Chapter 11
The Vertebrate Retina
Wallace B. Thoreson
1.
Briefly summarize the major steps in phototransduction in rods.
2.
What are the calcium-dependent steps in light adaptation?
3.
What are the major physiological features of the two most common types of retinal ganglion cells in the
primate retina?
4.
In mammalian retina, which capillary beds supply oxygen to the photoreceptors and which to the
remaining parts of the retina?
5.
For an ON-type ganglion cell, describe how the center-surround organization of visual receptive fields
improves edge detection.
6.
Describe the mechanisms responsible for directional selectivity in retinal ganglion cells.
7.
Describe the rod pathway used in scotopic vision.
8.
Describe the key enzymatic steps in the visual cycle converting bleached all-trans-retinal back into lightsensitive 11-cis-retinal.
9.
Describe the spatial redistribution of K+ by Müller cells.
10. Summarize the enzymatic mechanisms by which phototransduction is terminated after photostimulation.
Multiple choice:
11. Which of the following is the predominant glial cell type in the retina?
a. Astrocyte
b. Müller cell
c. Microglia
d. Schwann cell
e. RPE cell
12. In which layer of the retina do bipolar cell terminals contact ganglion cell dendrites?
a. Outer nuclear layer
b. Outer plexiform layer
c. Inner nuclear layer
d. Inner plexiform layer
e. Ganglion cell layer
13. Which of the following statements is MOST true about the fovea?
a. The fovea contains both rods and cones
b. The fovea is the region of the retina where ganglion cell axons exit the eye.
c. Cone axons in the fovea contain xanthophyll pigments.
d. The location of the fovea on the retina is not a fixed anatomical feature but varies with focus.
e. Light must first pass through overlying layers of retina to reach photoreceptor outer segments at the center
of the fovea.
14. Upon which chromosome are genes for M cone pigments located?
a. X chromosome
b. Y chromosome
c. Chromosome 3
d. Chromosome 7
e. Chromosome 10
11. The Vertebrate Retina
Wallace B. Thoreson
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15. Which of the following statements is most true about ON and OFF type retinal bipolar cells?
a. ON bipolar cells possess KA/AMPA receptors and OFF bipolar cells possess NMDA receptors.
b. ON bipolar cells possess NMDA receptors and OFF bipolar cells possess mGluR6.
c. ON bipolar cells possess mGluR6 and OFF bipolar cells possess NMDA receptors
d. ON bipolar cells possess mGluR6 and OFF bipolar cells possess KA/AMPA receptors
e. ON bipolar cells possess KA/AMPA receptors and OFF bipolar cells possess mGluR6
16. Which of the following is NOT considered to be a major role of Muller cells?
a. Spatial redistribution of potassium
b. Neurotransmitter uptake and removal
c. Homeostatic maintenance of retinal pH levels
d. Glycogen storage
e. Promoting retinal adhesion to the back of the eye
17. Which of the following is NOT considered to be a major role of RPE cells?
a. Migration to sites of injury.
b. Formation of blood-retinal barrier
c. Absorption of stray light
d. Photopigment recycling
e. Phagocytosis of outer segment discs
18. Which of the following proteins is a major constituent of synaptic ribbons?
a. Retinol dehydrogenase
b. Melanin
c. Ribeye
d. RGS/Gβ5
e. Bestrophin
19. Which of the following is the main reason that retinal detachment leads to blindness?
a. The retina dies from oxygen deprivation after separating the retina from the choroidal blood supply.
b. Separation of the RPE and photoreceptor outer segments prevents visual pigment recycling.
c. Retinal detachment shears off photoreceptor outer segments that remain covalently bound to the RPE.
d. Photoreceptors die because of the inability of the RPE to phagocytose photoreceptor outer segments.
e. Retinal detachment moves the retina out of the optical plane of focus.
20. What is the light-sensitive molecule found in intrinsically photo-sensitive retinal ganglion cells of
mammalian retina?
a. Melanopsin
b. Transducin
c. Rhodopsin
d. Cryptochrome
e. Peropsin
11. The Vertebrate Retina
Wallace B. Thoreson
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ANSWERS
1.
Briefly summarize the major steps in phototransduction in rods.
Absorption of a photon stimulates a conformational change in the chromophore, 11-cis-retinal (vitamin A
aldehyde), so that it assumes an all-trans configuration (all-trans-retinal). This conformational change in the
chromophore causes a comformational change in the G-protein-coupled receptor, rhodopsin, so that it achieves
its active conformation, meta-rhodopsin. Activated metarhodopsin activates the G protein alpha subunit,
transducin, which in turn stimulates cGMP-specific phosphodiesterase to cleave cGMP into GMP. This
reduction in [cGMP] causes cGMP-gated non-selective cation channels to close. Closure of these channels
reduces the influx of Na+ (and Ca2+) and thus hyperpolarizes the rod.
2.
What are the calcium-dependent steps in light adaptation?
In response to light, the influx of Ca2+ into outer segments through cGMP-gated channels channels diminishes
but its efflux by the Na+/Ca2+ exchanger continues. This causes the concentration of Ca2+ in the outer segment
to diminish in light. The reduction in Ca2+ levels has two major effects on phototransduction: 1) Decreased
[Ca2+] enhances the activity of guanylyl cyclase stimulating the production of cGMP. The resulting increase in
cGMP levels opens cGMP-gated channels causing photoreceptor cells to depolarize.
