Every image that the eye sees is produced by millions of photoreceptors in the retina of the eye. These photoreceptors, known as rods and cones, are specialized cells that are sensitive to light and convert light into nerve signals. Rods allow us to see in low light situations while cones provide us with color vision in bright light. The nerve signals produced by photoreceptors are transported to the brain, where the brain makes sense of these signals and produces the sense of vision.
The retina is a thin sheet of nervous tissue covering the posterior half of the eye. It is surrounded by the choroid and sclera of the eye and held in place by the clear, jelly-like vitreous body that fills the eye’s interior. Within the retina are millions of photoreceptor cells, bipolar cells, and ganglion cells that work together to detect light and send visual information to the brain.
Photoreceptors consist of two cell types: rod cells and cone cells. Rod cells are long, narrow cells that contain the photopigment rhodopsin. Rhodopsin is concentrated in a rod-shaped region of folded membrane discs on the exterior-facing end of the rod cells. The middle of the rod cell contains the nucleus, mitochondria, and most other organelles while the interior end widens to form a synaptic terminal. Cone cells are similar in shape to rod cells, except that the exterior-facing end of the cell is shaped like a cone. The cone is made of many folds of cell membrane and contains red, green, or blue opsin photopigment.
Rods and cones follow a common physiological pathway, but produce very different results due to differences in their structures. Rods have evolved for vision in low-light and are therefore very sensitive to tiny amounts of light. The discs in the rod are arranged so that a single photon of light with interact with the photopigment to produce visual stimulus. Cone cells, on the other hand, have folded membranes in their cones that are less sensitive to light, but produce respond faster to changes and are more resistant to the bleaching caused by intense light.
Both rods and cones contain photopigments, but their photopigments differ in structure and function. Photopigments are large molecules consisting of retinal, a chemical produced from vitamin A, and an opsin protein. Rods use the photopigment rhodopsin, which is sensitive to blue and green light. Cones use retinal and one of three different color-specific opsin proteins. Long-wave opsin detects light in the red, orange, and yellow range while medium-wave opsin detects light in the yellow, green, and cyan range. Short-wave opsin detects light in the blue to violet range. All three types of cones overlap their range of sensitivity to produce a continuous spectrum of visible light.
The distribution of rods and cones varies within the retina. While there are five to six million cones in the retina, there are a whopping 90 to 120 million rods. Most of the cones are found in the center of the retina in the macula lutea, particularly in the central fovea in the center of the macula. Rods are found more commonly in the peripheral parts of the retina surrounding the macula.
Displayed on other page