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23 Cards in this Set
- Front
- Back
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What are the 5 types of neurons present in the Retina?
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1. Photoreceptors
2. Horizontal cells 3. Bipolar cells 4. Amacrine cells 5. Retinal ganglion cells |
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What do photoreceptors of the retina do?
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Transduce light into neural activity. The process is phototransduction found only in rods and cones.
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What is the structure of the photoreceptors?
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1. Outer segment-site of visual transduction, contains photopigment and is stacks of membranous discs.
2. Inner segment-contains nucleus and mitochondria, etc. It renews and synthesizes visual pigments. 3. Synaptic terminal-synaptic vesicles, neurotransmitter-GLUTAMATE (of rods and cones) |
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Where are rods located? What are they important for?
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Throughout the entire retina and are responsible for night vision and scotopic vision. They mediate low-acuity, monochromatic vision in dim light.
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What are cones important for and where are they found?
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Cones detect high-acuity levels of illumination, resolve spatial detail well and are responsible for day vision (Photopic vision). They are found mainly in the macula lutea.
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How long does it take until visual acuity improves when in the dark? What is responsible for this?
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10-20 min, acuity improves because of a photopigment in rods that increases with time in the dark-->Rhodopsin
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What is phototransduction and how does it begin?
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It is the process by which light is converted into electrical signals in the rod cells, cone cells, and photosensitive ganglion cells of the retina.
-Begins with a photon of light being absorbed by a photopigment molecule (4 total photopigments; 3 in cones-red, blue, green and 1 in rods. ) -Next absorption of light caused by the conformational change in the pigment that causes activation of a G-protein (closes Na+ channels) -Note: Photoreceptors do NOT produce action potentials, only changes in membrane potential and release of glutamate. *** -Light induces cation channels to close, membrane hyperpolarizes toward K+ potential and transmitter concentration decreases. |
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What is the status of membrane potentials (electrophysiology) of the rods and cones of the eye?
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In darkness, Na+ channels are always open and always depolarized (@ about -40 mv) and this is called the "dark current".
-There is always some sort of NT being released (glutamate) -Changes in light will lead to an increase of a decrease in NT release from photoreceptors. -Darkness-steady NT, Na+ channels open, depolarized. -Light-Na+ channels close, hyperpolarization, decrease NT |
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What is the blind spot?
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No photoreceptors are present at optic disk.
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Photoreceptors excite or inhibit bipolar cells and bipolar cells inhibit or excite what?
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Bipolar cells inhibit or excite retinal ganglion cells and these axons then form the optic nerve.)
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Describe the visual field of a ganglion cell.
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Receptive fields are composed of 2 concentric, roughly circular zones.
-illumination of central area (on-center) field either increases or decreases background firing rate -illumination of the peripheral area (surround) has opposite effect ***Eg-ON-center and OFF-center receptor fields -Divisions: left, right, inferior, superior |
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What is the difference between the binocular zone and the monocular zone?
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Binocular zone is the overlap of visual fields of both eyes. The monocular zone is the visual field of 1 eye that does not overlap with the visual field of the other eye.
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Each lateral geniculate nucleus and cortex receives information from where?
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The contralateral visual field
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What are the 2 projections of the lateral geniculate to the visual cortex?
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1. Optic radiation-terminates in upper bank of V1 (calcarine sulcus)
2. Meyer's loop-axon terminates in lower bank of V1) |
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Where is the input from for the optic radiation?
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Input is from the superior retinal quadrants and is for seeing the inferior visual field.
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Where is the input for Meyer's loop?
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Input is from inferior retinal quadrants and is for superior visual space.
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What is the parvocellular division of the LGN?
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Layers 3-6 represent the parvocellular division where the primary input is from ganglion cells primarily from cones.
-Functions include: color perception and visual acuity |
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What is the magnocellular division of the LGN?
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Layers 1 & 2 (inferior LGN)
-input from large ganglion cells t/o the retina -functions: movement perception including contrast sensitivity) |
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What area of the brain is the visual cortex located?
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-all of occipital lobe
-parts of parietal -parts of temporal |
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What is the primary visual cortex?
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-aka: striate cortex
-calcerine sulcus (area 17-Brodmann area) of V1 -input from LGN -some of functions are to detect bars of light in periphery |
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What is the extrastriate cortex?
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-of visual cortex
-visual association cortex-and is all remaining visual cortex outside of V1 *V2, V3, V4, V5 and is heavily interconnected w/ area 17 -Areas 18, 19 together with related parts of parietal and temporal lobes -function: processing of complex information |
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There are 2 processing streams that V1 passes information along (visual cortex), what is the dorsal stream?
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Dorsal stream-
*object recognition processing stream (where pathway) that detects motion -Input comes from Parvocellular division of LGN and is important for visual acuity -Then projects to: V1->V2->V4->infratemporal cortex -cell responses are to specific objects |
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What is the ventral stream of V1?
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Information passed from V1 that receives information from Magnacellular division of LGN and projects to V1->V3->medial temporal cortex
-This stream is for motion/spatial processing (cells respond to changes in motion and direction) |
