Cone cell 534710 221218310 2008-06-23T15:48:48Z 130.76.96.16 /* Response to light */ [[Image:Cones SMJ2 E.svg|right|frame|Normalized [[responsivity]] spectra of human cone cells, S, M, and L types]] '''Cone cells''', or '''cones''', are [[photoreceptor cell]]s 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. A commonly cited figure of six million in the human eye was found by Osterberg<ref name="Osterberg">G. Osterberg (1935). “Topography of the layer of rods and cones in the human retina,” Acta Ophthalmol., Suppl. 6, pp. 1–103.</ref> 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 cell]]s in the human retina.{{Fact|date=September 2007}} Cones are less sensitive to light than the [[rod cell]]s in the retina (which support vision at low light levels), but allow [[color vision|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.<ref name="Kandel">{{cite book | first = E.R. | last = Kandel | authorlink = | coauthors = Schwartz, J.H, and Jessell, T. M. | year = 2000 | title = Principles of Neural Science | edition = 4th ed. | pages = 507-513 | publisher = McGraw-Hill | location = New York }}</ref> Because humans usually have three kinds of cones, with different [[photopsin]]s, which have different response curves, and thus respond to variation in color in different ways, they have [[trichromatic vision]]. Being [[color blindness|color blind]] can change this, and there have been reports of people with four or more types of cones, giving them [[tetrachromat]]ic vision. ==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 [[nanometre|nm]], 534–545 nm, and 420–440 nm, respectively.<ref name="Wyszecki">{{cite book | first = Günther | last = Wyszecki | authorlink = | coauthors = Stiles, W.S. | year = 1982 | title = Color Science: Concepts and Methods, Quantitative Data and Formulae | edition = 2nd ed. | pages = | publisher = Wiley Series in Pure and Applied Optics | location = New York | id = ISBN 0-471-02106-7 }}</ref><ref>{{cite book | author = R. W. G. Hunt | year = 2004 | title = The Reproduction of Colour | edition = 6th ed. | pages = 11–12 | publisher = Wiley–IS&T Series in Imaging Science and Technology | location = Chichester UK | id = ISBN 0-470-02425-9 }}</ref> 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]]. 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 (vision)|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.<ref>[http://education.guardian.co.uk/higher/medicalscience/story/0,9837,724257,00.html ''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)</ref> 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 cell]]s function, the sensitivity is greatest at a blueish-green wavelength. ==Structure== Cone cells are somewhat shorter than rods, but wider and tapered, 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 (geometry)|cone]]-like shape at one end where a pigment filters incoming light, giving them their different response curves. They are typically 40-50 [[micrometre|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. [[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.<ref>Roorda, A. and Williams, D.R. (1999). The arrangement of the three cone classes in the living human eye. ''Nature, 397,'' 520-522.</ref> 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]].<ref name="Kandel"/> The inner segment contains [[organelle]]s and the cell's [[cell nucleus|nucleus]], while the outer segment, which is pointed toward the back of the eye, contains the light-absorbing materials.<ref name="Kandel"/> 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 worn off at the end of the outer segment, to be consumed and recycled by [[phagocytosis|phagocytic]] cells. ==Response to light== Activation of a photoreceptor cell is actually a [[hyperpolarization]]; when they are not being stimulated, rods and cones [[depolarization|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 channel]]s (largely [[sodium channel]]s, 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. 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]]. The response of cone cells to light is also directionally nonuniform, peaking at a direction that receives light from the center of the pupil; this effect is known as the [[Stiles–Crawford effect]]. ===Table=== Comparison of rod and cone cells, from Kandel.<ref name="Kandel"/> {| class="wikitable" |- !Rods !Cones |- |Used for [[scotopic vision]] |Used for [[photopic 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 |} ==See also== *[[Color blindness]] *[[Color vision]] *[[Cone dystrophy]] *[[Tetrachromacy]] *[[Rod cell]] ==References== <references/> ==External links== * [http://webvision.umh.es/webvision/photo1.html Webvision's ''Photoreceptors''] {{Retina}} [[Category:Photoreceptor cells]] [[Category:Eye]] [[da:Tap (synet)]] [[de:Zapfen (Auge)]] [[et:Kolvikesed]] [[es:Cono (célula)]] [[fr:Cône (biologie)]] [[ko:원추세포]] [[is:Keila (ljósnemi)]] [[it:Cellula cono]] [[he:תא קולט אור#.D7.9E.D7.93.D7.95.D7.9B.D7.99.D7.9D]] [[lt:Kūgeliai]] [[nl:Kegeltje]] [[ja:錐体細胞]] [[no:Tapper]] [[pl:Czopki]] [[pt:Cone (célula)]] [[ru:Колбочки (сетчатка)]] [[sk:Čapík (oko)]] [[fi:Tappisolu]] [[sv:Tapp (biologi)]]