Lecture 11 - Retinal phototransduction and signal processing Flashcards Preview

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Flashcards in Lecture 11 - Retinal phototransduction and signal processing Deck (228):
1

Sclera:

A relatively spherical and avascular, white dense connective tissue that covers the globe posterior to the cornea.

2

What does the sclera provide?

a strong tough external framework to protect the delicate optic and neural structures.

3

The sclera maintains what?

the shape of the globe so that the retinal image is undisturbed and provides attachment for the extraocular muscles to rotate the globe and the ciliary muscle to accommodate the lens.

4

Cornea:

The window of the eye, it is a mechanically strong and transparent connective tissue that covers anterior 1/6 surface of the eye.

5

What is the most powerful focusing element of the eye?

Cornea, roughly twice as powerful as the lens.

6

Lens:

Specialized epithelial tissue that is responsible for fine-tuning the image that is projected on the retina.

7

Where is the lens?

Lies inside the eye surrounded by aqueous humor. It is transparent and has high refractive power.

8

What stabilizes the lens and allow accommodation to occur?

Elastin based zonular fibrils

9

Uveal tract consists of what three structures

the choroid, the ciliary body and the iris.

10

What is the Choroid?

Capillary bed nourishing the photoreceptors and outer retina

11

What are the parts of the Ciliary body?

1) ciliary muscle 2) vascular component

12

the ciliary muscle does what?

controlling the refractive power of the lens

13

a vascular component of the ciliary body does what?

that produces the aqueous humor filling the anterior chamber.

14

Iris:

Colored portion of the eye seen through the cornea.

15

What does the iris contain?

two sets of muscles with opposing actions that allow the size of the pupil (opening at the center) to be adjusted by neural control.

16

Anterior chamber:

Volume behind the cornea and in front of the lens. Filled with aqueous humor.

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Posterior chamber:

Region between the vitreous and the lens.

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Aqueous humor:

Clear watery liquid that nourishes the cornea and lens. It is produced by the vascular component of the ciliary body.

19

Vitreous humor:

Thick gelatinous substance filling the space between the back of the lens and the surface of the retina.

20

Retina:

Contains neurons that absorb light and process visual information in the images and send that information to the brain.

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Macula:

Oval spot containing a yellowish pigment (xantophyl). Supports high acuity.

22

Fovea:

Small depression at the center of the macula - has highest spatial acuity.

23

Optic disk:

Whitish circular area where retinal axons leave the eye and travel through the optic nerve to targets in the midbrain and thalamus.

24

What is the site where blood vessels supply the inner retina enter the eye?

Optic disk

25

Blood supply for the eye

The ocular vessels are all derived from the ophthalmic artery (OA), a branch of the internal carotid artery.

26

What are the two distinct vascular systems that supply the eye?

a) the anterior segment (iris and ciliary body) and b) the retinal systems.

27

Vascularization of the anterior segment

originates from the anterior ciliary arteries and the long posterior ciliary arteries. Penetrating vessels through the sclera vascularize the iris and the ciliary body.

28

Retinal supply

Delivery of metabolic substrates and oxygen to the retina is accomplished by two separate vascular systems

29

What are the two separate vascular systems of retinal supply?

the inner retinal and choroidal. The retinal and choroidal vessels differ morphologically and functionally from each other.

30

View with an ophthalmoscope shows what?

the surface of retina and the vasculature of the inner retina

31

What is cataracts?

Clouding of the lens that affects vision

32

Most cataracts are related to what?

Aging

33

Cataracts are the leading cause of what?

Blindness worldwide

34

Cataracts by age 80

more than half of all Americans have a cataract or have had cataract surgery

35

What are the risk factors for cataracts?

Aging, diabetes, sunlight and smoking

36

What are the symptoms for cataracts?

Hazy vision, poor night vision, glare and faded colors

37

What is the treatment for cataracts?

Surgical removal of the cloudy lens and replacement with an artificial lens. Very little recovery time after surgery

38

What is the process known as cataract formation?

Disruption of the order of the organization of the lens cell fibers or aggregation of the proteins within them can destroy transparency of the cell

39

Presbyopia causes what?

Loss of lens elasticity with age causes this inability to focus on near objects

40

Total refractive power =

power emmetropic eye + power of accommodation

41

What is glaucoma?

Group of diseases that damage the eye’s optic nerve and can result in vision problems

42

What are the types of glaucoma?

