4. 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.
9. 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.
Iris Functions:
Constricts or dilates to adjust the amount of light
entering the eye.
12. 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.
13.
14. The pupil allows light to enter the posterior
segment of the eye.
The iris constricts or dilates to adjust size of the pupil.
15.
16. Vitreous Humor
Transmits light within the posterior
segment.
Supports the lens posteriorly.
Holds the retina in place.
Contributes to intraocular pressure.
18. 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
19. Retina
Fovea Centralis
Contains only closely
packed cones
Provides acute color
vision in bright light
Macula Lutea
Contains more widely
spaced cones
Other areas of Retina
Contain only rods
Provide night, dim light
& peripheral vision
Shades of grey only
Optic Disc
Contains no receptors
Blind spot
24. 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
26. 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
27. Lens
Refracts (bends) light
Focuses precise image on the retina
(fovea) through accommodation (changing
thickness)
28. Accommodation and associated
disorders
28
Accommodation of the lens is limited and age
dependent
With age, lens becomes stiffer and less
compliant.
Age related loss of accommodation
called presbyopia
Accommodation accompanied by adaptive
changes in size of pupil
29.
30. Myopia (Nearsighted)
Eyeball too long
Distant objects focused in front of retina
Image striking retina is blurred
Correction:
• Concave lens or
• laser surgery to slightly flatten the cornea
31.
32. Hyperopia (Farsighted)
Eyeball too short, lens too thin or too stiff.
Nearby objects are focused behind
retina.
Image striking the fovea is blurred.
Correction:
• Convex lens
33.
34.
35.
36. Astigmatism
Irregular Curvature in parts of the cornea
or lens
Causes blurry image
This may be corrected by specially
ground lenses which compensate for the
irregularity or laser surgery.
37. Cararact
Clouding of lens due to aging, diabetes
mellitus, heavy smoking, frequent
exposure to intense sunlight or congenital
factors
Treatment: Lens Implant
39. Glaucoma
Most common cause of blindness.
Increasing intraocular pressure
compresses retina, optic nerve & blood
vessels.
Late symptoms include blurred vision &
halos around bright objects
Canal of Schlemn
41. 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
What numbers can you see in each of these?
42. Night Blindness
Impaired vision at night or in dim light
situations
Rhodopsin deficiency affecting rods
Most common cause - prolonged Vitamin A
deficiency
Rods degenerate
43. Macular Degeneration
Most common cause of vision loss after
65.
Progressive deterioration of macula
causing loss of central vision
•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
Editor's Notes
As you go through the notes associated with each slide in this unit, left click the mouse or depress either the right arrow key or the down arrow key to advance the animation or the slide each time you see this symbol *. Depress the left arrow key or the up arrow to return to the previous slide or animation. *
If you were to touch the surface of your eye with your finger, * you would be touching the conjunctiva. * * *
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. *
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. *
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. *
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. *
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. *
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. *
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. *
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. *
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 posterior segment * of the eye is the area behind the lens. The posterior segment is filled with a jelly-like substance, the vitreous humor. *
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. *
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. *
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. *
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. *
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. *
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. *
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. *
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 * 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. *
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.
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. *
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. *
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. *
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. *
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 * 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. * *
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. *
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. *
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. *
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. *
