CHAPTER 21
Ear
415
Auditory System
CN V
V
III
III
Tensor
Tensor
tympani
tympani
muscle
muscle
Apical turn
Apical turn
Middle turn
Middle turn
Basal turn
Basal turn
Base
Base
of cochlea
of cochlea
Cochlear
duct
Modiolus
Apex
of cochlea
Auditory
tube
Cartilage “C”
Basal turn
Middle turn
Apical turn
Base
of cochlea
Scala
tympani
Scala
vestibuli
Scala
media
Osseous
spiral lamina
Spiral
ganglion
Basilar
membrane
Tensor
tympani
muscle
CN VIII
A
Scala
vestibuli
Spiral
ganglion
Vestibular
(Reissner)
membrane
Stria
vascularis
Cochlear
duct
Organ
of Corti
Spiral
limbus
Spiral
ligament
Basilar
membrane
Osseous
spiral
lamina
Scala
media
Scala
tympani
B
Figure 21-4A.
Cross section of cochlea.
H&E,
3
22
The
cochlea
consists of a spiral tunnel in the temporal bone
and associated membranous structures within that tunnel.
The tunnel makes two and three-quarter turns as it pro-
ceeds from the wide
base
of the cochlea to its
apex
. The
cross section shows the cochlea in its approximate anatomi-
cal orientation (plane of section is indicated by the
blue line
through the cochlea in Fig. 21-3A). The tunnel is lined with
endosteum and is larger at the base, becoming progressively
narrower toward the apex. It is divided into two sections,
the
scala
(“staircase”)
vestibuli
and the
scala tympani
. These
two sections are separated by the membranous
cochlear duct
(Figs. 21-2 and 21-3A,B), which encloses the
scala media
.
The cochlear duct contains the sensory receptors of the
cochlea. The vestibule opens into the scala vestibuli, and
sound waves are transmitted from the oval window to the
sensory receptors by this route. The
scala vestibuli
is contin-
uous with the scala tympani at the cochlear apex via a small
opening, the
helicotrema
(see Fig. 21-6A). The
scala tym-
pani
extends from the helicotrema to the round window of
the tympanic cavity. The
auditory tube
connects the middle
ear cavity with the nasopharynx to allow air pressure in the
middle ear to equilibrate with that of the surrounding envi-
ronment. The auditory tube runs in a groove in a C-shaped
band of cartilage. The structures associated with the scalae
vestibuli, tympani, and media (
dashed rectangle
) are shown
at higher magni±
cation in Figure 21-4B.
Figure 21-4B.
Scala media and organ of Corti.
H&E,
3
84
It is traditional to represent the internal structures of the
cochlea as if the basilar membrane is horizontal. This pho-
tomicrograph has, therefore, been rotated 90 degrees coun-
terclockwise from its position in Figure 21-4A. The
cochlear
duct
(
dotted line
) is a roughly triangular structure that lies
between the
scala vestibuli
and the
scala tympani
. The fl uid-
± lled space within the cochlear duct is the
scala media
(
yel-
low shading
). In this photomicrograph, the cochlear duct is
bounded by the
bony labyrinth
and
basilar membrane
below,
the
stria vascularis
on the right, and the
vestibular membrane
(Reissner membrane)
above. The basilar membrane supports
the
organ of Corti
, which is described more fully in follow-
ing ± gures. The
stria vascularis
(see Fig. 21-5) is a special-
ized, thickened region of strati± ed epithelium. In contrast to
most types of epithelium, it is highly vascularized by a dense
meshwork of capillaries. The stria vascularis is instrumental
in maintaining the high K
+
concentration of the endolymph in
the scala media. Lateral to the stria vascularis, the endosteum
is much thickened and forms the
spiral ligament
, to which the
outer edge of the basilar membrane connects. The
vestibular
membrane
consists of two layers of squamous epithelial cells
on either side of a basal lamina. The
tectorial membrane
(
not
labeled
) normally rests on the hair cells of the organ of Corti
(see the illustration in Fig. 21-5). However, it is often dis-
torted or damaged during tissue processing.
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