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Encyclopedia > Cone cell
Normalized responsivity spectra of human cone cells, S, M, and L types
Normalized responsivity spectra of human cone cells, S, M, and L types

Cone cells, or cones, are photoreceptor cells in the retina of the eye which function best in relatively bright light. The cone cells gradually become more sparse towards the periphery of the retina. Image File history File links Cones_SMJ2_E.svg‎ Simplified human cone response curves, based on Dicklyons PNG version, itself based on data from Stockman, MacLeod & Johnson (1993) Journal of the Optical Society of America A, 10, 2491-2521d (log E human cone response, via http://www. ... Responsivity: In a photodetector, the ratio of the electrical output to the optical input. ... This article is about cellular photoreceptors. ... Human eye cross-sectional view. ... For other uses, see Eye (disambiguation). ... This article does not cite any references or sources. ...


A commonly cited figure of six million in the human eye was found by Osterberg[citation needed] 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.[citation needed] Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. ...


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. Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. ... Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they reflect or emit. ... This article or section does not cite its references or sources. ... Normalised absorption spectra of human cone (S,M,L) and rod (R) cells Trichromatic color vision is the ability of humans and some other animals to see different colors, mediated by interactions among three types of color-sensing cone cells. ... Color blindness in humans is the inability to perceive differences between some or all colors that other people can distinguish. ... A tetrachromat is an organism for which the perceptual effect of any arbitrarily chosen light from its visible spectrum can be matched by a mixture of no more than four different pure spectral lights. ...

Contents

Types

Humans normally have three kinds of cones. The first responds most to light of long wavelengths, peaking in the yellow region; this type is designated L for long. The second type responds most to light of medium-wavelength, peaking at green, and is abbreviated M for medium. The third type responds most to short-wavelength light, of a violet color, and is designated S for short. The three types have peak wavelengths near 564–580 nm, 534–545 nm, and 420–440 nm, respectively.[2][3] The difference in the signals received from the three cone types allows the brain to perceive all possible colors, through the opponent process of color vision. A nanometre (American spelling: nanometer, symbol nm) is a unit of length in the metric system, equal to one thousand-millionth of a metre, which is the current SI base unit of length. ... Opponent colors based on experiment. ...


The color yellow, for example, is perceived when the L cones are stimulated slightly more than the M cones, and the color red is perceived when the L cones are stimulated significantly more than the M cones. Similarly, blue and violet hues are perceived when the S receptor is stimulated more than the other two.


The S 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.[4] At moderate to bright light levels where the cones functions, the eye is more sensitive to yellowish-green light than other colors because this stimulates the two most common of the three kinds of cones almost equally. At lower light levels, where only the rod cells function, the sensitivity is greatest at a blueish-green wavelength. Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens. ... The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eyes optical power [1]. Together with the lens, the cornea refracts light and, as a result, helps the eye to focus. ... For other uses, see Ultraviolet (disambiguation). ... Aphakia is the absence of the lens of the eye, due to surgical removal, perforating wound or ulcer, or congenital anomaly; causes a loss of accommodation, hyperopia, and a deep anterior chamber. ... Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. ...


Structure

Cone cells are larger than rods, and are much less numerous than rods in most parts of the retina, but greatly outnumber rods in the fovea. 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 S cones are a little larger than the others. The fovea, a part of the eye, is a spot located in the center of the macula. ... This article is about the geometric object, for other uses see Cone. ... A micrometre (American spelling: micrometer, symbol µm) is an SI unit of length equal to one millionth of a metre, or about a tenth of the diameter of a droplet of mist or fog. ... The fovea, a part of the eye, is a spot located in the center of the macula. ...


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.[5] Photobleaching is the photochemical destruction of a fluorophore. ...


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] Illustration of the major elements in a prototypical synapse. ... As a part of the retina, the bipolar cell exists between photoreceptors (rod cells and cone cells) and ganglion cells. ... Not to be confused with Psyllium. ... Schematic of typical animal cell, showing subcellular components. ... HeLa cells stained for DNA with the Blue Hoechst dye. ...


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 worn off at the end of the outer segment, to be consumed and recycled by phagocytic cells. Look up cell membrane in Wiktionary, the free dictionary. ... A transmembrane protein is a protein that spans the entire biological membrane. ... Steps of a macrophage ingesting a pathogen: a. ...


