Chapter 12: Retina

Question 1

What is the retina composed of?

Answer 1

The retina is composed of two laminar structures that are derived from the invagination of the embryonic outer cup that folds an ectodermal layer into apex-to-apex contact with itself (thus creating a subretinal space)
- Outer RPE and Inner Neurosensory retina are created

Question 2

What cell types are located in the Retina?

Answer 2

• Photoreceptors (Rods/Cones)
• Bipolar Cells (Rod-On, Cone-On, and Cone-Off)
• interneurons (horizontal and amacrine cells)
• Ganglion cells and their axons
• Supporting cells (astroglia, oligodendroglia, schwanna cells, microglia, vascular endothelium, pericytes)

Question 3

What is phototransduction?

Answer 3

process of cathcing light and converting its energy into a neural response

occurs within the outer segment of photoreceptors

Question 4

Why do rods have a higher sensitivity to light then cones?

Answer 4

They have a higher light sensitivity than cones b/c they contain more membrane than cones

Question 5

What is the Rod Outer Segment composed of?

Answer 5

composed primarily of plasma-membrane organized into flattened sacs, each containing 1x10^6 rhodopsin molecules (membrane bound)

There are approx. 1000 sacs per rod, each containing the protein machinery to capture and amplify light energy

Question 6

What is the purpose of rhodopsin in the outer segment of photoreceptors?

Answer 6

Rhodopsin captures the lights energy, and responds to a single quantum of light.

Question 7

What type of light is rhodopsin best at absorbing and why? What type of light is it insensitive to, and why is this significant?

Answer 7

Best at absorbing green light, Lamdba= 510 nm
also able to absord blue and yellow light
Insensitive to red light due to longer wavelength. Significant b/c explains why red light doesnt bleach out photorecepters at night, adn therefore why its best to use a red flashlight in the dark

Question 8

What happens to rhodopsin after absorbing a quantum of light?

Answer 8

The opsin molecule undergoes structural changes and becomes Metarhodopsin II, which controls the inflow of cations into the rod outer segment via cGMP-gated cationic channel on the outer membrane (opens channel)
Rhodopsin also actives transducin, which helps to amplify this reaction

Question 9

How do rod sacs differ from cone sacs?

Answer 9

Cone sacs are connected to the outer membrane, rod sacs are not.

Question 10

What unique proteins are found on the membrane of the outer segments of the photoreceptors?

Answer 10

Peripherin and Rod Outer Regment Protein 1

Fx= play a role in development and maintenance of sacs curvature (also found in cones)

Question 11

What is the ABC protein? where is it found? What is its function?

Answer 11

ATP binding casette protein, is unique to Rods, and functions as a transporter of all-trans-retinol

Question 12

How do the outer segments produce energy?

Answer 12

Via glycolsysis. (b/c mitochondria are all contained within the inner segments of the photoreceptors)

Question 13

How does rod phototransduction compare to cone phototransduction?

Answer 13

Compared to rod phototransduction, cone phototransduction is insensitive, but is faster and able to adapt to ambient levels of illumination.

The greater the ambient illumination, the faster and more temporally accurate the response

Question 14

Why do higher levels of illumination decrease sensitivity?

Answer 14

B/c they bleach away photopigments

Question 15

Describe the process of cone phototransduction

Answer 15

Light activates cone Opsins-> enzymatic cascade-> hydrolysis of cGMP-> closure of cGMP-gated cation channels

Question 16

What kind of feedback do the photoreceptors have?

Answer 16

Neurally-mediated negative feedback (horizontal cells of the inner nuclear layer synapse antagonistically bac onto cones and release GABA, which inhibits the cones)

Question 17

What happens when light hyperpolarizes a cone?

Answer 17

Cone hyperpolarized–> hyperpolarizes neighboring horizontal cells–> inhibits horizontal cells-> disinhibition of cones

Increases fusion flicker frequency in cones (around 100 Hz) compared to rods (30 Hz)

Question 18

How do primates have trivariant color vision?

Answer 18

Through the presence of a 3 cone mechansim Red-Green color vision is enabled
L-type cones and M-type cones: contribute to both achromatic and chromatic contast, and are more numerous than S-type cones in the human retina

S-type cones

Question 19

Why do most visual defects involve red-green color discrimination?