2) Decreased
intracellular [Ca2+] increases the affinity of cGMP for cGMP-gated cation channels and thus enhances the
opening of these channels. The membrane depolarization that results from re-opening of cGMP-gated cation
channels restores the operating range of the photoreceptor cell by allowing it to hyperpolarize in response to
another flash of light.
3.
What are the major physiological features of the two most common types of retinal ganglion cells in the
primate retina?
Magnocellular ganglion cells have large cell bodies and large dendritic arborizations resulting in large receptive
fields. Because of their large receptive fields, M cells do not contribute to fine feature analysis. Instead, their
output is primarily related to motion and other changes in illumination.
Parvocellular ganglion cells have small cell bodies with small dendritic arborizations resulting in small
receptive fields. P cells are also wavelength-selective. P cell output thus contributes to fine feature analysis and
color vision.
4.
In mammalian retina, which capillary beds supply oxygen to the photoreceptors and which to the
remaining parts of the retina?
Choroidal capillaries provide oxygen to photoreceptors. Retinal capillaries in the INL and ganglion cell layer
provide oxygen to the remainder of the retina.
5.
For an ON-type ganglion cell, describe how the center-surround organization of visual receptive fields
improves edge detection.
The center-surround arrangement of the receptive field is due to the presence of an annular inhibitory region
surrounding a central excitatory region. A small spot of light illuminating only the excitatory center strongly
excites the cell whereas an annulus of light illuminating the inhibitory surround strongly inhibits it. By contrast,
stimulation of both the excitatory center and inhibitory surround by full field illumination produces relatively
small changes in ganglion cell output. A bar or edge of light illuminating the entire excitatory center along
with small portions of the inhibitory surround will evoke a stronger response than full field illumination, but not
as strong as a spot of light illuminating just the excitatory center. The net result of this center-surround
receptive field arrangement is that cells respond more strongly to spots, bars and edges than to full field
illumination.
6.
Describe the mechanisms responsible for directional selectivity in retinal ganglion cells.
Directional selective ganglion cells are excited by stimuli moving along one axis of the receptive field but show
little response to stimuli moving in the opposite direction. The directional selectivity in ganglion cells results
from the fact that cholinergic excitation from starburst amacrine cells occurs earlier than GABAergic inhibition
with visual stimuli moving in the preferred direction, but GABAergic inhibition precedes excitation with stimuli
moving in the opposite, non-preferred direction. Because the preceding GABAergic inhibition dampens the
cell’s responses to movement in the non-preferred, directional selective ganglion cells respond more strongly to
movement in the preferred direction.
11. The Vertebrate Retina
Wallace B. Thoreson
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7.
Describe the rod pathway used in scotopic vision.
Rods contact ON-type rod bipolar cells. These rod ON bipolar cells do not contact ganglion cells directly but
instead contact AII amacrine cells. AII amacrine cells form gap junctions with cone ON bipolar cell terminals
as well as inhibitory, sign-inverting glycinergic synapses with cone OFF bipolar cells. Thus, AII amacrine cells
feed rod signals into ON and OFF cone bipolar cells driving ON and OFF center ganglion cells, respectively.
8.
Describe the key enzymatic steps in the visual cycle converting bleached all-trans-retinal back into lightsensitive 11-cis-retinal.
After the bleaching of 11-cis-retinal to all-trans-retinal by light, all-trans-retinal converts rapidly to all-transretinol (all-trans-vitamin A) in the photoreceptor outer segment. All-trans-retinol is transported out of
photoreceptor cells, through the interstitial space, and into the RPE through a process involving interstitial
retinoid binding proteins (IRBP).
In the RPE, all-trans-retinol is converted to 11-cis-retinol by retinol
isomerase (RPE65) and 11-cis-retinol is converted back to 11-cis-retinal (11-cis-vitamin A aldehyde) by 11-cisretinol dehydrogenase. Regenerated 11-cis-retinal is transported back to photoreceptors via IRBP.
9.
Describe the spatial redistribution of K+ by Müller cells.
When K+ levels to increase in the IPL and OPL due to neuronal depolarization, excess K+ ions enter K+
channels localized to Müller cells processes. This K+ influx in the plexiform layers is accompanied by a
simultaneous efflux through K+ channels clustered at the vitreal surface and along blood vessels. Conversely, a
reduction in K+ accompanying neuronal hyperpolarization is accompanied by an efflux of K+ out of Müller cells
into the plexiform layers.
10. Summarize the enzymatic mechanisms by which phototransduction is terminated after photostimulation.
Inactivation of the light-activated form of rhodopsin, meta-rhodopsin, begins with its phosphorylation by
rhodopsin kinase. Phosphorylation of meta-rhodopsin by rhodopsin kinase allows arrestin to bind and arrest
further enzymatic activity. Light-activated transducin is shut down by the intrinsic GTPase activity of
transducin that converts GTP into GDP. The G protein, transducin, is de-activated by converting GTP into
GDP. The intrinsic GTPase activity of transducin is accelerated by binding to an accessory protein, RGS/Gβ5.
Multiple Choice
11.
b
12.
d
13.
c
14.
a
15.
d
16.
e
17.
a
18.
c
19.
b
20.
a