Functionally what is the purpose of rods and what is the purpose of cones Cones packed in fovea few cones as move away from fovea Cones for detailed vision acuity bright light More rods than cones 90 mil vs 4 5 million Rods for dim light Agh So fovea is blind in dim light Rods and cones are unequally distributed within retina Cones bright light color vision fovea each retina has about 4 6 million Rods dim light peripheral retina each retina has 90 120 million In what way are rods and cones unequally distributed in the retina Fovea location of sharpest focus only cones clearer light path In what way are cones unequal in number in distribution S for short wavelength blue Only 5 10 of cones are S cones No S cones in center of fovea dichromatic More L cones than M cones maybe by a factor of 2 varies b t as opposed to trichromatic individuals What are photopigments Where are they made where are they stored What makes different ones sensitive do different wavelengths of light aka photopigment made in inner segment stored in Visual pigments outer segment Metabotropic receptor 2 parts retinal and opsin Retinal A chemical chromophore that absorbs light photon releases energy activates G protein the wavelengths it responds to a membrane protein whose structure determines Opsin Exact structure of opsin molecule determines maximal sensitivity to wavelengths of light Long wavelength red light Medium wavelength green light Rods rhodopsin Short wavelength blue light What is going on in the dark what is the dark current Slide 6 ITS ALL BACKWARDS In the dark so no light stimulus the Na channel is open Sodium is coming in on the dark current There is also an outward K current That Na keeps a voltage gated Ca open so calcium is in the terminal Ca allows the neurotransmitter glutamate to be released Glutamate binds to metabotropic glutamate receptors on the bipolar cells and inhibits them the 2nd messenger closes non specific cation channels So in the dark no message is sent to the retinal ganglion cells What are the steps in phototransduction Slide 7 Photon absorbed by rhodopsin The energy from the photon is transferred to the retinal which causes activation of a 2nd messenger cascade 2nd messenger closes sodium channel that is open in the dark photoreceptor is hyperpolarized Hyperpolarization closes voltage gated Ca2 channel at terminal less neurotransmitter glutamate released at synapse Bipolar cell is disinhibited sends signal to reintal ganglion cell AP in optic nerve fiber How does hyperpolarization of the photoreceptor lead to an action potential in the optic nerve ie what happens at each successive layer of nerve cells in the retina Slides 7 8 At synapse b t photoreceptor cell bipolar cell amount of glutamate is inversely proportional to number of photons being absorbed more light less glutamate So get graded potentials in bipolar cells Light reduced release NT But an inhibitory NT Disinhibited Activation of a photopigment by a photon leads to a sodium channel and then a calcium channel being closed in the photoreceptor cell In the dark in the absence of photons the photoreceptor was releasing an inhibitory neurotransmitter well it was releasing glutamate which was acting in an inhibitory fashion inhibiting the bipolar cell no info was going to the brain The photon therefore inhibits the photoreceptor cell leading to the disinhibition of bipolar cells Bipolar cell activates the retinal ganglion cell an AP is sent to the What is lateral inhibition what is it good for contrast and what cells brain accomplish it Lateral Inhibition regions of the retina In other words input from neighbors is inhibitory Achieved primarily by Horizontal Cells Antagonistic neural interaction b t adjacent Amacrine cells also part of lateral pathway Connect bipolar cells other amacrine cells and ganglion cells Exact function unclear Photoreceptors connected directly to bipolar cell Photoreceptor activated hyperpolarized disinhibition of bipolar cell bipolar is depolarized also Photoreceptors connected to each other via horizontal cell Center photoreceptors stimulated inhibition of peripheral photoreceptors via horizontal cell So if shine a light on center of receptive field center which gets bright light is activated and periphery which gets dimmer part of periphery of light is inhibited Contrast What are the different types of bipolar cells What are the different types of ganglion cells How are they connected to each other What types of photoreceptors do they get info from What part of LGN do they project to How is this all related to the sensitivity acuity tradeoff How is it related to the sie of visual receptive fields Bipolar cells Different types Different wiring Peripheral vision Diffuse bipolar cell One bipolar cell gets input from many 50 photoreceptors pools info sends along to ganglion cell All rods and peripheral cones Great for increasing sensitivity terrible for acuity Fovea Midget bipolar cell One bipolar cell gets info from 1 photoreceptor cone Great for acuity bad for sensitivity Pic on next slide Also see web activity 2 5 Acuity versus Sensitivity Ganglion cells 1 250 000 ganglion cells in each human retina 1 hundredth the of photoreceptors P ganglion cells Info from midget bipolar cells Info to parvocellular layer of LGN thalamus M ganglion cells Info from diffuse bipolar cells Info to magnocellular layer of LGN P ganglion cells have smaller dendritic trees than M ganglion cells Center surround receptive fields Review of ganglion cells on BB Left some cones converge on a few midget bipolars which converge on a couple of P ganglion cells Notice the small dendritic trees of the P ganglion cells Right ignore the 2 sets of numbers on each side just focus on the fact that many more cells are involved at each level of processing compared to the image on the left TONS of rods converge on a lot of diffuse bipolars which ignore the amacrine cells for our purposes converge on M ganglion cells Notice the very large dendritic trees of the bipolars and the M ganglion cells Imagine the much smaller receptive fields of the P ganglion cells compared to the M ganglion cells Acuity sensitive slide on BB Understand the center surround nature of ganglion cell receptive fields What would increase the firing of an on center off surround retinal ganglion cell What would decease it s firing What would cause no change in its firing rate The region on the retina in which visual stimuli influence the neuron s firing rate
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