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GSU NEUR 3000 - NEUR 3000 - Chapter 9

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Slide 1The eye is the beginning of the visual systemGross anatomy of the eyeGross anatomy of the eyeGross anatomy of the eyeImage formationImage formationImage formationAnatomy of the retinaAnatomy of the retinaAnatomy of the retinaAnatomy of the retinaPhototransductionPhototransductionPhototransductionPhototransductionPhototransductionPhototransductionRetinal processingRetinal processingRetinal processingRetinal processingRetinal processingRetinal processingRetinal processingRetinal processingRetinal processingRetinal processingRetinal processingSlide 30THE EYENEUR 3000Dr. Joseph J. NormandinTHE EYE IS THE BEGINNING OF THE VISUAL SYSTEM•The eye allows us to focus electromagnetic energy and transduce this into an electrical signal•Humans can see a small portion of the entire spectrum of electromagnetic energy•The visual spectrumGROSS ANATOMY OF THE EYEGROSS ANATOMY OF THE EYEGROSS ANATOMY OF THE EYEIMAGE FORMATIONIMAGE FORMATION•The lens helps to focus light through a process called accommodation•The pupil assists in image formation be limiting or enhancing the amount of light entering the eye as well as adjusting depth of focusIMAGE FORMATION•We can only see what is in front of us, and what light our eyes can collect•The extent of our environment that we can see is the visual field•Each eye has its own visual field, a part of which overlaps with the other•The ability of the eye to distinguish two points is called visual acuity•Dependent on the density of visual receptor cells (photoreceptors) on the retinaANATOMY OF THE RETINA•Photoreceptors on the retina transduce light into neural activity•Photoreceptors synapse with bipolar cells which in turn synapse with ganglion cells whose axons form the optic nerve•Horizontal cells and amacrine cells communicate in between the other cell typesANATOMY OF THE RETINAANATOMY OF THE RETINA•There are 125 million photoreceptors on the back of the retina•Contain pigments that are sensitive to particular wavelengths of light•Photoreceptors are divided into two general types•Cone photoreceptors (cones)•Require substantial stimulation•Bright-light vision•3 types of pigments•Color•Rod photoreceptors (rods)•100x more sensitive than cones•Low-light vision•1 type of pigment•No color (or one?)ANATOMY OF THE RETINA•Distribution of rods and cones on the retina is non-homogenous•Peripheral vision has high sensitivity for low light•Central vision has high acuity in bright lightPHOTOTRANSDUCTION•Phototransduction in rods•When not transducing light there is a “dark current” in photoreceptors•Guanylyl cyclase constantly produces cGMP which opens sodium channels on the photoreceptor membrane •There is a constant Na+ conductance that results in a resting membrane potential of about -30 mV•At this membrane potential, glutamate is released into the synaptic cleft•The absence of light can be considered the “preferred stimulus”•Light reduces cGMP, and Na+ channels close•What is the effect on the membrane potential?•Light = hyperpolarizationPHOTOTRANSDUCTIONPHOTOTRANSDUCTION•Phototransduction in rods•Hyperpolarization is initiated when light interacts with the pigment rhodopsin•Rhodopsin is a rod-specific opsin protein responsive to 500 nm wavelength light•Opsin proteins contain a vitamin A-derived protein called retinal•Retinal absorbs light, causing a conformational change in the opsin•The conformation change activates a G-protein called transducin•Transducin activates phosphodiesterase (PDE) which breaks down cGMP•Consequence?•In bright light the cGMP levels drop to a level where no more cGMP can be de-activated•Consequence?PHOTOTRANSDUCTIONPHOTOTRANSDUCTION•Phototransduction in cones•Transduction mechanism is the same as rod except the opsins are different•Opsin in cones require more energy to activate•Active in bright light•The three types of opsins in cones are selective for different wavelengths of light•Blue: activated maximally by 430 nm light•Green: 530 nm•Red: 560 nmPHOTOTRANSDUCTION•Phototransduction in cones•The colors that we perceive are largely determined by relative contributions of the blue, green, red cones to retinal signals•Young-Helmholtz trichromacy theory•The brain assigns colors based on the comparison of readouts from the three cone typesRETINAL PROCESSING•Rods and cones release glutamate•When are they releasing glutamate?•The photoreceptors synapse with bipolar cells and horizontal cells•These cell types work together to process information sent to the ganglion cellsRETINAL PROCESSING•Retinal bipolar cell receptive fields•Bipolar cells can be categorized by their response to glutamate release from photoreceptors•OFF bipolar cells•Depolarize when “light is off”•No photons interacting with presynaptic photoreceptors•Photoreceptors are releasing glutamate•The bipolar cell has ionotropic glutamate receptors (selective for Na+) that depolarize the membrane•Depolarization in response to glutamate•Therefore, depolarized when light is OFFRETINAL PROCESSING•Retinal bipolar cell receptive fields•Bipolar cells can be categorized by their response to glutamate release from photoreceptors•ON bipolar cells•Depolarize when “light is on”•Photons interact with presynaptic photoreceptors•Photoreceptors are not releasing glutamate•The bipolar cell has metabotropic glutamate receptors that (somehow) hyperpolarize the membrane•Hyperpolarization in response to glutamate•Therefore, depolarized when light is ONRETINAL PROCESSING•Retinal bipolar cell receptive fields•Bipolar cells receive synapses from one (fovea) or many photoreceptors•Horizontal cells provide information from these and other photoreceptor cells to bipolar cells•The receptive field of the bipolar cells is an area of the retina, that when stimulated with light, changes the bipolar neurons membrane potentialRETINAL PROCESSINGRETINAL PROCESSING•Output from retinal ganglion cells•Ganglion cells receive input from bipolar cells and amacrine cells•Ganglion axons are the only output from the retinaRETINAL PROCESSING•Retinal ganglion cell receptive fields•Ganglion cells also have a center-surround receptive field that results from input from similarly-typed bipolar cells and interactions with amacrine cellsRETINAL PROCESSING•As a light-dark edge passes over a receptive field, ganglion cells will have


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