7.4: Retina

We have seen that light from an object being viewed passes through the optic media (conjunctiva, cornea, aqueous humor, lens, and vitreous humor) and the pupil so that it is focused and its intensity is adjusted. In this way, clear and accurate images are formed on the retina. The retina is a thin layer that lines the rear portion of the inner cavity of the eye. It extends back from the edge of the ciliary body and thus partially surrounds the vitreous humor (Figure 7.2).

Layers and Regions

The retina consists of two main layers of cells. The inner layer is called the sensory retina (Figure 7.5a, Figure 7.5b). It is in contact with the vitreous humor and contains several layers of neurons. The neurons in the deeper region are called photoreceptors because they respond to light by starting impulses in the form of action potentials. The neurons in the surface region, closest to the vitreous humor, use their synapses to process these impulses. The impulses are then passed to the optic nerve, which begins near the back of the retina at a spot called the optic disk. This nerve carries the impulses to the brain.

Figure 7.5a Structure of the retina and associated eye components. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Figure 7.5b Structure of the macula and associated eye components of the retina. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Blood vessels in the retina nourish the neurons that process impulses; these vessels pass through the optic disk. The photoreceptors are nourished by vessels behind the retina in the choroid layer, as is the outer layer of the retina, the pigmented epithelium.

The photoreceptors in the sensory retina are of two main kinds. The cones are clustered together in a small circular region at the very back of the eye. This region is in line with the center of the cornea and the lens and is called the macula lutea or simply the macula. In the center of the macula is a slightly depressed area that contains a very high concentration of cones. This central area is called the fovea centralis or simply the fovea.

The outer layer of the retina is called the pigmented epithelium because it is a darkly colored thin layer of cells. A noncellular membrane (Bruch’s membrane) lies behind the pigmented epithelium (Figure 7.5a, Figure 7.5b) The pigmented epithelium and apparently Bruch’s membrane regulate the exchange of materials between the choroid and the photoreceptors of the sensory retina.

Cones

There seem to be three types of cones, and each type is most sensitive to light of a different color, either blue, green, or red (Figure 7.5a, Figure 7.5b). If only one type of cone is stimulated, the person sees the corresponding color, while stimulation of various combinations of cone types allows a person to see many other colors and shades of color. This principle explains how people can see many colors on a television screen or computer screen that employs patterns of red, blue, and green dots to form images.

Cones initiate impulses because of specific chemical reactions that occur when light strikes pigmented molecules within them. Cones are not particularly sensitive to light; therefore, relatively high intensities of light must strike them before they initiate impulses.

When a person views a scene, the image of what is in the center of the field of view is focused on the fovea. Since the fovea has a very high concentration of cones, it allows the person to see the object in the center of the scene in color and with maximum visual acuity (the amount of detail that can be seen).

Objects immediately surrounding the center of the field of view are focused on the outer regions of the macula. Since the cones are less concentrated there, these objects are seen with less visual acuity but still are seen in color.

Rods

While the regions of the retina immediately surrounding the macula contain some cones, most of the photoreceptors are of the second type, called rods (Figure 7.5a, Figure 7.5b). The concentration of rods diminishes steadily at greater distances from the edge of the macula, and the front edge of the retina has few rods. Therefore, images from very peripheral objects are focused on retinal areas with low concentrations of photoreceptors and are seen with little visual acuity.

Rods are of only one type. Images focused on rods are seen as black and white images with shades of gray. Though rods do not permit us to see color, they allow us to see in dim light because they are more sensitive to light than are cones. As in the cones, the rods produce impulses when light causes specific chemical reactions within them.

Age Changes

Beginning at age 40, cones decrease in length and many are lost. This decrease in number is greatest in the fovea. The cones that remain in the retina widen to fill the spaces left by degenerated cones.

Since the level of visual acuity and color vision are directly related to the concentration of cones in the retina, the declining number of cones with aging contributes to a gradual drop in visual acuity and a diminishing ability to distinguish colors.

Rods

Aging causes little if any change in the number of rods. However, the rods lengthen, causing them to be bent into irregular shapes to fit within the confines of the sensory retina. These age changes further diminish an aging person's ability to see in dim light.

Processing Neurons

The sensory retina loses not only photoreceptors but also neurons in its inner layer that process impulses from the photoreceptors. The loss is greatest in the neurons that serve the macula. The declining number of neurons processing impulses probably decreases the quality of perceived images and the ability to interpret those images.

Pigmented Epithelium and Bruch's Membrane

Cell numbers and cell functioning also diminish in the pigmented epithelium of the retina. Furthermore, the exchange of materials between the sensory retina and the pigmented epithelium decreases, resulting in diminished servicing of the photoreceptors by the pigmented epithelium.

Age changes in Bruch's membrane seem to further inhibit the exchange of materials required by the cones and rods. Changes in this membrane also cause a reduction in capillaries in the choroid. These capillaries normally provide nutrients for the cones and rods and carry away wastes.

Free radicals in the eye

A unifying factor contributing to several age-related changes in the eye is damage from free radicals (*FRs) and reactive oxygen species (ROS), especially H₂O₂. Free radicals in the eye are produced by radiation (e.g., UV light); atmospheric oxygen; air pollutants; normal metabolism (e.g., mitochondria); and impulse generation.

The *FR damage may be from a combination of age-related increases in *FRs and ROS in the eye; age-related decreases in *FR and ROS defenses in the eye; and accumulation of damage from *FRs and ROS due to slow turnover of some eye components. Slow turnover seems especially relevant in the lens and the retina. The damage from *FRs and ROS may lead to cataracts, glaucoma, decreasing sensitivity of the retina to light and to color, and age-related maculopathy.