410
UNIT 3
Organ Systems
Figure 21-11C
Clinical Correlation: Otitis Media
Figure 21-12A
Clinical Correlation: Vestibular Schwannoma
Figure 21-12B
Clinical Correlation: CT and MRI of Inner Ear Structures
Synopsis 21-1
Pathological and Clinical Terms for the Ear
Introduction and Key Concepts
for the Ear
The
ear
is a complex structure that serves two important sen-
sory functions,
hearing
(through the
auditory system
) and
bal-
ance
(through the
vestibular
system
). Sensory receptor organs
that serve the two functions are supplied by two distinct
branches of cranial nerve (CN) VIII, the
acoustic branch
and
the
vestibular branch
. The ear can be divided into three general
regions, the
outer ear
,
middle ear
, and
inner ear
(Fig. 21-1A).
(1) The
outer ear
consists of a
pinna
(
auricle
), an irregularly
shaped structure with a core of cartilage covered on both sides
by thin skin, and an
external auditory meatus
that conducts
sound to the middle ear. (2) The
middle ear
includes the
tym-
panic membrane
, the
tympanic cavity
containing the
ossicles
,
and the
auditory tube
(Fig. 21-1A,B). The
tympanic membrane
,
the landmark between the outer ear and middle ear, covers the
medial end of the external auditory meatus and converts sound
waves in the air to mechanical vibrations. The
tympanic cavity
is an air-± lled space that contains the
ossicles
, three tiny bones
that conduct the mechanical vibrations of the tympanic mem-
brane to the
oval window
of the cochlea. The tympanic cavity is
connected to the nasopharynx by the
auditory tube
(eustachian
tube), thereby allowing the air pressure on each side of the tym-
panic membrane to be equalized when the ambient air pressure
changes (e.g., by changes in altitude). (3) The
inner ear
consists
of structures contained within the
bony labyrinth
, a system of
tunnels and cavities in the petrous portion of the
temporal bone
,
the hardest bone in the body (Figs. 21-2 and 21-3A). The struc-
tures include the
cochlear labyrinth
or
cochlea
(Latin for “snail
shell”), which subserves hearing. It contains a spiral, fl uid-±
lled
tunnel within which a membranous tube, the
cochlear duct
, is
suspended (Fig. 21-3A,B). The sensory receptors that detect
sound are located in a strip of specialized epithelium, the
organ
of Corti
(
spiral organ
), in the cochlear duct. The
vestibular lab-
yrinth
consists of a complex group of fl uid-± lled tunnels and
cavities in the temporal bone that contain a group of intercon-
nected membranous structures, the
semicircular ducts
,
utricle
,
and
saccule
(Figs. 21-2 and 21-3A). The sensory receptors that
are involved in balance are located in specialized regions of
the semicircular ducts (
rotation
) and in the utricle and saccule
(
static head position
and
acceleration
).
Auditory System
Sound
is normally produced by compression and rarefaction
waves in air, of various frequencies, that impinge upon the
tympanic membrane where they are converted into mechani-
cal vibrations in the ossicles. The mechanical vibrations, in
turn, are transferred to the fl
uid of the
vestibule
at the
oval
window
(Fig. 21-2). This fl uid is
perilymph
, a sodium-rich fl
uid
that is similar in composition to cerebrospinal fl uid and extra-
cellular fl
uid. The resulting vibrations, or pressure waves, in
the
perilymph spread into the
scala vestibuli
of the cochlear
labyrinth and act upon mechanoreceptors in the
cochlear duct
to produce the sensation of
hearing
(Fig. 21-6A). The cochlear
duct is a membranous tube, triangular in cross section, that
coils inside the spiral tunnel in the cochlea (Fig. 21-3A,B). It is
suspended within the cochlear labyrinth so that it divides the
labyrinth into two tunnels, the
scala vestibuli
(above) and the
scala tympani
(below), which are connected with each other by
a small opening, the
helicotrema
(Figs. 21-4A,B and 21-6A).
The cochlear duct encloses the
scala media
, a space contain-
ing
endolymph
, a potassium-rich fl
uid similar in composition
to intracellular fl
uid. The scala media is bounded above by the
vestibular membrane
, below by the
basilar membrane
, and
externally by the
spiral ligament
and
stria vascularis
(Fig. 21-5).
The sensory receptors for hearing are specialized epithelial cells
(
hair cells
) in the
organ of Corti
. The hair cells get their name
from clumps of
stereocilia
that project from their apical sur-
faces and contact an overlying gelatinous structure, the
tectorial
membrane
. The organ of Corti sits on the basilar membrane
and extends its entire length, from the base of the cochlea to
the apex. It contains a single row of
inner hair cells
and three or
four rows of
outer hair cells
(Figs. 21-5 and 21-7A). In humans,
there are about 3,500 inner hair cells and 12,000 outer hair
cells, yet 95% of the afferent axons in the auditory nerve con-
tact only inner hair cells. The primary function of the inner hair
cells appears to be basic frequency and loudness discrimination,
whereas the outer hair cells appear to be primarily concerned
with the ±
ne-tuning of frequency discrimination in the cochlea
(Figs. 21-6C and 21-7A,B). When sound waves cause pressure
waves to occur in the scala vestibuli and scala media, the basilar
membrane vibrates up and down, and a shearing force is gener-
ated between the surface of the organ of Corti and the tectorial
membrane. The shearing force bends the stereocilia of the hair
cells, leading to the release of neurotransmitters by the hair cells
and the initiation of action potentials in auditory nerve axons
(Fig. 21-6A,B).
Vestibular System
The sense of
balance
is critical to our ability to walk, run, jump,
or even just stand still with eyes closed. One important source
of neural signals that aid in controlling such behaviors is the
peripheral vestibular apparatus
. This includes the
vestibular lab-
yrinth
, consisting of the
vestibule
, a cavity within the temporal
bone, and three
semicircular canals
, curved tunnels that connect
with the vestibule (Fig. 21-2). One canal is in approximately the
horizontal plane; the other two are in approximately the ver-
tical plane and at right angles to each other. These
cavities in
the bone are ± lled with
perilymph
. Floating within the vestibule
are two membranous saclike structures, the
utricle
and
saccule
.
Within each of the semicircular canals is a membranous tube
called a
semicircular duct
, which joins the utricle at each of its
ends (Figs. 21-2 and 21-3A). The utricle, saccule, and semicir-
cular ducts contain
endolymph
. The
semicircular ducts
detect
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