Chapter 15
The Special Senses
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15
allows Na
1
and Ca
2
1
to enter, depolarizing the cell to its
dark
potential
of about
2
40 mV. In the light, cGMP breaks down, the
cation channels close, Na
1
and Ca
2
1
stop entering the cell, and
the cell hyperpolarizes to about
2
70 mV.
Tis arrangement can seem bewildering, to say the least.
Here we have receptors built to detect light that depolarize in
the dark and hyperpolarize when exposed to light! However, all
that is required is a signal and hyperpolarization is just as good
a signal as depolarization.
Information Processing in the Retina
How is the hyperpolarization of the photoreceptors transmit-
ted through the retina and on to the brain?
Figure 15.18
il-
lustrates this process. As you study this sequence, notice that
the photoreceptors do not generate action potentials (APs),
and neither do the bipolar cells that lie between them and the
ganglion cells. Photoreceptors and bipolar cells only generate
graded potentials—excitatory postsynaptic potentials (EPSPs)
and inhibitory postsynaptic potentials (IPSPs).
Tis is not surprising if you remember that the primary func-
tion of APs is to carry information rapidly over long distances.
Retinal cells are small cells that are very close together. Graded
potentials can serve quite adequately as signals that directly
combination breaks down, allowing retinal and opsin to
separate. Te breakdown of rhodopsin to retinal and opsin
is known as the
bleaching of the pigment
.
3
Pigment regeneration:
Once the light-struck all-
trans
-
retinal detaches from opsin, enzymes within the pig-
mented epithelium reconvert it to its 11-
cis
isomer.
Ten, retinal heads “homeward” again to the photore-
ceptor cells’ outer segments. Rhodopsin is regenerated
when 11-
cis
-retinal is rejoined to opsin.
Te breakdown and regeneration of visual pigments in cones
is essentially the same as for rhodopsin. However, cones are
about a hundred times less sensitive than rods, which means
that it takes higher-intensity (brighter) light to activate cones.
Light Transduction Reactions
What happens when light
changes opsin’s shape? An enzymatic cascade occurs that ul-
timately results in closing cation channels that are normally
kept open in the dark.
Figure 15.17
illustrates this process in
more detail, but in short, light-activated rhodopsin activates a
G protein called
transducin
. ±ransducin, in turn, activates
PDE
(
phosphodiesterase
), the enzyme that breaks down
cyclic GMP
(cGMP)
. In the dark, cGMP binds to cation channels in the
outer segments of photoreceptor cells, holding them open. Tis
Retinal absorbs light
and changes shape. Visual
pigment activates.
Visual pigment
activates transducin
(G protein).
Transducin
activates
phosphodiesterase
(PDE).
PDE converts
cGMP into GMP,
causing cGMP
levels to fall.
As cGMP levels fall,
cGMP-gated cation
channels close, resulting
in hyperpolarization.
Visual
pigment
Light
Transducin
(a G protein)
All-
trans
-retinal
11-
cis
-retinal
cGMP-gated
cation channel
open in dark
Phosphodiesterase (PDE)
cGMP-gated
cation channel
closed in light
cGMP
GMP
cGMP
Na
+
Na
+
Ca
2
+
Ca
2
+
Light (1st
messenger)
Receptor
G protein
Enzyme
2nd
messenger
Recall from Chapter 3 that
G protein signaling mechanisms
are like a molecular relay race.
5
4
3
2
1
Figure 15.17
Events of phototransduction.
A portion of photoreceptor disc membrane is
shown. The G protein conversion of GTP to GDP has been omitted for clarity. For simplicity, the
channels gated by cyclic GMP (cGMP) are shown on the same membrane as the visual pigment
instead of in the plasma membrane.
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