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Lecture

Light.doc

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Department
Neuroscience
Course
NSCI 410
Professor
Greg Matlashewski
Semester
Fall

Description
Light ● light consists of radiant energy similar to radio waves – oscillates as it is transmitted from source ● wavelength – distance between adjacent waves of radiant energy; in vision, most clearly associated with perceptual dimension of hue ● wavelegnth for visible light ranges from 380-760 nanometres (380 – violet, 760 – red) ● entire range of wavelength is called electromagnetic spectrum and part our eyes can detect is visible spectrum The Eye and Its Function ● cornea – transparent tissue covering front of eye; admits light ● sclera – tough white membrane that is outer layer of eye ● iris – pigmented muscle of eye that controls size of pupil – contains two bands of muscle  brain controls these muscles and regulates size of pupil ● aqueous humour - space behin cornea is filled with (watery fluid)  constantly produced by tissue behin cornea that filters fluid from blood  nourishes cornea and other portions of eye in place of blood vessels – must circulate  if produced too quickly or if passage that returns it to blood becomes blocked, pressure within eye can increase and cause damage to vision (glaucoma)  no blood vessels – transparency of cornea ● lens – transparent organ situated behin iris of eye; helps focus an image on retina (inner surface of back of eye)  image is upside down and reversed from left to right  brain compensates for this alteration and interprets information appropriately  contains no blood vessels – functionally dead tissue  shape is flexible – special set of muscles can alter shape so eye can obtain images of nearby or distant objects  accommodation- change in shape of lens to adjust for distance ● length of eye normally matches bending of light rays produced by cornea and lens so that image of visual scene is sharply focused on retina  for some people length of eye doesn not match bedning of light rays so iamge is out of focus – need extra lens in front of eye  near sighted - eyes too long (front to back) so need concave lens to correct focus  far sighted – eyes too short so need convex lens  as people age, lens becomes less flexible and becomes difficult to focus on objects close to them – need convex lens ● retina – tissue at back of inside surface of eye that contains photoreceptors and associated neurons (performs sensory functions of eye)  embedded in retina are over 130 million photoreceptors  photoreceptors – receptive cell for vision in retina (rod or cone); specialized neurons transduce light into neural activity ■ information from photoreceptors is transmitted to neurons that send axons toward one point at back of eye (optic disc) ■ optic disc – circular structure located at exit point from retina of axons of ganglion cells that form optic nerve; all axons leave eye at this point and join optic nerve which travels to brain ■ no photoreceptors in form of optic disc so that portion of retina is blind  retina has three layers – light passes successively through ganglion cells (front), bipolar cells (middle) and photoreceptor layer (back) ■ cells located above photoreceptors are transparent ● Johannes Kepler – astronomer suggested that retina, not lens, contained receptive tissue of eye ● Christopher Scheiner – 1625 proved that lens is only focusing device ● photoreceptors respond to light and pass info to bipolar cells by means of transmitter substance  bipolar cells – neuron in retina that receives information form photoreceptors and passes it on to ganglion cells, from which axons proceed through optic nerves to brain ● bipolar cells transmit information to ganglion  ganglion cells – neuron in retina that receives informaiton from photoreceptors by means of bipolar cells and form which axons proceed to optiv nerves to brain ● photoreceptor – responds to light that reaches its immediate vicinity ● ganglion cells – responds to information from many different photoreceptors ● retina also contains neurons that interconnect both adjacent photoreceptors and adjacent ganglion cells – indicates that some kinds of information processing are performed in retina ● retina – contains 2 types of photoreceptors: 125 million rods and 6 millions cones ● rods – sensitive to light but cannot detect changes in hue; function in dim light ● cones – responsible for acute daytime vision and for colour perception; function when light is bright enough to see things clearly ● fovea – small pit near centre of retina containing densley packed cones; responsible for most acut and detailed vision; 1 mm in diameter  cones connected to one ganglion each  when we look at point in visual field, we move our eyes so that image of that point falls directly on cone-packed fovea ● rods – increase as move further from fovea; connected to many ganglion (up to 100 each)  ganglion receiving info from many rods is sensitive to very low levels of light  rods are responsible for sensitivity to dim light but visual information lacks sharpness Transduciton of Light by Photoreceptors ● molecule responsible for transduction is derived from vitamin A, in absence of light it is attached to protein and forms photopigment  photopigment – complex molecule found in photoreceptors; when struck by light, splits apart and stimulates membrane of photoreceptor in which it resides ● photoreceptors of human eye contain 4 kinds of pigment (1 for rods, 3 for cones) ● when photon strikes photopigment it splits apart into 2 molecules causing series of chemical reactions that stimulate photoreceptor and causes it to send message to bipolar cell and form synapse ● bipolar cell sends message to ganglion cell – sends message to brain ● intact pigment has characteristic colour (Rhodopsin, in rod, is pink) but are bleached once split apart ● Franz Boll – discovered this when he removed eye from animal, pointed it toward light and found image on retina was still there but faded where light had fallen – led investigators to suspect chemical reaction ● after photopigment is split, energy from photoreceptor's metabolism cause 2 moelcules to r
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