Lectures 5 & 6 - The Eye Flashcards Preview

SMP - Neurophysio Exam 2 > Lectures 5 & 6 - The Eye > Flashcards

Flashcards in Lectures 5 & 6 - The Eye Deck (164):
1

What are 2 other names for the primary visual cortex?

Striate cortex, V1

2

What is vision?

Our perceptual experience of light: light is transduced by photoreceptor neurons and the signal is transmitted across many synapses in retina before reaching the optic nerve, which transmits signals to the brain

3

What are the 4 CNS structures for vision?

1. Retina
2. Thalamus
3. Primary visual cortex
4. Extrastriate cortex

4

What is another name for the extrastriate cortex?

Association cortex

5

Is visual system activity determined by absolute light levels?

NOPE, relative light levels

6

What is the dark current?

In the absence of light, photopigment proteins are inactive and therefore photoreceptors are depolarized and have current: cGMP opens cGMP gated cation channels (Na+ and Ca++ flow in)

7

What is the relative light level dependence of visual system activity due to?

Receptive field properties that begin in the retina

8

What are the 2 properties of light waves? What does each correspond to?

1. Amplitude: brightness
2. Wavelength: color

9

What is the wavelength related to?

Inverse of frequency

10

What is the range of visible light?

400-700 nm

11

What is the visible light with the highest energy?

400 nm = blue light

12

What is the visible light with the lowest energy?

700 nm = red light

13

What are the 3 possible behaviors of light rays on different media?

1. Reflection
2. Absorption
3. Refraction

14

When an object absorbs light, what light do we see?

The light that is NOT absorbed

15

What changes when light gets refracted?

Its velocity

16

What is velocity?

Direction + speed

17

If you have a red reflector strip on a bike, what behaviors of light are taking place when we look at it?

All 3:
- Reflection of red light by strip
- Absorption of some light by strip
- Refraction within the eye

18

What are the 3 tissue layers of the eye?

1. Fibrous tunic
2. Vascular tunic
3. Retina

19

What are the 2 parts of the fibrous tunic of the eye? What does each constitute?

1. Sclera (posterior 2/3rds)
2. Cornea (anterior 1/3rd)

20

What are the 3 parts of the vascular tunic of the eye?

1. Iris
2. Ciliary body
3. Choroid

21

List the 6 layers of the retina from superficial to deep.

NEURAL:
1. Ganglion cell layer
2. Inner plexiform layer
3. Inner nuclear layer
4. Outer plexiform layer
5. Outer nuclear layer
PIGMENTED
6. Pigmented epithelium layer

22

What is the sclera of the eye?

- Opaque connective tissue
- Posterior 5/6th of fibrous tunic
- Part of the eye commonly known as the “white”

23

What is the insertion point for the extraocular eye muscles?

Sclera

24

What is the cornea of the eye?

Transparent and convex anterior 1/3rd of the fibrous tunic

25

What is the iris of the eye?

Pigmented muscular curtain near the front of the eye, between the cornea and the lens, that is perforated by an opening called the pupil

26

What is the role of the ciliary body?

Secretes fluid into the eye

27

What is the choroid of the eye?

Posterior majority of vascular tunic made of vascularized connective tissue

28

What causes blood shot eyes?

Choroid becomes red

29

What branches off the ciliary body? Role?

Ciliary processes: secrete aqueous humor

30

What part of the eye is responsible for the pupillary light reflex?

Iris

31

What part of the eye is responsible for the accommodation reflex?

Ciliary body

32

How many intraocular muscles in the iris? What are they? What is each innervated by? What do they do?

1. Sphincter (constrictor) muscle - Parasympathetic NS (CN III) -> constricts pupil

2. Dilator muscle -Sympathetic NS -> dilates pupil

==> Control pupil size

33

What kind of muscles are the intraocular iris muscles?

Smooth muscles

34

What are the zonula fibers of the eye?

Fibers that suspend the lens in position during accommodation

35

Describe the process of accommodation to near vision.

Near source => parasympathetic bilateral afferents in CN II => synapse at lateral geniculate nuclei of thalamus => synapse at visual cortex => synapse at visual association cortex => synapse at Edinger-Westphal nuclei of the midbrain => parasympathetic preganglionic neurons (CN III) => synapse at ciliary ganglia => postganglionic neurons => intraocular contraction of ciliary muscles (both eyes) => relaxation of zonula fibers => relaxation of lens => increases refractive power

36

What are the 2 effects of the pupillary reflex test? Describe each.

