GSU NEUR 3000 - NEUR 3000 - Chapter 9 (30 pages)

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NEUR 3000 - Chapter 9



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NEUR 3000 - Chapter 9

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Pages:
30
School:
Georgia State University
Course:
Neur 3000 - Hon Principles of Neuroscience
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THE EYE NEUR 3000 Dr Joseph J Normandin THE 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 spectrum GROSS ANATOMY OF THE EYE GROSS ANATOMY OF THE EYE GROSS ANATOMY OF THE EYE IMAGE FORMATION IMAGE 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 focus IMAGE 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 retina ANATOMY 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 types ANATOMY OF THE RETINA ANATOMY 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 light PHOTOTRANSDUCTION 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 hyperpolarization PHOTOTRANSDUCTION PHOTOTRANSDUCTION 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 PHOTOTRANSDUCTION PHOTOTRANSDUCTION 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 nm PHOTOTRANSDUCTION 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 types RETINAL 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 cells RETINAL 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 OFF RETINAL 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 ON RETINAL 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 potential RETINAL PROCESSING RETINAL PROCESSING Output from retinal ganglion cells Ganglion cells receive input from bipolar cells and amacrine cells Ganglion axons are the only output from the retina RETINAL 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 cells RETINAL PROCESSING As a light dark edge passes over a receptive field ganglion cells will have different responses The maximal responses reveal that there is a contrast between light dark on the receptive field but not how light or how dark RETINAL PROCESSING Ganglion cells are not only ON or OFF center cells but also characterized by size M type Magnocellular Large receptive field bursts of rapidly conducted APs Sensitive to low contrast stimulation P type parvocellular 90 of ganglion cells Sustained discharge of APs Sensitive to wavelength of light nonM nonP type Sensitive to wavelength of light RETINAL PROCESSING P and nonM nonP type ganglion cells are responsive to color Called color opponent cells Wavelength in center can be cancelled by another wavelength in surround Two types red vs green blue vs yellow How would you get this specificity of response to particular


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