Term
| what are the functions of the eye? |
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Definition
| to regulate the amount of light reaching the photosensitive surface, to focus on near/far objects, to convert the pattern of incoming light to interpretable signals, and interpretation of patterns. |
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Term
| what are the 3 layers of the eye? |
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Definition
| sclera, choroid and retina |
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Term
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Definition
| the dividing line between the white sclera and the clear cornea |
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Term
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Definition
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Term
| what is the spherical shape of the eye maintained by? |
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Definition
| a balance of secretion and reabsorption of the aqueous humor |
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Term
| what is aqueous humor circulation? |
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Definition
| ciliary epithelium (secretion from between ciliary muscle and lens @ 2 ul/min) -> posterior chamber (between iris and lens) -> anterior chamber (behind the cornea) -> trabecular meshwork (resists outflow at a pressure of ~ 15 mm Hg) -> canal of schlemm (communicates w/venous drainage of eye) |
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Term
| what is the pathway of image processing? |
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Definition
| cornea -> anterior chamber -> lens -> vitreous body -> retina (photons are captured and are transmitted into the signal to the optic nerve) |
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Term
| what are photoreceptors? types? |
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Definition
| specialized nerve cells with light sensitive compounds in them – when light strikes them, they change their chemical makeup and initiate a cascade. rods: more sensitive to light and allow sight in dim/monochromatic environments. cones: color vision in bight light, allowing detail and visual acuity (can be packed closer together). |
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Term
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Definition
| rods to see by starlight (several rods send signals to a single bipolar cell which then gets sent through an amacrine cell = concentrating information from several photoreceptors) |
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Term
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Definition
| rods and cones in moonlight (between which gap junctions open and the rods communicate through the cones' denser bipolar connections) |
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Term
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Definition
| cones for brighter than moonlight |
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Term
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Definition
| photopigment: rhodopsin, more sensitive and slower response |
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Term
| what characterizes cones? |
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Definition
| photopigment: cone pigment (type of opsin), less sensitive and quicker response |
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Term
| how does phototransduction take place? |
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Definition
| the photoreceptors are formed by stacked disks w/photoreceptive compounds (opsin) embedded in the membranes which light passes through and as it does, the signal is amplified by a factor of 100,000+ at each level. specifically, the photosensitive compounds being stimulated (opsin converts 11-cis retinaldehyde to all -trans) leads to closing of Na+ channels = hyperpolarization and lower release of glutamate. |
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Term
| what is the response of the photoreceptors to dark? |
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Definition
| opsin deactivates or is metabolized and is reconstituted w/ "fresh" 11-cis retinal, leading to Na+ channels to be opened, increasing membrane potential (depolarization) and causing an increase in glutamate release - as light decreases, glutamate release increases. |
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Term
| what is the theory for why light stimulation causes a decrease rather than an increase in NT (glutamate) release? |
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Definition
| because the system is prone to less “noise” - another way you can think of photoreceptors is they are actually “dark detectors” and not “light detectors”. |
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Term
| what characterizes dark adaptation? |
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Definition
| for the eyes to be most sensitive, all of the opsin molecules need to be activated (replenished with fresh 11-cis retinal) and ready to receive light. this process takes about 20-30 min. |
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Term
| what characterizes light adaptation? |
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Definition
| there has to be a balance between the breakdown and replenishment of pigments (several seconds) |
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Term
| what color pigments are found in cone photoreceptors? |
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Definition
| blue (short wavelength), green (medium wavelength) and red (long wavelength) |
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Term
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Definition
| no red pigment, X chromosome (most prevalent) |
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Term
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Definition
| no green pigment, X chromosome |
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Term
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Definition
| no blue pigment, chromosome 7 |
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Term
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Definition
| equal levels of red, blue and green |
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Term
| what is the distribution of cones in the retina? |
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Definition
| very tightly packed in the center (most color sensitivity and highest visual acuity), in an area called the fovea which comprises 2% of the retina and is surrounded the macula lutea. |
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Term
| what is the distribution of rods in the retina? |
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Definition
| covering more of the periphery around the center (though never as tightly packed as the cones) |
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Term
| what is the optic disk/papilla? |
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Definition
| where axons converge to form the optic nerve, there are no rods or cones = blind spot |
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Term
| what is saccadic eye movement? |
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Definition
| b/c the fovea is limited in size and b/c the visual system adapts to "ignore" areas which don't change over time, the eye makes a series of small, quick movements called saccades to "take in" the whole image. |
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Term
| what is the simplified sequence of retinal layers? |
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Definition
| ganglion cells on the outside of the retina (running to the optic nerve, divert around the fovea), below that bipolar cells and below them - the rods/cones. |
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Term
| what are the horizontal cells? |
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Definition
| interneuronal cells between the rods/cones and bipolar cells. |
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Term
| what are the amacrine cells? |
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Definition
| interneuronal cells between the bipolar and ganglion cells |
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Term
| what is the contrast detection mechanism in the retina (image processing at the retina)? |
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Definition
| each photoreceptor is part of a receptive field of which there are 2 kinds: ON center and OFF centers. in uniform light/dark, these centers are the same in terms of signal, but the center of the ON center fires and the periphery of the OFF center fires. if light is focused in the center of the ON center, it will fire, but not if on the periphery. if light is focused on the periphery in the OFF center, it will fire, but not if on the center. this allows for contrast to be more apparent at borders between dark and light. the excitatory/inhibitory interconnections provided by the horizontal cells allow this kind of process to occur. |
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Term
| can the balance of excitation/inhibition by neighboring receptors (contrast detection) help w/light adaptation in the eyes? |
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Definition
| yes - the majority of the important information in an image is not the overall level of light, but the changes in light from region to the next, so the basic shapes can be determined quickly by this method. |
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