Chapter 15
The Special Senses
577
15
3
2
1
Malleus
Incus
Auditory ossicles
Stapes
Oval
window
Scala vestibuli
Helicotrema
Cochlear nerve
Scala tympani
Cochlear duct
Basilar
membrane
Basilar membrane
Fibers of basilar membrane
Round
window
Tympanic
membrane
High-frequency sounds displace the basilar
membrane near the base.
(a) Route of sound waves through the ear
(b) Different sound frequencies cross the basilar membrane at
different locations.
Medium-frequency sounds displace the basilar
membrane near the middle.
Low-frequency sounds displace the
basilar membrane near the apex.
Base
(short,
stiff fibers)
20,000
2000
200
20
Frequency (Hz)
Apex
(long,
floppy
fibers)
1
Sound waves vibrate the
tympanic membrane.
2
Auditory ossicles vibrate.
Pressure is amplified.
3
Pressure waves created by the
stapes pushing on the oval window
move through fluid in the scala
vestibuli.
4a
Sounds with frequencies below
hearing travel through the
helicotrema and do not excite hair
cells.
4b
Sounds in the hearing range go
through the cochlear duct, vibrating
the basilar membrane and
deflecting hairs on inner hair cells.
4a
4b
Figure 15.30
Pathway of sound waves and resonance of
the basilar membrane.
The cochlea is depicted as if uncoiled. The
graph in the bottom panel of (b) represents fibers that span the
width of the basilar membrane. The stiffness of these fibers “tunes”
specific regions of the basilar membrane to vibrate at specific
frequencies.
Transmission of Sound to the Internal Ear
Hearing occurs when the auditory area of the temporal lobe
cortex is stimulated. However, before this can happen, sound
waves must be propagated through air, membranes, bones,
and fluids to stimulate receptor cells in the spiral organ
(Fig-
ure 15.30a)
.
Airborne sound entering the external acoustic meatus strikes
the tympanic membrane and sets it vibrating at the same fre-
quency. Te greater the intensity, the farther the membrane is
displaced in its vibratory motion. Te motion of the tympanic
membrane is amplified and transferred to the oval window by
the ossicle lever system, which acts much like a hydraulic press
or piston to transfer the same total force hitting the eardrum to
the oval window.
Because the tympanic membrane is 17–20 times larger than
the oval window, the pressure (force per unit area) actually ex-
erted on the oval window is about 20 times that on the tympanic
membrane. Tis increased pressure overcomes the stiffness and
inertia of cochlear fluid and sets it into wave motion. Tis situ-
ation can be roughly compared to the difference in pressure
relayed to the floor by the broad rubber heels of a man’s shoes
versus a woman’s tiny spike heels. Te man’s weight—say, 70 kg
(about 150 pounds)—is spread over several square inches, and
his heels will not make dents in a pliable vinyl floor. But spike
heels concentrate the same 70-kg force in an area of about 2.5
cm
2
(1 square inch) and
will
dent the floor.
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