| Cone cells | |
|---|---|
 Normalizedresponsivity spectra of human cone cells, S, M, and L types | |
| Details | |
| Location | Retina of vertebrates |
| Function | Color vision |
| Identifiers | |
| MeSH | D017949 |
| NeuroLex ID | sao1103104164 |
| TH | H3.11.08.3.01046 |
| FMA | 67748 |
| Anatomical terms of neuroanatomy | |
Cone cells orcones arephotoreceptor cells in theretina of the vertebrateeye. Cones are active in daylight conditions and enablephotopic vision, as opposed torod cells, which are active in dim light and enablescotopic vision. Most vertebrates (including humans) have several classes of cones, each sensitive to a different part of thevisible spectrum oflight. The comparison of the responses of different cone cell classes enablescolor vision. There are about six to seven million cones in a human eye (vs ~92 million rods), with the highest concentration occurring towards themacula and most densely packed in thefovea centralis, a0.3 mm diameter rod-free area with very thin, densely packed cones. Conversely, like rods, they are absent from theoptic disc, contributing to theblind spot.[1]
Cones are less sensitive to light than therod cells in the retina (which support vision at low light levels), but allow theperception of color. They are also able to perceive finer detail and more rapid changes in images because their response times tostimuli are faster than those of rods.[2] In humans, cones are normally one of three types: S-cones, M-cones and L-cones, with each type bearing a differentopsin:OPN1SW,OPN1MW, andOPN1LW respectively. These cones are sensitive to visible wavelengths of light that correspond to short-wavelength, medium-wavelength and longer-wavelength light respectively.[3] Becausehumans usually have three kinds of cones with differentphotopsins, which have different response curves and thus respond to variation in color in different ways, humans havetrichromatic vision. Beingcolor blind can change this, and there have been some verified reports of people with four types of cones, giving themtetrachromatic vision.[4][5][6]The three pigments responsible for detecting light have been shown to vary in their exact chemical composition due togenetic mutation; different individuals will have cones with different color sensitivity.
Most vertebrates have several different classes of cone cells, differentiated primarily by the specificphotopsin expressed within. The number of cone classes determines the degree ofcolor vision. Vertebrates with one, two, three or four classes of cones possessmonochromacy,dichromacy,trichromacy andtetrachromacy, respectively.
Humans normally have three classes of cones, designatedL,M andS for the long, medium and short wavelengths of the visible spectrum to which they are most sensitive.[7] L cones respond most strongly to light of the longer redwavelengths, peaking at about560 nm. M cones, respond most strongly to yellow to green medium-wavelength light, peaking at530 nm. S cones respond most strongly to blue short-wavelength light, peaking at420 nm, and make up only around 2% of the cones in the human retina. The peak wavelengths of L, M, and S cones occur in the ranges of564–580 nm,534–545 nm, and420–440 nm, respectively, depending on the individual.[citation needed] The typical human photopsins are coded for by the genesOPN1LW,OPN1MW, andOPN1SW. TheLMS color space is an often-used model of spectral sensitivities of the three cells of a typical human.[8][9]
Cone cells are shorter but wider thanrod cells. They are typically40–50 μm long, and their diameter varies from0.5–4.0 μm. They are narrowest at the fovea, where they are the most tightly packed. The S cone spacing is slightly larger than the others.[10]
Like rods, each cone cell has a synaptic terminal, inner and outer segments, as well as an interior nucleus and variousmitochondria. The synaptic terminal forms asynapse with a neuronbipolar cell. The inner and outer segments are connected by acilium.[2] The inner segment containsorganelles and the cell's nucleus, while the outer segment contains the light-absorbingphotopsins, and is shaped like acone, giving the cell its name.[2]
The outer segments of cones have invaginations of theircell membranes that create stacks of membranous disks.Photopigments exist astransmembrane 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 byphagocytic cells.
While rods outnumber cones in most parts of the retina, thefovea, responsible for sharp central vision, consists almost entirely of cones. The distribution of photoreceptors in the retina is called theretinal mosaic, which can be determined usingphotobleaching. 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 thatS cones are randomly placed and appear much less frequently than theM andL cones. The ratio ofM andL cones varies greatly among different people with regular vision (e.g. values of 75.8%L with 20.0%M versus 50.6%L with 44.2%M in two male subjects).[12]
The difference in the signals received from the three cone types allows the brain to perceive a continuous range of colors through theopponent process of color vision.Rod cells have a peak sensitivity at498 nm, roughly halfway between the peak sensitivities of the S and M cones.
All of the receptors contain the proteinphotopsin. Variations in its conformation cause differences in the optimum wavelengths absorbed.
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. S Cones are most sensitive to light at wavelengths around420 nm. At moderate to bright light levels where the cones function, the eye is more sensitive to yellowish-green light than other colors because this stimulates the two most common (M and L) 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.
Cones also tend to possess a significantly elevated visual acuity because each cone cell has a lone connection to the optic nerve, therefore, the cones have an easier time telling that two stimuli are isolated. Separate connectivity is established in theinner plexiform layer so that each connection is parallel.[13]
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 theStiles–Crawford effect.
S cones may play a role in the regulation of thecircadian system and the secretion ofmelatonin, but this role is not clear yet. Any potential role of the S cones in the circadian system would be secondary to the better established role ofmelanopsin (see alsoIntrinsically photosensitive retinal ganglion cell).[14]
Sensitivity to a prolonged stimulation tends to decline over time, leading toneural adaptation. An interesting effect occurs when staring at a particular color for a minute or so. Such action leads to an exhaustion of the cone cells that respond to that color – resulting in theafterimage. This vivid color aftereffect can last for a minute or more.[15]
List of distinct cell types in the adult human body
Equipped with four receptors instead of three, Mrs M - an English social worker, and the first known human "tetrachromat" - sees rare subtleties of colour.
A tetrachromat is a woman who can see four distinct ranges of color, instead of the three that most of us live with.
| Fibrous tunic (outer) |
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| Uvea / vascular tunic (middle) |
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| Retina (inner) |
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| Anatomical regions of the eye |
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