Sensory Physiology
213
and other organelles, and the synaptic terminal that connects
the photoreceptor to the next neurons in the retina. The two
types of photoreceptors are called
rods
and
cones
because of
the shapes of their light-sensitive outer segments. In cones, the
light-sensitive discs are formed from in-foldings of the surface
plasma membrane, whereas in rods, the disc membranes are
intracellular structures. Note that the light-sensitive portions
of the photoreceptor cells face
away
from the incoming light,
and the light must pass through all the cell layers of the ret-
ina before reaching and stimulating the photoreceptors. Two
pigmented layers, the choroid and the
pigment epithelium
of the back of the retina, absorb light that has bypassed the
photoreceptors. This prevents its refl ection and scattering back
through the rods and cones, which would cause the visual
image to blur. The rods are extremely sensitive and respond to
very low levels of illumination, whereas the cones are consider-
ably less sensitive and respond only when the light is bright.
The photoreceptors contain molecules called
photopig-
ments,
which absorb light. There are four unique photopig-
ments in the retina, one found in rods
(rhodopsin),
and
one in each of three different types of cones. Photopigments
contain membrane-bound proteins called
opsins,
which sur-
round and bind a
chromophore
molecule. The chromophore
in all types of photopigments is
retinal,
a derivative of vita-
min A. This is the part of the photopigment that is light-sen-
sitive. The opsin differs in each of the four photopigments.
Each type of opsin binds to the chromophore in a different
way. Because of this, each of the four photopigments absorbs
light most effectively at a specifi c part of the visible spectrum.
For example, one photopigment absorbs light most effectively
at long wavelengths (sometimes designated as “red” cones),
whereas another absorbs short wavelengths (“blue” cones).
The membranous discs of the outer segment are arranged
in parallel to the surface of the retina (
Figure 7–28
). This lay-
ered arrangement maximizes the membrane surface area. In
fact, each photoreceptor may contain over a billion molecules of
photopigment, providing an extremely effective trap for light.
The photoreceptor is unique because it is the only type
of sensory cell that is depolarized when it is at rest (i.e., in the
dark), and
hyperpolarized
in response to its adequate stimu-
lus (see Figure 7–28). In the absence of light, action of the
membrane-bound enzyme
guanylyl cyclase
converts GTP into
a high intracellular concentration of the second messenger mol-
ecule, cyclic GMP (cGMP). The cGMP maintains the ligand-
gated cation channels in the outer segment membrane in the
open state, and a persistent infl ux of sodium and calcium results.
Thus, in the dark, cGMP concentrations are high, and the pho-
toreceptor cell is maintained in a relatively depolarized state.
Phosphodiesterase
Photopigment
(opsin)
P
Retinal
Light
Intracellular fluid
of photoreceptor
GTP
cGMP
cGMP
cGMP
GMP
Transducin
Processes favored
in the dark
Processes activated
by light
Guanylyl
cyclase
Cation
channel
Na
+
/Ca
2+
Outer
segment
Disc
Inner
segment
Figure 7–28
Phototransduction in a cone cell. Retinal is the chromophore in the photopigment. Stimulation of the cGMP phosphodiesterase in the
cytoplasm of the disc produces the decrease in cGMP that closes the cation channels. For simplicity, the proteins are shown widely spaced in
the membrane. In fact, all of these proteins are densely interspersed within the cone disc membrane. Phototransduction in rods is basically
identical, except the membraneous discs are contained completely within the cell’s cystol (see Figure 7–27), and the cGMP-gated ion channels
are in the surface membrane rather than the disc membranes.
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