SPECIAL SENSES THE EYE

Conjunctiva If you were to touch the surface of your eye with your finger, * you would be touching the conjunctiva. * * *

Conjunctiva Covers the inner surface of the eyelids and the anterior surface of the eye. Membrane which produces mucous that lubricates the eye and prevents dryness. Protects the eye. The conjunctiva * covers the inner surface of the eyelids as well as the anterior surface of the eyeball itself. * As a serous membrane, it produces mucous to lubricate the eye and prevent dryness. * It also serves to protect the eye. *

Fibrous Tunic The eye is made up of three tunics or layers of material. * The outermost tunic is called the fibrous tunic. The fibrous tunic is made up of the opaque white sclera * which is the tough layer which covers most of the eye and is seen anteriorly as the white of the eye, * and the transparent cornea in the front. *

Fibrous Tunic Sclera Functions: Protects eye Shapes eye Anchors eye muscles Cornea Functions: Transparent window for light entry Refracts light The sclera: * * protects the inner structures of the eye, * gives the eye its shape, and * provides a tough surface to which the tendons from the muscles which move the eye may be anchored. * The cornea * * serves as a transparent window which bulges forward from the place where it joins the sclera to allow light to enter the eye. As the light travels through the cornea it is refracted (bent) * so the image may be directed to the area of the eye where the receptors are located. *

Vascular Tunic The middle tunic or layer is the vascular tunic or uvea which is pigmented. * Structures which make up the vascular tunic include: * the posterior choroid, * the ciliary body *, and * the anterior iris. *

Vascular Tunic Choroid Functions: Provides nutrients to all eye tunics. Absorbs light preventing reflecting & scattering of light within the eye. Ciliary Body Functions: Ciliary processes secrete aqueous humor. Suspensory ligaments hold lens in place. Ciliary muscles pull on the ligaments to change the thickness of the lens. The choroid is darkly pigmented and highly vascular. Its functions include: * providing O2 and nutrients to the three tunics of the eye, * absorbing light to prevent its reflection and scattering within the eye. * The ciliary body is a circular ring of tissue that encircles the lens. It * secretes a watery fluid within the eye called the aqueous humor and * anchors the suspensory ligaments which hold the lens in place. The * ciliary muscles, to which the suspensory ligaments attach, help to change the thickness of the lens by pulling on the ligaments. * The iris * is the visible colored part of the eye which constricts or dilates to adjust the amount of light that can enter the eye. * Iris Functions: Constricts or dilates to adjust the amount of light entering the eye.

Vascular Tunic Ciliary Muscles Ciliary Processes The ciliary muscles * * form the largest part of the ciliary body. They form a muscular ring around the inside of the ciliary body which contracts or relaxes to change the tension on the suspensory ligaments which hold the lens in place. * Changes in tension on the the suspensory ligaments changes the thickness of the lens. The ciliary processes * * are also a part of the ciliary body. They contain capillaries which secrete the fluid that fills the anterior part of the eye. * Ciliary Processes

The aqueous humor secreted by the ciliary processes The aqueous humor secreted by the ciliary processes * circulates in the area of the eye anterior to the lens as well as diffusing through the material which fills the inside of the eye behind the lens. The area which is filled with aqueous humor is referred to as the anterior segment of the eye, * and is divided into an anterior chamber in front of the iris and a posterior chamber where the lens is located. * As the production of aqueous humor is a continuous process, aqueous humor must be able to leave the eye at a rate equivalent to its production so that the pressure inside the eye remains the same. The aqueous humor leaves the eye and moves into the venous blood by flowing into the canal of Schlemm (scleral venous sinus) * which circles the eye at the junction between the sclera and the cornea. *

Aqueous Humor Helps support the eye internally due to the intraocular pressure it produces inside the eye. Supplies nutrients & oxygen to the cornea, lens and portions of the retina. Carries away metabolic wastes from the cornea, lens and portions of the retina. The aqueous humor: * helps support the eye internally due to the intraocular pressure it produces inside the eye, * supplies nutrients and O2 to the avascular cornea, lens and to parts of the retina and * carries off metabolic wastes produced by the cornea, lens and other structures inside the eye. *

The iris constricts or dilates to adjust size of the pupil. You are looking at an anterior view of the iris, the colored part of the eye that you can see from the outside of the eye. * The function of the iris is to constrict or dilate to adjust the size of the pupil which is the opening in the center of the iris. * Light must pass through the pupil to enter the posterior segment of the eye where the receptors are located. * The size of the pupil, determines how much light can enter the eye. * The iris constricts or dilates to adjust size of the pupil. The pupil allows light to enter the posterior segment of the eye.

