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week 8notes

by: Emma Notetaker

week 8notes NSCI 3320

Emma Notetaker
GPA 3.975

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week 8 lecture notes
Systems Neuroscience
Laura Schrader
Class Notes
25 ?




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This 9 page Class Notes was uploaded by Emma Notetaker on Friday March 4, 2016. The Class Notes belongs to NSCI 3320 at Tulane University taught by Laura Schrader in Spring 2016. Since its upload, it has received 36 views. For similar materials see Systems Neuroscience in Nutrition and Food Sciences at Tulane University.

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Date Created: 03/04/16
Monday, February 29, 2016 Week 8 Visual System - Eye and Retina • primary visual cortex: striate cortex (projections from LGN) • lots of visual information processed in extrastriate areas • dorsal and ventral streams of visual processing (from extrastriate areas) where sensory information comes together • • properties of light • travels in waves • wavelength (determines frequency) • shorter wavelength - higher frequency (blue light) • longer wavelength - lower frequency (red light) amplitude • • light rays are refracted (bent) as they pass through a surface • bends at perpendicular angle to surface that it is refracted by • convex lens: converging or positive (shortens focal length) • brings a series of rays to CONVERGE a single focal point • distance from lens to this point is focal length concave lens: lengthen focal length • • scatters light • theory used in glasses/contact lenses • diopter: 1/focal length (reciprocal of focal length) • anatomy of the eye • cornea: encases anterior chamber filled with aqueous humor concave - lots of refractive power (42 diopters - enough to get to the retina which is • typically 2.4 cm behind it) • nourished by aqueous humor and tear film • continually replenished • canal of Schlemm: behind cornea • holds vitreous humor allows aqueous humor to flow through chamber • • if blocked - fluid pressure buildup in anterior chamber—> glaucoma • lens : changes shape to bend light differently • does not have very much refractive power • zonulas connect to ciliary muscles • pupillary reflex allows lens to flatten or round • • ciliary muscles • contraction causes zonulas to flatten • sclera: white part • encases vitreous chamber • retina: sensory epithelium contains photoreceptors • • fovea: central part of retina • surrounded by macula • degradation here in macular degeneration - loss of central vision • optic disk: where retinal ganglion cell axons exit the eye 1 Monday, February 29, 2016 • arteries and veins important for blood supply for the eye enter and exit here • blind spot - no photoreceptors light enters the eye and focused on lens by the retina • • refracted 2x - through cornea first and lens second • in order to see object, focal point must lie on the retina (light hits retina) • hits retina when lens is flattened (emmitropic) • photoreceptors on retina transduce light into electrical signal • accommodation: changing the shape of the lens in response to distance of the object • • zonulas: pull on lens (between ciliary muscles and lens) • acts in opposition to ciliary muscles • tightened zonulas and flattened lens for distance (ciliary muscles relaxed) • larger pupil • slackened zonulas and round lens for close (ciliary muscles contracted) smaller pupil • • constriction of pupils increases depth of focus (like changing aperture on camera) • emmetropia: lens flat and light rays focused on retina • pathway: important for focusing on objects as they change position • axons from retina a primary visual cortex to superior colliculus (in tectum of midbrain) • axons from superior colliculus to Edinger-Westphal nucleus (cranial nerve parasympathetic nucleus) and oculomotor nuclei (sending somatic information to for medial rectus contraction) • projection to ipsilateral oculomotor to ciliary and pupillary constrictor muscles • constriction of pupil and increases curvature of lens • ipsilateral oculomotor to medial rectus muscle (nasal side of each eye) • contraction of medial rectus results in convergence of the eyes to object of interest • parasympathetic component from Edinger-Westphal nucleus in brainstem • just below tectum, deep within brainstem • send projections via oculomotor nerve to extra ocular muscles of eye (including medial rectus) • axons synapse in ciliary ganglia (to cause contractions in ciliary muscles of the eye) • oculomotor nuclei: • just below tectum • send projections via oculomotor nerve • send projections