1) normal tension 2) open angle 3) closed angle

43

What are the risk factors for glaucoma?

Elevated eye pressure, thin cornea, abnormal optic nerve anatomy, high blood pressure

44

What is the relationship between intraocular pressure and glaucoma?

Not all subjects with high IOP develop glaucoma

45

What are the symptoms for glaucoma?

None until its too late, loss of peripheral visual fields

46

What is the treatment for glaucoma?

Eye drops to decrease aqueous production and/or increase drainage. Surgery.

47

The circulating aqueous humor does what?

It nourishes the cornea and lens structures that must be transparent and therefore devoid of blood vessels.

48

Aqueous humor is secreted by what?

the ciliary epithelium lining the ciliary processes

49

Where does the aqueous humor flow?

around the lens and through the pupil into the anterior chamber

50

How does the aqueous humor leave the eye?

by passive flow at the anterior chamber angle

51

What are the risk factors for glaucoma?

High eye pressure resulting from poor drainage of aqueous humor.

52

Open angle glaucoma:

Slow development of pathology. Caused by obstruction of drainage canals.

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-Close angle:

Sudden increase in intraocular pressure. Closed or narrow angle between the iris and cornea.

54

How is intraocular pressure regulated?

At the front of the eye

55

Retina

A ‘window to the brain’, is part of the Central Nervous System. It is a thin neural tissue (200 microns thick) that lines the back of the eye.

56

What does incoming light first hit?

the ganglion cell layer, the rest of the inner retina, then the photoreceptor (PR) nuclei and inner segments before the PR outer segments, where phototransduction occurs.

57

What must light travel through before striking and activating the rods and cones?

the thickness of the retina

58

Subsequently the absorption of photons by the visual pigment of the photoreceptors does what?

It is translated into first a biochemical message and then into an electrical message that can stimulate all the succeeding neurons of the retina.

59

How is the retinal message concerning the photic input transmitted?

to the brain by the spiking discharge pattern of the ganglion cells via the optic nerve.

60

What are the layers of the retina?

3 nuclear (cell somas), 2 plexiform (synaptic connections), 1 nerve fiber layer (NFL).

61

What are the neuron cell types?

1) photoreceptors 2) horizontal cells 3) bipolar cells 4) amacrine cells 5) ganglion cells

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What are the glia cells?

1) Muller glia (radial) 2) microglia

63

Layers and neuron types:

1) Photoreceptor outer segments (OS), 2) Outer nuclear layer (ONL) 3) Outer plexiform layer (OPL) 4) Inner nuclear layer (INL) 5) Inner plexiform layer (IPL) 6) Ganglion cell layer (GCL) 7) Nerve fiber layer (NFL)

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Outer nuclear layer (ONL):

photoreceptor somas

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Outer plexiform layer (OPL):

photoreceptor / bipolar /horizontal cell synapses

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Inner nuclear layer (INL):

horizontal, bipolar and amacrine cell somas

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Inner plexiform layer (IPL):

bipolar/amacrine/ganglion cell synapses

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Ganglion cell layer (GCL):

ganglion cell somas

69

What are Pigmented epithelium (RPE):

melanin-containing cells behind the photoreceptors,

70

What is the function of pigmented epithelium?

acts as backstop for light, maintains phototransduction machinery of photoreceptors by recycling of PR discs, pigment regeneration and photoreceptor nourishment.

71

Retinal glia cells

including astrocytes (neurovascular), microglia (immune system) and Mueller (radial glia-like) cells (ionic milieu, guidance during development).

72

Different subclasses of horizontal, bipolar, amacrine and ganglion cells make distinct contributions to

visual function.

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What is the vertical information flow?

Photoreceptors to bipolar cells to ganglion cells

74

Lateral information flow is mediated by

horizontal cells and amacrine cells

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Cells along the vertical path (photoreceptors, bipolar and ganglion cells) release what?

glutamate

76

Cells mediating lateral information transmission (horizontal and amacrine cells) release what?

mostly GABA or glycineric (some exceptions exist..)

77

What are the two types of photoreceptors for the retina?

Rods and cones

78

What types of cells are photoreceptors?

they are ciliated cells.

79

The outer segment of photoreceptors are connected to what?

the inner segment via a connecting cilium.

80

Outer segment of the photoreceptor

houses the phototransduction machinery

81

Inner segment of the photoreceptor

housekeeping machinery (nucleus, mitochondria, Golgi apparatus, ER, etc).

