Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 195 of 367 Each photoreceptor in the retina oversees a certain receptive field. The retina holds about 130million photoreceptors, 6 million bipolar cells, and 1 million ganglion cells. Thus, a considerable amount of convergence occurs at the start of the visual pathway. The degree of convergence differs between rods and cones. As many as a thousand rods may pass information via their bipolar cells to a single ganglion cell. The ganglion cells that monitor rods are called M cells . M cells provide information regarding the general form of an object, motion, and shadows in dim lighting. Because so much convergence occurs, the activation of an M cell indicates that light has arrived in a general area rather than at a specific location. This kind of retinal processing within ganglion receptive fields improves the detection of the edges of objects within the visual field. Cones typically show very little convergence; in the fovea, the ratio of cones to ganglion cells is 1:1. The ganglion cells that monitor cones, called P cells , are smaller and more numerous than M cells. P cells are active in bright light, and they provide information about edges, fine detail, and color. Because little convergence occurs, the activation of a P cell means that light has arrived at one specific location. As a result, cones provide more precise information about a visual image than do rods. Color Vision An ordinary light bulb or the sun emits photons of all wavelengths. These photons stimulate both rods and cones. When all three types of cones are stimulated, or when rods alone are stimulated, you see a “white” light. Your eyes also detect photons that reach your retina after they bounce off objects around you. If photons of all colors bounce off an object, the object will appear white to you. If every photon is absorbed by the object (so that none reach the retina), the object will appear black. An object will appear to have a particular color if it reflects (or transmits) photons from one portion of the visible spectrum and absorbs the rest, with sensitivity to a different range of wavelengths. Their stimulation in various combinations is the basis for color vision. In a person with standard vision, the cone population consists of 16 percent blue cones, 10 percent green cones, and 74 percent red cones. Color discrimination occurs through the integration of information arriving from all three types of cones. Persons who are unable to distinguish certain colors have a form of color blindness . Color blindness happens when one or more classes of cones are nonfunctional. Light and Dark Adaptation The sensitivity of your visual system varies with the intensity of illumination. After 30 minutes or more in the dark, almost all visual pigments will have recovered from photobleaching and be fully receptive to stimulation. This is the dark-adapted state. When dark-adapted, the visual system is extremely sensitive. When the lights come on, at first, they seem almost unbearably bright, but over the next fewminutes, your sensitivity decreases as bleaching occurs. Eventually, the rate of bleaching is balanced by the rate at which the visual pigments re-form. This condition is the light-adapted state. Constriction of the pupil, via the pupillary constrictor reflex, reduces the amount of light entering your eye to one-thirtieth the maximum dark-adapted level. Dilating the pupil fully can produce a thirtyfold increase in the amount of light entering the eye. Processing by the Retina