via oculomotor nerve to extra ocular muscles of eye (including medial rectus) • issues: • presbyopia: loss of accommodation (with age) • lens not at plastic, muscles don’t work as well • astigmatism: misshapen cornea • farsightedness: hyperopia • focal point behind the retina • to treat - need to shorten focal length • corrected with convex lens • nearsightedness: myopia • focal point in front of retina • to treat - need to lengthen focal length • corrected with concave lens to scatter light 2 Monday, February 29, 2016 • retinal processing: • photoreceptors are the only sensitive cells in the retina depolarize in dark • • hyperpolarized by light • ganglion cells are the only source of output from the retina • retina: • laminar organization (3 layers) • ganglion cells only cells that project out of the retina (out of optic disk through optic tract, • ultimately to LGN) • some retinal ganglion cells that are sensitive to light - not involved in the regular circuitry • send signals back to retina • MOST inner layer in the retina bipolar cells • • inner nuclear layer • photoreceptors • sense light • most external in the retina • also horizontal and amacrine cells (these modify the signals) light has to pass through all the cells before hitting photoreceptors (exception: fovea) • • fovea: photoreceptors receive light directly • ganglion and bipolar cells pushed off to the side - light hits photoreceptors IMMEDIATELY • highest visual acuity • high concentration of cones (sense color) • fewer rods (for low light conditions) • huge concentration in cones around fovea, increase in rods as you get closer to fovea but then sharp decrease in actual fovea • multiple neurons CONVERGE onto single postsynaptic cell (in peripheral retina) • 15-45 photoreceptors onto single bipolar neuron • exception: fovea - relationship is 1:1 (cones: retinal ganglion cells) • multiple bipolar cells synapse onto a single ganglion cell • photoreceptors: rods and cones • sensitive to light HYPERPOLARIZE in response to light • most external in the retina - outer layer (at the back) • embedded in pigmented epithelium (provides nutrients and processes molecules important for phototransduction) • glutamate is neurotransmitter • 4 regions • outer segment • membranous disk with photopigments that absorb light • inner segment • cell body • synaptic terminal • in darkness, tonically release glutamate onto the bipolar cells • visual pigments - opsins: • rods have one: rhodopsin • cones have 3 that are excited at different wavelengths 3 Monday, February 29, 2016 • red: 560 nm • blue: 430 nm green: 530 nm • • phototransduction: • retinoid cycle in epithelium • rod in darkness • inactive rhodopsin • cGMP levels high Na in (called dark current), K out • • causes depolarization —> -40 mV • tonic release of glutamate onto bipolar neuron • rod in light: rhodopsin bleaching • light activates retinal (agonist for rhodopsin) and opsin (bleached pigment) • separation of opsin and retinal due to light transducin activated (GPCR) —> causes decreased levels of cGMP (which gates Na) • • caused by phosphodiesterase breaking down cGMP • Na channels close, K stay open • no more tonic influx of sodium —> hyperpolarization • membrane potential: -70 mV • decreased release of glutamate termination: once stimulus is gone • • activated rhodopsin (retinal) is phosphorylated • arresting binds rhodopsin and blocks activation of transducin Retina to Cortex • retina important for seeing contrast (differences in luminance) • photoreceptors depolarized in dark - tonic release of glutamate onto bipolar • photoreceptors hyperpolarize in response to light • reduction in glutamate release bipolar cells respond to glutamate • • bipolar cells • receptive field: area of the retina that when stimulated affects bipolar cells • center and surround antagonistic: • on center bipolar cell • light in center: horizontal cell releases glutamate onto photoreceptor terminals in center • • photoreceptor hyperpolarized so releases glutamate onto ON bipolar cell • bipolar cell GPCR hyperpolarized —> removal of inhibition causes depolarization • light in CENTER is stimulus • if light in the surround, off center • • • indirect pathway: important when difference in luminance in surround • contributes to light adaptation • 2 types of bipolar cells • off: activation of glutamate receptor causes depolarization and AP 4 Monday, February 29, 2016 • EPSP in darkness • dark is stimulus on: hyperpolarize in response to glutamate (IN THE DARK) • • light removes inhibition, which causes depolarization • GPCR respond to glutamate by CLOSING cGMP-gated channels —> HYPERPOLARIZATION • depolarization results from removal of inhibition • light is stimulus horizontal cells mediate opposite effect of light on center vs. surround • • get info from surrounding photoreceptors and synapse onto center photoreceptors • horizontal cells release GABA onto photoreceptor terminals • center surround passed on to the ganglion cells • light in center • cone is hyperpolarized on center bipolar cell depolarized —> excited on center ganglion cell • • off center bipolar cell hyperpolarized • dark in center • cone is depolarized • on center bipolar cell hyper polarized • off center bipolar cell depolarized —> excited off center ganglion cell ganglion cells (only type in the retina that fire AP) • • m type - magno (large) • 5% - small population of ganglion cells • larger receptive fields • rapid AP conduction • are sensitive to low contrast stimuli (between light on retina) • NOT sensitive to color (no color processing) • p type - parvo (small) • 90% - most of ganglion cells • wavelength sensitive (COLOR) • medium and long wavelength (green and red) • nonM non P • wavelength sensitive (color) • short wavelengths (blue) • receptive fields: • sensitive to differences in illumination that occur within receptive field • concentric broad band cells (center surround antagonism) • respond best to differences in contrast within receptive field • off center cell: • light in center - hyperpolarizes • dark in center - depolarized, so fires AP • if dark in BOTH, still AP but reduced due to dark in the surround • moving dark stimulus: • dark in surround: hyperpolarization • as stimulus falls on center, increase in AP • as entire receptive field covered, reduction in AP • center surround organization of receptive fields leads to a response that emphasized contras at light dark edges • send axons out of eye via optic disk 5 Monday, February 29, 2016 • color vision • cones have 3 opsins: blue, green and red (short - 430, medium - , long wavelengths) cones sense color and report info to bipolar cells • • probability that photon will be absorbed and activate phototransduction cascade in cones - varies by wavelength • color opponency: wavelengths perceived in combination • red vs green (long vs. medium) • opsins for these wavelengths on the X chromosome reason for color blindness - in men these can be mutated and less likely to be • cancelled out (because not another X chromosome) • on center red • red in center increases AP • red throughout field decreases AP (because overlap between red and green wavelengths) - decreased because of red light surround cancelled ONLY by green • • blue vs yellow (short vs. white) • coextensive single-opponent ganglion cells (detect blue/yellow) • information from s cones • inputs from s cones oppose combined inputs of long and medium cones throughout receptive fields blue: short • • yellow: NOT short, combination of red and green (long/medium) • **red and green light combine to make yellow** • opsins for medium and • retinal ganglion cells project to optic nerve • nasal part of retina (closer to nose) • left sees left visual field • right sees right visual field • crosses in optic chiasm • temporal part of retina • left sees right visual field • right sees left visual field • stays ipsilateral • synapses in LGN of thalamus • LGN processes contralateral visual field info • projects to primary visual cortex (area 17, striate cortex, V1): • geniculocalcarine projections • OR optic radiations (aka Meyer’s loop) • 4 targets: • LGN: perception • hypothalamic suprachiasmatic nucleus: circadian rhythms • tectum: pupil and lens reflexes (accommodation) • oculomotor and Edinger-Westphal nuclei (contracts ciliary muscles) • superior colliculus: eye and head movement • superior colliculus: • important for directing eye movement • 2 functionally segregated layers • superficial: more sensory • input from retina ganglion cells and visual cortex (sensory info) 6 Monday, February 29, 2016 • map of stimuli • outputs to thalamus stratum griesum superficial (SGS) • • stratum opticum (SO) • deep: for eye-directed movements • excitatory input from multimodal sensory, cortical and basal ganglia inputs (multimodal and motor) • inhibitory input from substantial nigra map organizing orienting activity and stimulus selection • • outputs to the brainstem gaze centers • stratum griesum intermediale (SGI) • stimulate superior colliculus • A: slow pursuit of stimulus • cells in superior colliculus fire prior to eye movement B: quick saccade • • LGN • processes contralateral visual field • central portion of binocular vision important for depth perception • layered • ipsilateral eyes synapse in 2, 3 and 5 contralateral eyes synapse in 1, 4 and 6 • • 1 and 2 get info from magnocellular • 3-6 get info from parvocellular Visual Striate and Extrastriate • receptive fields: left and right hemifields • concentric center/surround in retina • concentric center/surround in LGN • bars: orientation and direction selectivity LGN • • processes contralateral visual field • central portion of binocular vision important for depth perception • layered • ipsilateral eyes synapse in 2, 3 and 5 • contralateral eyes synapse in 1, 4 and 6 1 and 2 get info from magnocellular • • 3-6 get info from parvocellular • koniocellular layers get info fro nonM non P cells • major source of excitatory inputs from primary visual cortex - function?? • also brainstem reticular system - involved in alertness and attentiveness • modulates LGN responses to visual stimuli retinotopy: neighboring cells in retina send projections to the neighboring places in their target • structures • often distorted because visual space not sampled uniformly by cells • central views magnified • image of light on retina activates many cortical neurons 7 Monday, February 29, 2016 • when retina stimulated by light, activity in striate cortex is broad distribution with peak at corresponding retinotopic location cytoarchitecture of striate cortex: 6 layers • • pathways from retina to striate • magnocellular: motion • object motion and guidance of motor actions • starts with m ganglion cells in retina • goes to magnocellular layers of LGN —> layer IVC alpha • • —> layer IVB • —> binocular simple and complex, orientation and direction • parvocellular (interblob pathway) • detects shape • small receptive fields analysis of small object shape • • starts with p type ganglion cells of retina • project to parvocellular layers of LGN • —>layer IVC beta • —> interblob layer II and III • NOT direction selective orientation selective (simple or complex) • • koniocellular (blob pathway) • analysis of color • monocular, lack orientation selectivity • starts with nonM non P type cells • go to koniocellular regions of LGN • —>directly to blobs of layer II and III • layer IV receptive fields • small monocular center-surround receptive fields • IVC alpha: color INSENSITIVE • IVC beta: color sensitive • monocular neurons here clumped in columns (alternating inputs from either eye) • ocular dominance columns • inject radioactive proline into one eye • transported down axon to LGN, ultimately to cortex • inputs from each eye end up in alternating columns • blobs: • cytochrome oxidase blobs - mitochondrial enzyme as marer for cell metabolism • in layers II, III, IV, and V • blobs in rows within ovular dominance columns • get direct inputs from koniocellular regions of LGN and layer IVC • area between called interblob • VI neurons outside layer IVC • properties: • binocularity • orientation selectivity • most cells in VI outside IVC (except blobs) • respond best to bars and are orientation selective • function: analysis of object shape 8 Monday, February 29, 2016 • orientation column • direction selectivity respond best when bar of light move perpendicular to optimal orientation in one • direction, but not the other • subset of orientation selective neurons • function: analysis of object motion • interblobs: simple and complex receptive fields • blobs: color sensitive and monocular • • lack above properties • circular receptive fields • color opponent center-surround • hypercolumn: 360 orientation in 1mm squared • 2 streams of cortical processing dorsal: motion and action control • • magnocellular • MT: middle temporal lobe • V5 receive input from V3, V2 and cells of layer IVB of striate • large receptive fields and direction selective - respond to motion • MST: linear otion • • radial motion • circular motion • navigation • directs eye movements and motion perception • ventral: perception of world and recognition of objects • parvocellular • interblob and blob • V4: inputs from blob and interblob regions • orientation and color • shape and color perception • IT: visual perception and memory • responds strongly to faces • V2 • strong projections from blobs and interblobs • anatomically organized and projections of dorsal and ventral stream defined • beyond striate cortex: • ventral stream (VI, V2, V3. V4, IT, others) • area V4: • achromatopsia: clinical syndrome in humans • caused by damage to area V4 • partial or complete loss of color vision • area IT: major output of area V4 • receptive fields respond to wide variety of colors and abstract shapes 9


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