We all know how important our eyes are, but we may not be familiar with the variety of eye diseases that exist or the terms used to describe them. The following series of articles explains how the eye functions, the different eye diseases and their treatments, advances in ophthalmology, and the types of research being done in this field. As the first in the series, this article discusses the basics of how we see.
The main function of the eye is to convert light from the outside world into electrical nerve impulses. These impulses then travel to the part of the brain responsible for vision, where they are interpreted as a visual scene. In the eye, light traverses through the tear film, cornea, anterior chamber, pupil, lens, and vitreous to the retina, which sends the nerve impulses through the optic nerve to the brain. Vision is decreased if any one of these structures is abnormal, is irregularly sized, is not functioning adequately, or is not properly positioned in relation to the others.
To see clearly, the outermost layer, the tear film, must be intact and adequately lubricate the cornea. If the amount of tears produced is less than normal (dry eyes), the eyes will be uncomfortable and vision will be affected. The inadequate production of tears is not uncommon; it may be caused by an isolated problem or by a disease process that affects the entire body.
Beneath the tear film is the cornea, which provides a clear stable structure for the passage of light. The rounded shape of the cornea causes the light rays to bend as they pass through to the anterior chamber. If the cornea is misshapen or becomes cloudy and cannot allow enough light to pass through, then vision is severely hampered.
The anterior chamber is a space between the cornea and the iris (colored part of the eye). It contains fluids that bathe the structures in the front (anterior) part of the eye. If the anterior chamber is too shallow, the iris can move forward and touch the back of the cornea. This abnormal relationship changes the anterior chamber angle (where the cornea and iris meet) and impedes the flow of fluids between the anterior chamber (via the trabecular meshwork in the angle) and the anterior structures. If the fluid cannot enter and exit freely through the trabecular meshwork in the angle, pressure builds up in the eye and glaucoma develops.
In the center of the iris is the pupil, which is an opening that allows light through to the lens. The muscles of the iris regulate the size of the pupil. In bright light the muscles constrict and the pupil becomes smaller; in the dark, the muscles dilate and open the pupil larger to let more light come through. Certain drugs and diseases affect the ability of the pupil to open and close normally.
In back of the pupil is the lens, a clear, pliable structure that changes the angle of the light rays as they enter the eye to focus them on the retina. With aging, changes occur in the lens. It may lose its clarity, and a cataract develops. Commonly, the lens becomes less pliable in a person’s early to mid-forties, affecting the ability of the lens to focus the light rays, resulting in a need for reading glasses. It also has been shown that the lens helps to filter out ultraviolet light rays, which could damage the retina in the back of the eye.
Between the lens and the retina is another chamber, the vitreous cavity, which contains a clear jelly-like substance called the vitreous. Behind the vitreous is the retina, a paper-thin, delicate tissue that contains the photoreceptors that convert light into electrical signals, as well as other types of cells that process these electrical signals. The retina is often compared to the film in a camera: without it, the camera, or eye, is useless.
When light hits the retina, a photochemical reaction takes place. Electrical impulses develop in the light receptors (rods and cones), the impulses are processed by other cells within the eye, and then they are transmitted to the brain via the optic nerve, which is composed of a group of nerve fibers. When the impulses reach the brain, they are interpreted as a visual scene.
It is the ophthalmologist’s job to assess the integrity of all these structures. If you have a specific eye problem, you will be asked to describe it. Many times important clues about your problem can be obtained as you report when the problem began and how it affects your vision. The ophthalmologist then will check your near and distant vision; look at all the structures of your eye to be sure that they are healthy; and check the pressure inside your eye to determine if glaucoma is present.
Even if you don’t have a problem with your vision, regular checkups should be part of your normal health care. Examinations should be made by an ophthalmologist every two to three years to maintain healthy eyes. With the many research advances made in the past 20 years, more is able to be done now than in the past to preserve and maintain vision.