578
UNIT 3
Regulation and Integration of the Body
15
Resonance of the Basilar Membrane
As the stapes rocks back and forth against the oval window, it
sets the perilymph in the scala vestibuli into a similar back-and-
forth motion. A pressure wave travels through the perilymph
from the basal end toward the helicotrema, much as a piece of
rope held horizontally can be set into wave motion by move-
ments initiated at one end.
Fluids cannot be compressed. Tink of what happens when
you sit on a water bed—you sit “here” and it bulges over “there.”
In a similar way, each time the stapes forces the fluid adjacent
to the oval window medially, the membrane of the round win-
dow bulges laterally into the middle ear cavity and acts as a
pressure valve.
Sounds of very low frequency (below 20 Hz) create pressure
waves that take the complete route through the cochlea—up
the scala vestibuli, around the helicotrema, and back toward
the round window through the scala tympani (Figure 15.30a).
Tese low-frequency sounds do not activate the spiral organ
and so are below the range of hearing.
In contrast, sounds with frequencies high enough to hear
create pressure waves that take a “shortcut” and are transmitted
through the cochlear duct into the perilymph of the scala tym-
pani. As a pressure wave descends through the flexible cochlear
duct, it vibrates the entire basilar membrane.
Maximal displacement of the membrane occurs where the
fibers of the basilar membranes are “tuned” to a particular
sound frequency (Figure 15.30b). Tis characteristic of maxi-
mum movement at a particular frequency is called
resonance
.
Te fibers of the basilar membrane span its width like the strings
of a harp. Te fibers near the oval window (cochlear base) are
short and stiff, and they resonate in response to high-frequency
pressure waves (Figure 15.30b). Te longer, more floppy basilar
membrane fibers near the cochlear apex resonate in time with
Tip links
Stereocilia
Figure 15.31
Photo of stereocilia on a hair cell.
Note the
precise order of the stereocilia.
lower-frequency pressure waves. As a result, the resonance of
the basilar membrane mechanically processes sound signals be-
fore the signals ever reach the receptors.
Excitation of Hair Cells in the Spiral Organ
Te spiral organ, which rests atop the basilar membrane, is
composed of supporting cells and hearing receptor cells called
cochlear hair cells
. Te hair cells are arranged functionally—
specifically, one row of
inner hair cells
and three rows of
outer
hair cells
—sandwiched between the tectorial and basilar mem-
branes (see Figure 15.27c). Afferent fibers of the
cochlear nerve
[a division of the vestibulocochlear nerve (VIII)] coil about the
bases of the hair cells.
Te hair cells have numerous
stereocilia
(actually long mi-
crovilli) and a single
kinocilium
(a true cilium) protruding from
their apices. Te “hairs” (stereocilia) of the hair cells are stiff-
ened by actin filaments and linked together by fine fibers called
tip links
(Figure 15.31)
. Te stereocilia protrude into the K
1
-
rich endolymph, and the longest are enmeshed in the overlying,
stiff gel-like
tectorial membrane
(see Figure 15.27c).
±ransduction of sound stimuli occurs a²er localized move-
ments of the basilar membrane “tweak” or deflect the trapped
stereocilia of the hair cells. Bending the stereocilia toward the
kinocilium puts tension on the tip links, which in turn opens
cation channels in the adjacent shorter stereocilia like a rope
pulling on a trap door. Tis results in an inward K
1
(and Ca
2
1
)
current and a graded depolarization (receptor potential). Bend-
ing the cilia away from the kinocilium relaxes the tip links,
closes the mechanically gated ion channels, and allows repolari-
zation and even a graded hyperpolarization.
Depolarization increases intracellular Ca
2
1
and so increases
the hair cells’ release of neurotransmitter (glutamate), which
causes the afferent cochlear fibers to transmit a faster stream of
impulses to the brain for auditory interpretation. Hyperpolari-
zation produces the exact opposite effect. As you might guess,
activation of the hair cells occurs at points of vigorous basilar
membrane vibration.
Te vast majority of the sensory fibers of the spiral ganglia
(90–95%) service the inner hair cells, which shoulder nearly
the entire responsibility for sending auditory messages to the
brain. By contrast, most fibers coiling around the more numer-
ous outer hair cells are
efferent
fibers that convey messages from
brain to ear. So what do the outer hair cells do?
Te outer hair cells act on the basilar membrane itself. When
outer hair cells depolarize and hyperpolarize as the basilar
membrane moves, they contract and stretch in a type of cellular
boogie called
fast motility
. Teir strange behavior changes the
stiffness of the basilar membrane.
Outer hair cell motility serves two functions:
It increases the responsiveness of the inner hair cells by am-
plifying the motion of the basilar membrane—a kind of co-
chlear tuning.
It may help protect the inner hair cells from damage. Loud
sounds activate a negative feedback loop from the brain
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