412
UNIT 3
Organ Systems
A
B
Tympanic membrane
Stapes
(in oval
window)
Round
window
Tympanic
cavity
Stapes
Stapedius
muscle
Tympanic
membrane
Auditory tube
Cochlea
External auditory meatus
Semicircular canals
Temporal bone
Malleus
Malleus
Anterior malleal
ligament
Posterior incudal
ligament
Vestibular nerve
Facial nerve
Cochlear nerve
Vestibule
Incus
Cartilage
Incus
Pinna
Outer
ear
Middle
ear
Inner
ear
Insertion
of tensor
tympani
muscle
Figure 21-1A.
Overview of the ear: Outer, middle, and inner ear.
The
ear
is divided into three regions: the
outer ear, middle ear
, and
inner ear
. The
outer ear
includes the
pinna
and the
external auditory
meatus
. The
pinna
(or
auricle
) is an irregularly shaped structure of elas-
tic cartilage covered by a layer of
perichondrium
(connective tissue)
and thin skin. One important function of the pinna is to selectively
F lter higher frequency sound waves and, therefore, aid in spatial local-
ization of sounds in the environment. The
external auditory meatus
is
a tunnel that carries sound waves to the
tympanic membrane
, where
the compressions and rarefactions of air are converted into mechanical
vibrations. The outer third of the meatus is lined with a continuation of
the cartilage of the pinna and a continuation of the perichondrium and
thin skin that covers the pinna. In the inner two thirds of the meatus,
the skin adheres directly to the periosteum of the temporal bone. The
middle ear
is an air-F lled cavity (
tympanic cavity
) that is separated from
the outer ear by the
tympanic membrane
. It contains three tiny bones,
the
ossicles
, which are the smallest bones in the body. These ossicles
transfer sound-induced movement of the tympanic membrane to the
fl uid contained in the
cochlea
, where the vibrations are transduced into
nerve impulses. The middle ear is connected to the posterior region of
the nasopharynx by the
auditory tube
(
eustachian
tube
), which allows
the equalization of air pressure on each side of the tympanic mem-
brane. The
inner
ear
contains the
cochlea
and the
vestibular apparatus
.
The
cochlea
, the snail shell–shaped organ of hearing, includes a mem-
branous tube lying within a fl uid-F
lled tunnel in the temporal bone (see
±ig. 21-2).
Auditory hair cells
in the cochlea are excited by vibratory
movements of the fl uid and generate action potentials in auditory nerve
F bers. The
vestibular apparatus
is the body’s sensory organ for bal-
ance and consists of membranous structures contained within the three
semicircular canals
and the
vestibule
as well as some accessory struc-
tures (see ±ig. 21-2).
Vestibular hair cells
within the
semicircular ducts
(inside the semicircular canals) detect rotational movement of the head
in three dimensions. Vestibular hair cells within the
utricle
and
saccule
(inside the vestibule) detect static head position and linear acceleration
in the horizontal and vertical planes, respectively. The auditory and
vestibular branches of CN VIII innervate these structures.
Figure 21-1B.
Middle ear structures.
The
tympanic membrane
(viewed here from its medial
side) is a thin, semitransparent cone-shaped sheet of col-
lagenous F
bers and F
broblasts, covered on the outer side
by a very thin layer of skin and on the inner side by the
mucosa that lines the rest of the tympanic cavity. The F
rst
of the
ossicles
, the
malleus
(hammer), is attached to the
upper half of the tympanic membrane, the
incus
(anvil) is
attached to the malleus by a saddle-shaped synovial joint,
and the
stapes
(stirrup) is attached to the incus by a ball-
and-socket synovial joint. The footplate of the stapes is
attached to the oval window of the
vestibule
. When sound
waves cause the tympanic membrane to vibrate, the chain
of ossicles rotates about the
anterior malleal
and
posterior
incudal ligaments
, causing the footplate of the stapes to
rock on the
oval window
and therefore producing waves of
compression in the fl
uid that F
lls the
bony labyrinth
. This
lever arrangement increases the pressure on the oval win-
dow approximately 20-fold compared to the air pressure
on the tympanic membrane. The
tensor tympani muscle
contracts during very loud sounds, reducing the movement
of the ossicles and the oval window.
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