Vision (2) Flashcards
1
Q
What is the focal distance?
A
distance from the refractive surface to the point where parallel light rays converge
2
Q
A measure of the refractive power of a transparent surface is what?
A
diopter; reciprocal of the focal distance in meters
3
Q
The cornea and sclera present what functionally?
A
tough physical barrier to trauma and infection, protects internal structures of the eye.
4
Q
The refractive power of the cornea depends on what?
A
slowing of light at the air-cornea interface; most of the refractive power, 42 diopters
5
Q
Aqueous humor fills what? Produced by what?
A
anterior chamber;ciliary processes of the ciliary
body
6
Q
What function does aqueous humor serve?
A
Provides nutrients for avascular tissues ant. segment (lens, cornea), Removes waste, Maintains intraocular pressure, Contains very little protein, no blood, allowing light to pass w/o scattering
7
Q
What is the function of the retina?
A
Regulates amount of light falls on retina; iris dilator muscle contracts->pupil dilates (mydriasis) iris sphincter muscle contracts, pupil constricts (miosis).
8
Q
Sympathetic innervation to iris utilizes what NT? Sympatholytic substances do what? Sympathomimetic substances?
A
NE; constrict pupil - Hexamethonium; dilate pupil- cocaine
9
Q
Parasympathetic innervation to iris utilizes what NT? Parasympatholytic substances? Parasympathomimetic substances?
A
ACh, dilate pupil- Atropine, constrict pupil- Pilocarpine
10
Q
The size of the pupil determines what? Small opening? Large Opening?
A
cone angle of a bundle of rays that come to a focus in image plane; highly collimated parallel rays, sharp focus at the focal plane; uncollimated rays, sharp focus only for rays with certain focal length.
11
Q
What is the F-number?
A
focal length / aperture diameter; ratio of the focal
length of the eye (from cornea to retina) and physical
aperture (pupil opening); Each smaller F-number doubles area of opening & amount of light entering; Wide apertures=small depths of field
12
Q
When we accommodate we constrict our pupils, what happens to the F-number?
A
gets larger, aperture gets smaller and depth of field gets larger
13
Q
What is the function of the lens? When does it change?
A
focus light on the retina; shape of the lens can change in order to enable accommodation.
14
Q
What does a Convex (converging) lens do?
A
bring light rays together at a focal plane a focal distance away from the lens.
15
Q
What does a concave (diverging) lens do?
A
spread light rays apart so that the focal plane is a virtual plane in front of the lens.
16
Q
What is the function of the ciliary processes?
A
Produce aqueous humor, Site of attachment of lens zonules
17
Q
What is the function of the ciliary muscle?
A
Involved in accommodation, Site of drainage of some aqueous humor
18
Q
The lens is involved in the accommodation when?
A
formation of point images closer than about 9 meters.
rays from objects at this distance or closer diverge, greater refractive power is required to bring them into focus on the retina.
19
Q
What are the Steps in accommodation?
A
ciliary muscles contract, Suspensory ligaments attached to lens relax, lens becomes more spherical due to
inherent elasticity, curvature of lens surface increases, refractive power increases.
20
Q
Other changes occurring during accommodation include what?
A
eyes converge, pupils constrict, F-number gets larger, the depth of field gets larger
21
Q
What is myopia? Type of corrective lens?
A
nearsighted; diverging or concave
22
Q
What is hyperopia? Type of corrective lens?
A
farsighted; converging or convex
23
Q
What is astigmatism? Corrective lens needed?
A
irregularities in the curvature of the cornea or lens is corrected by cylindrical lenses
24
Q
What is presbyopia? Corrective lens?
A
hardening of the lens is corrected by bifocals or trifocals.
25
What is the theory of opponent colors?
six primary colors, coupled in three pairs: red-green, yellow-blue and white-black. Any receptor that was turned off by one of these colors, was excited by its coupled color
26
What are the different trichromats?