1. Direct: constriction of illuminated pupil
2. Consensual: constriction of contralateral pupil

37

What is a consensual response?

Any reflex observed on one side of the body when the other side has been stimulated

38

Describe the pathway of the pupillary light reflex.

Light => eyes => bilateral CN II parasympathetic afferents => synapse at both lobes of pretectal nucleus of midbrain => neurons from each PTN lobe to EW with crossing over from each PTN lobe => Edinger-Westphal nuclei of the midbrain => preganglionic parasympathetic neurons in the efferent axons in CN III going to both eyes => ciliary ganglia => postganglionic parasympathetic neurons => short fibers in ciliary nerve => sphincter muscles of iris

39

What is another name for the consensual effect?

Indirect effect

40

What causes constriction of the contralateral pupil during the pupillary reflex test?

Crossing over of neurons from the pretectal nuclei to the Edinger-Westphal nuclei of the midbrain

41

Describe the innervation pathway of the dilation of the pupil.

Bilateral input in low light => hypothalamus => neurons to cervical spinal cord => synapse with preganglionic sympathetic efferents in spinal nerve => inferior cervical ganglia => synapse in superior cervical ganglion => postganglionic sympathetic neurons => dilator muscles

42

What are the 3 important structures of the retina? Describe each.

1. Optic disc: 2mm hole in posterior retina = highly vascularized exit/entry point of optic nerve

2. Fovea: depression (1.5 mm diameter) of outer-most cellular neuronal layer of retina

3. Macula: 6 mm in diameter

43

What constitutes the blind spot of the eye? What does it mean?

The optic disc of the retina
No photoreceptors in this spot

44

What is an ophthalmoscope?

Tool used to view the retina through the pupil

45

Why does the fovea have improved acuity?

1. Because the depression allows for the light to directly reach the photoreceptors
2. Because its RGCs have a smaller receptive field size

46

What part of the eye has the majority of the refractive power? What other part helps?

Cornea

The lens has additional and VARIABLE refractive power

47

What innervates refraction in the eye?

Parasympathetic reflexive control (accommodation)

48

What requires greater refraction?

Greater divergence of light rays

49

What does relaxation of the ciliary muscle mean for refraction?

Less refraction

50

What does contraction of the ciliary muscle mean for refraction?

Greater refraction

51

Near vision: ciliary muscle contracted or relaxed?

Contracted

52

Far vision: ciliary muscle contracted or relaxed?

Relaxed

53

What is the pathway of light through the eye? Use anatomical terms and reflection, refraction, and absorption

Light reflected from object => refraction in cornea => refraction in aqueous humor => pupil => refraction in lens => refraction in vitreous humor => absorption in retina by photoreceptors and signal transduction => bipolar cells => ganglion cells

54

Does a distant object require higher or lower refractive power? What does this mean for the shape of the lens?

Lower
Less convex

55

Does a near object require higher or lower refractive power? What does this mean for the shape of the lens?

Higher
More convex

56

What is myopia? Too much or too little refraction? What is it caused by?

Near sightedness
Too much refraction
Caused by steep curvature of the cornea

57

What is hyperopia? Too much or too little refraction? What is it caused by?

Far sightedness
Too little refraction
Caused by flattening of the cornea

58

What is astigmatism?

Uneven curvature of the cornea causing uneven refraction

59

What are corrective lenses or laser surgery a treatment for?

Near and far sightedness

60

What leads to common visual impairments?

Errors in refractive power of cornea

61

Is visual acuity homogenous across the retina?

NOPE

62

What is the purpose of the Snellen Eye Chart?

To test our ability to discriminate letters and numbers at 20 feet

63

What does the denominator correspond to in the Snellen Eye Chart?

Related to the size of the letters, which increases as you move up chart

64

What does the nominator correspond to in the Snellen Eye Chart?

Distance between the subject and the chart (always 20 ft)

65

Definition of visual acuity?

Ability to distinguish light from nearby points in visual field

66

Normal visual acuity?

1.74 mm @ 20 feet

67

What are the 5 cell types of the nuclear layer of the retina?

