My aims here are to see how close we have come to a complete census, to review the principles by which the diverse cell types are organized, to illustrate some of the ways in which they create the retina’s abilities, and to forecast the path by which we may progress. I will begin by outlining three large rules that govern relations among the retina’s neurons. The retina’s processing of information begins with the sampling of the mosaic of rod and cone photoreceptors

by the bipolar and horizontal cells. The photoreceptors form a single CP-868596 mw sheet of regularly spaced cells. Rod photoreceptors, specialized for vision in dim light, outnumber cone photoreceptors by about 20-fold in all but a few mammalian retinas. All rods contain the same light-sensitive pigment, rhodopsin. With one known exception (so far), each cone contains one—and only one—of several cone opsins, each with a different spectral absorption; as will

be discussed later, these are the basis of color vision. Both rods and cones respond to light by hyperpolarizing. Rods and the chromatic classes of cones can be easily identified in intact retinas by morphology and by their expression of the different opsins. This review will pass lightly over the rod system, which molecular dating shows to have MEK activation been a late evolutionary addition to the retina’s tool kit. This is not to say that rods are unimportant, nor that they are uninteresting. Yet the retinal circuitry truly dedicated to rod function includes only four cell types: the rod itself, a bipolar cell that receives input only from rods (“rod bipolar cell”), an amacrine cell that modulates the bipolar

cell’s output, no and an amacrine cell that feeds the output of the rod system into the circuitry that processes information derived from cones. A second pathway from rods to ganglion cells exists in some animals (it involves gap junctions with cones), but in either case the strategy is the same: the late-evolving rods inject their signals into circuitry that had already developed to service the cones (Famiglietti and Kolb, 1975; Nelson, 1982; Nelson and Kolb, 1985; Sandell et al., 1989; Strettoi et al., 1990, 1994; Strettoi et al., 1992). The types of cones are structurally and, as far as is known, functionally similar. (This review pertains primarily to mammalian retinas.) Their functional types are defined by the opsin that each type expresses. A generic mammal expresses one short wavelength-sensitive cone and one long wavelength. Comparison of the two outputs forms the basis of most color vision. The numbers of rods and cones are known with great precision. They have been counted and their topography mapped for dozens of mammalian and nonmammalian species. These have been collected at http://www.retinalmaps.com.au (Collin, 2008). For humans and the common laboratory animals, the accounting of photoreceptor cells is complete.