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Synaptic terminal of the photoreceptor

contacts bipolar and horizontal cells

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Neurotransmitter for the photoreceptor

glutamate

84

How do photoreceptors respond to light?

With graded hyperpolarizations

85

In darkness, rods and cones are depolarized near what?

-40mV

86

When do photoreceptors fire action potentials?

They don’t, they respond with graded hyperpolarizations

87

Describe neurotransmitter release when rods and cones are depolarized.

Neurotransmitter is released continuously

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How do rods and cones compare to most other neurons?

rods and cones maintain a relatively depolarized membrane potential at rest.

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In darkness rods and cones are

depolarized near -40 mV and neurotransmitter (glutamate) is continuously released. When stimulated by light, photoreceptors do not fire action potentials but instead respond with graded hyperpolarizations.

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What happens to the photoreceptor hyperpolarization?

it then spreads passively to the synapse where it reduces the release of the neurotransmitter glutamate.

91

Photoreceptors in the dark

they have a circulating current in the dark

92

Describe Na+ and Ca2+ cations for Photoreceptors in the dark.

They flow inward through cGMP-gated channels at the outer segment

93

Describe K+ ions for Photoreceptors in the dark.

They flow outward through potassium-selective channels at the inner segment

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Na+ and Ca2+ inflow for Photoreceptors in the dark does what?

depolarizes the cell

95

K+ outflow for the photoreceptor does what?

acts to hyperpolarize the cell

96

In the photoreceptor what does the combined action of the depolarIzing and hyperpolarizing ionic currents result in?

a steady, depolarized membrane potential of -40 mV

97

How do photoreceptors respond to light?

by suppressing the inward cationic current

98

What does the absorption of light do for the photoreceptor?

it reduces cGMP levels in the outer segment

99

What is the intracellular transmitter of phototransduction?

cGMP

100

What is the effect of the decreased cGMP in the photoreceptors response to light?

1) cGMP-gated channels close reducing the inflow of Na+ and Ca2+ cations 2) K+ ions continue to flow out of the inner segment reducing the amount of positive charge in the cell 3) The photoreceptor becomes hyperpolarized and the amount of neurotransmitter

101

For the photoreceptors response to light, what does the reduction of inward current result in?

a reduction in intracellular calcium levels.

102

What are light induced reductions in calcium levels important for?

light adaptation

103

When does phototransduction begin?

When a pigment molecule absorbs a photon

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What is phototransduction?

it is the process by which light is converted into electrical signals.

105

In pigment activation, disk membranes are packed with what?

the primary light detector, the visual pigment protein rhodopsin (cone opsin in cones).

106

In pigment activation, what does the pigment consist of?

an opsin molecule that contains the light-absorbing chromophore 11-cis retinal (an aldehyde of vitamin A)

107

In pigment activation, what are opsins?

They are members of the G-protein coupled receptor (GPCR) superfamily of signal transducing receptors.

108

What does opsin do?

Opsin tunes the absorption of light to a particular region of the spectrum

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What is the effect of absorption of a photon?

it changes the conformation of retinal from 11-cis to all-trans

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What does the conformational change induced by the photon lead to?

The conformational change in retinal leads in turn to activation of rhodopsin (R to R* state)

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When activated what do opsins catalyze?

the activation of heterotrimeric G-protein known as Transducin

112

What does active rhodopsin (R*) initiate?

a series of biochemical reactions that lead to a reduction in cGMP levels

113

What does retinal bind to?

It binds covalently inside a pocket formed by the opsin molecule

114

What does all-trans retinal do?

it breaks the covalent bond and exits the pocket in the opsin molecule

115

How is the absorption of a single photon related to amplification?

The absorption of a single photon closes more than 200 channels.

116

What does the amplification of a single photon lead to?

This leads to a membrane hyperpolarization of ~ 1 mV. Careful observers can see light flashes so dim that only 1 in 100 rods absorb a single photon.

117

Pigment epithelium

The visual cycle is essential for maintaining the light sensitivity in photoreceptors. It occurs largely in the pigment epithelium.

118

What is involved in the restoration of retinal?

It gets restored to a form capable of signaling photon capture in a complex process known as the visual (or retinoid) cycle.

119

In the visual or retinoid cycle, what happens after absorbing a photon?

all trans-retinal dissociates from the opsin and is transported to pigment epithelium.