1. Bipolar cells
2. Horizontal cells
3. Amacrine cells
4. Photoreceptor cells
5. Ganglion cells

68

At what layer of the retina does pigmentation start?

Outer nuclear layer

69

Does transduction happen in the ganglion cell layer of the retina?

NOPE

70

In what retina cell layer are photoreceptors found?

Outer nuclear layer

71

In what retina cell layer are amacrine cells found?

Inner plexiform layer

72

In what retina cell layer are bipolar cells found?

Inner nuclear later

73

In what retina cell layer are horizontal cells found?

Outer plexiform layer

74

By what cells is light transduced?

Photoreceptor cells

75

What is the direct pathway of light transduction? What NT does it use?

Photoreceptors => bipolar cells => ganglion cells

Glutamate

76

What are the only retinal cells capable of firing APs?

Ganglion cells

77

What is the lateral pathway of light transduction? What NT does it use?

Inhibitory information from amacrine and horizontal cells to dampen the responsiveness of adjacent cells in direct pathway

GABA

78

How many photoreceptors in each eye?

100 million

79

What are the 2 types of photoreceptor cells? % for each?

1. Rods: 90%
2. Cones: 10%

80

List the 3 portions of photoreceptors cells.

1. Synaptic terminals
2. Inner segments with cell bodies
3. Outer segments

81

How do cones and rods differ?

1. Number of disks: rods have many, cones have few
2. Sensitivity to light: high for rods, 1000x lower for cones
3. Types of photopigment protein (opsin part): rhodopsin for rods and photopsin 1-3 for cones

82

In dim light are we primarily using rods or cones?

Rods

83

How are the photoreceptors distributed across the retina?

Heterogeneously

84

How does the fovea's photoreceptor composition differ to that of the periphery? Explain each.

1. More cones: better color vision
2. Less rods: lower light sensitivity
3. Smaller ganglion cells receptive field in fovea: higher visual acuity

85

How does receptive field size relate to visual acuity?

Smaller ganglion cells receptive field = high visual acuity

86

What is the RGC receptive field size?

Small circular patch of photoreceptors that send signals to a single retinal ganglion cell

87

What are the 3 types of vision during the day? Is each rod or cone mediated? Describe each.

1. Scotopic vision: rod mediated so poor acuity and high sensitivity
2. Mesopic vision: rod and cone mediated
3. Photopic vision: cone mediated so poor sensitivity and high acuity and color vision

88

When is scotopic vision used?

Overcast, no moon

89

When is mesopic vision used?

Moolight to early twilight

90

When is photopic vision used?

Early twilight to daytime (inside and outside)

91

Why is the photopic vision cone mediated?

Because the rods are saturated

92

Macular degeneration:
1. Definition
2. Symptom
3. Treatment

1. Progressive degeneration of photoreceptors in the macula
2. Loss of vision in the central portion of the visual field
3. None other than retinal transplant

93

What is the cause of macular degeneration?

Unknown: genetics, CVD, oxidative stress

94

What is the initial symptom of macular degeneration?

Difficulty reading

95

Definition of phototransduction?

Process by which the outer segments of photoreceptors transduce light to an electrical signal = hyperpolarization

96

What are the 2 parts of the photopigment proteins? Describe each and its role.

1. Retinal = aldehyde of VA: light-absorbing molecule
2. Opsin = GPCR

97

Describe the phototransduction pathway in rods.

Light hits retinal portion of rhodopsin => rhodopsin is now activated and facilitates the exchange of GDP for GTP on transducin => transducin activates phosphodiesterase to convert cGMP to GMP => cGMP gated cation channels close => hyperpolarization of photoreceptor cell => IPSP => inhibition of glutamate release

98

Where is rhodopsin found in rods?

Disk membrane

99

What is transducin?

G-protein that activates phosphodiesterase upon activation

100

How does light affect retinal on rhodopsin exactly? Explain.

It bleaches it: photo absorption breaks the double bond between C11 and C12 which permits rotation and conversion from 11-cis-retinal to all-trans-retinal

101

What is the initiating step in vision?

Retinal bleaching

102

Can retinal absorb more light after bleaching? What does this mean?

NOPE so regeneration of retinal is necessary

103

What does retinal regeneration require?

Enzymes in the pigment epithelium

104

How do the photopsin varieties differ?

Wavelength sensitivity

105

What is the trichromacy theory of color?