Normal- Can distinguish about 150 hues; Protanomalous-Reduced red sensitivity, Can distinguish only 5 to 25 hues; Deuteranomalous- Reduced green sensitivity; distinguish only 5 to 25 hues
27
What are the different dichromats?
Protanopes=No red sensitivity (red looks black to them)
Deuteranopes= No green sensitivity
Tritanopes= Rare individual who are insensitive to short wavelengths
of light.
28
What is a Monochromat?
totally colorblind, world is shades of gray
29
How is color vision tested?
ishihara plates
30
What are the eye field genes? What do they do?
Rx, Pax6, Six3, and Lhx2 delineation of an eye field in the
| developing neural plate
31
Where is the optic groove formed? When? What does it become? How does it remain connected to the brain?
either side midline at boundary of telencephalon and diencephalons; 3 weeks; evaginate from diencephalon form bilateral optic vesicles (3-4 weeks). continuous with third ventricle through the optic stalk
32
What are the six neuronal and one glial cell types in the retina? Match them with their developmental stage.
early stage of histogenesis (embryonic): retinal ganglion cells(RGCs), cone photoreceptors, 3 horizontal cells and amacrine cells; late stage of histogenesis (perinatal)
rod photoreceptors, bipolar cells and Muller glial cells
33
the outer nuclear layer contains what?
cell bodies of rod and cone photoreceptors
34
the inner nuclear layer contains what?
bipolar cells, horizontal cells, amacrine cells, and Muller glia.
35
The RGC layer consists of what?
RGCs and displaced amacrine cells
36
The outer and the inner plexiform layers represent what?
the fiber layers where synapses are located
37
What is the outermost cell and the innermost cell of the retina?
Photoreceptors- outermost (face the outer layer of the optic cup, the RPE); RGCs - innermost (toward the lens)
38
Light travels through what to reach photoreceptors?
RGC, plexiform, and nuclear layers
39
What are the characteristics of a rod/cone cells?
outer segment contains stack of membranous discs. which contain visual signal transduction machinery, renewed by diurnal shedding of discs- phagocytosed by
RPE.; inner segment contains nucleus and proteinsynth machinery. outer and inner connected by cilia
40
The light absorbing visual pigment in rods is what?
rhodopsin (contains non light absorbing opsin and light absorbing retinal)
41
Opsin is a transmembrane protein consisting of what?
348 amino acids; 11-cis retinal, obtained from the dietary vitamin A (11-trans retinol), attached covalently to residue 296 (lysine)
42
What are the functional characteristics of rods?
high sensitivity, rods are densely packed with photo
| pigments; few photons can evoke maximum response in a rod, (dim light), adapt slowly to changing light levels.
43
What are the three different kinds of cones?
blue-Short-wave,S cone; least numerous, most sensitive), green-middle-wave, M cone, and red-long wave or L cone; most numerous but least sensitive cones
44
What are the characteristics of cones?
lower sensitivity, less photo pigments than rods; hundreds of photons are needed to evoke maximum response, adapted for day phototopic vision. Cones adapt faster than rods to changing light levels; reading, face recognition
45
What are bipolar cells?
second order neurons in retina carry information vertically from photoreceptors to RGCs
46
What are the properties of ON bipolar cells?
Depolarizing; depolarize in response to direct
| illumination, due to presence of GPCR (=depolarized in light ON condition)
47
What are the properties of OFF bipolar cells?
hyperpolarize in response to direct illumination due to the presence of Glu-gated cation channel (=depolarized in light OFF condition)
48
Rods transmit information through which cells? How?
ON bipolar cells, which selectively express PKC; indirectly communicate to RGC thru AII amacrine cells & cone bipolar cells
49
Cones transmit information through which cells? How?
ON or OFF bipolar cells; directly synapse with RGC
50
Which cells constitute local circuit interneurons modulating the vertical transfer of information photoreceptors to RGCs?
