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NEUR 2000 > Lecture 10: Visual System > Flashcards

Lecture 10: Visual System Flashcards

1
Q

What are the 3 layers of tissue in the eye?

A
  • Retina
  • Uveal tract
  • Sclera
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2
Q

Describe the retina.

A
  • Contains all neural cells of the eye (e.g. neurons, photoreceptors)
  • innermost layer
  • Only part of eye that contains neurons sensitive to light and capable of transmitting visual signals to central targets
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3
Q

Describe the uveal tract. What 3 structures is it made up of?

A
  • 3 distinct but continuous structures
  • Middle Layer
  • Made of: choroid, ciliary body, and iris
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4
Q

Describe the choroid.

A
  • Largest component of uveal tract
  • Rich capillary bed that nourishes photoreceptors (contains light-absorbing melanin in ‘pigment epithelium’)
  • Keeps photoreceptors healthy
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5
Q

Describe the ciliary body.

A
  • Extends from choroid near front of eye
  • Ring of tissue that circles the lens
  • Muscular component (ciliary muscle) adjusts lens refractive power
  • Vascular component (ciliary processes) produces aqueous humour
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6
Q

Describe the iris.

A
  • Most anterior component
  • Coloured
  • Contains 2 sets of muscles with opposing actions to adjust size of pupil (opening in its centre) with neural control
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7
Q

Describe the sclera.

A
  • Opaque ‘white of the eye’
  • Continuous with transparent cornea at front of eye
  • Outermost tissue layer
  • Composed of tough, white fibrous tissue
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8
Q

What are the 2 separate fluid environments between the cornea and retina?

A
  • Aqueous humour
  • Vitreous humour
  • Light must pass through cornea and these fluid environments before reaching retina
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9
Q

Describe the aqueous humour.

A
  • In anterior chamber (behind cornea, in front of lens)
  • Clear watery liquid
  • Nourishes cornea and lens
  • Produced by ciliary processes in posterior chamber and flows into anterior chamber through pupil
  • Replaced about 12x a day
  • Drained by meshwork of cells junction of iris/cornea
  • Failure to drain leads to glaucoma
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10
Q

Describe the vitreous humour.

A
  • In posterior chamber
  • Between back of lens and surface of retina
  • Clear thick gelatinous substance
  • 80% of eye volume
  • Holds shape of eye
  • Phagocytic cells clear debris (e.g. blood, old retinal cells) that might interfere with light transmission
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11
Q

What is accommodation and what does it consist of?

A
  • Change of lens shape that alters how light is refracted when it enters the eye
  • Focusing on near objects: contraction of ciliary muscle, reduces tension in zonule fibers and allows elasticity of lens to increase its curvature
  • Thin lens = distant object
  • Thick lens = near object
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12
Q

What are the 2 opposing forces that determine the shape of the lens?

A
  • Elasticity of lens which tends to keep it rounded up

- Tension exerted by zonule fibers which tends to flatten it

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13
Q

How does the eye accommodate to viewing distant objects?

A
  • Force exerted by zonule fibers is greater (ciliary muscle is relaxed) than elasticity of lens
  • Flatter shape for distance viewing
  • Least refractive power
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14
Q

How does the eye accommodate to viewing close objects?

A
  • Contraction of the ciliary muscle relaxes tension of zonule fibers
  • Allows inherent elasticity of lens to increase its curvature
  • Highest refractive power
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15
Q

What is the relationship between age and capacity for accommodation?

A
  • As you age, capacity for accommodation is gradually diminished
  • Lens weakens
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16
Q

What is the fundus?

A
  • Surface of the retina
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17
Q

What is the optic disk/optic papilla?

A
  • Site where retinal axons leave the eye
  • Entry/exit point of blood supply
  • Lies nasally
  • Contains no photoreceptors
  • ‘blind spot’ (scotoma, white matter pathway)
  • Not dark, just can’t receive visual info here
  • Travel through optic nerve to reach target structures in brain
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18
Q

What is the macula lutea?

A
  • On fundus/inner retina
  • Circular region with yellow pigment
  • 3mm
  • Near centre of retina
  • High visual acuity
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19
Q

What is the fovea?

A
  • Depression in retina located in centre of macula lutea
  • 1.5mm
  • Greatest visual acuity
  • Dense concentration of small diameter cones
  • 1:1 relationship b/n cones and bipolar ganglion cells
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20
Q

What is the purpose of the optic nerve?

A
  • Transmits signals from retina to brain
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21
Q

What is the cornea?

A
  • Specialized transparent tissue that permits light rays to enter the eye
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22
Q

What are the primary functions of the optical components of the eye?

A
  • Efficiently transmitting light energy

- Achieve focussed image on surface of retina

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23
Q

What parts of the eye are primarily responsible for refraction/bending of light? What does this bending do?

