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Neuroscience > L11: Retina: Phototransduction & Signal Processing > Flashcards

L11: Retina: Phototransduction & Signal Processing Flashcards

1
Q

Sclera

A

CT that covers the eye posterior to cornea
protects retina
provides support for EOM & ciliaris

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2
Q

Cornea

A

“window of the eye”
mechanically strong
CT that covers the anterior eye
most powerful focusing element of the eye

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3
Q

lens

A 
specialized epithelial tissue 
responsible for fine-tuning image that is projected on retina
surrounded by aqueous humor
transparent
high refractive power
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4
Q

zonular fibrils

A

elastin that stabilize lens & allow for accomodation

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5
Q

What are the 3 components of the uveal tract?

A

1) choroid - capillary bed nourishing photoreceptors & outer retina

2) ciliary body - 2 parts
a) ciliary muslce -controls refractive power of lens by constricting or dilating
b) vascular - produces aqueous humor

3) iris - colored portion of eye seen thru cornea; contains 2 sets of muscles that control pupil dilation or constriction

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6
Q

anterior chamber

A

volume behind cornea, in front of lens

filled with aqueous humor produced by ciliary body (a part of uveal tract)

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7
Q

posterior chamber

A

btw vitreous & lens

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8
Q

aqueous humor

A

clear watery liquid nourishes cornea & lens

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9
Q

vitreous humor

A

thick gelatinous substance filling space btw back of lens & surface of retina

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10
Q

retina (CNS)

A

contains neurons that absorb light & process visual info in the images & send that info to brain

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11
Q

macula

A

oval spot containing yellowish pigment in retina; supports high acuity.

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12
Q

fovea

A

small depression at center of macula

highest spatial acuity

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13
Q

optic disk

A

whitish circular area where retinal axons leave the eye & travel thru optic nerve to midbrain & thalamus
*site where blood vessels supplying the inner retina would go

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14
Q

Photoreceptors are ciliated cells in retina are the rods and cones that turn light energy into electrical energy. Which ones will you find the most in the fovea?

A

the cones; there are 3 pigments (red, green and blue opsins)

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15
Q

There are 2 distinct vascular systems supplying the eyes.

A

1) anterior segment
- from anterior ciliary arteries
- long posterior ciliary arteries
- penetrating vessels thru sclera
2) retinal systems
- inner retinal
- choroidal

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16
Q

Explain the percentage distribution of focusing done by cornea and lens.

A

80% done by cornea

20% done by lens

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17
Q

Cataracts affect aging population and is the leading cause of blindness. Explain what the risk factors, symptoms and treatment.

A

-cataracts cloud the lens that affects vision due to protein aggregation

Risk factors: aging, diabetes, sunlight, smoking

Symptoms: hazy vision, poor night vision, glare & faded colors

Treatment: surgery

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18
Q

Glaucoma is caused by? Explain what the risk factors, symptoms and treatment.

A

Glaucoma is caused by damage to optic nerve.

Risk factors: elevated eye pressure due to poor drainage of aqueous humor, , thin cornea, abnormal optic nerve anatomy, HTN

Symptoms: loss of peripheral visual fields

Treatment: eye drops, surgery

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19
Q

Both lens and cornea are avascular. Since they need nutrients, where are they getting it?

A

Lens and cornea receive nutrients via aqueous humor produced by the ciliary body (part of the uveal tract), which drain to angle of eye into venous circulation.

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20
Q

What happens when there’s impaired drainage of aqueous humor?

A

will increase intraocular pressure, stressing retina & optic nerve; can cause irreversible loss of vision

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21
Q

open angle vs closed angle glaucoma

A

Open angle glaucoma = slow progressive obstruction of drainage canals

Close angle glaucoma = sudden increase in intraocular pressure

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22
Q

Explain what it means that the retina is a form of “reverse engineering”

A

The retina (“window to the brain”) contains layers of neurons. The last cell layers contain the photoreceptors that sense light and convert to chemical energy. (you would think it’d be the first layer)

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23
Q

List the layers of the retina

A 
anterior
1- nerve fiber layer
2- ganglion cell layer
3 - inner plexiform layer
4 - inner nuclear layer
5 - outer plexiform layer
6 - outer nuclear layer
7 - photoreceptors 
8 - pigment epithelium
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24
Q

There are 5 types of neurons found in retina. List them.

A 
1 - photoreceptors
2 - horizontal cells (spread laterally)
3 - bipolar cells
4 - amacrine cells (spread laterally)
5 - ganglion cells
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25
Q 
List the types of neurons found in the following layers of the retina:
1-ganglion cell layer
2-inner plexiform
3-inner nuclear
4-outer plexiform
5-outer nuclear
A

1- ganglion cell layer = ganglion cells
2- inner plexiform = ganglion cells, amacrine cells & bipolar cells
3 - inner nuclear = amacrine cells, bipolar cells & horizontal cells
4- outer plexiform = bipolar cells, horizontal cells, photoreceptors
5- outer nuclear = photoreceptors

26
Q

What is the pigment epithelium?

