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52 Cards in this Set
- Front
- Back
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What is the fovea?
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Area of the retina devoid of vessels where light gets focused
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How does the relative number of photoreceptors change near the fovea vs. farther away?
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Near fovea: cones > rods
Farther away from fovea: rods > cones |
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What is the optic disc?
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The area where all the retinal ganglion cells come together to go out to the eye
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What does the lens do?
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Adjusts thickness for focusing light on retina --> accomodation
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What does the cornea do?
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Provides most of the refraction for focusing images
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To what system does the retina belong?
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CNS
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What fills the anterior and posterior chambers?
What happens with insufficient drainage? |
Aqueous humor
Increases intraocular pressure = glaucoma |
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What is the Iris?
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Two sets of muscles that form the pupil and control the size of the pupil
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What leaves at the optic disc?
What enters? |
Axons and veins leave
Arteries enter |
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What is the Macula Lutea?
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A small circle of yellow/brown pigment called xanthophyls around the fovea
Located on cone synaptic processes and muller glial cell processes |
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What do xanthophylls do?
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Absorb shorter wavelengths, and thereby, protect cells
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What is the foveola?
What is special about it? |
Center of the fovea
Devoid of blood vessels, minimizes light scattering |
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What does the ciliary muscle do?
What happens when it contracts? |
Controls thickness of lens
When it contracts, lens relaxes, moves to more spherical shape --> allows you to focus close up |
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What is emmetropia?
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Normal vision
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What is myopia?
What causes it? (2) |
Near-sightedness
Eye cannot become flat enough because... 1. eye is too long 2. cornea too curved |
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What is hyperopia?
What causes it? (2) |
Farsightedness
Lens cannot become round enough because... 1. Eye too short 2. Cornea too flat |
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What makes up the outer segment of the photoreceptor cell?
Function? |
500-1000 flattened membranous discs
SItes of photon capture and reactions of visual transduction |
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What exists in the inner segment of the photoreceptor cell (3)?
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1. Cell body w/ lots of mitochondrion (photoreceptor cell uses lots of energy
2. nucleus |
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What does the synaptic terminus connect?
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photoreceptor cells w/bipolar cells and horizontal cells
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How do rods and cones differ in structure of outer segment?
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Rods = Longer, cylindrical outer segments containing more photopigment
Cones = Smaller, conical-shaped outer segments |
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How do outer segment discs differ between rods and cones?
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Rods = discs are free-floating organelles, separate from plasma membrane
Cones = Simple evaginations of plasma membrane, like one continuos sheet |
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What is the retinal pigment epithelium?
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Single layer of cells immediately adjacent to the retina
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What is retinal detachment?
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Separation of retina from RPE --> loss of vision
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What happens to the distal outer segment?
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Distal 10% of outer segment shed diurnally, phagocytosed by RPE
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How much of the human retinal photoreceptors are rods?
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95%
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What do rods specialize in?
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Higher sensitivity for low intensity or night vision
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What are the characteristics of the inactivation response and temporal dispersion of a rod?
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Slower inactivation response
Lower temporal resolution |
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What type of information do rods convey to the brain?
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Monochromatic light amplitude information
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What are cones' sensitivity to light?
Temporal resolution? |
Lower sensitivity to light
Faster temporal resolution |
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How does packing of cones lead to their specialized function?
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Dense packing in fovea --> high spatial resolution (permits reading fine print)
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What information do cones convey to they brain?
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Color (wavelength) and light intensity
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What is the state of photoreceptor channels in the dark?
What happens in the light? |
Dark: Na, Ca, and K channels open --> depolarization (-40 mv) --> more NT released
Light: Na and Ca channels close --> hyperpolarization (tries to move to Ek = -90) --> graded potentials --> less NT released |
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What is Rhodopsin?
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Photopigment in rods, GPCR
Made up of chromophore retinal coupled to opsins |
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What is the Rhodopsin "ligand"?
To what is it bound? |
11-cis-retinaldehyde
Bound to opsin apoprotein via Schiff base linkage |
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What is the visual cycle?
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photon absorbed by rhodopsin --> changes retinaldehyde and retinol from cis to trans --> activates many transducins
Trans retinol converted back to cis by RPE in outer rod segment |
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What is transducin?
What does it do? |
G-protein that is activated when rhodopsin binds a photon
Activates PDE to hydrolyze cGMP --> GMP |
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What are the three steps in which transducin carries out its functions?
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1. a-subunit of transducin exchanges its bound GDP for GTP --> activated
2. GTP-bound a-subunit dissociates from transducin 3. Activated a-subunit activates cGMP PDE --> hydrolysis of cGMP to GMP |
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How does amplification of a signal occur in the rods?
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Activation of one rhodopsin by one photon --> activation of several-hundred transducin molecules
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What is cGMP PDE?
How is it activated? |
Three subunit enzyme
Dissociation of catalytic ab-subunits of PDE from the inhibitory gamma-subunit when tranducin binds |
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What is the role of cGMP PDE?
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Catalyzes the hydrolysis of cGMP --> GMP, lower cGMP levels in the cell
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What is the role of cGMP on a photoreceptor cell?
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cGMP regulates open/closed probability of Na and Ca channels on a rod
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What happens to cGMP levels in the dark/light in a photoreceptor cell?
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Dark: cGMP levels high, channels relatively open --> photoreceptor depolarized
Light: cGMP levels low, channels close --> photoreceptor hyperpolarize |
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What causes retinitis pigmentosa?
What happens? (2) |
nonsense mutation in the a-chain gene of cGMP PDE
Photoreceptors die apoptotic death Vision loss due to loss of cones (a secondary mechanism when mutation is in rhodopsin and rods are dying) |
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Where does the visual transduction cascade occur?
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On surface of outer segment discs
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What are 3 important elements of the visual transduction cascade?
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1. Amplifier
2. Sensitive over 10,000 fold difference in background illumination 3. Able to detect rapid changes in light intensities (high temporal resolution) |
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What are 3 types of opsin? Wavelengths?
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1. Blue opsin (wavelength 437 nm)
2. Green (533 nm) 3. Red opsin (564 nm) |
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What chromophore do all opsins use?
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11-cis-chromophore
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What dictates the spectral sensitivity of different visual pigments?
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"Tuning" of chromophore by the surrounding protein, which is a function of the protein structure (genetic)
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What are the 3 cone types, and what color wavelength do they correspond to?
How about rods? |
Short = Blue
Medium = Green Long = Red Rods = green |
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How does the brain perceive color?
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Perceives differential stimulation of blue, green, and red photoreceptors, compares intensities
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What are the output channels of the retina?
Where do they go? |
Blue, green, and red cones
Go to lateral geniculate and visual cortex |
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What causes color blindness?
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Genetic alterations in red and green pigment genes
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