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Cone cells, or '''cones''', are Cells in the Retina of the Eye which only function in relatively bright Light . There are about 6 million in the human eye, concentrated at the Fovea . They gradually become more sparse towards the outside of the retina. TYPES The three kinds of cones typically respond most to yellowish-green light, which has long wavelength and is abbreviated L, bluish-green medium-wavelength light abbreviated '''M''', and blue-violetish short-wavelength light abbreviated or '''S''' light. They have peak wavelengths of 564 Nm , 534 nm, and 420 nm respectively. The difference in the signals received from the three kinds allows the brain to perceive a wide range and Gamut of different colors. The color yellow, for example, is perceived when the yellow-green receptor is stimulated slightly more than the blue-green receptor, and the color red is perceived when the yellow-green receptor is stimulated significantly more than the blue-green receptor. Similarly, blue and blue-like hues are perceived when the blue-violet receptor is stimulated more than the other two. The S bluish-violet cones are most sensitive to light at wavelengths around 420 nm. However, the Lens and Cornea of the human eye are increasingly absorbative to smaller wavelengths, and this sets the lower wavelength limit of human-visible light to approximately 380 nm, which is therefore called ' Ultraviolet ' light. The eye is more sensitive to green light than other colors because this stimulates two of the three kinds of cones almost equally. STRUCTURE Cone cells are larger and less numerous than rods. Structurally, cone cells have a Cone -like shape at one end where the pigment that filters incoming light, giving them their different response curves. They are typically 50 µm long, and their diameter varies from 1.0 to 4.0 µm, being smallest and most tightly packed at the center of the eye at the Fovea . The blue-sensitive cells are a little larger than the others, and an order of magnitude less common. Like rods, each cone cell has a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a Synapse with a neuron such as a Bipolar Cell . The inner and outer segments are connected by a Cilium . The inner segment contains Organelles and the cell's Nucleus , while the outer segment, which is pointed toward the front of the eye, contains the light-absorbing materials . Like rods, the outer segments of cones have invaginations of their Cell Membrane s that create stacks of membranous disks. Photopigments exist as Transmembrane Protein s within these disks, which provide more surface area for light to affect the pigments. In cones, these disks are attached to the outer membrane, whereas they are pinched off and exist separately in rods. Neither rods nor cones divide, but their membranous disks wear out and are sloughed off at the end of the outer segment, to be consumed and recycled by Phagocytic cells. Table Comparison of rod and cone cells, from Kandel . SEE ALSO REFERENCES EXTERNAL LINKS |
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