The actual visual receptors of the eye are the rods and clones. These are cells which are
stacked close together just beneath the surface of the retina.
1. The rods. There are approximatelly 100 milion rods in each eye. They are used chiefly for vision in dim lightand are extremely sensitive to light. The image produced by the rods is, hovewer, not a sharp one. The rods function in groups. In other words, a number of rods share a single nerve circuit to the brain. A single rod can initiate an impulse in that circuit but there is no way for the brain to determine which rod in the cluster was involved.
The dark-adapted rods are very sensitive to light, being capable of detecting a single photon. The rod is a specialized type of neuron.
The disks of rod cells are completely separated from the outer plasma membrane. The signal that is generated by the rod cells, unlike neurons, is not an all-or-none type response. The signal may be graded in intensity. The net effect of light is to reduce the rate of opening of the sodium channels with no effect on the rate of closing of the channels. Thus, if the rate of sodium channel opening is momentarily decreased, the concentration of sodium within the rods will decrease, leading to hyperpolarization.
In order for the rods to become as sensitive as possible (Ądark-adapted") it is necessary that
the rate of rhodopsin synthesis exceed the rate of its breakdown. This means that bright light
must be excluded from the eye. We all are aware of how difficult it is to see in a dimly
lighted room immediately after entering from a brightly lighted one. It takes some thirty
minutes in the dark for our eyes to become fully adapted to the dark.
2. The cones. We know less about how the cones work than about how the rods work. The cones are especially abundant (about 15,000 in each square millimeter) in a single region of the retina, the fovea, a region just opposite the lens. Unlike the rods, the cones operate only in bright light. Furthermore, they enable us to see colors. At least two kinds of cones must be present in order to detect any colors at all. Each must contain a pigment that absorbs a certain wavelenght best. With three kinds of cones, it would be possible to have full color vision if each kind contained a pigment that best absorbed one of the three primary colors: red, green, and blue. Theoretically, the brain could mix three primary color sensations to produce any of the more than 17,000 different hues that the well-trained eye can distinguish. A single cone contains only one of the three pigments (a red-absorbing pigment, a green-absorbing pigment, a blue-absorbing pigment). Working together, the red-, green-, and blue-absorbing cones in the fovea provide the basis for color vision.
The genes encoding the visual pigments have been mapped to specific chromosomes. The rhodopsin gene resides on the third chromosome, the gene encoding the blue pigment resides on the seventh chromosome, and the two genes for the red and green pigments reside on the X chromosome. In spite of their great similarity, the red and green pigments are distinctly different proteins.
There are some differences between the rods and the cones structurally. There is no evidence, however, that the mechanism for the chemical or the biochemical reactions are different. The disks of the cone cells do show some attachment to the outer plasma membrane, which may contribute to teh somewhat quicker response of the plasma membrane to hyperpolarization than in the rod cells.
The sensitivity and the response time of the rods is different from that of the cones. The cone
response is about four times faster than the rod response. Thus, the cones are better suited for
discerning rapidly changing visual events and the rods are better suited for low-light visual
acuity. A direct correlation between these functional differences and the structural differences
between the organelles and/or some of the proteins involved in the transduction and signal
amplification has not yet been made.
All the nerve impulses generated by the rods and cones travel back to the brain by way of neurons in the optic nerve. At the point on the retina where the approximately one million neurons converge on the optic nerve, there are no rods or cones at all. This spot, the blind spot, is thus insensitive to light. With the marks you can demonstrate the presence of the blind spot for yourself. The blind spots of our two eyes do not receive the same portions of the visual image, so that each eye compensates for the blind spot of the other.