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dental neuroscience > the visual system > Flashcards

the visual system Flashcards

1
Q

the visual system

A

~ 50% of the cortex is devoted to visual perception
- has the most complex circuitry of all sensory systems
- over 1 million fibers

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2
Q

anatomy of the human eye

A
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3
Q

the surface of the retina

A
  • has numerous vessels fan out over the surface
  • vessels arise from othalamic artery and vein
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4
Q

where does the opthalmic artery and vein enter the eye through?

A

optic disk

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5
Q

retinal axons leave the eye through

A

optic disk

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6
Q

the optic disk

A
  • has no photo receptors
  • blind spot
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7
Q

macula lutea

A
  • located near the center of the retina
  • contains yellow pigment
  • high visual acuity
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8
Q

where is acuity the greatest?

A

center of macula (fovea)

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9
Q

physical properties of light

A
  • electromagnetic radiation visible to eye
  • visible light consists of wavelengths 400-700nm
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10
Q

cornea

A
  • the site of most refractive power of the eye
  • responsible for the majority of the refractive power of the eye (40D)
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11
Q

refraction of the cornea

A
  • has refractive power of about 40 diopters
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12
Q

focal distance

A

distance from the refractive surface of the point where parallel light rays converge

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13
Q

greatest amount of refraction is at

A

the anterior surface of the cornea

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14
Q

corneal refractive power is

A

constant

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15
Q

lens

A
  • 20 D of refractive power
  • changes with accommodation
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16
Q

accommodation by the lens

A
  • lens adds refractive power by changing the shape of the lens
  • contraction of ciliary muscles releases tension in zonula
  • lens becomes rounded
  • greater curvature provides greater refraction
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17
Q

ciliary muscle is innervated by

A

parasympathetic fibers in CN III

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18
Q

amplitude of accommodation

A
  • the range of accommodation decreases with age
  • crystalline lens and ciliary muscles become less elastic
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19
Q

refractive errors

A
  • ametropia
  • myopia
  • hyperopia
  • presbyopia
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20
Q

ametropia

A

a refractive error is present

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21
Q

emmetropia

A

normal vision

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22
Q

myopia

A

near sightedness

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23
Q

hyperopia

A

far sightedness

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24
Q

correction of myopia and hyperopia

A
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25
Q

presbyopia

A

loss of accommodative ability of the lens resulting in difficulties with near tasks

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26
Q

where do the photoreceptor cells lie?

A

in the back of the retina
- light rays must pass through retina before reaching photopigment molecules to excite

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27
Q

the laminar organization of the retina

A

(furthest back toward front)
- pigment epithelium
- rods
- cones
- outer plexiform layer
- horizontal cells
- bipolar cells
- amacrine cells
- inner plexiform layer
- ganglion cells
- nerve fiber layer

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28
Q

retinal pigment epithelium

A
  • plays critical roles in maintenance of photoreceptors and photopigments
  • has melanin
  • absorbs any light not captured bu the photoreceptors
  • thus minimizes reflection of light that could blur image
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29
Q

retina: neural circuitry

A
  • light hits photoreceptors
  • sends signals to the bipolar cells
  • bipolar cells send signal to ganglion cells
  • ganglion cells send signal to the brain
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30
Q

photoreceptors include

A

rods and cones !!

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31
Q

rods

A
  • 100 million
  • light sensitive
  • found in periphery of retina
  • low activation threshold
  • achromatic
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32
Q

cones

A
  • 6 million
  • are color sensitive
  • found mostly in fovea
  • high acuity
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33
Q

the outer segments of a rod or cone contain

A

different photopigments

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34
Q

cones work under _____ light.
rods work under _____ light.

A

bright, dim

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35
Q

photopigments of rods are

A

rhodopsin

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36
Q

rhodopsin…

A

absorbs light in a very narrow range

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37
Q

photopigments of cones are

A

cone opsins (iodopsins)

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38
Q

3 isoforms of cones include

A
  • short WL
  • medium WL
  • long WL
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39
Q

short wavelength isoform of the cones:

A
  • absorb the blue or violet color region of visual spectrum
  • S or blue cone
    ~437nm
40
Q

medium wavelength isoform of the cones:

A
  • absorbs in the green region of visual spectrum
  • M or green cone
    ~533nm
41
Q

long wavelength isoform of the cones:

A
  • absorbs in the red region of the visual spectrum
  • L or red cone
    ~573nm
42
Q

rhopsodin is formed by

A
  • cis-retinal (aldehyde of vitamin A) bound to opsin
43
Q

iodopsin is formed by

A
  • the same cis-retinal bound to isoforms of opsins
44
Q

melanopsin in

A

intrinsive photosensitive RGC (ipRGCs)

45
Q

rods and cones are

A

not evenly distributed

46
Q

in fovea there are

A

only cones

47
Q

in peripheral retina there are

A
  • rods and cones
  • primarily rods
48
Q

convergence of rods

A
  • high convergence
  • high sensitivity
  • low spatial resolution
49
Q

convergence of cones

A
  • low convergence
  • low sensitivity
  • high spatial resolution
50
Q

visual acuity of the fovea

A
  • enhanced by displacement of neurons to minimize scattering of light
  • 1:1 relationship of cones to bipolar to retinal ganglion cells
  • high density of cones
51
Q

differences between rods and cones

A
52
Q

phototransduction

A
  • photoreceptors do no inhibit action potentials
  • light activation causes graded change in membrane potential
53
Q

much of the processing within the retina is mediated by

A

graded potentials

54
Q

shining light of photoreceptors leads to

A

hyperpolarization

55
Q

in the dark, the receptor is in a

A

depolarized stated and releases glutamate

56
Q

cyclic GMP gated channels in outer segment membrane of photoreceptors

A
  • cGMP allow Na and Ca to enter thru ion channel in the dark
  • cGMP does not bind in light and Na and Ca cannot enter
57
Q

phototransduction in rod photo receptors

A
  1. light stimulation of rhodopsin leads to activation of a G-protein transduction
  2. activated G-protein activates cGMP phosphodiesterase (PDE)
  3. PDE hydrolyzes cGMP reducing concentration
  4. closure of Na channels
58
Q

bipolar cells have 2 main groups

A
  • on (light on)
  • off (light off)
59
Q

on-center bipolar cells

A
  • have inhibitory glutamate receptor
    (metabotropic mGluR6)
60
Q

off-center bipolar cells

A
  • have excitatory glutamate receptors (ionotropic glutamate receptors)
61
Q

both cones and rods photoreceptors release what neurotransmitter?

