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27 Cards in this Set
- Front
- Back
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VISION
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Perhaps the most important of all the senses. It allows us to know more about our environment than the other four senses, & is extremely important for our survival. More than half of all the sensory receptors in the body are found in the eyes. A large part of the cerebral cortex is involved in the processing visual information
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Accessory structures of the eye
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a)Eyebrows/eyelashes-Help to sheild the eye from perspiration, foreign particles & direct rays of the sun b)Lacrimal apparatus-A group of structures that produce & drain lacrimal fluid(tears). Which protects, moistens, lubricates & cleans the eye c)Eyelids (palpebrae)-Both upper and lower shade the eye during sleep, protect the eye from damage arising from intense light & foreign particles, & spread lubricating secretions d)Extrinsic eye muscles-6 muscles move the eye, collectively known as extrinsic eye muscles, help the eye to move medially, laterally, superiorly & inferiorly. e) Conjuctiva-A thin protective mucous membrane lines the inside of the eyelids and covers the whites of the eyes(sclera)
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Fibrous tunic
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Is the outermost or superficial coat of the eyeball and consists of the CORNEA and SCLERA
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Vascular tunic
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Is the middle layer of the eyeball and is made up of the CHOROID,CILIARY BODY(ciliary processes and ciliary muscle)and IRIS(pupillary sphincter/circular muscles and pupillary dilator/radial muscles
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Retina
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The inner coat of the eyeball and line the posterior three-quarters of the interior of the eyeball.Its the beginning of the visual pathway. The retina consists of a Pigmented layer and a Neural layer
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Eyeball cavities
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A number of fluid filled cavities are found in the interior of the eyeball including the Anterior cavity and the Vitreous chamber
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Cornea
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Is a transparent, avascular caot that covers the iris. It is curved and helps to focus light on the retina
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Sclera
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This is the white part of the eye, & consists of dense connective tissue. It covers the entire eyeball, except the cornes & its ridgity gives shape to the eyeball & offers protection to the structures inside they eyeball
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Choroid
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This is the inside lining of the posterior sclera. It is highly vascularised, and supplies nutrients to the posterior surface of the retina. Mealnocytes produce melanin, which gives the choroid a black appearance, & absorbs light. This stops the scattering of light within the retina, so the image will remain sharp and clear. The black appearance of the choroid is what also gives the pupil its black appearance
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Ciliary body
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Ciliary body is formed from the choroid at the anterior portion of the vascular tunic. It consists of Ciliary processes-vascularised folds on its surface & Ciliary muscle-a circular band of smooth muscle that alters the shape of the lens. Zonular fibres are suspensory ligaments that extend from ciliary processes and attach the ciliary body to the lens
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Iris
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Is the coloured portion of the eye that is responsible for eye colour. The colour is determined by the pigment melanin. It regulates the amount of light entering the eyeball by determining the size of the pupil. It contains two sets of smooth muscles: pupillary sphincter/circular muscles-in bright light these contract, decreasing the size of the pupil. Pupillary dilator/radial muscles-in dim light these contract, increasing the size of the pupil
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Retina
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The retina lines the inside of the posterior three-quarters of the eyeball. The retina is the beginning of the visual pathway, as it is the location where the light is converted into electrical impulses that can travel to, & be interpreted by the brain. The area of retina where the optic nerve exists is called the optic disc. The retina consists of a-pigmented layer and a -neural layer. Other areas of the retina are the photoreceptors(rods and cones), macula lutea, and the central fovea
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RETINA-pigmented layer
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Consists of a sheet of melanin-containing epithelial cells. Its found between the choroid & the neural layer of the retina
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RETINA-neural layer
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Is a multilayers outgrowth of the brain, which receives & processes visual information, before sending nerve impulses via neurons tha form the optic nerve
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Photoreceptors of the retina
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There are 3 distinct layers of neurons in the retina, between these layers are two zones, which is where the neurons form synapses with each other. One of the layers consists of specialised receptor cells called \b photoreceptors\b0 , which begin the process of converting light into nerve impulses. There are two kind of photoreceptors they are-Rods and cones
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Macula lutea and the central fovea
