Each retina contains about 100 million rods and 3 million cones, yet the number of ganglion cells is only about 1.6 million. Thus, an average of 60 rods and 2 cones converge on each ganglion cell and the optic nerve fiber leading from the ganglion cell to the brain.
However, major differences exist between the peripheral retina and the central retina. As one approaches the fovea, fewer rods and cones converge on each optic fiber, and the rods and cones also become more slender. These effects progressively increase the acuity of vision in the central retina. In the center, in the central fovea, there are only slender cones—about 35,000 of them—and no rods. Also, the number of optic nerve fibers leading from this part of the retina is almost exactly equal to the number of cones, as shown to the right in Figure 1. This phenomenon explains the high degree of visual acuity in the central retina in comparison with the much poorer acuity peripherally.
Fig1. Neural organization of the retina, with the peripheral area to the left and the foveal area to the right.
Another difference between the peripheral and central portions of the retina is the much greater sensitivity of the peripheral retina to weak light, which occurs partly because rods are 30 to 300 times more sensitive to light than cones are. However, this greater sensitivity is further magnified by the fact that as many as 200 rods converge on a single optic nerve fiber in the more peripheral portions of the retina, and thus signals from the rods summate to give even more intense stimulation of the peripheral ganglion cells and their optic nerve fibers.
Retinal Ganglion Cells and Their Respective Fields
W, X, and Y Cells. Early studies in cats described three distinct types of retinal ganglion cells, designated W, X, and Y cells, based on their differences in structure and function.
The W cells transmit signals in their optic nerve fibers at a slow velocity and receive most of their excitation from rods, transmitted by way of small bipolar cells and amacrine cells. They have broad fields in the peripheral retina, are sensitive for detecting directional movement in the field of vision, and are probably important for crude rod vision under dark conditions.
The X cells have small fields because their dendrites do not spread widely in the retina, and thus the signals of X cells represent discrete retinal locations and transmit fine details of visual images. In addition, because every X cell receives input from at least one cone, X cell transmission is probably responsible for color vision.
The Y cells are the largest of all and transmit signals to the brain at 50 m/sec or faster. Because they have broad dendritic fields, signals are picked up by these cells from widespread retinal areas. The Y cells respond to rapid changes in visual images and apprise the central nervous system almost instantaneously when a new visual event occurs anywhere in the visual field, but they do not specify with great accuracy the location of the event, other than to give clues that make the eyes move toward the exciting vision.
P and M Cells. In primates a different classification of retinal ganglion cells is used and as many as 20 types of retinal ganglion cells have been described, each responding to a different feature of the visual scene. Some cells respond best to specific directions of motion or orientations, whereas others respond to fine details, increases or decreases in light, or particular colors. The two general classes of retinal ganglion cells that have been studied most extensively in primates, including humans, are designated as magnocellular (M) and parvocellular (P) cells.
The P cells (also known as beta cells or, in the central retina, as midget ganglion cells) project to the parvocellular (small cells) layer of the lateral geniculate nucleus of the thalamus. The M cells (also called alpha or paaliMath cells) project to the magnocellular (large cells) layer of the lateral geniculate nucleus, which, in turn, relays information from the optic tract to the visual cortex, as discussed in Chapter 52. The main differences between P and M cells are as follows:
1. The receptive fields for P cells are much smaller than they are for M cells.
2. P-cell axons conduct impulses much more slowly than do M cells.
3. The responses of P cells to stimuli, especially color stimuli, can be sustained, whereas the responses of M cells are much more transient.
4. The P cells are generally sensitive to the color of a stimulus, whereas M cells are not sensitive to color stimuli.
5. The M cells are much more sensitive than are P cells to low-contrast, black and white stimuli.
The main functions of M and P cells are obvious from their differences: The P cells are highly sensitive to visual signals that relate to fine details and to different colors but are relatively insensitive to low-contrast signals, whereas the M cells are highly sensitive to low-contrast stimuli and to rapid movement visual signals.
A third type of photosensitive retinal ganglion cell has been described that contains its own photopigment, melanopsin. Much less is known about this cell type, but these cells appear to send signals mainly to nonvisual areas of the brain, particularly the suprachiasmatic nucleus of the hypothalamus, the master circadian pace maker. Presumably these signals help control circadian rhythms that synchronize physiological changes with night and day.