The posterior segment. of the eye is the area behind the lens The posterior segment * of the eye is the area behind the lens. The posterior segment is filled with a jelly-like substance, the vitreous humor. *

Vitreous Humor Transmits light within the posterior segment. Supports the lens posteriorly. Holds the retina in place. Contributes to intraocular pressure. The vitreous humor * conducts light within the posterior segment, * supports the lens posteriorly, * holds the retina in place, and * along with aqueous humor, contributes to intraocular pressure within the eye. *

Sensory Tunic The third, or innermost tunic making up the eye is an outpocketing of the brain referred to as the sensory tunic. * The specific name given to the sensory tunic is the retina. * The retina is actually composed of two separate layers. *

Retina Pigmented Layer Absorbs light Carries out phagocytosis Stores Vitamin A Neural Layer Contains photoreceptors (rods and cones) for visual perception Contains bipolar cells & ganglion cells for visual impulse transmission The outermost pigmented layer of the retina lies adjacent to the choroid. It functions to: * absorb light so it will not be reflected within the eye, * carry out phagocytosis to remove cellular debris and other materials which might interfere with vision, and * store vitamin A. * The transparent inner layer of the retina is referred to as the neural layer. * Only the neural layer contains the photoreceptors that respond to light, * the bipolar cells and ganglion cells *which process visual stimuli and conduct nerve inpulses to the brain. *

Retina Other areas of Retina Fovea Centralis Contain only rods Provide night, dim light & peripheral vision Shades of grey only Optic Disc Contains no receptors Blind spot Fovea Centralis Contains only closely packed cones Provides acute color vision in bright light Macula Lutea Contains more widely spaced cones As the fovea centralis * contains the highest concentration of closely packed cones, * it provides the sharpest color images in bright light. * The macula lutea * also contains cones, but they are more widely spaced and are interspersed with rods. * As a result, the images produced by the macula lutea are not as sharp and clear as those produced by the fovea centralis. * Other areas of the retina besides the fovea centralis and the macula lutea only contain rods. * Because rods are very sensitive to bright light, they only function at night, in dim light situations or in peripheral vision. * Images produced by rods are somewhat blury and are only in shades of grey rather than in other colors. * As indicated previously, the optic disc * contains neither rods nor cones, and is therefore unable to respond to visual stimuli. * For this reason, it is referred to as the blind spot of the eye. *

Retina Optic Disc This is a view of the retina from the front of the eye. The macula lutea * surrounds the fovea centralis * directly behind the pupil through which light enters the posterior segment of the eye. The optic disc * * is on the left where the blood vessels and nerves exit the eye. *

Photoreceptors Notice the specific arrangement of cells in the fovea centralis. * Each cone * synapses with a single bipolar cell, * which synapses with a single ganglion cell * which sends information along its axon to the optic nerve. *

Cones Are located in macula lutea but are most highly concentrated in the fovea centralis. Are sensitive to bright light (daylight) situations in which light is very intense. Each cone synapses with a single bipolar cell which synapses with a single ganglion cell. The axons of ganglion cells form the optic nerve to conduct visual images to the brain. Provide acute (sharp) color images (vision). While cones are found scattered throughout the macula lutea, they are most highly concentrated in the fovea centralis. * They respond to bright visual stimuli such as one would encounter during daylight or to bright lights at other times. * Each cone synapses with a single bipolar cell which in turn synapses with a single ganglion cell. * The axons of the ganglion cells form the optic nerve which carries visual images from each cone to the brain. * Thus the visual stimuli sent to the brain from the cones sharp and in color. *

Optic Nerve Pathway

Cones There are three different types of cones. One type only responds to wavelengths of light which are found in the green area of the visible light spectrum.* These are called green cones. Another type of cone responds only to wavelengths of light in the red area of the visible light spectrum. These are the red cones. * Finally there are blue cones which respond only to the blue wavelengths of the spectrum. * As the brain receives visual stimuli from these colored receptors, the brain processes them to give us images in all the various colors. *

