.
Auditory Nerve
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Interior structures of the cochlea
Two membranes run its length – Reissner’s membrane and the basilar membrane
Basilar
membrane
Reissner’s
membrane
Reissner’s
membrane
Vestibular
Canal
Tympanic
Canal
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End view of basilar membrane – analogous to looking
south down Lookout Mountain.
Cilia
Action of basilar membrane and receptors
Movement of stapes causes pressure changes in the
vestibular/tympanic canals.
These pressure waves cause ripples on the basilar membrane –
like an earthquake moving down Lookout Mt. toward Fort
Payne
This movement causes the cilia attached to the hair calls to
bend, like seaweed bending in ocean currents.
The bending of the cilia causes hair cells to release
neurotransmitter substance which triggers action potentials in
neurons whose axons make up the auditory nerve and whose
dendrites are located near the base of the inner hair cells. See
G8, p. 272, Fig. 11.18. (Read the figure caption for more
detail.)
The action potentials of the auditory nerve neurons are the
neural sound stimulus.
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Hair Cell Details
Inner
Outer
Number of hair cells
4,500
15,500
Auditory nerve neurons activated
27,000
3,000
Ratio of hair cells to neurons
1:8
4:1
There is considerable evidence that the inner hair cells are the primary receptors with the outer hair cells
serving an amplifying/modulating function, described in some detail in G8, p. 272 and 277.
Show Virtual Lab 11-10 (Cilia Movement) and
Virtual Lab 11-13 (Cochlear Amplifier) here to show the action of the basilar membrane along with the
amplification associated with the outer hair cells described on p. 277.
Show how the hair cells are changing their lengths in 11-13.
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