Horizontal cells, along with amacrine cells
51
What are Type 1 horizontal cells?
dendrites contact only cones, axons that contact only rods.
52
What are type 2 horizontal cells?
short dendrite and axons and contact only cones.
53
Horizontal cells give off what type of signal? receive input and conduct where?
inhibitory; photoreceptors and conduct information laterally, feeding back on photoreceptor terminals, and also on the bipolar cells
54
What are the general features of an amacrine cell?
mostly without axons. many are “displaced” and their cell
| bodies are in RGC layer, varied NT phenotypes. GABA, glycine, dopamine and acetylcholine
55
AII amacrine cells, that connect rod bipolar cells indirectly to RGCs are what kind? How common?
glycenergic and constitute 10% of all amacrine cells.
56
What is the function of amacrine cells?
unknown; important component of scotopic (dark-adapted vision) or rod visual pathway. or cells modulate phototopic or cone visual pathway. or like horizontal cells, in inhibitory surrounds in receptor fields.
57
What is the function of RGCs?
sole output neurons in the retina, carrying information to midbrain and thalamus
58
What are the two kinds of RGCs?
parvocellular (P) and magnicellular (M)
59
What are the features of the P RGCs?
midget cells; most numerous (70% of total) connect to parvocellular division (layer 3-6) of LGN; mediate fine spatial and color discriminations because of small receptive fields and two P cells convey information from each foveal cones.
60
What are the features of M RGCs?
less numerous but large, connect with the magnocellular (layer 1 and 2) division of the LGN, large receptive field, poor discriminator of color.
61
Axons of ganglion cells project to where?
suprachismatic nucleus (circadian rhythm), pretectal area (pupillary reflex), superior colliculus (saccadic eye movements) and LGN (vision)
62
The visual information from LGN is conveyed to where?
area 17 in the occipital lobe, the primary visual cortex.
63
RGCs connected with suprachiasmatic nucleus contain what molecule? Why?
melanopsin,; sensing the general luminescence, presumably without rod/cone inputs, for entraining the
biological clock to day and night cycle.
64
The sole glial cell types in the retina are what? What do they develop from?
Muller glia (MG), generated by retinal stem cells,
65
Astrocytes and microglia found in the nerve fiber layer are from where?
optic nerve and embryonic mesoderm, respectively
66
MG extend processes to where forming what?
radially toward inner & outer retina; end feet form inner limiting membrane (ILM); outer limiting membrane (OLM) consisting of adherens junct. btwn Muller cells & photoreceptors; barrier btwn RPE & retina
67
What are the functions og the MG cells?
structural scaffold (ensheathment of cell bodies), barriers (ILM and OLM); induction of retinal blood vessels and blood retinal barrier, retinal homeostasis (Glu uptake, extracellular K uptake, reg. extracellular pH), reg. neuronal activities, stem cells ?
68
RPE cells are highly specialized pigment cells located where?
between photoreceptors & choroids, basal
surfaces rest on the Bruch’s membrane, apical
surfaces have microvilli that en-sheath outer segments of
photoreceptors
69
What are the essential roles of RPE in the maintenance of photoreceptor function?
promotes retinal attachment- active dehydrating sub retinal space, phagocytosing diurnally shed tips of the outer segments, forms the outer blood-retinal barrier
promotes transport of water, ion (Na-K ATPase apical), retinol (retinol binding protein receptor basal), reisomerizes 11-trans retinal to 11-cis retinal, and absorbs
scattered light and scavenges free radicals
70
What is the outer retinal blood barrier?
tight junctions btwn RPE cells, regulating exchange of molecules between the fenestrated choriocapillaries and the retina
71
What is the blood supply to the retina?
ophthalmic branches of internal carotid; outer 1/3 retina
supplied by choroid blood vessels (arising from the posterior ciliary artery), inner 2/3 supplied by central retinal or hyaloid artery
72
What portion of the eye receives the greates amount of blood? How much?
choroid, 65-80%, vital for maintenance of photoreceptors, remaining 20-30% to central retinal a.
