CHAPTER 20
Eye
405
Figure 20-14A and B.
A
representation of rods and cones.
Rods and cones
are photoreceptor cells in the retina. They are similar in structure and both have (1) a
synaptic region
(2) a
nuclear
region
(3)
inner segments
, and (4)
outer segments
. There are numerous
synaptic vesicles
in the synaptic regions, where the photorecep-
tors synapse with the dendrites of bipolar cells. The nuclear regions of rods and cones lie in the outer nuclear layer. The inner and outer
segments form the photoreceptor layer of the retina. The
inner segments
contain
mitochondria
,
rough endoplasmic reticulum
,
Golgi
apparati
, and other organelles that support the synthesis of proteins. The
outer segments
are in contact with the apical region of the
pigmented epithelium cells.
Outer segments of rods
are composed of a series of superimposed
disks
, which have individual membranes,
are stacked on each other, and are enclosed within the
plasma membrane
.
Outer segments of cones
have
disks
that are formed by invagi-
nations of the
plasma membrane
. The membranes of the disks are continuous with the plasma membrane of the cell. The inner region is
much wider than the outer region, which gives a cone appearance. Other differences between rods and cones are listed in Table 20-1.
D. Cui
Inner rod fiber
Synaptic vesicles
Synaptic
region
Nucleus
Nuclear
region
Outer rod fiber
Mitochondrion
Basal body
Disk
Plasma membrane
Modified cilium
Inner
segment
Outer
segment
A
D. Cui
Synaptic
region
Inner cone fiber
Nuclear
region
Nucleus
Mitochondrion
Basal body
Modified cilium
Outer cone fiber
Inner segment
Outer segment
B
Types of
Photoreceptor
Cells
Synaptic
Region
Nuclear
Region
Inner Segment
Outer Segment
Distribution in
the Retina
Main
Function
Rods
Spherule
synapses with
dendrites of
one bipolar
neuron
Small,
round
nucleus
Fewer
mitochondria than
cones; synthesized
proteins passed
only to newly
forming disks
Cylindrical in shape; disk
membranes are separate
and are not connected
to the outside plasma
membrane; disks contain
rhodopsin protein
Numerous in the
peripheral retina;
none in fovea
Vision in
dim light;
motion
detection
Cones
Pedicle
synapses with
dendrites of
several bipolar
neurons
Large,
ovoid
nucleus
More
mitochondria than
rods; synthesized
proteins passed
to entire outer
segment
Cone shaped; disk
membranes are connected
to the plasma membrane
and form invaginated
membranes; disks contain
iodopsin protein
Highly
concentrated in
the fovea and
macula lutea;
less numerous in
periphery
Color
vision;
perception
of ± ne detail
TABLE 20-1
Comparison of Rods and Cones
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