CHAPTER 21
Ear
421
C
Figure 21-10C.
Otoconia from the macula of the utricle.
SEM, scale bar = 10
μ
m
Otoconia
are crystals of almost pure calcium carbonate that
form in the region of the macular hair cells. The otoconia lie
on the surface of the otolithic membrane, attached by a pro-
teinaceous substance that is not well understood.
The number of
otoconia
present decreases with age, contribut-
ing to the balance difF culties often experienced by older indi-
viduals. In addition, crystals or small groups of crystals some-
times become detached from the otolithic membrane and drift
into a semicircular canal or even become attached to the hair
cells in a canal. This dislocation can disrupt the normal neural
signals from the labyrinth and produce a type of severe
vertigo
termed
benign paroxysmal positional vertigo
. Treatment that
involves sequences of different head positions and movements
can usually improve or eliminate the symptoms. The exact
maneuver depends upon which semicircular canal is involved.
Connective
Connective
tissue
tissue
Hair cells and
Hair cells and
support cells
support cells
Hair cells and
support cells
Connective
tissue
Otolithic
membrane
Axons of
utricular nerve
Otoconia
B
Figure 21-10B.
Macula of the utricle with otoconia.
H&E,
3
260
This photomicrograph shows a cross section of the
macular
region of the wall of the utricle
(an enlargement of the area indi-
cated by the
dashed rectangle
in ±ig. 21-8B). The
otoconia
, which
stain darkly in this H&E stain, lie on the surface of the
otolithic
membrane
; the otolithic membrane itself is almost transparent.
The support cells provide mechanical support to the hair cells
and also secrete the substance of the otolithic membrane.
Dizziness
is one of the most common reasons adults seek
medical care. The term has two general meanings: (1) A feeling
of
light-headedness
or “about to faint” or (2) a feeling that the
individual is spinning or that the room is spinning. This
latter feeling is properly called
vertigo
. Twenty percent of
all complaints of dizziness involve problems related to the
otoconia of the utricle and saccule (see ±ig. 21-10C).
Otolithic
membrane
Otoconia
Afferent
nerve axon
Type II
hair cell
Type I
hair cell
Support cell
A
Figure 21-10A.
Macula of the utricle.
The
utricle
and
saccule
are similar in structure. The walls consist
of an outer F brous layer, an intermediate layer of vascularized
connective tissue, and an inner epithelial lining. In both the
utricle and the saccule, there is a region of specialized epithe-
lium termed the
macula
(see ±ig. 21-2), which is 2 to 3 mm in
diameter. The macula contains two types of
sensory hair cells
,
classiF ed as
type I
and
type II
(see ±ig. 21-11A). The sensory
epithelium of the macula is overlaid by a gelatinous structure,
called the
otolithic membrane
, which is similar in makeup to the
cupula of the ampulla. The stereocilia and kinocilia of the hair
cells are embedded in the membrane. Hundreds of tiny crystals,
otoconia
, are attached to the surface of the otolithic membranes.
These crystals have a higher speciF c gravity than the surround-
ing
endolymph
and are consequently affected by gravity or
linear acceleration. Changes in head position or acceleration,
therefore, cause the otoconia-weighted otolithic membrane to
defl ect the cilia of the hair cells and, thus, trigger changes in the
frequency of nerve impulses generated by the hair cells.
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