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23 Cards in this Set
- Front
- Back
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compre the function of rods and cones
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rods=dim light, 100million; cones= day time, color, 5 million
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How do cones and rods differ in terms of saturation
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cones do not saturate, rods do, at normal room light vision is based on cones, 5% of photoreceptors
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How do rods and cones differn in terms of integration time and spatial resolution
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rods=slow, poor resolution cones=fast, high resoultion because of 1:1 receptor:ganglion ratio
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what is the response of the photoreceptor in response to light
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hyperpolarize
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Describe the principle of univariance
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Photoreceptors cannot register the wavelength of the photons they catch, only the fact that they caught one. The output depends on quatum catch but not upon what quanta are caught
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Describe the excitatory cascade that occurs when light hits a photoreceptors
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Absorption of light activs the photopigment (cis=>trans), the active pigment stimulates a G protein (Transducin) which then activates cGMP phosphodiesterase. This catalyzes the breakdown of cGMP, the decrease causes the channels to close. Closed channels result in a fall of Na and the cell hyperpolarizes
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define the receptive field of a neuron of the visual system
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every neuron that is higher order than the photoreceptors has a receptive field that is the field of the photoreceptors that provide input to that neuron. The receptive field of a neuron is the retinal area of photoreceptors that, when stimulated, influences the activity of that neuron
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T/F photoreceptors produce action potentials
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false, photoreceptors do not produce action potentials but rather respond to light in a graded fashion with a graded release NT
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why are photoreceptors callled off center
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light that is centered on the photoreceptor turns it off, that is light hyperpolarizes photoreceptors and causes then to release LESS NT
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Describe how ON center bipolar cells respond to light stimulated photoreceptors
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On center/ metabotropic=sign reverings, the off-center character of the cone output is reverse in sign to on center. When light hits the cone it turns the cone off and the cone reelases less glutamate. Despite its usual excitatory role, glutamate from photoreceptors hyperpolarizes on center bipolar cells. Turning the cone off removes the inhibition from the on center bipolar cell and the on center bipolar cell turns on (depolarizes)
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Describe how OFF center bipolar cells respond to light stimulated photoreceptors
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OFF center/inotropic= sign converving. When light hits the photoreceptor, the photoreceptor hyperpolarizes and stops releasing glutamate. Glutamate excits off center biopolar cells so when the photoreceptor stops releasing glutamate, the off center bipolar cell hyperpolaizes and releases less NT iteself
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How does glutamate differentially affect On and OFF center bipolar cells.
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OFF center bipolar cells respond to glutamate in the normal way, they are excited by glutamate and depolarize in response. On center cells are opposite (they must be if they are going to sign reverse) and they are inhibited by glutamate (they hyperpolarize). So, when light strikes a photoreceptor and hyperpolarizes the it, the photoreceptor releases less glutamate. This removes the inhibition of the ON center cell so he ON center cell depolarizes. The decrease in glutamate release by the photoreceptor removes the excitatory simulus form the OFF center cell so the OFF center cell hyperpolarizes
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Describe the affect that a dark light would have on an on center bipolar cell
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Dark light would depolarize the photoreceptor and cause it to release more glutamate. Glutamate inhibits ON center bipolar cells. The ON center bipolar cell would hyperpolarize
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Describe the affect that a dark light would have on an off center bipolar cell
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Dark light would depolarize the photoreceptor and cause it to release more glutamate. Glutamate excits OFF center cells so the OFF center cell would depolarize
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Why do cones have two parallel channels but the rod system only has one
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The parallel system (on center/ off center) allow sthe cones to resond to simuli that are brighter than background (ON center) and darker than background (off center)
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Which bipolar cells are used to resond to simuli that are brighter than the background
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on center (because the light will hyperpolarize the cone causing it to release less glutamate removing the inhibition from the on center cell, depolarize the on center cell, signal to ganglion cell)
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which bipolar cells respond to stimuli that are darker than the background
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off center (because the light will depolarize the cone causing the cone to release glutamate. Glutamate excits off center cells causing the off center cell to depolarize and signal to the ganglion cell)
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what is the signifiance of melanospin ganglion cells
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photosensitive ganglion cells, may be linked to SAD, manipulate pathway with light/ drugs to treat
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What is the role of horizontal cells
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sets up an inhibitory relationship between cones and their neighboring cones. Whne a cone is excited (depolarized!!, in the DARK) it depolarizes its horizontal cells. The excited horizontal cells then inhibit neighboring cones (hyperpolarize).
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Imagine a right of light surronding a cone so that the center of the cone is in the dark but the cones around it are in the light. The central cone will turn on (depolarize) for two reasons. Why does the central cone turn on?
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1. The dark hitting the central cone depolarizes it 2. The light hitting the surrond cones hyperpolarizes them and turns them off. When the surronding cones are off they are not stimulating their horizontal cells so the horizontal cells can't inhibit the center cone so the center cone turns on
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Conceptually, it makes sense to refer to photoreceptors as "on surround" even though we call them off center. WHy is this true
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Light falling on the region around a photoreceptor turns the photoreceptor on. This is because the surronding light hyperpolarizes the surronding cones causing them to stop signaling to their horizontal cells. Turning off the horizontal cells releaese their inhibitory effect on the central cone and allows the central cone to depolarize. This action works synergenistically with the depolarizing action of the dark on the center cone.
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describe rod monochromacy
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rare hereditary condition due to absence of cone based vision, pts. have photophobia and poor acuity, fixation, nystagmus, VFD's and serious refractive errors, caused by mutations in CNG channel subunits or GNAT2 (transducing) so cones are unable to hyperpolarize in response to light
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Describe red-green dichromacy
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missing the function of one of the three cone types, protanopia and deuteranopia are sex linked, the disruptions at the L/M array include gene deletions and missense mutatiosn
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