Net activity across 3 cone types is analyzed by brain to produce all color varieties

106

Describe the 3 types of photopsin.

1. Long wavelength: red
2. Medium wavelength: green
3. Short wavelength: blue

107

Does the wavelength sensitivity of cones overlap?

YUP, mainly between green and red ones

108

How does the sensitivity of photoreceptors vary with light variations?

1. Dark adaptation = sensitization
2. Light adaptation = desensitization

109

What are the 2 mechanisms of dark adaptation?

1. Dilation of pupils (minor)
2. Regeneration of unbleached retinal (major)

110

Does dark adaptation go faster in rods or cones? Why?

Cones because they have a smaller outer segment and same size pigmented epithelium to be able to regenerate a smaller amount of bleached retinal

111

What are the 2 mechanisms of light adaptation?

1. Bleaching retinal (immediate)
2. Decrease in intracellular Ca++

112

Describe the mechanism of decrease in intracellular Ca++ during light adaptation?

Light => decreases Ca++ conductance => less Ca++ in cells to inhibit the GC => more cGMP => cGMP gated channels open => opposes effect of light

113

Describe the receptive field characteristics of bipolar and retinal ganglion cells.

Special center-surround receptive field characteristics:
- Their response to light in the center receptive field is either excitatory (ON) or inhibitory (OFF)
- Their response to light in the surround receptive field opposes the effect of light in the center receptive field which is further inhibited by GABAergic horizontal and amacrine cells

114

Are the center-surround receptive field characteristics of non-transducing cells in the direct pathway inherited by cells in target structures?

YUP

115

What do the center-surround receptive field characteristics of non-transducing cells in the direct pathway allow us to do?

Sense contrast

116

Describe how an ON bipolar cell works (dark and light).

DEPOLARIZES w/ light in CENTER receptive field:

- Dark => glutamate released by photoreceptors => mGluR6 activated => K+ efflux => hyperpolarization
- Light => no glutamate released by photoreceptors => mGluR6 inactive => no K+ efflux => depolarization

117

Describe how an OFF bipolar cell works (dark and light).

HYPERPOLARIZES w/ light in CENTER receptive field:

- Dark => glutamate released by photoreceptors => AMPA active => Na+ influx => depolarization
- Light => no glutamate released by photoreceptors => AMPA not activated => NO Na+ influx => hyperpolarization

118

What receptors do ON bipolar cells express?

mGluR6

119

What receptors do OFF bipolar cells express?

AMPA

120

What NT do horizontal cells release?

GABA

121

What NT do photoreceptors release?

Glutamate

122

Describe the transduction pathway from photoreceptors to ON bipolar cells in both the center and surround receptive fields.

- Light in center receptive field => photoreceptor hyperpolarization => no glutamate release => bipolar cell depolarized

- Light in surround receptive field => center photoreceptors release glutamate and surround ones do not => horizontal cells inhibit center photoreceptors with GABA => hyperpolarization of bipolar cells

123

What forms the optic nerve?

Axons of retinal ganglion cells

124

What are the 2 basic types of RGC receptive fields?

1. ON-center/OFF-surround
2. OFF-center/ON-surround

125

When is the maximal response in an ON-center/OFF-surround ganglion cell achieved?

When the entire center of the receptive field is illuminated

126

When is the minimal response in an ON-center/OFF-surround ganglion cell achieved?

When the entire surround of the receptive field is illuminated (ring of light)

127

Which effect is stronger on the RGC: center or surround?

Center

128

What happens when you illuminate the entire receptive field of an ON-center/OFF surround RGC? Why?

Response will be just above baseline because the center effects are slightly stronger than surround

129

When is the maximal response in an OFF-center/ON-surround ganglion cell achieved?

When the entire surround of the receptive field is illuminated (ring of light)

130

When is the minimal response in an OFF-center/ON-surround ganglion cell achieved?

When the entire center of the receptive field is illuminated

131

Describe the transduction pathway from photoreceptors to OFF bipolar cells in both the center and surround receptive fields.

- Light in center receptive field => photoreceptor hyperpolarization => no glutamate release => bipolar cell is hyperpolarized

- Light in surround receptive field => center photoreceptors release glutamate and surround ones do not => horizontal cells do not inhibit center photoreceptors with GABA => bipolar cell is depolarized

132

What are the 2 basic types of RGCs?

