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psya01 - vision

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Steve Joordens

PSYAO1 Notes: VISION Eye and its Functions Means of Protection - enclosed within bony slot - eyelid - cover from dust and dirt - eyebrow - inhibiting sweat from exposing open eye - reflex mchsms - upon obj. hitting face or surface of eye, they activate - cause eyelid to close automatically, head withdrawl Eye Anatomy - Cornea: eye tissue; transparent, covers eye's front side - "bulge" that allows in light - fixed shape - Sclera: outer eye layer; tough, "whiteness" of eye - rest of eye covered by this - Iris: muscle; shows a pigment phenotype, controls pupil size - double muscle band that controls amt of light allowed in - adjustment thro. dilation/constriction depending on light conditions in environmt. - dark (=dim): dilate (stretch) eye - bright: constrict (tighten) eye - Aqueous humour: fluid that fills region right behind cornea - tissue behind cornea constantly prod. this - removes the fluid from blood & gets it to this eye region - provides nutrients for cornea and other frontal parts of eye - circulation and renewal key for using this fluid effectively (ex. cornea req. it for it to remain transparent) Diagnosed with Glaucoma! Why? Eye disorder (vision dmged) by a) rapid prod. of aqueous humour b) clogging of passageway that returns aqueous humour to blood => exposed to too much pressure by high [] of aq. humour w/in eye. - Lens: transparent organ behind iris; facilitates img focus on retina - way in which lens and cornea curved makes imgs focus on inner surface at eye's back region (=retina) - incoming imgs reversed in both up/down and left/right, but brain able to make up for this and interpret info. properly. - right behind iris - comprised of no blood vessels b/c must remain transparent - dead tissue - Lens: - flexible - thus able to accomodate: special group of muscles able to alter its shape to make it compatible for eye to obtain imgs of those obj's either near or close Vision Problems - Normal conditions: - matching of eye length w/ bending of light rays created by cornea & lens - allows for incoming img to focus sharply on retina - Problem? - immatching of eye length w/ bending of light rays - retina = out of focus -> Sightedness problems [eye status; artifical lens req. to refocus img] - NOTE: artifical lens (ex. eyeglasses, contact lenses) - farsightedness - too long - concave lens - nearsightedness - too short - convex lens - natural (coming from aging) - lens losing flexibility - harder to focus on imgs closeby - reading glasses + convex lenses OR bifocals + glasses - Retina - tissue; loc. at back internal eye surface, holds the photoreceptors (receptive cells) & other neuron groups that assoc. w/ them. - performs sensory fcns of eye - holds >130mil photoreceptors - Photoreceptor - visual receptor cell in retina (=rod/cone) - special neuron group doing transduction [light -> neural.E] - send info to neurons that fire their axons towards optic disc - Optic Disc - circle-shaped structure; back of eye - one end of eye from retina where axons depart from eye and join up w/ optic nerve - optic nerve sends info to brain Blind spot - apparent b/c no photoreceptors placed directly at optic disc's front end Early Views about Eye - Pre 17th century: lens are structure for sensing coming of light - Kepler: Brought forth evidence that it was retina, not lens that sensed light using photoreceptors - Schiener: lens was focusing device Retinal Layers - 3 main layers [1] ganglion cell layer (frontal) [2] bipolar cell layer (mid-region) [3] photoreceptor layer (back, at deepest part of retina) - Incoming light passes thro - [1] -- > [2] --> [3] - Transparent cells above photoreceptors Photoreceptor Response and Three-Cell Chain - respond to light and pass info via transmission, photoreceptor cell -> bipolar cell --> ganglion cell --> brain (via optic nerve) - \ bipolar cells: retinal neuron that picks up info from photoreceptors & passes it to ganglion cells - \ ganglion cells - retinal neuron that picks up info from bipolar cells > picked up info. goes to optic nerves, that goto brain Further Investigation of Retinal Cells - one photoreceptor = responds to only light in immed. surroundings - one ganglion cell = picks up info from various, numerous photoreceptors - linker neurons - neurons that connect side-by-side photoreceptors and ganglion cells together. - having neural circuits like this show evidence that retinal pre-processing occurs w/ info that came in Retina and its Photoreceptors 2 main types: rods - fcn primarily in dim light (dark conditions) > sensitive to light, insensitive to color variations > 125mil. rods in retina cones - fcn primarily in bright light (able to see clearly) > key for colour vision > most of them come as particular amts connected to ganglion cell fovea - ~1mm diameter, cone-only region at retina's back > (prev >), b/c of this, fovea key for prod. most detailed vision Going Farther from Fovea - less cones, more rods - about 100 rods assoc. w/ one ganglion cell - these ganglions sensitive to low light lvl conditions - visual info SHARPNESS provided by rods lacking b/c connected to 1 gangalion from diff. retinal regions Transduction of Light by Photoreceptors - chemical-wise, light sensory organs same - vitamin-A derived molec.: > primary ingredient for transduction processes (converting light.E -> neural impulse) > no light,then attaches to protein, forming => photopigment Photopigments(aka. Vampires) - photopigment - sophisticated molec. w/in photoreceptors that when hit by light, splits & stimulates its photoreceptors' memb that it is in. > 4 kinds: rods (1), cones (3), though basic mechanics same > 1 photoreceptor = 1000s of photopigments > "bleached"ness prop. to light intensity Photopigment hit by Photon! [1] > splits into unit molec's, which triggers chem. r'xn series that ultimately cause photoreceptor to stimulate and make it send msg to bipolar cell which it synapses w/ [2] > bipolar --> ganglion --> brain (info. flow) [3] > upon splitting, photopigment loses color (ie. bleached) [4]> E derived from metabolic pathways used to fuel photoreceptor recomb. r'xn
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