Photoreceptors Rods are primarily found in the retina, outside of the fovea centralis. Although there are some rods scattered among the cones in the outer parts of the macula lutea, most of our vision from rods comes from other areas of the retina. * Images from rods are not as sharp as those from cones because of their arrangement with bipolar and ganglion cells. Notice from the diagram that several rods synapse with one or more bipolar cells. * * Thus the information received by the bipolar cells from rods may include stimuli from several rods. * More than one bipolar cell may synapse with a single ganglion cell, * which sends the information from many rods to the brain. *

Rods Most highly concentrated in the retina outside the macula lutea Many rods synapse with a single bipolar cell Many bipolar cells may synapse with a single ganglion cell which carries stimuli to brain More sensitive & function only in dim light, night and peripheral vision Images are blurry and only in shades of gray Rods * are most highly concentrated in the retina outside of the macula lutea. * Several rods may synapse with a single bipolar cell. More than one bipolar cell * may synapse with a single ganglion cell which carries the information to the brain along its axon. * Rods are much more sensitive to light than cones. * As a result, in daylight and bright light situations, the rods are overwhelmed and are thus inactivated. For these reasons, the rods only function effectively at night, in dim light situations and in peripheral vision. * The images from rods are fuzzy and are only in shades of gray. Thus night vision is not colored. *

Visual Pigments Composed of two components Retinal - light absorbing molecule (made from Vitamin A) Opsin (four types made from protein) Opsin combined with retinal = visual pigment OPSIN + RETINAL = Visual Pigment Visual pigments are the chemicals within rods and cones which respond to various wavelengths of light. * Each visual pigment is composed of two components: * Retinal which is synthesized from vitamin A, and * Opsin which is a special kind of protein synthesized within photoreceptors of which there are four distinct types. * The physical binding of an opsin and a retinal molecule creates the visual pigment. To summarize: * opsin + * retinal * = * visual pigment * * Depending on the type of opsin which is combined physically with retinal within the rod or cone, the specific type of visual pigment that is characteristic of each type of receptor is prepared to respond only to certain wavelengths of light. The visual pigment found in rods which responds in dim light situations, while the in cones there are three different kinds of visual pigments which respond only to red, green and blue wavelengths of light respectively. * Depending on the type of opsin retinal is bound to, each of the four pigments will only absorb certain wavelengths of light.

Visual Pigments: RODS Retinal + Opsin = Rhodopsin (visual purple) Absorbs light throughout entire visible light spectrum (most sensitive to green) Functions only in dark, dim light & peripheral vision Light causes Retinal to change shape & separate from opsin causing nerve impulse Regenerate only in dark or dim light situations The specific type of visual pigment used by rods is called Rhodopsin. * It is a combination of retinal plus the special opsin produced by rods. Rhodopsin is sometimes called visual purple. * While rhodopsin is sensitive to wavelengths of light throughout the visible light spectrum, it is most sensitive to green. * Since rhodopsin absorbs low intensity light, it functions only in the dark, in dim light situations and in peripheral vision. * When light reacts with rhodopsin, it causes the molecule to undergo a physical change in shape which causes the retinal and the changed opsin to physically separate. This separation causes the rod to send an impulse to the bipolar cell that may result in a nerve impulse by the ganglion cell. * Because of its sensitivity to light, activated opsin can only combine with retinal in the rod if little or light is available to trigger the reaction. To review the reaction ,* when rhodopsin is exposed to light, * * the light causes the opsin and retinal to separate, * which may generate a nerve impulse. * * (Light) Impulse OPSIN RETINAL RHO DOPSIN

Visual Pigments: Cones Retinal + Red, Green or Blue Opsin = Red, Green or Blue visual pigments Each Opsin absorbs light only in the area of the visible light spectrum it is sensitive to, ie, red cones, green cones & blue cones Function only in bright light (daylight) Provide sharp color images Visual pigments in cones react to light in a similar way, * except each visual pigment reacts to different wavelengths of light. * Each opsin absorbs light only in the area of the spectrum it is sensitive to: red, green or blue. * To stimulate a reaction in cones the light must be bright. Thus, cones only are stimulated in daylight situations or by light at night that is sufficiently bright. * Because of the arrangement of each cone synapsing with a single bipolar cell, the images are sharp and in shades of red, green or blue. * To review the reaction, a red cone when activated by light of sufficient intensity * will react, * causing the red opsin to separate from the retinal. * This will result in a nerve impulse to the brain in shades of red. * * If the wavelength of light is in the green part of the spectrum, * it will stimulate green cones to react * causing an impulse in shades of green to be sent to the brain. * (Light) Impulse Red Opsin RETINAL Red Cone Impulse Green Opsin RETINAL Green Cone

