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Chapter 9

Chapter 9

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Matthias Niemeier

Chapter 9: The Eye Introduction o About half of the human cerebral cortex is involved with analyzing the visual world o Retina: at the back of the eye; contains photoreceptors specialized to convert light energy into neural activity  Specialized to detect differences in intensity of light falling on different parts of it o Optic nerve -> lateral geniculate nucleus -> cerebral cortex Properties of Light Light o Electromagnetic radiation that is visible to our eyes o Wave with wavelength frequency (energy) and amplitude o Wavelengths 400-700nm  Mix of wavelengths appear white and light of single wavelength appears as colour Optics o In a vacuum it travels in a straight line as is a ray o Interaction with atoms, molecules and objects  Reflection: bouncing of light rays off a surface  Absorption: transfer of light energy to a particle or surface  Coloured objects reflect what colour they are and absorb the other colours  Refraction: the bending of light rays that can occur when they travel from one transparent material to another  How images are formed in the eye The Structure of the Eye o Eye: organ specialized for detection, localization and analysis of light Gross Anatomy of the Eye o Pupil: the opening that allows light to enter the eye and reach the retina  Dark due to light absorbing pigments o Iris: pigmentation that provides eye colour  Contains two muscles one that makes it smaller when it contracts and the other that makes it larger o Cornea: glassy transparent external surface of the eye o Sclera: the white of the eye that forms tough wall of eyeball o Eye orbit: bony eye socket that the eyeball sits in o Extraocular muscles: in the sclera; three of them that move the eyeball in the orbit o Conjunctive: a membrane that folds back from the inside of the eyelids and attaches to the sclera  Blocks the extraocular muscles from view o Optic nerve: carries axons from the retina, exits the back of the eye, passes through the orbit and reaches base of the brain Opthalmoscopic Appearance of the Eye o Ophthalmoscope: device that lets one look into the eye through the pupil to the retina o Optic disk: pale circular region where retinal vessels originate and where optic nerve fibres exit the retina  Cant perceive light here as there are no photoreceptors  Also no vision where blood vessels exit as the cast shadows on the retina o Macula: darker-coloured region with a yellowish hue at the middle of the retina  The part of the retina for central (not peripheral) vision  Absence of large blood vessels; improves quality of central vision  Blood vessels arc from optic disk to macula  Optic nerve fibres arc from macula to optic disk o Fovea: a dark spot approx 2mm in diameter  Retina is thinner here  Marks the center of the retina Cross-sectional anatomy of the eye o Aqueous humor: nourishes the cornea which doesnt have blood vessels o Lens: transparent  Suspended by ligaments  Ciliary muscles: attached to ligaments and sclera and form a ring inside the eye  Change in shape allows eyes to change their focus to different viewing distances  Divides the into two compartments  Aqueous humor: watery fluid that is between cornea and lens  Vitreous humor: between lens and retina  Its pressure keeps the eyeball spherical Image Formation by the Eye Refraction by the Cornea o As light passes into a medium where it speed is slowed it bends toward a line perpendicular to the border o Light rays that hit cornea bend so they converge on the back of the eye o Focal distance: the distance from the refractive surface to the point where parallel light rays converge  Depends on the curvature of the cornea; tighter curve shorter focal distance o refractive power in diopter: reciprocal of the focal distance in meters  Cornea has refractive power or 42 diopters; so parallel light rays striking surface will be focused 2.4cm behind it  Most prescription glasses have power of only a few diopters  Refractive power depends on the difference of mediums between air and eye which is why we cant see well under water Accommodation by the Lens o Lens contributes another 12 or so diopters to refractive power o Lens is more important in forming sharp images of objects <9m from eye o Accommodation: process of changing shape of lens to give additional focusing power to bring diverging rays into focus  Ciliary muscles contract and well making areas in muscle smaller and decreasing tension in suspensory ligaments  This makes lens rounder and thicker increasing curvature and therefore refractive power  Ability to do this changes with age The Pupillary Light Reflex o Pupil continuously adjusts for different light levels o Pupillary light reflex: connections between the retina and neurons in the brain stem that control the muscles that constrict the pupils  Consensual: causes adjustment of both eyes even if light is presented to only one eye o Constricting pupil increases depth of focus The Visual Field o Visual field: the area you can actually see o Image is inverted; LVF falls on right side and RVF falls on left side Visual Acuity o The ability of the eye to distinguish two nearby points o Depends on spacing of photoreceptors in retina and precision of eyes refraction o Visual angle: describes the distance across the retina  20/20 vision: when you can recognize a letter that has a crosses a visual angle of 0.083 Macroscopic Anatomy of the Retina o Photoreceptors -> bipolar cells -> ganglion cells -> optic nerve o Horizontal cells: receive input from the photoreceptors and projects laterally to influence surrounding bipolar cells and photoreceptors o Amacrine cells: receive input from bipolar cells and project laterally to influence ganglion cells, bipolar cells and other amacrine cells o Photoreceptors: the only light sensitive cells in the retina  All other cells are influenced by light only by direct and indirect synaptic interactions with photoreceptors  Ganglion cells are the only source of output from the retina The Laminar Organization of the Retina o Laminar organization: cells are organized in layers  Light passes from vitreous humor through ganglion cells and bipolar cells to reach photoreceptors  Cells above are relatively transparent so minimal distortion  Beneficial as pigmented epithelium under the photoreceptors is critical in maintaining photoreceptors and photopigments and in absorbing light that can blur image  Ganglion cell layer: contains cell bodies of ganglion cells  Inner plexiform layer: contains the synaptic contacts between bipolar cells amacrine cells and ganglion cells  Inner nuclear layer: contains the cell bodies of the bipolar cells and horizontal and amacrine cells  Outer plexiform layer: where photoreceptors make synaptic contact with the bipolar and horizontal cells  Outer nuclear layer: contains the cell bodies of the photoreceptors  Layer of photoreceptor outer segments: contains light sensitive elements of the retina  Outer segments are embedded in pigmented epithelium Photoreceptor Structure o Four regions  Outer segment: contains stack of membranous disks in which light sensitive photopigments absorb light that triggers changes in photoreceptor membrane potential  Inner segment  Cell body  Synaptic terminal o Two types  Duplex: retina as have a scotopic retina using only rods and photopic retina using mainly cones  Rod photoreceptors: have a long cylindrical outer segment with many disks  More than 1000x more sensitive to light  In dark only these contribute to vision  All have the same photopigment  20:1 rods:cones  Cone photoreceptors: have shorter tapering outer segment with fewer disks  Do most of the work in day light  Are three types of pigments that make them sensitive to different wavelengths (colour) Regional Differences in Retinal Structure o Peripheral retina  More sensitive to light as it has a higher ratio of rods to cones and has higher ratio of photoreceptors to ganglion cells  Poor at resolving fine details in daylight  Good visual acuity requires low ratio of photoreceptors to ganglion and more cones than rods o Fovea: most highly specialized for high re
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