Chapter 50 .pptx

UNIT 10 Chapter 50: The Eye: I. Optics of Vision Slides by Thomas H. Adair, PhD Copyright © 2021 by Saunders, an imprint of Elsevier Inc. Physiological Anatomy of the Eye Suspensory fibers that connect ciliary muscle with lens White outer layer. Continuous with cornea at front. Transparent, jel...

UNIT 10 Chapter 50: The Eye: I. Optics of Vision Slides by Thomas H. Adair, PhD Copyright © 2021 by Saunders, an imprint of Elsevier Inc. Physiological Anatomy of the Eye Suspensory fibers that connect ciliary muscle with lens White outer layer. Continuous with cornea at front. Transparent, jellylike tissue. Contains photosensitive cells. 2/3 of refractive power of eye. Resculptured in LASEKS/LASIKS surgery Visual acuity is highest. Cones only. Optic disc Aka, Optic nerve head or blind spot. Exit point for axons. Entry point for retinal blood vessels. Nasal to fovea. Free flowing, watery fluid. Fills anterior and posterior chambers Needed for accommodatio n. Vascular layer between retina and sclera. Feeds outer layers of retina, ciliary body, and iris. CN II. Contains retinal ganglion cell axons and glial cells. About 1.5 million axons. Refractive Index Speed of light in air 300,000 km/sec. Light speed decreases when it passes through a transparent substance. The refractive index is the ratio of speed in air to speed in the substance. For example, if speed in a substance = 200,000 km/sec, R.I. = 300,000/200,000 = 1.5. Light rays bend when passing through an angulated interface with a different refractive index. The degree of refraction increases as the difference in R.I. increases and the degree of angulation increases. Structures of the eye have different R.I. and cause light rays to bend. These light rays are eventually focused on retina. Polycarbonate RI: 1.58 Figure 50-1 Refractive Principles of a Lens Convex lens focuses light rays Concave lens diverges light rays. TMP14, Figure 50-2 TMP14, Figure 50-3 Note that a point source of light has a longer focal length compared to light from a distant source; this is why an object comes into focus as it moves closer to the eye in a person with myopia (nearsightedness, long eyeball). Refractive Power of the eye - Diopter • • 2/3 of refractive power of eye resides in anterior surface of cornea. This refraction is virtually eliminated when swimming under water since water has refractive index close to that of cornea; hence, you get a blurry image underwater. Lens has less refractive power, but it’s adjustable. – a diopter is a measure of the power of a lens. – 1 diopter is the ability to focus parallel light rays at 1 meter. – the retina is about 17 mm behind refractive center of eye. – hence, the eye has a total refractive power of 59 diopters (1000/17). 1000/17 = 59 diopters 17 mm TMP14, Figure 50-8 TMP14, Figure 50-9 Accommodati on Refractive power of lens is 20 diopters. Refractive power can be increased to 34 diopters by making lens thicker; this is called accommodation. Accommodation is necessary to focus image on retina. An untethered lens is almost spherical in shape. Lens is held in place by suspensory ligaments (zonule fibers) which under normal resting conditions causes the lens to be almost flat. Contraction of ciliary muscle decreases tension in the suspensory ligaments, allowing the lens to become more spherical (thicker); this increases the refractive power of lens. Under control of parasympathetic nervous system. Presbyopia: Also called age-related farsightedness. It’s the inability to accommodate. The lens gets harder and less flexible with age because of decreased levels of Power of accommodation α-crystallin. decreases with age: Child, 14 diopters (34-20) 50 years old, 2 diopters 70 years old, 0 diopters TMP14, Figure 50-8 TMP14, Figure 50-10 Errors of Refraction Normal vision corrected with convex lens Far sightedness Near sightedness Guyton, Figure 50-12 Astigmatism: unequal focusing of light rays due to an oblong shape of the cornea. corrected with concave lens Hyperopia and Myopia ciliary muscle relaxed “farsightedness” Ciliary muscle relaxed Ciliary muscle contracted HYPEROPIA (farsightness) caused by a short eyeball or sometimes a weak lens. contraction of ciliary muscle increases strength of lens (i.e., reduces focal distance). • So, a farsighted person can focus distant objects on retina because of accommodation. • If there is sufficient accommodation left, a farsighted person can also focus close objects on retina. MYOPIA (nearsightness) • caused by a long eyeball or sometimes too much refractive power in lens system. Genetic and ciliary muscle relaxed environmental factors contribute to myopia – too much close work can promote myopia. No mechanism to focus distant objects on retina (contraction of ciliary muscle would make distant “nearsightedness” objects even more out of focus). • Objects come into focus as they move closer to As an object moves toward the eye. eye, the rate of parasympathetic stimulation increases, causing the ciliary muscle to contract. What happens to the lens? Cataracts leading cause of blindness worldwide • Cataracts – cloudy or opaque area of the lens caused by coagulation of lens proteins – Accounts for about half the cases of blindness in the world. – UV solar radiation is major factor in production of cataracts Surgical implantation of plastic lens can usually restore vision. ~6 million per year. Visual Acuity • 20/20 – ability to see letters of a given size at 20 feet (normal vision) • 20/40 – what a normal person can see at 40 feet, this person must be at 20 feet to see. • 20/200 – what a normal person can see at 200 feet, this person must be at 20 feet to see. • 20/15 – Means what? Ans. can see at 20 feet what a person with 20/20 vision could only see at 15 feet Snellen chart Fluid System of the Eye Intraocular fluid keeps eyeball round and distended. 2 fluid chambers. aqueous humor, in front of lens. (freely flowing fluid). vitreous humor, behind lens (gelatinous mass with little fluid flow). Produced by ciliary body at rate of 2-3 microliters/min. (~3-4 mL/day) Flows through pupil into anterior chamber; then between cornea and iris, through meshwork of trabeculae to enter the canal of schlemm which empties into extraocular veins . TMP14, Figure 50-19 Intraocular Pressure Normally 15 mm Hg (range: 2-20 mm Hg). Level of pressure is determined by resistance to outflow of aqueous humor in canal of Schlemm. Rate of production of aqueous humor is constant under normal conditions (can be increased in systemic hypertension). Long-term hypertension is a risk factor for glaucoma. Increased pressure can cause blindness due to compression of axons of optic nerve as well as blood vessels. Hall, Figure 5021 Glaucoma 2nd leading cause of blindness worldwide (after cataracts) • Usually caused by high intraocular pressure (IOP). Increased IOP compresses blood vessels and axons of optic nerve at optic disc; this leads to poor nutrition of nerve fibers. Two main types of glaucoma • Open angle and closed angle (angle refers to area between iris and cornea). Open angle glaucoma (also called Chronic glaucoma) • 90% of cases in U.S. • Insidious – no pain initially • reduced flow through trabecular meshwork (tissue debris, WBC, deposition of fibrous material, etc). Types of Glaucoma Eye Drops Closed angle glaucoma • 10% of cases in U.S. – sudden closure of iridocorneal angle with sudden ocular pain. A medical emergency. • Treatment: Laser peripheral iridotomy (LPI), where an opening in the iris is made using a Prostaglandin analogs - increase outflow of fluid from eye. Beta blockers – decrease production of intraocular fluid. Alpha agonists - decrease fluid production and increase drainage. Carbonic anhydrase inhibitors (CAIs) – decrease