To understand how and why macular degeneration affects vision, one must know how the human eye works.
The eye can be compared to a camera.
The cornea is the transparent, curved front layer of the eye, where light is first focused before it passes through the pupil.
The pupil, behind the cornea, is a hole in the colored membrane called the iris. Tiny muscles in the iris change the size of the pupil – like the aperture apparatus of a camera – to control the amount of light getting into the back of the eye.
There is a small, powerful lens behind the pupil which changes shape, based on the pull of muscles in the eye, in order to further focus the light on the retina, which is a light-sensitive membrane lining the inside back of the eye.(Incidentally, cataracts are a deterioration of the human lens, and cataract surgery removes this lens and replaces it with a clear, artificial one.)
Like film in an older camera or sensors in a digital camera, the retina is a light-sensitive surface where light energy from the outside world is displayed and initiates a reaction that leads to the formation of an image. In a camera, film must be exposed to chemicals in order to permanently fix the image for future print processing. In the retina, millions of microscopic photoreceptor cells, known as rods and cones, are activated by light and, through a biochemical process, turn light into electric impulses. These impulses travel the optic nerve (a cable built of thousands of nerve fibers) to the visual processing center of the brain, where they are interpreted as vision.
The biochemical process of transforming light into electricity that occurs in the retina (known as phototransduction) makes it the most metabolically active tissue in the body. Just as the chemicals used to process film — or to print from film onto materials such as paper — have to be regularly removed and refreshed in order to maintain the purity of the image-making process, the protein-fat biological waste generated by photoreceptors has to be constantly removed, and fresh nutrients have to be resupplied to the rods and cones. This cleanup and nutrient supply process is carried out by the retinal pigment epithelium (RPE), a single layer of tightly joined cells that lies behind the photoreceptors and in front of Bruchs membrane, and the capillary-rich choroid. Specialized white blood cells, called macrophages, aid in the cleanup.
While scientists are still learning about the variety of factors that can disrupt this cleanup-and-refresh process, the common byproduct of disruption is a druse — a clump of proteins and fats that can grow and connect to other drusen. When drusen become so large that they cannot be cleared away by macrophages, they can push the RPE and photoreceptors away from the back of the eye, creating distortion in the images formed in the brain.
The components of the retina
The inside surface of the retina is made up of light-sensitive cells, called rods and cones.
Rods are sensitive to light and dark, shapes and movement. They function in low light, becoming more active as daylight fades, and allow us to see in dark conditions. There are many more rods than cones, and they are concentrated in the outer retina.
Cones account for only 5% of the retina’s photoreceptors but we rely on them more heavily for our daily activities because they are sensitive to fine detail and to color. Cones are highly concentrated in the macula, which is where our visual perception is most acute — especially in the fovea, at the center of the macula, which is 100% cones.
The retina also contains other types of neural cells — retinal ganglion cells, amacrine cells and bipolar cells — whose function is to convey information from the rods and cones.
The macula is the name for a tiny oval near the center of the retina, where cones are in high concentration and visual perception is most acute. The macula is responsible for receiving the sharp, straight-ahead images which are used in reading, driving, facial recognition, color perception and so on. It is a hundred times more sensitive to detail than the rest of the retina. The fovea is the center of the macula and has the highest concentration of cones anywhere on the retina.
The retina’s photoreceptors rest on Bruch’s membrane. Behind this membrane is the retinal pigment epithelium (RPE), which supplies nutrients to the photoreceptor cells. Behind these layers of the retina is the choroid; this is connective tissue and is rich in tiny blood vessels called capillaries. Behind the choroid is the sclera – the supporting structure of the eye.