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U N I T 1 0
Nervous System
hemorrhage and the development of opacities; separa-
tion of the pigment and sensory layers of the retina (i.e.,
retinal detachment); and macular degeneration. Because
the retina has no pain fibers, most diseases of the retina
do not cause pain, nor do they produce redness of the eye.
The retina is composed of two layers: an outer pig-
ment (melanin-containing) epithelium and an inner
neural layer
16,22
(Fig. 38-6A). The outer surface of the
pigmented layer, a single-cell–thick lining, abuts the
choroid and extends anteriorly to cover the ciliary body
and the posterior side of the iris. Its pigmented epithelial
cells, like those of the choroid, absorb light and prevent
it from scattering. The pigment layer also stores large
quantities of vitamin A, which is an important precursor
of the photosensitive visual pigments.
The inner light-sensitive neural retina covers the inner
aspect of the eyeball.
16,22
The neural retina is composed
of three layers of neurons: a posterior layer of photore-
ceptors (rods and cones), a middle layer of bipolar cells,
and an inner layer of ganglion cells that communicate
with the photoreceptors (see Fig. 38-6B). Light must
pass through the transparent inner layers of the sensory
retina before it reaches the photoreceptors. Impulses
produced in response to light spread from the photore-
ceptors to the bipolar neurons and other interneurons
and then to the ganglionic cells, where action potentials
are generated. The interneurons, which have cell bodies
in the bipolar layer, play an important role in modu-
lating retinal function. The optic disk, where the optic
nerve exits the eye, is the weak part of the eye because
it is not reinforced by the sclera. It also forms the blind
spot because it is not backed by photoreceptors, and
light focused on it cannot be seen. People do not notice
the blind spot because of a sophisticated visual func-
tion called “filling in,” which the brain uses to deal with
missing visual input.
Two types of photoreceptors are present in the ret-
ina: rods capable of black–white discrimination and
cones capable of color discrimination.
16,22,24
Rod-based
vision is particularly sensitive to detecting light, espe-
cially moving light stimuli, at the expense of clear pat-
tern discrimination. Rod vision is particularly adapted
for night and low-level illumination. Cone receptors,
which are selectively sensitive to different wavelengths
of light, provide the basis for color vision. Three types
of cones, or cone-color systems, respond to the blue,
green, and red portions of the visible electromagnetic
spectrum. Cones do not have the dark adaptation
capability of rods. Consequently, the dark-adapted eye
is a rod receptor eye with only black–gray–white expe-
rience (scotopic or night vision). The light-adapted
eye (
photopic vision
) adds the capacity for color
discrimination.
Both rods and cones contain chemicals that change
configuration on exposure to light and, in the process,
generate electric signals that lead to the action potentials
generatedbytheganglioniccells.The light-sensitivechem-
ical in rods is called
rhodopsin
, and the light-sensitive
chemicals in cones are called
cone
or
color pigments
.
Both types of photoreceptors are thin, elongated, mito-
chondria-filled cells with a single, highly modified cil-
ium (Fig. 38-6C). The cilium has a short base, or inner
Optic disk
Optic
nerve
Sclera
Layer
of rods
and
cones
A
B
C
Bipolar
cell
layer
Ganglion
cell
layer
Axons of retinal
ganglion cells
Pigment
epithelium
Neural
layer
Direction
of light
Synaptic body
Nucleus
Inner
segment
Connecting
structure
Outer
segment
Pigment
epithelium
FIGURE 38-6.
Organization of the retina.
(A)
Organization of the retina showing the inner neural
layer and the outer pigment epithelium.
(B)
The three layers of the neural retina: a posterior layer
of photoreceptors (rods and cones), a middle layer of bipolar cells, and an inner layer of ganglionic
cells.
(C)
Photoreceptor structure: a retinal rod, showing its component parts and distribution of
organelles.