73
Photoreceptors are depolarized in the dark because?
cGMP-gated channel (in the outer segment membrane),
| permeable to Na+ and Ca++ remains open, continuously release neurotransmitter glutamate
74
What is the resting membrane potential of a rod?
-40mV
75
The K+ selective channel plays what role in the photoreceptor?
stabilizes the membrane potential at the
| reversal potential of K+ (-70mV)
76
The Na+-K+ exchanger helps do what in a photoreceptor?
maintain the steady intracellular concentration of Na+, K+, and Ca++.
77
Photoreceptor cGMP gated channel is closed in response to what?
incident light, preventing the entry of Na+, membrane is relatively hyperpolarized, reducing release of glutamate
78
The signal transduction by light and adaptation for the next round of signal generation involves what?
regulation of the cGMP channel by G-protein coupled
| receptor, rhodopsin/ cone opsin
79
What are the three steps involved in generation of the visual signal?
Activation of visual pigments by the incident light, Activation of cGMP-phosphodiesterase (PDE), Closure of
cGMP-gated channel
80
How is Activation of visual pigments by the incident light achieved?
light absorbed by 11-cis retinal and chromophore in opsin, isomerizes into 11-trans retinol, conformational change receptor shape, converting to active meta-rhodopsin II state, alters interaction with trimeric G9
protein (transducin), dissociation & activation of subunit of transducin by exchanging bound GDP for GTP.
81
How is Activation of cGMP-phosphodiesterase (PDE) achieved?
activated α-transducin activates PDE ->hydrolysis of cGMP, lowering intracellular [cGMP]
82
How is closure of cGMP-gated channel achieved?
decrease [cGMP] leads to closure of cGMP-gated cation
| channel-> hyperpolarization of membrane-> reduction in glutamate release.
83
How is restoration of the dark state achieved?
Inactivation meta-rhodopsinII: cont. activation transducin by metarhodopsin II prevented by rhodopsin kinase (Grk1) and arrestin. Phosphorylation metarhodopsin by rhodopsin kinase allows arrestin to bind, terminating activation. Recoverin, active in low [Ca++], also participates in termination of receptor activation
84
Restoration of cGMP concentration is achieved how?
closure cGMP-gated channel following photoactivation prevents entry of Ca++ in the cell; same time Na/Ca-K exchanger continue to extrude Ca++.drop in intracellular [Ca++] activates GC1 and GC2 via GC activating proteins
(GCAP1 and GCAP2)-> restore cGMP levels-> re-opening of the cGMP-gated channel.
85
retinal ganglion cells (RGCs) facilitating their
| detection of contrast differences by what?
center surround receptive fields
86
What is a receptive field in the retina? What are the two kinds?
area of retina that, when illuminated with light, changes the membrane potential of bipolar cells in that area; center and surround
87
What is a center receptive field?
circular area of retina where a cluster of photoreceptors have direct input to either an ON or OFF bipolar cell
88
In an ON-center receptive field what happens?
illumination photoreceptors hypolarized, Glu release
| reduced at the synapse, depolarizes (activate) the bipolar cell; opposite for shadow;
89
What happens in an Off-center receptive field?
illumination hyperpolarizes photoreceptors which hyperpolarizes (inhibit) the bipolar cell; opposite if a shadow
90
What is the surround receptive field?
circular area that surrounds the center of a receptive field; ring of photoreceptors connected indirectly to photoreceptors in the center via horizontal
cells, which are inhibitory
91
What is the intervention of horizontal cells the effect on membrane potential of bipolar cells?
antagonistic to center illumination when surround photoreceptors are illuminated
92
ON-center and OFF-center ganglion cells receive input from the what type of bipolar cells?
corresponding; ON-center ganglion cell fire AP when light is projected in middle of On-center receptive field. OFF-center ganglion cell fire AP not in response to light but shadow, cast in center of OFF-center receptive field.
