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Sight and Vision

Advanced Health Presentation for 3rd Quater
by

Kaitlynn Stone

on 20 February 2013

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Transcript of Sight and Vision

The Human Eye Definition: A part of the Nervous System; the body structure that receives and transforms information about objects into neural impulses that can be translated by the brain into visual images Development the eye begins to form when we are embryos, near the end of the third week of development the major structures of the eye are formed by the fifth month of development; these include the retina, lens, and eyeball coats for the remainder of the pregnancy the eye grows larger, more mature, and increasingly complex when a child is born, his or her eyes are about two thirds the size of an adult's eyes however, until after the first month of life, the eye normally lacks complete retinal development, especially in the area responsible for visual acuity this is what causes infants the inability to focus their eyes properly, which typically results in a vacant stare most of the subsequent eye growth occurs rapidly during the first year of life; eye growth then progressively slows until it stops developing sometime around puberty when the human eye is fully developed it weighs approximately 7.5 grams and measures approximately 24.5 millimeters in diameter Structure & Function to capture pictures from the environment and transform them into neural impulses that are processed by the brain into visual images All movement of the eyeball, or globe, is accomplished by six voluntary muscles; One voluntary muscle elevates the upper lid. the retina is what receives the light, to send the image to the brain. The retina, with its light sensitive cells, acts similarly to a camera. It "puts the picture on film" while neural processing in the brain "develops the film" and forms the image that in meaningful and informative for the individual. the lense focuses light onto the retina the ciliary body provides nourishment to the anterior structures of the globe the iris is the pigmented portion of the eye, and gives the eye it's color the choroid provides oxygen and nutrients to the retina through an extensive network of capillaries (branching blood vessels) the pupil is the small opening into the posterior parts of the eyeball; it lies in the center of the iris the anterior chamber regulates the amount of light entering the eye by adjusting the size of the pupil. the cornea is the clear front window of the eye, which transmits and focuses light into the eye the sclera (the white part) is the opaque outer layer of the eye; it is the part of our eye that moves, and gives the eye its shape the vitreous humor is the clear, gel-like substance that fills the body of the eye between the retina and the lens the optic nerve transmits electrical impulses from the retina to the brain LIGHT How it Works Crystalline Lens the crystalline lens is elastic in nature, which allows it to vary its thickness to change the focusing power of the eye The eye can refract (bend) light rays when light passes through the cornea and the crystalline lens to reach the retina. The curved surfaces of the eye will refract the rays to a greater or lesser degree depending on the steepness of the curve (the steeper the curve, the greater the refracting power). How much the light is refracted determines the diopter of power. Light rays emitted from a distant point of light enter the eye in a basically parallel pattern, and are bent to intersect perfectly at the retina, forming the image. Focusing If the point of light is near the eye, the rays that are emitted are divergent in pattern, and must be refracted to meet at a point on the retina. Because objects closer to the eye require more refracting this means that a steeper curved surface is needed. Accommodation the process by which the eye automatically adjusts the thickness of the crystalline lens to focus the image perfectly on the retina Photoreceptor Cells the eye focuses the light so that the photoreceptor cells, rods and cones, on the retina can absorb the light energy and transform it into electrical signals that are carried to the brain Cones: specialized for color or daylight vision; have greater visual acuity than rods Rods: specialized for black and white or nighttime vision The Fovea a pin-sized depression in the center of the retina contains only cone cells in very high concentration, this makes it the point on the retina of greatest visual acuity (the area with the most distinct vision) when the eye focuses on an object, the image falls on the retina in fovea Macula Lutea- contains a relatively high concentration of cones; less visual acuity Periphery- the area with the fewest number of cones and the least visual acuity the concentration of cones is greatest at the fovea and declines towards the periphery the concentration of rods is greatest in the peripheral areas of the retina the retina of each eye contains about 100 million rod cells, and about 300 million cone cells Optic Nerve carries impulses from the photo receptors to the brain the central location where this nerve is located is called the blind spot Individuals are typically not aware of this blind spot because the brain compensates for the missing information when some part of the peripheral image falls across this part of the retina. Refracting The eye refracts the light rays to
focus images correctly. Photoreceptors 3 main parts - an outer segment that detects light - an inner segment that provides energy for the cell - & a synaptic terminal that transmits the visual signal to the next nerve cell in the pathway leading to the brain *in rods this segment is shaped like rods; in cones it is shaped like cones; this is where the photoreceptors get their names* Photopigment Molecules in the outer segment of the photoreceptors these molecules undergo chemical changes when activated by light different photopigments absorb some light better than others the light that is absorbed best by a photopigment is called its absorption maximum only absorbed light produces the photochemical reaction that results in vision Disorders Glaucoma Astigmatism Farsightedness Cataracts Nearsightedness a disease that causes damage to the optic nerve Glaucoma is a condition of that causes increased pressure within the eyeball this results in gradual loss of sight. At age 50 about 2% of the US population has glaucoma, increasing to 4% by age 70 and 24% for those over age 80 Often times, a person with glaucoma is unaware that they have it, because there are no symptoms. This is one reason it is important to get a dilated eye exam every two years. With early diagnosis there are treatments that can often protect against irreversible damage to the optic nerve. If left undetected Glaucoma can lead to complete loss of vision. Astigmatism is the condition that results from the curvature of the cornea being irregular. Because of the irregular curvature the eye is unable to focus light rays correctly when it reaches the retina. This results in a blurred image at all distances. This condition is normally present at birth, but can form after a person has undergone eye surgery or has had an eye injury. Cataracts are cloudy areas that appear in the lens of the eye which is suppose to be clear Cataracts can develop in one or both eyes. If they develop in both eyes, one will be more severely affected than the other. A normal, clear lens allows light to pass through to the retina where the image is focused. If a part of the lens becomes opaque, light does not pass through easily and the image becomes blurry. A person's vision with Cataracts is similar to a person with perfect vision looking through a glass of cloudy water or through a fogged up window. The more opaque (or cloudy) the lens is, the worse that the patient's vision becomes because more of the light is distorted, People who are farsighted see things at a distance more easily than they see things up close. If you are farsighted, close objects may be so blurry that you can't do tasks such as reading or sewing without glasses. Farsightedness occurs when light entering the eye is focused behind the retina instead of directly on it. This is happens when an eye is too short, the cornea is not curved enough, or the lens sits farther back in the eye than normal. Many times, as we get older, our eye sight weakens, and we lose the ability to focus images properly. This results in farsightedness. Most often, farsightedness begins in childhood, but normal growth corrects the problem. If the child still experiences symptoms after the eye is fully grown, the process of accommodation usually makes up for it. Presbyopia This condition is called Presbyopia. As presbyopia gets worse, near and distance vision will become blurred, until you need glasses or contacts for both. People with nearsightedness have difficulty seeing distant objects, but can clearly see objects that are near. A person who is nearsighted may not be able to make out highway signs until they are just a few feet away. In nearsighted people, the eyeball is too long or the cornea has too much curvature, so the light entering the eye is not focused correctly. Images focus in front of the retina, rather than directly on the retina, causing blurred vision. Nearsightedness runs in families and usually appears in childhood. Usually the condition plateaus, but it can worsen with age. Glasses, contact lenses, or refractive surgery can correct nearsightedness. Nearsighted prescriptions are represented as a negative number (such as -3.00). The higher the number, the stronger the prescription is. The prescription of eye glasses and contact lenses helps the eye focus light on the retina, clearing up the vision. There are also refractive surgeries available that improve the eye's ability to refract the light onto the retina. This would reduce or eliminate a patient's dependency on glasses or contacts.
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