Visual Receptors

The photoreceptors of the eye are modified neurons of two distinct kinds. One group of receptor cells, called rods, have long, thin projections at their terminal ends. The cells of the other group, called cones, have short, blunt projections. The retina contains about 100 million rods and 3 million cones.

Rods and cones are found in a deep layer of the retina, closely associated with a layer of pigmented epithelium (see figs. 12.33 and 12.34). The projections from the receptors extend into the pigmented layer and contain light-sensitive visual pigments.

The epithelial pigment of the retina absorbs light waves that the receptor cells do not absorb, and together with the pigment of the choroid coat, the epithelial pigment keeps light from reflecting off the surfaces inside the eye. The pigment layer also stores vitamin A, which the receptor cells use to synthesize visual pigments.

Concave surface

Converging Diverging light waves light waves

Concave surface

Converging Diverging light waves light waves

Figure 12.37

(a) A lens with a convex surface causes light waves to converge. (b) A lens with a concave surface causes them to diverge.

Figure 12.37

(a) A lens with a convex surface causes light waves to converge. (b) A lens with a concave surface causes them to diverge.

Object

Figure 12.38

The image of an object forms upside down on the retina.

Researchers can grow retinal epithelial cells in laboratory cultures, and the cells retain their pigment. This means that someday scientists may be able to grow tissue that can be implanted into a person's eye to treat some forms of blindness.

The visual receptors are only stimulated when light reaches them. Thus, when a light image is focused on an area of the retina, some receptors are stimulated, and send impulses to the brain. However, the impulse leaving each activated receptor provides only a small portion of the information required for the brain to interpret a total scene.

Rods and cones function differently. Rods are hundreds of times more sensitive to light than are cones, and as a result, rods provide vision in dim light. In addition, rods produce colorless vision, whereas cones can detect colors.

Cones provide sharp images, whereas rods produce more general outlines of objects. This difference is due to the fact that nerve fibers from many rods may converge, and their impulses may be transmitted to the brain on the same nerve fiber (see chapter 10, p. 385). Thus, if light stimulates a rod, the brain cannot tell which one of many receptors has actually been stimulated. Such a convergence of impulses occurs to a much lesser degree among cones, so when a cone is stimulated, the brain is able to pinpoint the stimulation more accurately (fig. 12.39).

The area of sharpest vision, the fovea centralis in the macula lutea, lacks rods but contains densely packed cones with few or no converging fibers. Also, the overlying layers of the retina, as well as the retinal blood vessels, are displaced to the sides in the fovea, which more fully exposes the receptors to incoming light. Consequently, to view something in detail, a person moves the eyes so that the important part of an image falls upon the fovea centralis.

The concentration of cones decreases in areas farther away from the macula lutea, whereas the concentration

Albinism is an inherited condition in which a missing enzyme blocks pigment synthesis, causing very pale, highly sun-sensitive skin. More severe forms of albinism also affect the eyes, making vision blurry and intolerant to light. A person may squint even in very faint light. The extrasensitivity is due to the fact that light reflects inside the lenses, over stimulating visual receptors. The eyes of many people with albinism also dart about uncontrollably, a condition called nystagmus.

-Rods

Pigmented epithelium

Figure

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