Vision: Histophysiology of the Eye
Histophysiology refers to the collective functioning of cells and tissues. In the case of the eye, these cells and tissues function in vision.
The human eye is sensitive to light having wavelengths between 397 and 723 nanometers (a nanometer is 10-9 meters). The light entering the eye is focused by a lens to produce an image. The image is projected onto the back surface of the eye, a region called the retina. Here, specialized cells known as rods convert the light signal into an electrical signal. Chemically, this is accomplished by the bleaching out by the light of a rod protein called rhodopsin. Rods are important in lower-light vision, where color and detail are not as prominent. Color vision and the addition of visual detail is the concern of other specialized cells in the retina, which are known as cones. A trio of photopigments in cones cells called iodopsin are responsible for the selective detection of light whose wavelength corresponds to blue, red and green.
The light to electrical energy conversion in the rod and cone cells is transmitted to a bundle of nerves in one region of the retina (the optic nerve). The signals representing the visual image are conveyed by the optic nerve to the brain for interpretation.
The eye is designed to function in varying intensities of light. A lens operates very much like a camera lens, to let in a greater amount of light in dark conditions or less light in brighter conditions. Unlike a camera lens, the human eye is capable of adaptation to a change from relatively bright to less intense light, and visa versa.
Another difference between the human eye and a camera concerns the lens. While the focal length of a camera lens can be changed, the shape of the lens is permanent. However, the lens of the eye can be changed in shape, through the ciliary muscles that hold the lens in place. Contraction of the muscles shortens the lens and causes it to bulge, giving it a more convex shape. This permits the focusing of objects that are closer to the eye. When focusing on an object that is further away, the ciliary muscles relax. This relaxation allows the lens to adopt a thinner, more concave shape, which increases the focal length.
The information passing through the left eye is focused on the rightward region of the retina while the right eye focuses information on the leftward side of the retina. To provide a complete picture, the cells and tissues of the eye are designed so that the visual field of one eye overlaps the visual field of the other eye. Furthermore, as visual information passes to the brain, the information from the left retina flows along both of the two optical nerves leading to the brain, while information from the right retina flows along only one of the optic nerves. This different routing provides one of the physiological bases of depth perception.
Physiological abnormalities in the eye can cause visual impairment. Abnormal focusing, due to the shape of the lens, which is termed astigmatism, cane corrected by external lenses. Another condition called presbyopia decreases the ability of the eye to adapt, because the lens becomes less elastic. The loss of water in the cornea or the lens can produce opaque regions termed cataracts.
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