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Chapter 2

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University of Toronto Scarborough
Matthias Niemeier

Chapter 2 – First Steps in Vision: Seeing Stars A Little Light Physics  Light is form of electromagnetic radiation – energy produced by vibrations of electrically charged material  2 ways to conceptualize light o Wave: oscillation that travels through medium by transferring energy from one particle or point to another without causing any permanent displacement of medium o Photon: quantum of visible light or other form of electromagnetic radiation demonstrating both particle and wave properties  Visible light waves have wavelengths between 400 and 700 nanometers o Colour we observe changes from violent at about 400 nm through rainbow up to red at 650 nm  In empty space, electromagnetic radiation from a star travels in straight line at speed of light (186 000 miles per second)  Once reaches atmosphere, some of starlight’s photons will be absorbed by encounters with dust, vaporized water, and so on; and some of light will be scattered by particles  Absorbed: to take up light, noise, or energy and not transmit it at all  Scatter: disperse light in irregular fashion  Reflected: to redirect something that strikes surface – especially light, sound or heat – usually back toward its point of origin  Transmit: to convey something from one place to another.  Refracted: 1. To alter course of wave of energy that passes into something from another medium, as water does to light entering it from air. 2. To measure degree to refraction in lens or eye Eyes That See Light  In order to see stars or anything else, need some type of physiological mechanism for sensing light  An eye can form an image of outside world, enabling animals that possess eyes to use light to recognize objects, not just to determine whether light is present and what direction it is coming from  Image: picture or likeness  Cornea: transparent “window” into eyeball. Firs tissue light from star will encounter  Transparent: allowing light to pass through no interruption so that objects on other side can be clearly seen  Cornea is transparent because it’s made of highly ordered arrangement of fibers and because it contains no blood vessels or blood  Cornea has high supply of transparent sensory nerve endings, which are there to force eyes to close and produce tears if cornea is scratched, preserving its transparency  External layers of cornea regenerate every 24 hours  Aqueous humor: watery fluid in anterior chamber of eye. Fluid derived from blood, fills space immediately behind cornea, supplying oxygen and nutrients to, and removing waste from cornea and crystalline lens  Crystalline lens: lens inside eye that enables changing focus  Pupil: dark circular opening at center of iris in eye, where light enters eye. Controls amount of light that reaches retina  Iris: colored part of eye, consisting of muscular diaphragm surrounding pupil and regulating light entering eye by expanding and contracting pupil  Francis Bacon showed that stepping to sun with eyes closed and didn’t sneeze. o Guessed that sun’s light makes eyes water and that moisture seeps into and irritates nose o Current thinking suggests that photic sneeze reflex is result of crossed wires in brain  After passing through lens, light enters vitreous chamber, where it will be refracted for 4 and final time by vitreous humor  Vitreous humor: transparent fluid that fills vitreous chamber in posterior part of eye o Longest part of journey through eyeball; this chamber comprises 80% of internal volume of eye  Retina: light sensitive membrane in back of eye that contains rods and cones, which receive an image from lens and send it to brain through optic nerve  Good deal of light becomes lost in eyeball, so only about half a starlight that arrives at the cornea actually reaches retina o Role of retina is to detect light and “tell brain abut aspects of light that are related to objects in world” Shining Starlight onto Retina  Because cornea is highly curved and has higher refractive index than air (1.376 vs. 1), it forms most powerful refractive surface in eye  Accommodation: process by which eye changes focus (in which lens gets fatter as gaze is directed toward nearer objects) o Accomplished through contraction of ciliary muscle o When ciliary muscle is relaxed, zonules are stretched and lens is relatively flat – eye focused on distant objects o To focus on closer things, ciliary must contract as it reduces tension on zonules and enables lens to bulge o Enables power of lens to vary by as much as 15 diopters  Ability to accommodate declines with age starting at 8 and lose 1 diopter of accommodation every 5 years up to age 30  By 40-50, find arms are too short because they can no longer easily accommodate 2.5 diopters  Presbyopia: literally “old sight”. Loss of near vision because of insufficient accommodation o Main reason to look forward to presbyopia is because lesn becomes sclerotic (harder) and capsule that ecircles lens (enabling I to change shape) loses its elasticity  Lens is normally transparent because crystallins are packed together very densely and therefore very regular  Cataracts: opacity of crystalline lens. Caused by irregularity of cyrstallins o Congenital cataracts are relatively rare; but if are dense, can have devastating effecs on normal visual development if not treated as early as possible  Most cataracts discovered by 50, and