CHAPTER 21
Ear
411
rotational movements of the head. Each duct has an enlargement
at one end where it joins the utricle. This swelling is called the
ampulla
and contains the sensory receptors that are stimulated
by rotational movements (Fig. 21-9A). A short wall of connec-
tive tissue, the
crista ampullaris
, extends across part of each
ampulla. Hair cells, similar to those of the organ of Corti, cover
the upper surface of the crista. Their
stereocilia
and
kinocilia
are embedded in a gel-like structure, the
cupula
, which blocks
the ampulla. When the head turns, the inertia of the endolymph
in the semicircular ducts causes the fl uid to push against the
cupula and defl ect the cilia of the hair cells, thereby initiating
action potentials in axons in the vestibular nerve (Figs. 21-8B
and 21-9A,B).
Vestibular hair cells
are also clustered in a small
region of the
utricle
, the
macula utriculi
(Figs. 21-8B and
21-10A,B). The stereocilia and kinocilia of these hair cells are
embedded in a gelatinous structure, the
otolithic membrane
(Fig.
21-10A). Thousands of tiny calcium carbonate crystals,
otoco-
nia
, are clustered on the surface of the otolithic membrane (Fig.
21-10A–C). These crystals are heavier than the surrounding
endolymph.
Gravity
or
linear acceleration
, therefore, exerts a
force on the otoconia, causing the underlying cilia to be defl ected
and consequently sending a neural signal to the central nervous
system (CNS) related to head position or acceleration. The
sac-
cule
contains a similar region, the
macula sacculi
.
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