Convex lens increase light convergence and so shorten the focal length. Uneven cylindrical lens needed to correct. In the retina there are two types of photoreceptors . Neural layers of retina are a forward extension of the brain, and so the light must pass through several layers before reaching the photoreceptors. Bipolar cells connect them to the photoreceptors.
Rods contain the purple pigment rhodopsin .
The dissociation causes changes in ionic permeability of the cell and so allows the creation of nerve impulses. In cones, the trichromatic theory of color vision suggests that our color perception dpends on stimulation of only 3 cone types. The fovea centralis is within the macula lutea , and there is the greatest concentration of photoreceptors. Olfaction involves actively dividing receptor neurons , with receptors linked with G-proteins. Anosmia can follow head injuries that damage the olfactory nerves or infections that damage the cells, and will contribute to ageusia because of the contribution of smell to taste.