 Dear students, in this topic we shall discuss the visual receptor cells of vertebrates. The visual receptor cells or photoreceptors capture the energy of light and transduce it into neuronal signals. They are located in the retina of vertebrate eye. They fall into two classes, rods and cones. First we shall discuss the rods, which are more sensitive to light. They enable the animal to see in the dark, that is they enable night vision. However, they cannot distinguish colors, so they provide a chromatic vision that is black and white only. Cones, in comparison to rods, they are less sensitive. They function best in the presence of bright light, however they provide better high resolution with color vision. They contribute very little to night vision. There are three types of cones. Each type of cone cell has different sensitivity across the visual spectrum. They are active in the ranges of blue, green and red colors. Dear students, the number of rods and cones varies in animal retinas. This number correlates with the extent to which an animal is active in night. Most fishes, amphibians, reptiles and birds have well-developed color vision because they have large number of cone cells. However, most mammals have reduced capacity to see colors, especially the nocturnal mammals which are active during the night. These nocturnal mammals have high proportion of rods in their retinas that give them very much better night vision. Large mammals, humans and primates have well-developed color vision. Human retina contains about 125 million rods and 6 million cones. Dear students, in many mammals, including humans, the center of the visual field is about 1 millimeter square central part of retina. This central part of retina is known as fovea centralis. This part has very high density of cones but it has no rods. It provides very detailed information about the visual field. This highly detailed visual field information is called high visual equity. This high visual equity is the importance of humans and only a few other mammals. Since there are only cones in fovea centralis, there is no rod in it, but as we move away from fovea to retina, we see that the ratio of cones decreases and the concentration of rods increases. So that the peripheral regions of the retina have only rods but no cones. Dear students, now we shall discuss the structural features of rods and cones. The general cell structure of both these types of, that is rods and cones, cell structure is basically similar. However, they are narrower and longer than the cones. There are three major functional segments of rods and cones. The outer segment, the inner segment and the synaptic body. First, we shall discuss the outer segment. The outer segment of both rods and cones contains the light sensitive photochemicals, that is, rodopsins or photopsins. This outer region has a large number of disks, which may be up to 1000. These disks are formed by the infolding of cell membrane. Cell membrane is infolding and making such disks. The lumen of lamellae of cones is open to the exterior. These are made of shelves, which are made of cones. Whereas, the lumen of rods are stacked on each other and form flattened disks. Photopigments are incorporated as trans membrane proteins in the membranes of disks. These are the primary steps of transduction. The vertebrate photoreceptor cells contain a rudimentary selium, that connects the outer segment to the inner segment. The inner segment contains the cytoplasm, with organelles and nucleus. The third part of the rods and cones is the synaptic body, which is present at the end of the inner segment. This segment connects the rods or cones with the neuronal cells.