Anatomy & Physiology

Anatomy & Physiology Study Guide special organic compounds called visual pigments . The absorption of photons by visual pigments is the first key step in the process of photoreception—the detection of light. Photoreception The plasma membrane in the outer segment of the photoreceptor contains chemically gated sodium ion channels. Because the channels are open, the transmembrane potential is approximately –40 mV, rather than the –70 mV typical of resting neurons. At the –40 mV transmembrane potential, the photoreceptor is continuously releasing neurotransmitters (in this case, glutamate) across synapses in the inner segment. Sodium ions are constantly extracted from the cytoplasm by the inner segment. The transportation of sodium ions into the outer segment to the inner segment and out of the cell is known as the dark current. 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 D rk 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 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.

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