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Chapter 2

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University of Toronto Scarborough
Matthias Niemeier

Chapter 2- The First Steps in Vision: From Light to Neural Signals See companion website for practice quizzes and activities! There are 2 ways to conceptualize light: 1. Wave: An oscillation that travels through a medium by transferring energy from one particle or point to another without causing any permanent displacement of the medium 2. Photons: A quantum of visible light or other form of electromagnetic radiation demonstrating both particle and wave properties *Light is made of waves when it moves around the world, and made up of photons when it is absorbed *Visible wavelengths are between 400-700 nanometers *As a wavelength varies in the visible spectrum, the colour we observe changes from violet (400 nm) through the whole spectrum of the rainbow up to red (650 nm) Absorb: To take up light, noise, or energy and not transmit it at all Scatter: To disperse light in an irregular fashion Reflect: To redirect something the strikes a surface- especially light, sound or heat- usually back toward its point of origin Transmit: To convey something (eg light) from one place or thing to another Refract: (1) To alter the course of a wave of energy that passes into something from another medium, as water does to light entering it from the air. (2) To measure the degree of refraction in a lens or eye Transparent: Allowing light to pass through with no interruption, so that objects on the other side can be clearly seen Parts of the Eye Cornea: The transparent “window” into the eyeball (located at the very front of the eye) Aqueous Humor: The watery fluid in the anterior chamber of the eye that supplies the cornea with oxygen and nutrients. It also removes waste from the cornea and crystalline lens. (Located directly behind the cornea). Iris: The coloured part of the eye, consisting of a muscular diaphragm surrounding the pupil and regulating the light entering the yee by expanding and contracting the pupil (located directly behind the aqueous humor) Pupil: The dark, circular opening at the center of the iris in the eye, where light enters the eye. It is the hole that the iris controls to allow in more or less light, called the pupillary light reflex (located in the middle of the iris) Crystalline Lens: The lens inside the eye that enables the changing of focus (Controlled by the ciliary muscles that contract the zonules of zin, which makes the crystalline lens fatter or thinner). When the crystalline lens is fatter when looking at close objects, and is thinner when focusing on far away objects. (Located right behind the iris) Vitreous Humor: The transparent fluid that fills the vitreous chamber in the posterior part of the eye. Light is refracted for the 4 th and final time here. This space encompasses 80% of the space of the eye. (Located right behind the crystalline lens) Retina: A light sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve. (Located right behind the vitreous humor) *The sclera is the white of the eyes and is located behind the retina and continues around to the front of the eye Accommodation: The process by which the eye changes its focus (in which the lens gets fatter as gaze is to near objects) Presbyopia: Literally “old sight”. The loss of near vision because of insufficient accommodation - Happens to everyone - Happens because the lens becomes sclerotic (harder), and the capsule encircling the lens loses elasticity making it harder to change shape Cataract: An opacity of the crystalline lens. Cataracts block light from entering the eye * The cornea is the most powerful light refractor of the eye, the aqueous and vitreous humors also help refract light 3 optics of the human eye that are caused by length of the eye Emmetropia: The state in which there is no refractive error, because the refractive power of the eye is perfectly matched to the length of the eyeball - In order to see distant objects the refractive power of the 4 optical components of the eye must match perfectly to the length of the eyeball - See figure 2.3 a for picture - This is normal (meaning the eye is the proper length) Myopia: Nearsightedness, a common condition in which light entering the eye is focused in front of the retina and distant objects cannot be seen sharply - The eye is too long - Can be corrected with negative (minus) lenses - See figure 2.3 b Hyperopia: Farsightedness, a common condition in which light entering the eye is focused behind the retina and accommodation is required in order to see near objects clearly - Can be corrected through accommodation if not too severe. If severe can be corrected with positive (plus) lenses which converge the light before it arrives at the eye - See figure 2.3 d Astigmatism: A visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea. Can be corrected by lenses that have 2 focal points that can provide different amounts of focusing power in the horizontal and vertical planes The Retina - Process of seeing begins with the retina - This is where light energy is transduced into neural energy - Is a layered sheet of clear neurons, with another layer of darker cells called the pigment epithelium directly behind the last layer - Contains 5 major classes of neurons: photoreceptors, horizontal cells, bipolar cells, and amacrine cells, and ganglion cells Transduce: To convert from one form of energy to another (e.g. from light to neural electrical energy, or from mechanical movement to neural electrical energy) Fundus: The back layer of the retina- what the eye doctor sees through an ophthalmoscope. It is also the best place to view the tree of arteries on the retina Optic Disk: The point where the arteries and veins that feed the retina enter the eye, and where the axons of ganglion cells leave the eye via the optic nerve. t\his area contains no photoreceptors, and consequently it is blind, causing a small blind spot in human vision Photoreceptors: A light-sensitive receptor in the retina. This is where the transduction of light energy into neural energy first begins in the back layer of the retina - When photoreceptors sense light they can stimulate neurons in the intermediate layers, including bipolar cells, horizontal cells, and amacrine cells. These cells then connect to the front-most part of the retina made up of ganglion cells whose axons pass through the optic nerve to the brain Light Transduction by Rod and Cone Photoreceptors The human retina contains 2 types of photoreceptors: 1. Rods: A photoreceptor specialized for night vision 2. Cones: A photoreceptor specialized for daylight vision, fine visual acuity, and colour - Rods and cones have different shapes Duplex: In reference to the retina, consisting of 2 parts: the rods and cones, which operate under different conditions All photoreceptors consist of an outer segment, inner segment, and synaptic terminal: Outer Segment: The part of a photoreceptor that contains photopigment molecules. It is adjacent to the pigment epithelium Inner segment: The part of the photoreceptor that lies between the outer segment and the cell nucleus. Visual pigments are made in the inner segment and are stored in the outer segment where they are incorporated into the membrane - Visual pigment molecules consist of a protein (opsin) which determines which wavelength of light it absorbs, and a chromophore: The light catching part of the visual pigments of the retina Synaptic Terminal: The location where axons terminate at the synapse for transmission of information by the release of a chemical transmitter Each photoreceptor has only 1 of the 4 types of visual pigments found in the human retina: Rhodopsin: The visual pigment found only in rods - Each cone has one of the other 3 pigments, rods only contain rhodopsin Recent evidence suggests there may be a third photoreceptor located in the ganglion cells that is involved in our biological rhythms to match the day and night of our external world. They contain the photopigment melanopsin Melanopsin: A photopigment that is sensitive to ambient light Photoactivation: Activation by light - When a photon makes its way into the outer segment of a rod and is absorbed by a molecule of rhodopsin, it transfers its energy to the chromophore portion of the visual pigment molecule. This process is photoactivation. It initiates a biochemical cascade of events eventually resulting in
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