558
UNIT 3
Regulation and Integration of the Body
15
In rods, the discs are discontinuous—stacked inside a cylinder
of plasma membrane like pennies in a coin wrapper. In cones,
the disc membranes are continuous with the plasma membrane,
so the interiors of the cone discs are continuous with the extra-
cellular space.
Te outer segments contain an elaborate array of
visual pig-
ments (photopigments)
that change shape as they absorb light.
Tese pigments are embedded in areas of the plasma membrane
that form discs (Figure 15.15b). Folding the plasma membrane
into discs increases the surface area available for trapping light.
(a) The outer segments of rods and cones are
embedded in the pigmented layer of the retina.
Rod discs
Visual
pigment
consists of
• R
etinal
• Opsin
(b) Rhodopsin, the visual pigment in rods, is
embedded in the membrane that forms discs
in the outer segment.
Process
of bipolar
cell
Light
Light
Light
Out
er
fiber
Apical
micro
villus
Discs
containing
visual pigments
Inner segment
Melanin
granules
Discs being
phagocytiz
ed
Pigment
cell
nucleus
Inner
fibers
Rod
cell
body
Cone
cell
body
S
ynaptic
terminals
Rod cell
body
Nuclei
Mitochondria
Connecting
cilia
Outer segment
Pigmented layer
Basal lamina
(border with
choroid)
Figure 15.15
Photoreceptors of the retina.
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