Vision is a process by which light striking the retina is converted into electrical impulses that are transmitted to the brain.
The impulses produced by light reaching the photoreceptor cells are conveyed to the brain by an elaborate network of nerves. The conversion of the incident light into nerve impulses is called visual processing and involves two basic steps:  Step 1 is a photochemical reaction that occurs in the outer segment of the rod and cone receptors as absorbed light energy causes conformational changes in the chromophores. The activated opsin molecules interact with a G-protein called transducin. Transducin then activates phosphodiesterase, which breaks down cyclic GMP (cGMP). In the dark, high levels of cGMP in the photoreceptor cells are bound to the cytoplasmic surface of Na channels, causing them to stay open. As a result, photoreceptor cells have a low membrane potential.
 Step 2 consists of a decrease in the concentration of cGMP within the cytoplasm of the inner segment of the photoreceptor cells. These changes, which are activated by light energy, decrease the Na permeability of the plasma membrane. When fewer cGMP molecules are bound to Na channel proteins, the photoreceptor becomes hyperpolarized, resulting in a decrease of neurotransmitter (glutamate) secretion. This decrease in glutamate secretion is detected by bipolar cells of the retina, which initiate electrical impulses that are conveyed to the brain.
In rods, absorbed light energy causes conformational changes in retinal, converting it to retinol.
The conversion of retinal to retinol results in its release from scotopsin (a reaction called bleaching). The energy for this process is provided by the mitochondria located in the inner segment. Mler’s cells and pigment epithelial cells
FIGURE 24.12 • Electron micrographs of portions of the inner and outer segments of cones and rods. a. This electron micrograph shows the junction between the inner and outer segments of the rod cell. The outer segments contain the horizontally flattened discs. The plane of this section passes through the connecting stalk and cilium. A centriole, a cilium and its basal body, and a calyceal process are identified. 32,000. b. Another electron micrograph shows a similar section of a cone cell. The interior of the discs in the outer segment of the cone is continuous with the extracellular space (arrows). 32,000. (Courtesy of Dr. Toichiro Kuwabara.) also participate in the interconversion of retinal and retinol and the reactions necessary for the resynthesis of rhodopsin.
During normal functioning of the photoreceptor cells, the membranous discs of the outer segment are shed and phagocytosed by the pigment epithelial cells (Fig. 24.13). It is estimated that each of these cells is capable of phagocytosing and disposing of about 7,500 discs per day. The discs are constantly turning over, and the production of new discs must equal the rate of disc shedding.
Discs are shed from both rods and cones.
In rods, after a period of sleep, a burst of disc shedding occurs as light first enters the eye. The time of disc shedding in cones is more variable. The shedding of discs in cones also enables the receptors to eliminate superfluous membrane. Although not fully understood, the shedding process in cones also alters the size of the discs so that the conical form is maintained as discs are released from the distal end of the cone.