prevalence of cataracts increases with age so that by70 almost everyone has some loss of transparency  To focus on retinal, refractive power of 4 optical components of eye must be perfectly matched to length of eyeball  Emmetropia: condition in which there isn’t refractive error, because refractive power of eye Is perfectly matched to length of eyeball  Refractive errors occur when eyeball is too long or too short relative to power of optical components o If eyeball is too long for optics image of star will be focused in front of retina, and star will thus be see as blur rather than a spot of light o Myopia: condition in which light entering eye is focused in front of retina and distant objects can’t be seen sharply o Corrected with negative (minus) lenses, which diverge rays of starlight before they enter eye o If too short, image will be focused behind retina o Hyperopia: condition in which light entering eye is focused behind retina o Corrected by increasing power of eye and positive (plus) lenses, which converge rays of starlight before enter eye  Astigmatism: visual defect caused by unequal curving of one or more refractive surfaces of eye, usually cornea The Retina  Process of seeing begins with retina where light from star is traduced into neural energy that can be interpreted by brain  Transduced: referring to conversion from one form of energy to another  Opthalmoscope used to look at back surface of patients’ eyes  Fundus: back layer of retina – what eye doctor sees through an ophthalmoscope  White circle is known as optic disc where arteries and veins hat feed retina enter eye, and where axons of ganglion cells leave eye via optic nerve o Contains no photoreceptors, and consequently it’s blind o Only place in body where arteries and veins directly seen  Reason don’t normally notice this large blind spot in visual field is that visual system “fills it in” with info from surrounding area  Photomicrograph used to get good structure of retina  Transduction of light energy into neural energy begins in backmost layer of retina, which is made up of photoreceptors  Photoreceptors: light sensitive receptors in retina o When sense light, can stimulate neurons in intermediate layers, including bipolar cells, horizontal cells and amacrine cells  Arrangement requires that light pass through ganglion, bipolar, horizontal and amacrine cells before making contact with photoreceptors Retinal Info Processing Light Transduction by Rod and Cone Photoreceptors  Retina contains roughly 100 million photoreceptors o Neurons that capture light and initiate act of seeing by producing chemical signals  Human retina contains 2 types of photoreceptors o Rods: photoreceptors specialized for night vision o Cones: photoreceptors specialized for day light vision fine visual acuity, and color  Because human retinas have both rods and cones, considered to be duplex retinas  Duplex: consisting of 2 parts: rods and cones, which operate under different conditions o Rats and owls have mostly rods while lizards have cone  Both types of photoreceptors consist of o Outer segment: part of photoreceptor that contains photopigment molecules o Inner segment: part of photoreceptor that lies between outer segment and cell nucleus o Synaptic terminal: location where axons terminate synapse for transmission of info by release of chemical transmitter  Molecules (visual pigments) are made in inner segment (factory filled with mitochondria) and stored in outer segment, where they’re incorporated into membrane  Visual pigment molecules consist of protein, structure of which determines which wavelengths of light they absorb, and a chromophore o Chromophore: light catching part of visual pigments of retina  Each receptor has only 1 of 4 types of visual pigments found in human retina o Rhodopsin: visual pigment found in rods, concentrated mainly in stack of membranous discs in outer segment  Each cone has 1 of other 3 pigments – respond to long, medium and short wavelengths respectively rd  Evidence suggests that there may be 3 type of photoreceptor – “lives” among ganglion cells, that is involved in adjusting biological rhythms to match day and night of external world  When photon makes it way to outer segment of rod and absorbed by molecule, transfers energy to chromophore portion of visual pigment molecule o Photoactivation: activation by light o Initiates biochemical cascade of events eventually resulting in closing of channels in cell membrane that normally allow ions to flow into rod outer segment  Hyperpolarization: increase in membrane potential where inner membrane surface becomes more negative than outer membrane surface o Closes calcium channels at synaptic terminal, reduces concentration of neurotransmitter (glutamate) molecules at synaptic terminals, and change signals to bipolar cell that rod has captured a photon  Amount of glutamate present in photoreceptor bipolar cell synapse at any one time is inversely proportional to number of photons being absorbed by photoreceptor  Graded potentials: electrical potential that can vary continuously in amplitude  Humans have more rods (90 million) than cones (4-5 million) and 2 types of cells have very different geographic distributions on retina o Rods completely absent from center of fovea, and density increases to peak at 20 degrees, then declines o Cones are most concentrated in center of fovea and their density drops off dramatically with retina
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