Lens Refracts (bends) light Focuses precise image on the retina (fovea) through accommodation (changing thickness) The lens is a transparent, flexible, biconcave structure which is held in place by the suspensory ligaments. * Its primary job is to refract light passing through it so that an image may be focused on the fovea centralis. * Because the lens is flexible, tension exerted by the ciliary muscles through the suspensory ligaments changes the thickness of the lens. * This adjustment in the thickness of the lens to focus images precisely on the fovea is called accommodation.

Steps of Vision Light enters the eye through the cornea. Light passes through the aqueous humor of the anterior cavity. 3. Light enters the posterior chamber of the anterior cavity through the pupil.

Steps of Vision The light passes through the lens, causing the light to be refracted. The refracted light passes through the vitreous humor of the posterior cavity. The light converge and becomes focused on the photoreceptor layer of the neural portion of the retina.

Steps of Vision The light causes isomerization of the retinal photopigament. Isomerization (cis to trans conversion) of retinal activates an enzyme that breaks down cGMP.

Steps of Vision 9. The break down of cGMP caused cGMP gated Na+ channels to close. Closing the channels slows the influx of Na+ . The reduction in Na+ influx hyperpolarizes the membrane.

Steps of Vision The hyperpolarization decreases the release of the inhibitory neurotransmitter glutamate. 14. The decrease in the release of glutamate excites the bipolar cells.

Steps of Vision The bipolar cells synapse with the dendrites of the ganglion cells. 16. Nerve impulses (action potentials) propagate along the axons of the ganglion cells toward the optic disc.

Steps of Vision The axons of all retinal ganglion cells in 1 eye exit the eyeball at the optic disc and form the optic nerve on that side. At the optic chiasm, nerve fibers from the temporal half of each retina do not cross but continue directly to the later geniculate nucleus of the thalamus on the same side. Nerve fibers from the nasal half of each retina cross and continue to the opposite thalamus.

Steps of Vision Each optic tract consists of crossed and uncrossed axons that project from the optic chiasm to the thalamus on one side. 21. The axons of thalamic neurons form the optic radiations as they project to the primary visual area of the occipital lobe of the cortex on the same side.

Myopia (Nearsighted) Eyeball too long Distant objects focused in front of retina Image striking retina is blurred Myopia * is a condition where the eyeball is longer than normal. As a result, the image is focused in front of the fovea rather than directly on it. * Thus the actual image striking the fovea is not in sharp focus. * * Individuals who are near sighted can see nearby objects clearly because the lens can accommodate sufficiently to adjust the focal point. That is not the case for things farther away. * This condition may be corrected by using glasses which have concave lenses* which are ground precisely to correct the problem. * The condition may also be corrected by laser surgery which flattens the cornea to adjust the focal point for distant objects. * Correction: Concave lens or laser surgery to slightly flatten the cornea

Hyperopia (Farsighted) Eyeball too short, lens too thin or too stiff. Nearby objects are focused behind retina. Image striking the fovea is blurred. Hyperopia or farsightedness * is due to the eye being too short, the lens too thin, or the lens being too stiff to focus the image precisely on the fovea. * As a result, the image of a nearby object is focused behind the fovea * causing the image striking the fovea to be blurry. * * Farsighted individuals can see distant objects clearly. This condition * may be corrected by using a convex lens * to bring the image of nearby objects forward far enough * for the image to be focused precisely on the fovea. * Correction: Convex lens

Astigmatism Irregular Curvature in parts of the cornea or lens Causes blurry image Irregular curvatures of different parts of the lens or the cornea * may cause portions of the image on the fovea to be blurry. * This is referred to as an astigmatism. * * This condition may be corrected through the use of specially ground cylindrical lenses to compensate for the differences in curvature of the cornea or lens or through laser surgery. * This may be corrected by specially ground lenses which compensate for the irregularity or laser surgery.

