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rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than the other type of visual photoreceptor, cone cells. Rods are concentrated at the outer edges of the retina and are used in peripheral vision. On average, there are approximately 90 million rod cells in the human retina.〔Curcio, C. A., K. R. Sloan, et al. (1990). "Human photoreceptor topography." The Journal of Comparative Neurology 292(4): 497-523.〕 More sensitive than cone cells, rod cells are almost entirely responsible for night vision. However, because they have only one type of light-sensitive pigment, rather than the three types that human cone cells have, rods have little, if any, role in color vision (which is why colors are much less apparent at night).
==Structure==
Rods are a little longer and leaner than cones but have the same structural basis. The opsin or pigment is on the outer side, lying on the Retinal pigment epithelium, completing the cell's homeostasis. This epithelium end contains many stacked disks. Rods have a high area for visual pigment and thus substantial efficiency of light absorption.
Like cones, rod cells have a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with another neuron, for example a bipolar cell. The inner and outer segments are connected by a cilium,〔 which lines the distal segment.〔"Photoreception" McGraw-Hill Encyclopedia of Science & Technology, vol. 13, p.460 2007〕 The inner segment contains organelles and the cell's nucleus, while the rod outer segment (abbreviated to ROS), which is pointed toward the back of the eye, contains the light-absorbing materials.〔
A human rod cell is about 2 microns in diameter and 100 microns long.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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