Answer 19

b/c they are located tandem on the x chromosome, related to unequal crossing of L- and M- cone opsin genes, and creating a different spectral absorption function

Question 20

What is the most common AD mutation of rhodopsin that causes ADRP?

Answer 20

P23H, causes an abnormal folding of a protein that accumulates in RER
accounts for 10% of RP cases in US

Question 21

What do defects in the L- and M- cone opsins cause?

Answer 21

Blue cone monochromatism, which occurs in males 2/2 location on X chromosome.

Question 22

What do defects in all 3 cone opsin genes cause?

Answer 22

Achromotopsia (= Rod Monochromatism)

Question 23

What do defects in L- or M- cone opsins cause?

Answer 23

Red-Green color deficiencies, almost exclusively occurs in males 2/2 location on X Chromosome.

Question 24

What cell types are located in the Inner Nuclear Layer of the Retina?

Answer 24

Neurons: Horizontal, Amacrine, Bipolar

Glial Cell: Muller Cells

Question 25

How many types of bipolar cells exist?

Answer 25

Rods and cones have separate bipolar cells, and cones have two types of bipolar cells–On-type and Off-type

However, S cones only have one type of bipolar cell (similar to Rods)

Question 26

How do bipolar neurons interact with photoreceptors?

Answer 26

On bipolar cells are inhibited/Off are excited by release of glutamate

Question 27

What are horizontal cells?

Answer 27

antagonistic interneurons that inhibit photoreceptors by releasing GABA when depolarized

Dendrites synapse on cones (one class modulates L- and M-types, another S-types)

Provide negative feedback onto rods/cones (photoreceptors release glutamate, and horizontal cells release GABA back onto the photoreceptors)

Question 28

What are amacrine cells?

Answer 28

interneurons that mediate antagonistic interactions among on-bipolar, off-bipolar, and ganglion cells

Question 29

How many types of retinal ganglion cells exist?

Answer 29

Two types: On type and Off type
On Type: center cells excited
Off Type: center cells inhibited by light in the center of their receptive field

Question 30

What are the main sub-types of retinal ganglion cells?

Answer 30

Tonic cells (driven by L- and M-cones), Tonc cells (driven by s-cones), and Phasic cells

Question 31

What are the tonic cells driven by L- and M-type cones?

Answer 31

include small cells concentrated in fovea, are responsible for high acuity vision

Project to parvocellular layers of the LGN and mediate high spatial resolution and color vision

Question 32

What are the tonic cells driven by S-cones?

Answer 32

Designed to detect successive color contractq

Question 33

What are the phasic cells?

Answer 33

Project to the magnocellular layer of the LGN and mediate movement detection

Question 34

What are Muller cells?

Answer 34

glial cells that play a supportive role to neural tissue extending from inner segments of photoreceptors to the ILM

Also buffer ionic concentration in extracellular space, seal off the subretinal space by forming the ELM, and play a role in cone vitamin A metabolism

Question 35

What are the functions of the macroglia/microglia?

Answer 35

Function is to provide physical support, respond to retinal injury, regulate ionic/chemical composition of ECM, participate in blood-retina barrier, form myelination for ON, guide neuronal migration during development, and exchange metabolites with neurons

Question 36

What are pericytes?

Answer 36

Cells that surround retinal vessels and play a role in autoregulation, and structurally support endothelium/suppress proliferation

Are an early target in DM

Question 37

What happens if pericytes are injured, such as in diabetes?

Answer 37

Damage to pericytes leads to increased vessel permeability and microaneurysm formation

Question 38

What is the difference between the responses of cones and rods to light hyperpolarization?

Answer 38

Cones respond rapidly, turn off while light is still on, and overshoot the dark potential

Rods have a prolonged response that turns off slowly

Question 39

What affect does the dark have on cones? Rods?>

Answer 39

Cones- depolarized by darkness

Rods- little effect from darkness

Question 40

What channel is responsible for photoreceptor conductibity?

Answer 40

Photoreceptor conductivity of Na+ and K+ ions is facilitated by cGMP-gated cation channel