Diseases of the Eye

Treatment: Lens Implant Cararact Clouding of lens due to aging, diabetes mellitus, heavy smoking, frequent exposure to intense sunlight or congenital factors To refract light and focus sharp images on the fovea, the lens must be transparent. * The lens may become cloudy or opaque as a result of aging, diabetes mellitus, heavy smoking, frequent exposure to bright sunlight or congenital factors. * If the light cannot pass through all parts of the lens, the image may be dim and unfocused. * Treatment for cataracts * involves the replacement of the lens. * Treatment: Lens Implant

Conjunctivitis Inflammation of the conjunctiva by: Bacteria, fungi or viruses Trauma Conjunctivitis * is inflammation of the outer covering of the cornea (conjunctiva). * The most common causes of conjunctivitis include * infection by bacteria, fungi or viruses, and * trauma. * *

Glaucoma Most common cause of blindness. Increasing intraocular pressure compresses retina, optic nerve & blood vessels. Late symptoms include blurred vision & halos around bright objects Glaucoma * is the most common cause of blindness in the U.S. It is caused by increasing intraoccular pressure inside the eye, * which compresses the retina, optic nerve and the blood vessels that supply blood to the eye, shutting off the supply of nutrients. Although glaucoma may develop rapidly, * late symptoms may include blurred vision and halos around bright objects. * Aqueous humor * is constantly being produced by the ciliary processes within the eye. As long as an equivalent amount of aqueous humor can drain from the anterior segment through the mesh network at the junction between the cornea and the sclera * and into the canal of Schlemn, * the intraocular pressure remains constant. * However, if the aqueous humor drains more slowly than it is produced, pressure builds up within the eye gradually reducing the flow of blood and nutrients. Open angle glaucoma * is caused by the aqueous humor draining too slowly. * In angle closure glaucoma, there is such a sharp angle between the iris and the mesh network through which the aqueous humor must drain to reach the canal of Schlemn that the aqueous humor cannot drain due to the iris being pressed against and covering the area the drainage must occur through. * * Canal of Schlemn

Glaucoma The top figure shows the optic disc in a normal eye. * The bottom figure shows the effect of increased intraoccular pressure on the optic disc caused by glaucoma. *

What numbers can you see in each of these? Color Blindness Congenital lack of one or more cone types Deficit or absence of red or green cones most common Sex-linked trait Most common in males Colorblindness * is due to a congenital lack of one or more cone types. * The most common type of colorblindness is red-green, due to the lack of cones sensitive to red or green wavelengths of light. * This is a sex-linked trait which is carried on the X chromosomes. * Since males inherit only one X chromosome, the condition occurs most commonly in males. * Look at the three figures shown. * What numbers can you see in each of these? Charts like this are used to determine if a person has the cones which are necessary to distinguish the colors used to form the numbers. * What numbers can you see in each of these?

Night Blindness Impaired vision at night or in dim light situations Rhodopsin deficiency affecting rods Most common cause - prolonged Vitamin A deficiency Rods degenerate Night blindness * is impaired vision at night or in dim light situations. Since rods are the receptors which function at night or in dim light situations, * night blindness is primarily due to a rhodopsin deficiency. * The most common cause of night blindness is a prolonged Vitamin A deficiency. * If adequate amounts of Vitamin A are not available for an extended period of time, the rods degenerate and the condition becomes permanent. *

Macular Degeneration Most common cause of vision loss after 65. Progressive deterioration of macula causing loss of central vision Degeneration of the macula lutea * is the most common cause of vision loss after the age of 65. * It is due to progressive deterioration of the macula. * Since the images of things we look directly at are focused on the macula and the fovea centralis, * deterioration of the macula affects the quality of images in the central area of our vision. * Here you can see that the center of the image is not as sharp and distinct as the peripheral areas. This is typical of macular degeneration. Macular degeneration typically occurs in two primary forms. * The first is called the dry form, and is due to an over accumulation of debris from visual pigments which have reacted to light in cones. Normally the pigmented layer of the retina would remove these by phagocytosis. However, if the build up of debris occurs too rapidly, the pigmented cells will not be able to remove the debris fast enough and degeneration of the macula will occur. * The second type of macular degeneration is caused by invasion of the macula with new blood vessels from the choroid. This causes scarring of the macula and may result in separation of the pigmented and neural layers of the retina. This separation is called retinal detachment. * Dry Form - due to accumulation of pigments in macula due to reduced phagocytosis of cone debris by pigmented layer Wet Form - due to invasion of macula with new blood vessels from choroid causing scarring & retinal detachment