422
UNIT 3
Organ Systems
CLINICAL CORRELATION
Figure 21-11C.
Otitis Media.
Otitis media
is a bacterial or viral infection involving the
middle ear. The mucosal lining of the middle ear produces
serous exudate and pus when infl
amed. In children (younger
than age 3 years), the auditory tube is not fully developed and
drainage of infection is problematic. The consequent buildup
of fl
uid in the middle ear may cause severe pain (
otalgia
) and
a temporary
conductive hearing loss
. The tympanic mem-
brane becomes
erythematous
and bulges outward from the
fl uid pressure (
left
). (Because of the limited F
eld of view of the
operating microscope, only the anterior half of the membrane
is visible.) Most cases resolve spontaneously, although anti-
biotics are commonly prescribed to speed recovery. In cases
of severe and repeated infections, a ventilation tube may be
inserted into an incision in the tympanic membrane to relieve
pressure in the middle ear (
right
). These tubes usually stay
in place for about 1 year before they spontaneously extrude.
Very rarely, middle ear infections may spread locally and pro-
duce
mastoiditis
or
labyrinthitis
.
Kinocilium
Basal body
Calyx
nerve
ending
Afferent
nerve
fiber
Efferent
nerve
fibers
Afferent
nerve
fiber
Type I hair cell
Type II hair cell
Stereocilia
A
Figure 21-11A.
Vestibular hair cells.
Type I hair cells
are
pyriform
(pear shaped) and have basally
located nuclei. They are almost completely surrounded by a single,
chalice-shaped synaptic terminal (
calyx
) of a large afferent nerve
F ber. Each type I hair cell is innervated by a single nerve axon,
and each axon branches to innervate only a few hair cells. Type I
hair cells receive few efferent endings, which contact the afferent
nerve endings rather than the hair cell itself.
Type II cells
are more
cylindrical in shape, with more centrally located nuclei. These
cells are contacted by multiple small boutonlike synaptic endings
associated with both afferent and efferent nerve F
bers. Type II
cells receive axon terminals from multiple nerve F
bers, each of
which branches to contact many type II hair cells. Both types of
vestibular hair cells have a single
kinocilium
(with a typical
basal
body
and a ring of nine double microtubules) on one side of the
apical surface. A group of 40 to 100
stereocilia
of various lengths
are arranged in a hexagonal array next to the kinocilium.
Action potentials
AB
C
B
Figure 21-11B.
Excitation and inhibition in hair cells.
Vestibular hair cells
continuously release a small amount of
neurotransmitter at the afferent terminal synapses, producing a
moderate frequency of action potentials in afferent nerve F
bers in
the resting state (
B
). When movement of endolymph causes the
cupula or otolithic membrane to defl
ect the hair cell stereocilia
toward
the kinocilium, the amount of neurotransmitter released
goes up and the frequency of action potential discharge
increases
(
A
). When the stereocilia are defl
ected
away from
the kinocilium,
less neurotransmitter is released and the frequency of the action
potentials
decreases
(
C
). Hair cells in different regions of the
peripheral vestibular apparatus have their kinocilia and stereo-
cilia oriented in different directions. The CNS integrates informa-
tion from the various hair cells to form a central representation of
the position of the head in space, the direction of any movement
of the head, and the rate of change (acceleration) of any move-
ment.
Inflamed and bulging
tympanic membrane,
viewed through operating
microscope
Silicone ventilation
tube in place
C
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