120

In the visual or retinoid cycle, what happens after the trans-retinal dissociates from the opsin and is transported to pigment epithelium?

In the pigment epithelium it is re-isomerized and converted back to 11-cis retinal

121

In the visual or retinoid cycle, what happens to the chromophore 11-cis retinal?

it is transported back to the outer segment where it recombines with opsin to form pigment.

122

What is dark adaptation?

Restoration of sensitivity after exposure to illumination

123

In the fovea, what happens to give it higher acuity?

The ganglion cells, IPL, INL are pushed away and are populated by cones while the periphery is populated largely by rods

124

What does the higher convergence of rods onto rod bipolar cells lead to?

1) high sensitivity 2) low resolution

125

Midget bipolar cells

1 cone to 1 cone bipolar cell; 1) low sensitivity 2) high resolution

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Fovea:

1) Visual angle subtended is 0.5 deg – same as full moon or thumb nail at arm distance 2) Involves ~0.01% of retinal area, but 10-50% of the optic nerve, no S cones, no rods.

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Rods

1) distributed in periphery – not in fovea 2) very sensitive to light

128

Which is more sensitive to light, rods or cones?

Rods

129

What is the distribution of rods and cones?

1) Rods = periphery 2) Cones = fovea

130

Which has the ability to respond to a single photon?

Rods

131

What is required to elicit a response in cones?

About 100 photons are required to elicit a response

132

What is the relationship between rods and cones to their respective bipolar cells?

1) rods – high convergence onto rod bipolar cells 2) cones – 1:1 (midget bipolar cell)

133

Describe the sensitivity and spatial resolution for rods and cones?

1) Rods trade off high sensitivity for low spatial resolution 2) cones tradeoff high spatial resolution for low sensitivity

134

Which mediates high acuity vision – rods or cones?

Cones

135

Which is responsible for color vision – rods or cones?

Cones

136

Scotopic

(rod-only) vision: high sensitivity, low acuity, no color

137

Phototopic

(cone-only) vision: low sensitivity, high acuity, color

138

Mesopic vision

cone and rod acute vision, active together

139

What is activated in dim light – rod or cone?

Rod

140

What is activated in bright light – rod or cone?

Cone

141

What is retinitis pigmentosa?

A group of genetic eye conditions that leads to incurable blindness, 1:4000 (400,000 in US)

142

What are the symptoms of retinitis pigmentosa?

1) night blindness 2) tunnel vision 3) usually legally blind by age 40. Loss of ERG.

143

What is the cause of retinitis pigmentosa?

Mutation of genes for rhodopsin and other rod proteins (PDE, cGMP-gated channels, RDS/peripherin) leading to degeneration of rods and eventually cones. Current count of gene defects exceeds 100

144

What is the treatment of retinitis pigmentosa?

None. Unclear why the cones die

145

What is ERG?

The electroretinogram (ERG) provides a measure of retinal function. The ERG is a brief change in potential (<1mV) evoked by a large-field flash of light to the eye and recorded with a contact lenses electrode.

146

What does the ERG reflect?

‘mass’ activity of retinal neurons. Valuable tool in diagnosis of retinal pathologies.

147

What is the leading cause of vision loss?

Age related macular degeneration (AMD) – 10% of people over 50, 33% of people over 75

148

What is wet AMD?

Abnormal blood vessels behind the retina grow under the macula, leaking and rapidly damaging the retina (choroidal neovascularization)

149

What is dry AMD?

The retinal pigment epithelium and photoreceptors of the macula degenerate, accumulation of drusen

150

Which is more common wet or dry AMD?

Dry AMD is 85% of AMD cases

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What is drusen?

(dry AMD) yellow deposits – cellular debris

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What are the symptoms for AMD?

loss of central vision and acuity

153

What are the risk factors for AMD?

Aging, smoking, inheritance (local inflammation and the complement system may also be implicated)

154

What is the treatment for the wet form of AMD?

Proliferation of blood vessels in macula. Treat with laser coagulation of vessels and intravitreal injection of anti-neovascular agents

155

What is the treatment for the dry form of AMD?

Gradual atrophy of central retina. Antioxidants slow the progression of disease

156

What is diabetic retinopathy?

Retinal damage from complications of diabetes mellitus

157

What is the risk for diabetic retinopathy?

Up to 80% of all patients who have had diabetes for 10 years or more

158

What are the symptoms for diabetic retinopathy?