A

melanin containing cells behind photoreceptors (in btw back of eye & photoreceptors)

27
Q

Explain the vertical info flow in terms of the neurons in the retina

A

photoreceptors –> bipolar cells –> ganglion cells

28
Q

which cells are involved in lateral info flow in the retina?

A

horizontal & amacrine cells

29
Q

Both rods and cones have outer & inner segments connected by connecting cilium & synaptic terminal. Explain these segments and the synaptic terminal found in these photoreceptors. Also, what is the NT released by photoreceptors?

A

outer segment = phototransduction machinery

inner segment = housekeeping machinery

synaptic terminal =contacts bipolar and amacrine cells

NT = glutamate

30
Q

Explain what is going on with the rods and cones in darkness. Are they depolarized, hyperpolarized? Explain what’s going on based on ion channels and which segments of the rods and cones are involved.

A

In darkness, rods & cones are depolarized at -40mv. Glutamate is continuously released.

Depolarization is caused by Na+ and Ca2+ flow inward thru cGMP-gated channels in outer segment (phototransduction machinery). K+ flow outward thru K+ channels in inner segment (house keeping machinery)

**overall = more depolarized than hyperpolarized = steady depolarized state at -40mV = continual glutamate release

31
Q

Explain what happens to rods and cones when exposed to light. Are they depolarized, hyperpolarized? Explain what’s going on based on ion channels and which segments of the rods and cones are involved.

A

In light, rods & cones DO NOT fire A.Ps; they respond with graded hyperpolarizations, which spread passively to synapse (decrease release of glutamate)

Absorption of light decreases cGMP levels in outer segment, closing cGMP gated channels, decreasing influx of Na+ and Ca2+ = not as depolarized

K+ continues to flow out thru K+ channels in inner segment

**overall = more hyperpolized than depolarized = hyperpolarized state leading to decrease glutamate release

32
Q

cGMP’s role in photoreception

A

cGMP is an intracellular transmitter of phototransduction

cGMP in outer segment of rods & cones can bind cGMP-gated channels, opening Na+ and Ca2+ channels, allowing inward flow & depolarization. Depolarized state will cause continuous release of glutamate (darkness, rods and cones are depolarized)

33
Q

Which ion plays an impt role in light adaptation?

A

light-induced reductions in Ca2+ levels is impt for light adapation

34
Q

The phototransduction machinery lies In the outer segments of both the rods & cones. More specifically, the disk membranes in the outer segment are packed with primary light detectors. What are the names of the primary light detectors/visual pigment proteins in the cones and rods?

A

Rods: the disk membranes in outer segment are packed with primary light detectors known as RHODOPSIN

Cones: the disk membranes in outer segment are packed with primary light detectors known as CONE OPSINS

35
Q

What is Rhodopsin and what are the 2 parts that make it up?

A

Rhodopsin is the primary light detector found in the outer segment of rods. It is composed of:

1) opsin = G-protein coupled receptor
2) chromophore = Vitamin A (retinal)

36
Q

The 2 parts of rhodopsin are opsin & chromophore. What happens when the person is deficient in vit. A?

A

Vitamin A = chromophore

no chromophore = cannot absorb/respond to light

37
Q

Explain what happens when rhodopsin absorbs light (photon)

A

1 - absorption of photon
2 - conformational change from 11-cis to all-trans retinal (breaking the bond btw retinal + opsin), activating rhodopsin
3 - activated rhodopsin: the opsin can catalyze activation of transducin (a g-protein)
4 - series of biochem rxns causing a decrease in cGMP
5 - less cGMP-gated channels opening = less inflow of Na+ & Ca2+ = less depolarized
6. cell is hyperpolarized

38
Q

T/F: absorption of a single photon closes more than 200 cGMP-gated channels

A

True, which is why this is a high amplification transduction pathway

39
Q

When there’s light, the 11-cis retinal gets converted to all-trans retinal. There’s a mechanism associated to restore the retinal into 11-cis. Where does it occur and what are some of the mechanisms involved? *this is called the retinoid cycle/dark adaptation restoration of sensitivity after light exposure

A

-occurs in pigment epithelium (melanin-containing layer behind the photoreceptors)

  • background: rhodopsin absorbs light, causing a conformational change of 11-cis retinal to all-trans
  • this conformational change causes dissociation btw retinal & opsin (happening in outer segment)
  • all-trans retinal transports to pigment epithelium
  • in pigment epithelium, all-trans retinal is re-isomerized & converted back to 11-cis retinal
  • 11-cis retinal is transported back into outer segment where it re-combines with opsin to form rhodopsin (visual pigment protein in rods)
40
Q

How does fovea have such high acuity?

