How the Retina Works

Retinal Cross section (photo)

Light rays scattered from an object pass through the cornea and lens of the eye where refraction bends them into an inverted image on the retina. At the retina the light is converted into electrical signals which are passed to the brain through the optic nerve. (Show image on Retina from Nilsson book.)

Light comes into the retina through the retinal blood supply and and its nerve axons. It then passes through two transparent layers of neurons: the ganglion cells, and bipolar cells, before finally reaching the rods and cones which actually convert the light into an electrical impulse. Electrical impulses are the language of the brain. (Image )

There are 100 million rods and cones on the retina of each eye. Rods are sensitive to dim light but insensitive to color. (On a moonlit night the light is too dim to see color.) Cones on the other hand operate in bright light and discriminate between colors. Humans have three different cone cell types, One most sensitive to red light, one to green and one to blue. The rods outnumber the cones 10 to 1 except in the center of our view, near the macula where the cones predominate. (This is why you have to look to the side of a dim star to see it.)(Rod Cell image p11)

In rod cells the receptor protein is rhodopsin, it crosses the cell membrane of the portion of the rod known as the disk seven times. A part of the rhodopsin protein is retinal, a derivative of vitamin A. The light actually interacts with the retinal.

Here are the details:

Step 1.

A photon hits a rod or cone cell. In this cell there are 100 million (same number as above!) molecules of rhodopsin embedded in the membrane the part of the cell furthest from the cornea, in this portion of the cell the membrane forms a stack of disks. Each snakelike molecule of rhodopsin crosses the cell membrane seven times. In the dark, the retinal fits snugly into a binding pocket in the rhodopsin molecule. When the retinal absorbs light it straightens out. This alters the three-dimensional structure of the rhodopsin molecule activating it and starting a biochemical cascade.

Step 2.

Activated rhodopsin stimulates transducin a G protein, this activates an enzyme, that breaks down cyclic GMP. Cyclic GMP carries signals from the disks to the cells surface membrane. The membrane contains channels which are kept open by the cyclic GMP when the cell is in the dark. positive ions of sodium and calcium move through these channels into the cell to keep its inside voltage equal to the voltage outside the cell. In the light the cyclic GMP is removed and the channels close. Without the flow of positive ions through the channels into the cell the center of the cell becomes negative relative to the outside.

Step 3.

When the inside of the cell becomes negative it reduces the amount of neurotransmitter released from the base of the rod cell. This change in neurotransmitter communicates the absorption of a photon to bipolar nerve cells adjacent to the rod cell.

Detecting Color

After light is detected by the cones neurotransmitters carry the signal to the next layer in the retina. Here nerve cells called "opponent" cells compare the activity of the red versus the green cones. Then this combined "yellow" signal is compared with the blue cone by a second set of "opponent" cells. The result of these color differences is then sent to the brain. At the present time much controversy exists about color processing after the retina.

Color Blindness

7% of men and 0.4% of women see red and green differently from the rest of the population. They are red-green color blind. The recipe for the red and green cones reside in genes on the X chromosome. Women have two of these while men have only one. The DNA sequences for the red and green receptors differ by only 2%, evidence that they diverged recently. (New world monkeys diverged from old world monkeys about 40 million years ago, they have only 1 receptor.) The exhibit Seeing Yellow allows you to compare your color vision with that of another person.

Some of the above material was excerpted from

Seeing, Hearing, and Smelling the World

by the Howard Hughes Medical Institute