ON or OFF cells

133

What kind of receptors do RGCs have?

AMPA

134

What is the color-opponent processing in RGCs?

The receptive fields of ganglion cells innervated by cones have a center-surround organization also and the center and surround are also color-opponent, meaning the ganglion cell is excited by one color in the center and inhibited by the "opposite" color in the surrounding area

135

What are the opposite colors of the color-opponent processing in RGCs?

Yellow/Blue
Red/Green

136

What does total color blindness lead to?

Monochromacy

137

What does partial colorblindness lead to? What are the 2 types?

Dichromacy = loss of one cone type

1. Red-green color blindness: lack green or red cones

2. Yellow-blue color blindness: lack blue cones

138

On what gene is the red-green color blindness? What does this mean?

X chromosome

Men are more prone to it

139

On what gene is the yellow-blue color blindness?

Non-sex chromosomes

140

What are the 3 ways of classifying RGCs?

1. Appearange: large (Magno) or small (Parvo) or other (non-M non-P)

2. Connectivity: to rods or cones

3. Electrophysiology: sensitivity to light, adaptation rate, and conduction velocity

141

Describe the 3 characteristics of M-type RGCs. What % of the total RGC population do they constitute?

1. Large with a large receptive field
2. Connected to rods via bipolar cells
3. Highly sensitive, fast adaptation (shorter AP), and rapid conduction

5%

142

What is another way of saying an RGC shows fast adaptation?

It has a transient response

143

Can each type of RGC be ON or OFF type?

YUP

144

Describe the 3 characteristics of P-type RGCs. What % of the total RGC population do they constitute?

1. Small with a small receptive field
2. Connected to cones via bipolar cells
3. Low sensitivity to light and high sensitivity to color, slow adaptation (longer AP), and slow conduction

90%

145

Describe the 1 characteristicsof non-M non-P-type RGCs. What % of the total RGC population do they constitute?

Connected to cones (little else known)

5%

146

What does tunnel vision mean?

Loss of peripheral vision

147

Give an example of rods being more sensitive to light than cones?

1 photo can change the membrane potential in rods, but not in cones

148

What kind of layers are the inner and outer plexiform layers?

Synaptic layers

149

How does the macula's photoreceptor composition differ to that of the periphery?

Smaller ganglion cells receptive field in macula

150

What is the visual field?

The environmental space viewed by one eye or both eyes

151

How is visual field different from receptive field?

Visual field is the space around us and the receptive field corresponds to the number of photoreceptors sending signals to each bipolar cell/RGC

152

How do acuity and sensitivity relate to each other?

Inverse relationship: the higher the acuity, the lower the sensitivity

153

Do the photoreceptors absorb all of the light that reaches the retina?

NOPE, they absorb the light that the pigmented epithelium did not absorb

154

How do horizontal cells work? What is their role?

Horizontal cells are activated by glutamate secreted by either the center or surround photoreceptors and secrete GABA on GABAa receptors on either the center or surround photoreceptors to further inhibit them and strengthen the contrast of the center/surround receptive field

155

What happens when you illuminate the entire receptive field of an OFF-center/ON surround RGC? Why?

Response will be just below baseline because the center effects are slightly stronger than surround

156

What is a diopter?

Unit of refractive power that is equal to the reciprocal of the focal length (in meters) of a given lens

157

What is the focal length?

Distance between the cornea and retina

158

What is a fundus exam of the eye?

Physician looks through opthalmoscope to look at fundus and uses drops to dilate pupil to have a better view

159

What is the fundus of the eye?

The interior lining of the eyeball, including the retina, optic disc, and the macula

160

What is papilledema? Usually bilateral or unilateral?

Bulging of the optic disc due to swelling that is caused by increased intracranial pressure

Usually bilateral

161

What other non-ocular structure can a papilledema affect?

Put pressure on the olfactory bulb

162

What is nystagmus?

Involuntary movement of the eye

163

Role of sudomotor nerve? Where does it come from?

Innervate face to control vasodilation and sweating
and smooth muscles of eyelid

Same pathway as sympathetic innervation of the pupil but branches off at the external carotid artery

164

How is a ON or OFF RGC similar to that in bipolar cells? How is it different?

RGC (ON and OFF): only express AMPA receptors

Bipolar cells: mGluR6 or AMPA (depending on whether ON or OFF)