418
UNIT 3
Organ Systems
Figure 21-7A.
Inner and outer hair cells.
SEM,
3
1,300
The arrangement of
inner
and
outer hair cells
is illustrated in these scan-
ning electron micrographs. The row of
stereocilia
of the inner hair cells is
separated from the outer hair cells by the heads of the inner pillar cells.
The “V” or “W” pattern of the stereocilia of the outer hair cells is clearly
seen. Outer hair cells are particularly important for
frequency discrimina-
tion
. They possess the unique property of being able to actively change
their physical length in response to changing electrical F
elds. This property
contributes to the frequency selectivity of hair cell responses. The
inset
shows the thin processes of the outer
phalangeal cells
that fl
atten out to
form the
reticular lamina
. This lamina serves to isolate the endolymph in
the scala media from the perilymph in the scala tympani. The upper sur-
faces of the hair cells are smooth, whereas the surrounding processes of
phalangeal cells are covered with
microvilli
.
Stereocilia
Stereocilia
Heads of inner
Heads of inner
pillar cells
pillar cells
Inner
Inner
hair cells
hair cells
Outer
Outer
hair cells
hair cells
Process of
phalangeal cell
Reticular
lamina
Microvilli
Stereocilia
Reticular
lamina
Heads of inner
pillar cells
Outer
hair cells
Inner
hair cells
A
Tip links
Phalangeal
cell
process
Microvillus
Hair cell
Stereocilia
B
Figure 21-7B.
Stereocilia of an outer hair cell.
SEM,
3
5,000
The
stereocilia of outer hair cells
are typically
arranged in three rows, with the tallest row on the
outside of the “V.” Each cilium is narrower at its
base than in its body (
inset
). The cilia are connected
at their tips by F ne F laments (
tip links
), which play
a critical role in changing the ionic permeability of
the cell membrane when the stereocilia are bent. The
permeability change initiates a sequence of events
that results in the release of transmitter molecules,
leading to action potentials in the afferent nerve
F bers. Outer hair cells vary in length along the basi-
lar membrane, with the shortest at the basal end of
the cochlea and the longest at the apical end.
CLINICAL CORRELATION
Figure 21-7C.
Sensorineural Hearing Loss.
SEM,
3
376
Extended exposure to loud sounds can impair hearing. These
scanning electron micrographs compare a normal mammalian
organ
of Corti
(
left
) with one that was subjected to high-intensity sound
for several days (
right
).
Outer hair cells
are more subject to damage
than inner hair cells. Durations of loud noise as short as a few min-
utes can produce detectable damage to stereocilia; longer duration
exposure (as illustrated here) causes death of hair cells. Damaged
hair cells are not replaced in mammals, although they are replaced
in some birds and reptiles. In mammals, the holes left by dying
hair cells are F lled by the processes of phalangeal cells in order to
maintain the barrier between the endolymph and perilymph. Hair
cells are normally lost with advancing age (
presbyacusis
). Hair-cell
damage can also be produced by prolonged exposure to high dos-
ages of aminoglycoside antibiotics (e.g., streptomycin, neomycin),
some diuretics (e.g., furosemide), and some chemotherapy agents.
When only outer hair cells are damaged, there is an overall loss of
sensitivity and a profound loss of frequency discrimination (e.g.,
ability to understand speech). A loss of both inner and outer hair
cells leads to complete deafness, which cannot be ameliorated by
hearing aids.
IHCs
IHCs
OHCs
OHCs
Normal organ of Corti:
Inner hair cells (IHCs) and
outer hair cells (OHCs)
Inner and outer hair cells
damaged by prolonged noise
C
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