A
  • Cornea and lens

- Necessary for formation of focussed images on photoreceptors of retina

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24
Q

What tool is useful for visualizing inner surface of retina (or fundus) through the pupil?

A
  • Ophthalmoscope
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25
Q

How are blood vessels organized on the inner surface of the retina?

A
  • Fan out

- Arise from ophthalmic artery/vein

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26
Q

Where do the ophthalmic artery and vein enter the eye?

A
  • Through whitish circular area called optic disk/papilla
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27
Q

Why must humans move their eyes and head around when looking at things?

A
  • Direct fovea to objects of interest

- High acuity of vision is restricted to a very small portion of the retina

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28
Q

What are saccades?

A
  • Rapid/orienting movements of the eyes
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29
Q

What do retinal bipolar cells consist of?

A
  • Concentrated, few dendrites at one end
  • Cell body
  • Axon extending from other side of cell body
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30
Q

What do retinal ganglion cells consist of?

A
  • Widespread dendrites
  • Cell body
  • Axon extending from other side
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31
Q

What do retinal amacrine cells consist of?

A
  • Lots of dendrites interwoven
  • Extend from cell body
  • No axon
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32
Q

What is the simplest path that retinal cells take?

A

Photoreceptors -> Retinal bipolar cells -> Retinal ganglion cells
- Horizontal and amacrine cells provide lateral interactions and maintain visual system sensitivity to contrasting light changes

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33
Q

How many neuron classes can be seen in inner nuclear, outer nuclear and ganglion cell layers?

A

5

  • Photoreceptors
  • Retinal bipolar cells
  • Retinal ganglion cells
  • Horizontal
  • Amacrine cells
34
Q

Where are retinal processes and synapses located?

A
  • Inner plexiform and outer plexiform layers
35
Q

What kind of cells must light travel through in order to reach outer segments of photoreceptors?

A
  • Non-light-sensitive cells
36
Q

What is phototransduction?

A
  • Light absorption initiates intracellular cascade which changes membrane potential of receptor
37
Q

What are photoreceptors embedded in?

A
  • Pigment epithelium
  • Composed of disks to nourish photoreceptors
  • Outermost layer of retina
38
Q

What do epithelial processes do?

A
  • Extend down b/n outer segments of photoreceptors
39
Q

How long do the membranous disks in the pigment epithelium last?

A
  • About 12 days
40
Q

Where are new outer segment disks being formed? What disks are removed and how?

A
  • Formed near base, then grow outward toward pigment epithelium
  • Oldest disks (those closest to outer pigment epithelium) are removed/phagocytosed
41
Q

How are the membranous disks phagocytosed?

A
  • Disks curl
  • Tip becomes spherical
  • Tip separates from rod
  • Tip is engulfed by pigment epithelium
  • Broken down/recycled for parts
42
Q

Does light stimulation lead to depolarization or hyperpolarization?

A
  • Hyperpolarization
43
Q

Do different levels of light stimulation lead to different effects on neurons? Why?

A
  • Yes: more/less hyperpolarization
  • Show a graded change, changes in light
  • There is continuous ntx release because they have a higher resting MP
  • More stimulus = less cell excitement
44
Q

What happens on the molecular level in the dark?

A
  • cGMP levels in outer segment membrane are high
  • cGMP binds to Na+-permeable channels in membrane, keeping them open
  • Allows Na+ and other cations to enter, depolarizing cell
45
Q

What happens at the molecular level in the light?

A
  • Absorption of photons leads to a decrease in cGMP levels
  • Closes cation channels
  • Results in receptor hyperpolarization
46
Q

What is seen dependent on light absorbed?

A
  • Stimulation-specific graded change in how much ntx is being released
47
Q

Does the direction of K+ movement change with light absorption?

A
  • NO

- Always an efflux

48
Q

What are the two types of photoreceptors?

A
  • Rods and cones
49
Q

What does the outer segment of photoreceptors consist of?

A
  • Membranous disks

- Contain light-sensitive photopigments

50
Q

What does the inner segment of photoreceptors consist of?

A
  • Cell nucleus

- Gives rise to synaptic terminals that contact bipolar and horizontal cells

51
Q

What is phototransduction?

A
  • Process of absorbing light and creating a response

- Change in amount of ntx released on target neurons

52
Q

What is the photopigment in rods? What does it consist of?

A
  • Rhodopsin
  • 7 transmembrane domains of protein portion (opsin) traverse membrane bilayer
  • Opsin forms pocket where light absorbing portion (retinal molecule) resides
53
Q

How do rods and cones differ?

A
  • Protein portion
54
Q

What do the different forms of retinal do?

A
  • Absorb different wavelengths of light
55
Q

What does a photon do to retinal?

A
  • Breaks its double bond, which leads to a cascade of events
56
Q

What does transducin look like in its inactivated state?

A
  • Bound to GDP
57
Q

What happens when transducin is activated?