Response to light

Activation of a photoreceptor cell is actually a hyperpolarization; when they are not being stimulated, rods and cones depolarize and release a neurotransmitter spontaneously, and activation of photopigments by light sends a signal by preventing this. Depolarization occurs due to the fact that in the dark, cells have a relatively high concentration of cyclic guanosine 3'-5' monophosphate (cGMP), which opens ion channels (largely sodium channels, though Calcium can enter through these channels as well). The positive charges of the ions that enter the cell down its electrochemical gradient change the cell's membrane potential, cause depolarization, and lead to the release of the neurotransmitter glutamate. Glutamate can depolarize some neurons and hyperpolarize others, allowing photoreceptors to interact in an antagonistic manner. Hyperpolarization has several meanings: In biology, hyperpolarization occurs when a cells membrane potential dips below its resting level. ... In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ... Chemical structure of D-aspartic acid, a common amino acid neurotransmitter. ... Cyclic guanosine monophosphate (cGMP) is a second messenger derived from GTP. Cyclic guanosine monophosphate (cGMP) is a cyclic nucleotide derived from guanosine triphosphate (GTP). ... Ion channels are pore-forming proteins that help to establish and control the small voltage gradient that exists across the plasma membrane of all living cells (see cell potential) by allowing the flow of ions down their electrochemical gradient. ... Sodium channels are integral membrane proteins that exist in a cells plasma membrane and regulate the flow of sodium (Na+) ions into it. ... For other uses, see Calcium (disambiguation). ... ... In cellular biology, an electrochemical gradient refers to the electrical and chemical properties across a membrane. ... Membrane potential (or transmembrane potential or transmembrane potential difference or transmembrane potential gradient), is the electrical potential difference (voltage) across a cells plasma membrane. ... Glutamate is the anion of glutamic acid. ...


When light hits photoreceptive pigments within the photoreceptor cell, the pigment changes shape. The pigment, called iodopsin (rhodopsin is found in rod cells) consists of a large protein called opsin (situated in the plasma membrane), attached to which is a covalently-bound prosthetic group: an organic molecule called retinal (a derivative of vitamin A). The retinal exists in the 11-cis-retinal form when in the dark, and stimulation by light causes its structure to change to all-trans-retinal. This structural change causes it to activate a regulatory protein called transducin, which leads to the activation of cGMP phosphodiesterase, which breaks cGMP down into 5'-GMP. Reduction in cGMP allows the ion channels to close, preventing the influx of positive ions, hyperpolarizing the cell, and stopping the release of neurotransmitters (Kandel et al., 2000). Though cone cells primarily use the transmitter substance acetyl choline, rod cells use a variety. The entire process by which light initiates a sensory response is called visual phototransduction. Transducin is the name given to the G-protein alpha-subunits that are naturally expressed in vertebrate retina rods and cones. ... Visual phototransduction is a process by which light is converted into electrical signals in the rod cells and cone cells of the retina of the eye. ...


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 to only 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

Nyctalopia (literally night blindness) is a condition making it difficult or impossible to see in the dark. ... Blindness can be defined physiologically as the condition of lacking sight. ...

See also

Color blindness in humans is the inability to perceive differences between some or all colors that other people can distinguish. ... Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they reflect or emit. ... Cone cells are photoreceptors responsible for both central and color vision. ... A tetrachromat is an organism for which the perceptual effect of any arbitrarily chosen light from its visible spectrum can be matched by a mixture of no more than four different pure spectral lights. ... Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. ...

References

  1. ^ 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. 
  2. ^ 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. 
  3. ^ R. W. G. Hunt (2004). The Reproduction of Colour, 6th ed., Chichester UK: Wiley–IS&T Series in Imaging Science and Technology, 11–12. ISBN 0-470-02425-9. 
  4. ^ 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)
  5. ^ Roorda, A. and Williams, D.R. (1999). The arrangement of the three cone classes in the living human eye. Nature, 397, 520-522.

External links

  • Webvision's Photoreceptors

  Results from FactBites:
 
Cone cell - Wikipedia, the free encyclopedia (830 words)
Cone cells, or cones, are cells in the retina of the eye which only function in relatively bright light.
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.
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.
Cone cell - definition of Cone cell in Encyclopedia (386 words)
Cone cells, or cones, are cells in the retina which only function in relatively bright light.
Cones are less sensitive than the rod cells in the retina (which support vision at low light levels), but allow the perception of color because there are [normally] three kinds of cones, with different photopsins, which have different response curves (that is, they respond to variation in color in different ways).
The S (bluish-violet) cones are sensitive to light at wavelengths shorter than 400 nm, but the lens and cornea of the human eye are increasingly absorbative to these wavelengths, and this sets the lower wavelength limit of human-visible light to approximately 380 nm (the onset of ultraviolet light).
  More results at FactBites »

 
 

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