A

glutamate

62
Q

horizontal cells

A
  • play crucial role in creating receptive fields and lateral inhibition
    why? because of inhibitory connections
63
Q

amacrine cells

A
  • 30 types
  • mediate lateral interactions between bipolar cell terminals and the dendrites of ganglion cells
64
Q

glial cells of the retina

A
  • muller cells
  • microglia
65
Q

muller cells

A

the principal glial cell of the retina

66
Q

microglia

A
  • enter the retina coincident with the mesenchymal precursor of retinal blood vessels in development
  • ubiquitous in retina (found in every layer)
67
Q

retinal ganglion cells

A
  • 10-15 types
  • 1.5 million that form optic nerve (CN II)
  • connect eye to brain
68
Q

% of RGCs in primate retinas

A
  • M cells = 10%
  • P cells = 70%
  • K cells = 8%
  • intrinsically photosensitive RGC (1-2%)
69
Q

M RGCs

A
  • origin of magnocellular pathway
  • broadband, achromatic signal to magnocellular layer of LGN
70
Q

the cells in magnocellular pathway

A
  • generally have large receptive fields
  • fast axonal conduction velocities
  • are found in the periphery of the retina
71
Q

P RGCs

A
  • origin of parvocellular pathway
  • convey color signal to the parvocellular layers of LGN
72
Q

the cells of parvocellular pathway

A
  • small receptive fields
  • slow axonal conduction velocities
  • mostly found in the fovea (high acuity)
73
Q

intrinsically photosensitive RGCs (ipRGC)

A
  • 1-3% of RGCs
  • contain melanopsin
  • axons innervate suprachiasmatic nucleus (SCN)
  • contribute to photic entrainment of circadian rhythm
  • project to olivary pretectal nucleus
    PULPILLARY LIGHT REFLEX
74
Q

the receptive field

A

the area of the retina that, upon illumination, enhances or inhibits signaling

75
Q

RGC receptor fields

A
76
Q

center surround organizaiton

A
  • serves to emphasize areas of difference (contrast)
  • visual system detects intensity not the absolute amount of light
77
Q

visual pathways (main)

A
  • retinal signal travels to lateral geniculate nucleus (LGN) of thalamus
  • then to primary visual cortex (V1)
78
Q

lateral geniculate nucleus (LGN)

A
  • major target of RGCs
  • receives input form both eyes and relays these messages to the primary visual cortex via optic radiation
79
Q

central projections of retinal ganglion cells

A
  • optic nerve and optic disc
  • RGC exits retina through optic disc
  • bundle of all the cells form optic nerve
80
Q

central visual pathway includes what structures?

A
  • optic nerve
  • optic chiasm
  • optic tract
  • LGN
  • optic radiations
  • primary visual cortex
81
Q

lateral geniculate nucleus (LGN)

A
  • 6 layers
  • each layer receives input from one eye
  • 2 inner layers = magnocellular
  • 4 outer layers = parvocellular
82
Q

koniocellular sublayers of LGN

A
  • ventral
  • small as dust
83
Q

primary visual cortex (V1)

A
  • neurons from LGN carrying visual information synapse with cortical neurons in medial portion of the occipital lobe
  • area 17 of Brodmann
  • AKA striate cortex
84
Q

cortical retinoptic map

A
  • info received at adjacent portions of retina remain adjacent in visual cortex
  • about 25% of V1 is dedicated to input from fovea
  • upper fields are mapped below calcarine fissure and lower fields are above
85
Q

striate cortex input

A
  • LGN projections to the striate cortex are segregated
  • axons from magnocellular layers terminate principally within sublamina (4Ca)
  • axons form parvocellular layers terminate principally within sublamina (4Cb)
  • axons from koniocellular layers terminate in layers 2 and 3
86
Q

striate cortex cells

A
  • simple
  • complex
87
Q

simple cells

A
  • elongated receptive fields
  • respond best to oriented elongated stimuli
  • bars and edges
  • on and off regions
  • direction selective
88
Q

complex cells

A
  • respond only to appropriately oriented stimuli
  • don’t have stimulatory and inhibitory zones
  • direction-selective
  • NO on or off regions
  • orientation and direction
89
Q

orientation of columns in V1

A

????????

90
Q

distribution of axons from the eye are distributed into patches called

A

ocular dominance columns

91
Q

ocular dominance columns respond to

A

either left or right eye… NOT BOTH

92
Q

orientation columns respond to lines of

A

the same angle

93
Q

hypercolumns contain

A

orientation columns that respond to complete 180 degrees

94
Q

cytochrome oxidase blobs respond to

A

color

95
Q

extrastriate visual areas form 2 processing pathways

A
  1. dorsal pathway
  2. ventral pathway
96
Q

dorsal pathway

A
  • leads to parietal lobe
  • spatial vision
  • neurons selective to speed and direction of movement
97
Q

ventral pathway

A
  • leads to temporal lobe
  • cognition
  • selective to color, texture, shape

dental neuroscience (24 decks)

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