Regulation and Integration of the Body
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
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
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
Photo of stereocilia on a hair cell.
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
—sandwiched between the tectorial and basilar mem-
branes (see Figure 15.27c). Aﬀerent ﬁbers of the
[a division of the vestibulocochlear nerve (VIII)] coil about the
bases of the hair cells.
Te hair cells have numerous
(actually long mi-
crovilli) and a single
(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
. Te stereocilia protrude into the K
rich endolymph, and the longest are enmeshed in the overlying,
(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
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
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
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
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
. 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-
It may help protect the inner hair cells from damage. Loud
sounds activate a negative feedback loop from the brain