Early diabetic retinopathy often has no early warning signs. Blurry vision with macular edema. New vessels bleed into the retina and block vision

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What is non-proliferative diabetic retinopathy?

Hyperglycemia induced pericyte death leads to incompetence of the vascular walls, microaneurysms and dot and blot hemorrhages. Vascular bleading and cotton wool spots (ischemia)

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What is the proliferative stage for diabetic retinopathy?

New, fragile vessels grow, which leak blood

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What is the treatment for diabetic retinopathy?

Laser surgery to reduce edema and injections with anti-neovascular factors

162

Spatial Contrast

in sensory systems most sensory neurons respond best to some form of CONTRAST (differences in stimuli) rather than constant, uniform stimuli.

163

Contrast though can be of many types:

spatial/luminance, color, spatial/color, motion, depth, optic flow, curvature, differences in face shape, etc.

164

Receptive field (RF):

That part of sensory stimulus space that, when stimulated, causes a change in the activity of the neuron (stimulus response).

165

For a visual neuron what would the RF definition mean?

that position in visual space where a change in light (visual stimulus) causes a change in the activity of that visual neuron.

166

ON center cells

increase their discharge rate to luminance increments in the receptive field center.

167

OFF center cells

increase their discharge to luminance decrements in the receptive field center.

168

Receptive fields of ganglion cells have what?

A center and an antagonistic surround

169

What is the ON center / OFF surround receptive field?

1) Light in the center is excitatory (depolarization) 2) light in surround is inhibitory (hyperpolarization)

170

What is the optimal excitatory stimulus for the ON center / OFF surround receptive field?

It is a spot of light against a dark background

171

What is the optimal inhibitory stimulus for the ON center / OFF surround receptive field?

Dark spot against a light background

172

What is the OFF center / ON surround receptive field?

1) light in the center is inhibitory (hyperpolarization) 2) light in the surround is excitation (depolarization)

173

What is the optimal excitatory stimulus for the OFF center / ON surround receptive field?

A dark spot against a light background

174

What is the optimal inhibitory stimulus for the ON center / OFF surround receptive field?

A light spot against a dark background

175

Neurons whose firing rate is most affected by the edge are where, and ones who are least afffected?

those whose receptive fields lie along the border. Neurons whose receptive fields are completely illuminated are less affected.

176

Receptive Field in the dim light

spontaneous activity (note that in the text book this condition is referred to as being in the dark, however GCs have very small spontaneous activity in the complete darkness. To be consistent with the firing rates shown in the figure the edge probably tr

177

Illumination falls on the inhibitory surround

reduction in the discharge rate of the Ganglion Cell

178

Border of illumination falls on the middle of the Receptive Field

excitatory and inhibitory contributions cancel out – no change in response rate is observed

179

The center of the Receptive Field is fully illuminated but a portion of the surround remains in the dark area providing only partial inhibition results in what?

strong discharge rate

180

Where do the functional differences between ON and OFF center GCs arise?

At the synapse between photoreceptors and bipolar cells (BC) in the OPL (outer plexiform layer) the first synapse in the visual system

181

ON bipolar cells

depolarize in response to light, hyperpolarize in response to dark

182

OFF bipolar cells

hyperpolarize in response to light, depolarize in the dark

183

What determines the ON or OFF center properties of the bipolar cells and those of the ganglion cells they innervate?

The glutamate receptors

184

What leads to increase glutamate release at bipolar cell synapses and depolarization of ganglion cells they contact?

Graded depolarization of bipolar cells

185

Where does extracting contrast information begin?

at the first synapse in the visual system

186

How are the center-surround receptive fields constructed?

Antagonistic surround of GC receptive fields is a product of lateral interactions that occur in the OPL by way of horizontal cells (HCs) and in the IPL by way of amacrine cells.

187

What is responsible for the antagonistic surround of GC receptive fields in the outer plexiform layer?

Horizontal cells

188

What is responsible for the antagonistic surround of GC receptive fields in the inner plexiform layer?

Amacrine cells

189

Horizontal cell inputs establishes what?

the surround component of the Receptive field of a bipolar cell (and thus that of the ganglion cell to which it is attached).

190

Cones synapse on what?

On bipolar cells to form RF centers and H-cells to form RF surrounds.

191

The antagonistic surround inhibit what?

the center response.

192

Lateral inhibition

Because inhibition spreads laterally from the surround to the center

193

Input from Horizontal Cells does what?