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A region of the retina called the macula lutea sits in the exact centre of the back of the retina.In its centre is a small depression called the central fovea (or fovea centralis), which contains only cones(densest area). The CF is also the area where our vision is the sharpect, & the reason we move our eyes when looking at an object, or reading, is to focus the image at this point
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RODS
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Rods allow us to see in DIM LIGHT and are responsible for BLACK AND WHITE vision.They are more neumerous towards the periphery of the retina, and reduce in number as they get closer to the central fovea. This explains why we can see faint distant objects like the stars more easily when you look slightly to one side of an object rather than directly at it
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CONES
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Allow us to see in BRIGHT LIGHT and are responsible for COLOUR vision. The central fovea contains only cones
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LENS
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The lens sits behind the pupil and the iris, in the cavity of the eyeball. It is avascular and consists of proteins. Usually the lens is completely transparent.Its function is to focus images onto the retina to ensure clear vision. This is done by changing the shape of the lens to focus the image clearly on the retina. The shape of the lens is controlled by the ciliary body of the vascular tunic. The contraction or relaxation of the ciliary muscle changes the tension on the suspensory ligament, which laters the shape of the lens
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Cavities of the internal eyeball
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Within the eyeball are two main cavities, which are separated by the lens: The anterior cavity which is filled with a fluid called the aqueous humour & is divided into two chambers: the anterior chamber-lies between the corneA and the iris the posterior chamber-lies behind the iris and in front of the zonular fibres and lens. The vitreous chamber is a large cavity that lies between the lens and the retina. It contains a transparent gel-like substance, called the vitreous body which holds the retina firmly in place against the choroid and helps maintain the shape of the eyeball
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Aqueous humour
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Aqueous humour helps maintain the shape of the eyeball, & supplies oxygen & nutrients to the lens and cornea. This fluid filters out of the capillaries of the ciliary processes, passes into the posterior chamber, & then flows into the anterior chamber through the pupil. In the anterior chamber aqueous humour drains back into the blood via the scleral venous sinus.In this was the aqueous humour is completely replaced every 90 minutes
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IMAGE FORMATION
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Image formation refers to the way that the eye processes light so that we can clearly see what we are lookng at. Light must come into complete focus on the retina for clear vision, & the eye is structurally adapted to ensure that this happens. In many ways the eye functions like a camera, as it focuses the image on a surface(retina) & controls the amount of light that enters(by constricting or dilating the pupil
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REFRACTION
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Image formation on the retina involves the refraction (bending) of light rays by the cornea and lens. This process focuses an upside-down image on the central fovea of the retina.
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ACCOMMODATION
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When viewing close objects the lens increases its curvature(this is called accommodation) and the pupil contricts to prevent light entering the eye through the periphery lens.The curvature of the lens is controlled by the ciliary muscle as follows-when viewing distant objects the ciliary muscle relaxes, which increase the tension of the zonular fibres on the lens, causing the lens to become flatter.-When viewing close objects the ciliary muscle contracts which reduces the tension of the zonular fibres on the lens causing the lens to become more convex(because it is elastic
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CONVERGENCE
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This refers to the way that the eyeballs move medially so that they are both directed towards the object at which we are looking. This action is coordinated by intrinsic eye muscles. Convergence allows binocular vision, which is responsible for depth perception & the viewing of objects as three dimensional. The nearer the object the greater the degree of convergence required to maintain binolcular vision
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PHYSIOLOGY OF VISION
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As the light hits the back of the retina, it stimulates the photoreceptors(rods&cones)These cells contain photopigments which change their structure when exposed to light. Difference between pigments will determine which wavelength (colour) of the light spectrum they respond to.Absence of certain pigments in cones=colour blindness.When stimulated by light a series of events causes a change in how these receptors normally communicate with other neural cells.Increased excitation of cells in contact with rods&cones leads to generation of nerve impulses to the brain.Cones regenerate their pigment very quick, but the regeneration of pigment is slower in rods.Reason why we adapt to bright light quick&dark light slowly.
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VISUAL ACRUITY
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Visual acruity refers to the keeness of vision. The general measure of acruity is 20/20. 20/20 vision means that the individual can see at 20 feet what an average person can see at 20 feet. Therefore a person with 20/200 vision can only see at 20 feet what an average individual could see at 200 feet
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