93
most ganglion cells are responsive to what over what?
differences in illumination that occur within the receptive fields, changes in illumination
94
In both types of receptive fields the response to stimulation of the center is affected how by surround?
canceled by the response to stimulation of the surround
95
What is the division of the fibers in the optic tract?
small medial root (10% of fibers in the tract) and a larger lateral root (90% of fibers in the tract).
96
The medial root terminate where?
superior colliculus, pretectal area and suprachiasmatic nucleus
97
The lateral root terminates where?
synapses the lateral geniculate nucleus of
| the thalamus.
98
The RGC inputs create a map of the contralateral visual field where? This projects where?
on the superficial layer of the SC; deeper layer, where inputs from auditory and somatosensory systems also terminate
99
The pathway to SC is involved in what?
saccadic eye movements (movements that shift the gaze from one point to another) and visual reflex grasp (reflex that fixes the object of interest
on fovea that was discovered in peripheral vision)
100
RGCs project to what area rostral to SC? Fibers in this area do what?
pretectal; project bilaterally to PS neurons in Edinger-Westphal nuclei, which controls the pupillary constrictor muscle via the ciliary ganglion-third cranial nerve
101
RGCs, particularly those with melanopsin, project
| to where? why?
suprachiasmatic nucleus for the entrainment of the circadian rhythm
102
Axons of RGCs, carried in the large lateral root,
| synapse where?
LGN in such a way that there is visuatopic representation of the contralateral half of the visual field.
103
LGN consists of six layers (layers 1-6 from ventral to dorsal) of cell bodies separated by what?
intervening layers of axons and dendrites.
104
Layers 1 and 2 are called what? Receive inputs from where?
magnocellular layers; magnocellular RGCs (less
| numerous, large receptive field, poor discriminator of color)
105
Layers 3 to 6 are called what? Which receive inputs
| from where?
parvocellular layers, parvocellular RGCs (more numerous, small receptive field, discriminator of color).
106
Fibers from the contralateral hemiretina synapse where?
layers 1,4,6
107
Fibers from the ipsilateral hemiretina synapse where?
layer 2, 3 and 5.
108
Cells in M and P pathway differ in what way?
sensitivity to color contrast, luminance contrast
(discriminating brightest and darkest part of stimulus), spatial frequency (discriminating narrowly spaced dark and bright bars), temporal frequency (discriminating bars turned off and on at a higher rate)
109
The visual information from LGN is carried in fibers
| that pass through what?
retrolenticular portion of the internal capsule and forms the optic radiation on way to the primary visual cortex
110
Fibers in the inferior portion of the optic radiation carry information from where? Travel how? Terminate? What is this pathway?
lower retinal quadrant (=upper quadrant of the visual field), sweep anterior direction in temporal lobe, move posteriorly, terminate lower bank of calacarine fissure, lingual gyrus; Meyers loop
111
Fibers in the superior portion of the optic radiation carry information from where? Travel how? Terminate?
upper retinal quadrants (=lower quadrants of the visual fields), travel under cortex of parietal lobe, terminate upper bank of calcarine fissure, the cuneus gyrus
112
primary visual cortex in each cerebral hemisphere receives information exclusively from where? Half of the area is devoted to what?
contralateral half of the visual field, information coming from foveal and parafoveal retina.
113
What is anopsia?
loss of one or more quadrants of the visual field
114
What is homonymous in terms of visual lesion?
visual field loss similar for both eyes, loss of overlapping visual field
115
What is heteronymous in terms of visual lesion?
loss of non-overlapping visual field
116
Vascular accident involving posterior cerebral artery leads to what loss?
contralateral homonymous hemianopsia with macular sparing; larger representation in cortex receives blood supplies from MCA
117
Axons from M cells (M channel or path)
| synapse predominantly in what layer? P cells?
4Ca, 4Cb