Opposes changes in the membrane potential of photoreceptors that are induced by phototransduction

194

Color facilitates what?

Detecting borders of objects, a trichromatic system facilitates distinguishing red and green objects

195

Trichromacy

The three cone types, L, M, S (nominally red, green, blue), are the basis for trichromatic color vision among Old World primates (including humans).

196

• Cone opsins:

Like rhodopsin, cone opsins belong to the G-protein coupled receptor (GPCR) superfamily of signal transducing receptors.

197

Describe the structure of GPCR cone opsins

They have have seven transmembrane domains that traverse the disk membranes in outer segment forming a pocket in which the chromphore resides.

198

What is the chromophore in the cone opsin?

11-cis retinal

199

What does the opsin sequence do?

it tunes the absorption of light to a particular region of the spectrum.

200

In the opsin sequence, where do significant sequence differences exist?

between Rho and S-opsin and between S-opsin and M-opsin.

201

M- and L-opsin have

relatively few sequence differences, they have high degree of sequence homology.

202

Cone mosaic:

The retinal cone mosaic is far from uniform, regular or evenly populated by L, M and S cones. Only 5-10% of cones are S-cones. Interestingly, large differences in the ratio of M and L cone types do not seem to have a severe impact on color perception.

203

What is the impact on the non uniform distribution of L, M and S cones on color perception?

They do not seem to have a severe impact on color perception

204

Trichromatic vision

compares activity of three sets of cones with different absorption spectra.

205

In trichromatic vision the absence of one or more cone classes are responsible for what?

common color blindness: (no L cones: protanope, no M cones: deuteranope, no S: tritanope).

206

Protanopes and deuteranopes

have difficulty detecting differences between red-and green colors and are the most common of dichromacy affecting 6-8% of males and 0.4% of females (because it is X-linked)

207

What are the distinct anatomical parallel pathways from the GC to the brain?

1) parvocellular 2) magnocellular 3) koniocellular

208

Where do the parvocellular and magnocellular pathways originate?

in the P and M ganglion cells

209

Where do the koniocellular pathway originate?

it originates in small bistratified yellow-blue ganglion cells

210

M cells (periphery):

1) large receptive fields 2) Good light and contrast sensitivity and temporal resolution (sensitive to motion) 3) not color sensitive 4) large cells - receive input from a relatively large number of photoreceptors 5) Origin of the magnocellular pathway

211

P cells :

1) small receptive fields (midget cells) 2) provide high acuity and color sensitivity 3) poor light and contrast sensitivity 4) opponent color receptive fields 5) in the fovea a P ganglion cell receives input from a single bipolar cell that receives input

212

What does a P ganglion cell receive input from?

A single bipolar cell that receives input from a single cone

213

K cells:

bi-stratified ganglion cells. Carry short wavelength (blue) information.

214

What is meant by Color opponent receptive fields:

The outputs of the three cone types are encoded in the retinal circuitry into two color opponent pathways: red vs green and blue vs yellow (green + red).

215

What is the only way to determine color/wavelength information?

can only be determined through comparisons (i.e. subtractions, differences) between two or more cone types, hence the term “opponency”.

216

P cells of the monkey retina (presumably also of human retina) are known to respond to what?

light with an opponent chromatic organization in their surround.

217

That is, P ganglion cells of central retina, when recorded from electrophysiologically, have what?

the smallest receptive fields,

218

How are the P ganglion cells organized?

as red-cone ON or OFF center, and green-cone ON or OFF center.

219

Each P-ganglion cell type has what?

a larger surround of the opposite polarity and the opponent color.

220

Magnocellular cells

luminance encoding

221

Parvocellular cells

color opponent ganglion cells

222

Koniocellular cells

blue ON/ yellow OFF

223

Intrinsically photosensitive retinal ganglion cells contain what?

Contain primitive opsin

224

Intrinsically photosensitive retinal ganglion cells response to light.

Autonomous response to bright lights

225

Intrinsically photosensitive retinal ganglion cells morphology

have vast dendritic trees

226

What are the roles of intrinsically photosensitive retinal ganglion cells?

1) Papillary control 2) synchronization of circadian rhythms 3) sleep 4) learning

227

Why is it thought that intrinsically photosensitive retinal ganglion cells play a role in papillary control?

Because they project to the pretectum

228

Why is it thought that intrinsically photosensitive retinal ganglion cells play a role in the synchronization of circadian rhythms?

They project to the suprachiasmatic nucleus