408
UNIT 3
Organ Systems
L
a
m
in
in
a
Lamina
Subarachnoid
Subarachnoid
space
space
(filled with CSF)
(filled with CSF)
Retinal nerve
fibers
(nonmyelinated)
Sclera
Sclera
Retina nerve
fibers
Neuroglial cells
Optic disk
(optic papilla)
Subarachnoid
space
(filled with CSF)
Pia mater
Optic nerve
Optic nerve
(myelinated)
(myelinated)
Optic nerve
(myelinated)
Sclera
cribrosa
c
r
ib
r
o
s
a
cribrosa
A
Optic Nerve
Figure 20-17A.
Optic nerve and optic disk.
H&E,
3
22; inset,
3
83
The
optic nerve
is a nerve f ber trunk Formed by the convergence oF
retinal ganglion cell axons at the posterior pole oF the eye. ±rom there,
they leave the globe on their way to the brain. Each optic nerve con-
tains about one million
myelinated axons
and even more
neuroglial
cells
. The surFace oF the optic nerve is covered by pia mater, which is
continuous with that on the surFace oF the brain. The
optic disk
(
optic
nerve papilla
) is the small circular site in the retina where the retinal
nerve f ber layer (
nonmyelinated nerve f bers
) continues into the optic
nerve. The nonmyelinated nerve f bers begin to acquire myelin at the
level oF the
lamina cribrosa
(
thin dotted line
), a perForated, sievelike
region oF the sclera through which optic nerve f bers and blood ves-
sels pass. The myelinated segments oF ganglion cell axons, thereFore,
Form the optic nerve. The
neuroglial cells
include
oligodendrocytes
,
which produce myelin For axons in the CNS, and
astrocytes
, which
perForm several nutritive and supportive Functions.
CLINICAL CORRELATION
Figure 20-17B.
Papilledema.
Papilledema
is a noninfl
ammatory swelling oF the optic disk. It is
produced by
increased intracranial pressure
that is transmitted
along the optic nerve sheath as elevated
cerebrospinal fl
uid
pres-
sure, and it is, thereFore, a sign oF a potentially liFe-threatening
condition. The increased pressure disrupts blood circulation
within the optic nerve, causing leakage oF water, protein, and
other contents into the extracellular spaces oF the optic disk. The
most noticeable symptoms oF increased intracranial pressure are
headache and vomiting. Clinical f
ndings include a
swollen and
elevated optic disk
(papilledema),
engorged and tortuous retinal
veins
, and Focal hemorrhages.
Cerebral
tumors
,
subdural
hema-
toma
,
malignant
hypertension
, and
hydrocephalus
are the most
common causes oF increased intracranial pressure and papille-
dema. Because papilledema is a sign oF many systemic intracra-
nial and spinal diseases, the correct diagnosis oF the underlying
disease is essential For proper treatment.
D. Cui
Swollen and elevated
optic disk
Normal
CSF
Optic
disk
Elevated
CSF
pressure
Engorged and
tortuous central vein
B
Blepharitis
: Infl ammation oF the eyelids and especially oF their margins (±ig. 20-3C).
Ptosis
:
Drooping or Falling down, usually oF the upper eyelid, because oF muscle weakness or paralysis (±ig. 20-4C).
Open angle (glaucoma)
: Raised intraocular pressure in which obstruction to aqueous fl
ow occurs at the ultrastructural
level, within the walls oF the tiniest interstices oF the trabecular meshwork (±ig. 20-12C).
Closed angle (glaucoma)
: The fl
ow oF aqueous fl uid is blocked by the root oF the iris. This can be a generalized closure
(hyperopic eyes) or in multiple Focal adhesions caused by infl
ammation (peripheral anterior synechiae [±ig. 20-12C]).
Cupping of the optic disk
: Glaucoma destroys retinal axons as they traverse the optic disk in the walls oF a central, cup-
shaped conduit. As the walls disappear, the cup enlarges (±ig. 20-12C).
Dry macular degeneration
: Scattered drusen damage the pigment epithelium oF the macula, gradually decreasing visual
acuity (±ig. 20-16A).
Wet macular degeneration
: Abnormal capillaries break through the retinal pigment epithelium and leak underneath the
retina. When close to the Fovea, they can decrease visual acuity (±ig. 20-16A).
Limbu
s: An anatomic transition zone between cornea and sclera (corneoscleral junction), which is an important landmark
For eye surgery procedures (±ig. 20-11A).
Ora serrata
: A denticulate border (junction) between the ciliary body and the retina; this is an important anatomic landmark
For the ophthalmologist (±ig. 20-12B).
SYNOPSIS 20-2
Clinical Terms for the Eye
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