The Human Eye - Anatomy and Physiology 2 Review Sheet

Anatomy and Physiology 2 Review Sheet Kimberly Denise Vigee’ N.D. THE HUMAN EYE Size: approximately 1 inch deep, 1 inch wide, and 1 inch tall. Location: Orbital cavity of the face; bony structure protects the eyes Other structures that protect the eyes: - Fat surrounding the...

Anatomy and Physiology 2 Review Sheet Kimberly Denise Vigee’ N.D. THE HUMAN EYE Size: approximately 1 inch deep, 1 inch wide, and 1 inch tall. Location: Orbital cavity of the face; bony structure protects the eyes Other structures that protect the eyes: - Fat surrounding the eye = cushioning - Eyelids = blinking - Eyelashes and eyebrows = protect from particles - Tears secreted by the lacrimal glands : Flow of tears: Lacrimal gland Excretory lacrimal ducts Superior or Inferior lacrimal canal Lacrimal sac Nasolacrimal duct Nasal cavity This is why your nose runs when you cry! 3 Tunics (layers) of the Eye 1. Sclera – outermost layer - AKA “white part of the eye” - Maintains the shape of the eye - Anterior extension of the sclera is the cornea (transparent and avascular part) - Cornea - convex outermost surface is responsible for the bending of light during its path to the retina - Cornea Transplants - because of its avascularity the cornea can be readily transplanted from one person to another. - Without blood vessels, the transplanted cornea cannot be “attacked” by the immune system - The cornea is well innervated with pain fibers 2. Choroid- AKA “Uveal tract” – middle layer - Contains the blood vessels that supply blood to structures of the eye - Anterior part of the choroid contains two structures: 1. Ciliary body - muscular area that is attached to the lens. It contracts and relaxes to control the size of the lens for focusing. 2. Iris - colored part of the eye. The color of the iris is determined by the color of the (1) connective tissue and (2) pigment cells. 1 Anatomy and Physiology 2 Review Sheet Kimberly Denise Vigee’ N.D. Blue eyes = less pigment Brown eyes = a large amount of pigment The iris is an adjustable diaphragm around an opening called the pupil. Pupil size is controlled by 2 muscles: 1. Dilator muscle – makes the iris smaller therefore the pupil larger = more light into the eye 2. Sphincter - muscle makes the iris larger and the pupil smaller = less light into the eye 3. Retina – innermost nervous layer - Lined with black melanin pigment – just like the inside of your camera to lessen the reflect of light - Light-sensing (photoreceptive) portion of the eye = function of the sensory nervous system!!!! - Contains: 1. Rod cells – responsible for black and white vision in low dim light 2. Cone cells – responsible for color and detail in bright light 3. Macula – located in the back of the eye and the center of the retina 4. Fovea Centralis – area contains only cones and used for seeing fine detail and color - This area has the finest acuity 5. Rhodopsin – AKA “Visual Purple” - Rhodopsin is a mixture of 2 proteins: scotopsin and 11 cis retinal (derived from vitamin A) - Rhodopsin is a chemical that converts light into electrical impulses that the brain interprets as vision. - Optic nerve – a collection of the retinal fibers at the posterior part of the eye that connect and send impulses to the brain. - Optic disk – AKA “Blindspot of the retina” - The site of the where the optic nerve and blood vessels exit the retina so no cones and rods here === hence blind spot. - You do not know there is a blindspot because your eyes overcompensate for this When a doctor looks at the back of your eye through an ophthalmoscope, here's the view: 2 Anatomy and Physiology 2 Review Sheet Kimberly Denise Vigee’ N.D. Inside the Eyeball - 2 Fluid Filled Portions Separated by the Lens – Humors 1. Vitreous Humor - clear and gel-like - Located posteriorly 2. Aqueous Humor – clear and water-like - Located anteriorly - Aqueous Humor is produced by the ciliary body of the choroid layer of the eye - Aqueous Humor is drained through the canal of Schlemm. When this drainage is blocked, a disease called glaucoma can result. Lens - Clear and biconcave - The lens changes shape because it is attached to muscles in the ciliary body. The lens is used to fine-tune vision. Conjunctiva - The mucous membrane covering on the inside surface of the eyelids and sclera, which helps to keep the eye moist - Conjunctivitis = “pink eye” 6 Muscles Attached to the Sclera that Move the Eye Muscle Primary Function Medial rectus moves eye towards nose Lateral rectus moves eye away from nose 3 Anatomy and Physiology 2 Review Sheet Kimberly Denise Vigee’ N.D. Superior rectus raises eye Inferior rectus lowers eye Superior oblique rotates eye Inferior oblique rotates eye - These muscles are innervated by cranial nerves III, IV and VI 1. III- oculomotor nerve 2. IV- trochlear nerve 3. VI- abducens nerve Perceiving Light - Refer to your figure 20.4 - When light enters the eye, it first passes through the cornea, then the aqueous humor, lens and vitreous humor. Ultimately it reaches the retina, which is the light-sensing structure of the eye. The retina contains two types of cells, called rods and cones. - When light enters the eye, it comes in contact with the photosensitive chemical rhodopsin (also called visual purple). - When light comes in contact with rhodopsin it decomposes. It decomposes because light causes a physical change in the 11-cis-retinal portion of the rhodopsin, changing it to all-trans retinal. - The 11-cis-retinal is an angulated molecule, while all-trans retinal is a straight molecule. This makes the chemical unstable. - This chemical causes electrical impulses that are transmitted to the brain and interpreted as light. Color Vision - The color-responsive chemicals in the cones are called cone pigments and are very similar to the chemicals in the rods. 2 pigments are: 1. Retinal portion (same as in the rods) 2. Photopsins (scotopsin is replaced with photopsins) - There are 3 kinds of color-sensitive pigments: 1. Red-sensitive pigment 2. Green-sensitive pigment 3. Blue-sensitive pigment 4 Anatomy and Physiology 2 Review Sheet Kimberly Denise Vigee’ N.D. - Each cone cell has one of these