Cone cell
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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. This figure, commonly cited, was found by Osterberg in 1935. Oyster's textbook (1999) cites work by Curcio et al. (1990) indicating an average closer to 4.5 million cone cells and 90 million rod cells in the human retina. The sampling methodology in these modern papers include computer models and statistical samples. In this latter study several retinas were used and there was significant variability in the densities of the cells in each region of the retina. The cone cells gradually become more sparse towards the periphery of the retina.
Cones are less sensitive to light than the rod cells in the retina (which support vision at low light levels), but allow the perception of color. They are also able to perceive finer detail and more rapid changes in images, because their response times to stimuli are faster than those of rods.[1] Because humans usually have three kinds of cones, with different photopsins, which have different response curves, and thus respond to variation in color in different ways, they have trichromatic vision. Being color blind can change this, and there have been reports of people with four or more types of cones, giving them tetrachromatic vision.
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[edit] 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 S. They have peak wavelengths of 564 nm, 534 nm, and 420 nm respectively.[2] 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. People with aphakia, a condition where the eye lacks a lens, sometimes report the ability to see into the ultraviolet range.[3] The eye is more sensitive to green light than other colors because this stimulates two of the three kinds of cones almost equally.
[edit] Structure
Cone cells are larger and less numerous than rods. Structurally, cone cells have a cone-like shape at one end where a pigment filters incoming light, giving them their different response curves. They are typically 40-50 µm long, and their diameter varies from .50 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.
Photobleaching can be used to determine cone arrangement. This is done by exposing dark-adapted retina to a certain wavelength of light that paralyzes the particular type of cone sensitive to that wavelength for up to thirty minutes from being able to dark-adapt making it appear white in contrast to the grey dark-adapted cones when a picture of the retina is taken. The results illustrate that S cones are randomly placed and appear much less frequently than the M and L cones. The ratio of M and L cones varies greatly among different people with regular vision.[4]
Like rods, each cone cell has a synaptic terminal, an inner segment, and an outer segment as well as an interior nucleus and various mitochondria. The synaptic terminal forms a synapse with a neuron such as a bipolar cell. The inner and outer segments are connected by a cilium.[1] The inner segment contains organelles and the cell's nucleus, while the outer segment, which is pointed toward the back of the eye, contains the light-absorbing materials.[1]
Like rods, the outer segments of cones have invaginations of their cell membranes that create stacks of membranous disks. Photopigments exist as transmembrane proteins 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.
[edit] Table
Comparison of rod and cone cells, from Kandel.[1]
| Rods | Cones |
|---|---|
| used for night vision | used for day vision |
| very light sensitive; sensitive to scattered light | not very light sensitive; sensitive only to direct light |
| loss causes night blindness | loss causes legal blindness |
| low visual acuity | high visual acuity; better spatial resolution |
| not present in fovea | concentrated in fovea |
| slow response to light, stimuli added over time | fast response to light, can perceive more rapid changes in stimuli |
| have more pigment than cones, so can detect lower light levels | have less pigment than rods, require more light to detect images |
| stacks of membrane-enclosed disks are unattached to cell membrane | disks are attached to outer membrane |
| 20 times more rods than cones in the retina | |
| one type of photosensitive pigment | three types of photosensitive pigment in humans |
| confer achromatic vision | confer color vision |
[edit] See also
[edit] References
- ^ a b c d Kandel, E.R.; Schwartz, J.H, and Jessell, T. M. (2000). Principles of Neural Science, 4th ed., New York: McGraw-Hill, 507-513.
- ^ Wyszecki, Günther; Stiles, W.S. (1982). Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed., New York: Wiley Series in Pure and Applied Optics. ISBN 0-471-02106-7.
- ^ Let the light shine in: You don't have to come from another planet to see ultraviolet light EducationGuardian.co.uk, David Hambling (May 30, 2002)
- ^ Roorda, A. and Williams, D.R. (1999). The arrangement of the three cone classes in the living human eye. Nature, 397, 520-522.
[edit] External links
Photoreceptor cells (Cone cell, Rod cell) → (Horizontal cell) → Bipolar cell → (Amacrine cell) → Ganglion cell
Giant retinal ganglion cells | Photosensitive ganglion cell
Inner limiting membrane - Nerve fiber layer - Ganglion cell layer - Inner plexiform layer - Inner nuclear layer - Outer plexiform layer - Outer nuclear layer - External limiting membrane - Layer of rods and cones - Retinal pigment epithelium