A
  • Exchanges GDP for GTP
  • Alpha subunit activates phosphodiesterase in disk membrane, which hydrolyzes cGMP thus lowering cGMP concentration in outer segment
58
Q

What happens when the concentration of cGMP falls?

A
  • Molecule no longer binds to and holds open ion channels on surface of outer segment membrane
59
Q

What does the flow of cations in the dark look like?

A
  • Ion channels are open
  • Influx of Na+ and Ca2+ depolarizes cell
  • Efflux of K+ opposes, as a hyperpolarizing influence
  • Combined result = depolarization of photoreceptors
  • Continuous release of NTX from synaptic terminals of photoreceptors
60
Q

How does light affect ion flow?

A
  • Light reduces cGMP concentration
  • Na+ channels close
  • K+ flows out faster than Na+ or Ca2+ can flow in
  • Hyperpolorization
  • Decreases NTX release
  • Enormous signal amplification
  • Net change of about 1 mV per rhodopsin molecule
61
Q

What is responsible for limiting amplifying cascade and restoring molecules to inactivated state?

A
  • Proteins in photoreceptor
62
Q

When are molecules converted back to a form that can absorb light again?

A
  • In the dark
63
Q

What is the optic chiasm?

A
  • 60% of fibres (nasal retina) cross to contralateral side
  • 40% (temporal retina) continue ipsilaterally to thalamus and midbrain targets
  • Anterior to stalk of pituitary gland
64
Q

What is the optic tract?

A
  • Contains ganglion cell fibres from both eyes (contralateral and ipsilateral nerves)
  • Major target is dorsolateral geniculate nucleus of thalamus (primary relay nucleus for visual processing by cerebral cortex)
  • Secondary target of pretectum
  • Information from left visual field is carried in right optic tract and vice versa
65
Q

What is optic radiation?

A
  • Axons from lateral geniculate nucleus to the striate cortex (part of the primary visual cortex, or V1, in occipital cortex)
66
Q

What is the pretectum?

A
  • Responsible for pupillary light reflex via circuitry interacting with oculomotor nerve (CNIII)
  • Managed by projections from optic nerve
  • Collection of neurons b/n thalamus and midbrain
67
Q

What are 2 other projections/targets of optic nerve?

A
  • Suprachiasmatic nucleus of the hypothalamus
    • Retinohypothalamic pathway
    • Circadian rhythms (comes mostly from light sources)
  • Superior colliculus
    • Coordination b/n head and eye to visual targets
    • Orienting toward/away from visual stimuli
  • Includes ganglion cell types distinct from rod/cone light sensitivity
68
Q

How are images formed on the retinal surface?

A
  • Light rays pass through pupil

- Results in images that are inverted and left-right reversed

69
Q

What is a visual field?

A
  • Each eye sees a part of the visual space that defines its visual field
  • Divisions of nasal v temporal and superior v inferior
  • Portion of the world you can see without moving your head or eyes
70
Q

What does monocular mean?

A
  • Only one eye can see

- Ex. peripheral vision

71
Q

What is the binocular field?

A
  • Where left/right visual fields overlap (central portion)
  • Projection of binocular field onto 2 retinas and its relation to the crossing of fibres in optic chiasm (topographical organization)
  • Left half of visual world is represented in right half of brain and vice versa (regardless of what eye it originates in)
72
Q

Which part of the retina receives info from the right visual hemifield?

A
  • Temporal left retina

- Nasal right retina

73
Q

What is periphery vision and what is it mediated by?

A
  • Monocular

- Mediated by most medial portion of nasal retina

74
Q

What does the centre of the visual field project to?

A
  • Foveal region of retina
75
Q

What does the right lateral geniculate nucleus receive information from?

A
  • Left visual field
  • Nasal left retina
  • Temporal right retina
76
Q

How do axons containing info about the superior portion of the visual field travel?

A
  • Sweep around lateral horn of ventricle in temporal lobe before reaching occipital lobe
  • “Meyer’s loop”
77
Q

How do axons containing info about inferior portions of the visual field travel?

A
  • Travel in parietal lobe
78
Q

Where is the fovea represented in the visual cortex?

A
  • Posterior part
  • Large portion
  • Everything else (more peripheral) in anterior regions
79
Q

Do inputs of the two eyes converge?

A
  • Yes, at cortical levels
80
Q

Where does the V1/striate cortex project to?

A
  • Others of cerebral cortex (extrastriate) involved in complex visual perception
81
Q

Where does the dorsal stream travel?

A
  • Leads from striate cortex to parietal lobe
  • Spatial aspects of vision
  • Analysis of motion and positional relationships
82
Q

Where does the ventral stream travel?

A
  • From striate cortex to inferior part of temporal lobe

- Responsible for high resolution form vision and object recognition

NEUR 2000 (10 decks)

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