pigments so that it is sensitive to that color. - The human eye can sense almost any gradation of color when red, green and blue are mixed. Color Blindness - Color blindness is the inability to differentiate between different colors. People with this problem are not completely unable to see red or green, but often confuse the two colors. The inability to see any color, or seeing only in different shades of gray, is very rare. - It occurs when either the red or green cones are not present or not functioning properly. - The most common type is red-green color blindness. - This occurs in 8 percent of males and 0.4 percent of females. - This is an inherited disorder and affects men more commonly since the capacity for color vision is located on the X chromosome. (Women have two X chromosomes, so the probability of inheriting at least one X with normal color vision is high; men have only one X chromosome to work with. Vitamin A Deficiency - When severe vitamin A deficiency is present, then night blindness occurs. - Vitamin A is necessary to form retinal, which is part of the rhodopsin molecule. When the levels of light-sensitive molecules are low due to vitamin A deficiency, there may not be enough light at night to permit vision. During daylight, there is enough light stimulation to produce vision despite low levels of retinal. Emmetropia AKA “Normal Vision” - Vision or visual acuity is tested by reading a Snellen eye chart at a distance of 20 feet. - By looking at lots of people, eye doctors have decided what a "normal" human being should be able to see when standing 20 feet away from an eye chart. - If you have 20/20 vision, it means that when you stand 20 feet away from the chart you can see what a "normal" human being can see. In other words, if you have 20/20 vision your vision is "normal" -- a majority of people in the population can see what you can see at 20 feet. - If you have 20/40 vision, it means that when you stand 20 feet away from the chart you can only see what a normal human can see when standing 40 feet from the chart. That is, if there is a "normal" person standing 40 feet away from the chart, and you are standing only 20 feet away from the chart, you and the normal person can see the same detail. - 20/100 means that when you stand 20 feet from the chart you can only see what a normal person standing 100 feet away can see. 20/200 is the cutoff for legal blindness in the United States. - You can also have vision that is better than the norm. A person with 20/10 vision can see at 20 feet what a normal person can see when standing 10 feet away from the chart. - Hawks, owls and other birds of prey have much more acute vision than humans. A hawk has a much smaller eye than a human being but has lots of sensors (cones) packed into that space. This gives a hawk vision that is eight times more acute than a 5 Anatomy and Physiology 2 Review Sheet Kimberly Denise Vigee’ N.D. human's. A hawk might have 20/2 vision! Refraction - When light rays reach an angulated surface of a different material, it causes the light rays to bend. This is called refraction. When light reaches a convex lens, the light rays bend toward the center: - When light rays reach a concave lens, the light rays bend away from the center: When everything is working correctly, light makes it through to the retina in perfect focus. Errors of Refraction - Normally, your eye can focus an image exactly on the retina: - Nearsightedness and farsightedness occur when the focusing is not perfect. - When nearsightedness (myopia) is present, a person is able to see near objects well and has difficulty seeing objects that are far away. : Light rays become focused in front of the retina. - This is caused by an eyeball that is too long, or a lens system that has too much power to focus. 6 Anatomy and Physiology 2 Review Sheet Kimberly Denise Vigee’ N.D. : Nearsightedness is corrected with a concave lens. This lens causes the light to diverge slightly before it reaches the eye, as seen here: - When farsightedness (hyperopia) is present, a person is able to see distant objects well and has difficulty seeing objects that are near. : Light rays become focused behind the retina. - This is caused by an eyeball that is too short, or by a lens system that has too little focusing power. : This is corrected with a convex lens, as seen here: CLINICAL APPLICATIONS 1. Cataracts 1. Certain conditions cause an increase in the opacity of the lens 2. When this occurs the transparency of the lens diminishes 3. Causes of cataracts: a. smoking b. overexposure to sunlight; especially UV light c. diabetes 4. Repair of cataracts - surgical removal of the lens and replacement with a prosthetic cataract implant. 2. Glaucoma 1. Common cause of blindness - in elderly person 2. It is generally caused by the failure of the aqueous humor- to properly drain from the anterior chamber which causes 7 Anatomy and Physiology 2 Review Sheet Kimberly Denise Vigee’ N.D. and increase in intraocular pressure. 3. Certain drugs or surgery can increase the drainage of aqueous humor. 3. Nyctalopia AKA “Night Blindness” 1. Condition is typically due to a deficiency of Vitamin A. 2. Vitamin A therapy- usually reverses the condition. 4. Diabetic Retinopathy 1. Occurs in the blood vessels of the retina 2 Treatment = laser coagulation of leaky vessels is one treatment 3. The best control - is the maintenance of normal blood glucose levels. 5. Macular Degeneration AKA “ Senile macular degeneration or SMD” 1. The leading causes of blindness in adults. 2. Disorder has been linked to a variety of causes: a. poor diet b. smoking c. connective tissue disorder 4. Repair - antioxidant therapy Clinical Terms 1. Astigmatism – defective curvature of the cornea or lens of the eye resulting in a distorted image on the retina. 2. Diplopia – double vision 3. Tonometry – measurement of the tension or pressure within the eye, which is useful in detecting glaucoma 4. Strabismus – “Cross-eyed” – a condition in which the muscles of the eye do not maintain alignment with each other. It prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision. *** Images from: http://health.howstuffworks.com/eye.htm 8