A

b/c the retinal layers are pushed away

41
Q

Rods vs Cones: which one is more sensitive to light?

A

Rods

42
Q

Rods vs Cones: which one has high convergence onto bipolar cells and which has the 1:1 ratio?

A

rods have high convergence onto rod bipolar cells

cones have 1:1 ratio to cone bipolar cells

43
Q

Rods vs Cones: which one is most numerous?

A

Rods

44
Q

Rods vs Cones: which one has higher spatial resolution?

A

Cones have higher spatial resolution; lower sensitivity to light

Rods have lower spatial resolution; higher sensitivity to light

45
Q

Rods vs Cones: which one is more localized in the fovea? In the periphery?

A

Cones are found in fovea while rods are mostly in the periphery

46
Q

Rods vs Cones: which one is being used in the dark/at night? What type of vision is it: scoptopic or photopic?

A

At night, we are using rods

-Rod vision is active in dim light (scoptopic vision)

47
Q

Rods vs Cones: which one is used in light? What type of vision is it: scoptopic or photopic?

A

In light, we are using cones

-Cone vision is active in bright light (photopic vision)

48
Q

Define:
Mesopic vision
Scoptopic vision
Photopic vision

A

Mesopic vison: using both rods & cones (not too dark; not too light)

Scoptopic vision: using rods in the dark (high sensitivity to light, low spatial resolution)

Photopic vision: using cones in the light (low light sensitivity, high spatial resolution)

49
Q

Retinitis Pigmentosa is a genetic disorder that leads to incurable blindness. Where is the genetic defect? Are cones or rods affected first? List some of the symptoms

A

Caused by mutations of genes for rhodopsin & other rod proteins

Degeneration of rods first, and then cones

Symptoms: night blindness, tunnel vision

50
Q

Age related Macular Degeneration (AMD) is the leading cause of vision loss (loss of central vision & acuity). Compare and contrast wet AMD & dry AMD.

A

Most of the cases (85%) are dry AMD

Dry AMD: pigment epithelium & photoreceptors of macula degenerate; accumulation of drussen yellow deposits

Wet AMD: abnormal blood vessels behind retina grow under macula, leaking & rapidly damaging retina (this is more severe, but also more treatable)

51
Q

Diabetic Retinopathy (eye problems secondary to diabetes). 80% of pts with 10+ yrs h/o of diabetes are at risk. List some of the symptoms and what are the differences btw proliferative & non-proliferative diabetic retinopathy.

A

Symptoms: blurry vision w/ macular edema, new vessels bleed into retina & block vision

non-proliferative = microanerysms 
proliferative = new fragile blood vessels grow which leak blood
52
Q

What allows eyes to be such good detectors of changes in light (spatial contrast)?

A

retinal ganglion cells & bipolar cells are on-center/off-surround, off-center/on-surround cells.

53
Q

In an on-center, off-surround cell, what happens when light shines at on-center?

A

when light shines at on-center, action potentials will fire, cell will depolarized = excitatory

54
Q

In an off-center, on-surround, what happens when light shines at off-center?

A

when light shines at off-center, decrease in action potential firing, hyperpolarized = inhibitory.

55
Q

What determines On or Off center properties of bipolar cells and those of ganglion cells?

A

glutamate receptors on bipolar cells at synapse btw photoreceptors and bipolar cells

56
Q

An on-center, off-surround cell -how is the off-surround constructed?

A

due to lateral interactions that occur in outer plexiform layer via horizontal cells & in inner plexiform layer via amacrine cells

57
Q

Input from horizontal cells can do what?

A

can inhibit the on-center aka oppose changes in membrane potential of photoreceptors that are induced by phototransduction.

58
Q

what are the 3 cone types?

A 
s-cones = short cones (blue)
m-cones = medium cones (green)
l-cones = long cones (red)
59
Q

Cone mosaic: what is the general distribution of the 3 different cone types?

A

s-cones = only 5-10% of cones

m-cones & l-cones = varied, random distribution btw these 2

60
Q

Partial color blindness is not a progressive retinal disease. It is characterized by abnormal green-red vision. The 2 most common X-linked partial color blindess are 1) Deuteranopia & 2) Protanopia; which opsin is missing in those two conditions?

A

1) deuteranopia = no m-opsin (no green)

2) protanopia = no L-opsin (no red)

61
Q

There are ganglion cells that send information to 3 different pathways. List them and briefly describe their functions

A

1) parvocellular pathway = color
2) magnocellular pathway = light & contrast sensitivty
3) koniocellular pathway = color

62
Q

T/F: In addition to rods and cones, there are also intrinsically photosensitive retinal ganglion cells.

A

True; they are very scarce

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