Cone or cone cell , anatomically neuron coniferum ( Latin conifer , `` cone bearing ''), is the name of a type of photoreceptor in the retina of the vertebrate eye with a cone-shaped extension, the cone , anatomically Conus retinae (Latin conus , cone '). Cone cells are neurons that serve as specialized sensory cells for photopic vision in daylight and are necessary for the perception of colors .
Cones and color vision
A cone is a type of light-sensitive cell in the retina of the eye. There are photoreceptors of vertebrates, together with the chopsticks , the vision allow. Cones are only active when there is sufficient illuminance as they are not very sensitive to light. Since most vertebrates have at least two types of cones with different spectral sensitivity, they are usually able to perceive colors. Humans have three different types of cones whose absorption maxima are around 455 nm , 535 nm and 563 nm; these wavelengths correspond to the colors blue-violet, emerald green and yellow-green. The middle cone type is relatively new in evolution and improves the ability to distinguish between yellow, red and green tones. The predominant activation of certain types of cones results in corresponding color tones, balanced irritation of all cone types, on the other hand, results in the impressions gray to white. In the case of excessive illuminance - both in individual areas as gloss or in the entire field of vision - the cones are overexcited (saturated) and glare occurs .
The rods are already saturated at much lower light intensities, much more sensitive in terms of brightness , which is why they do not contribute to the visual impression in bright daylight. Since there is only one type of rod in humans, no color impression can result from the rod. When the amount of light decreases, the cones are no longer exposed to sufficient light, which means that the color tones seem to disappear, for example during dusk. With the low exposure intensity, only rods are sufficiently stimulated.
The term "cone" was used differently in historical contexts and referred to the staphyloma on the eye.
Types of cones in humans
There are three different types of cones in humans.
S hort receptor wavelength or K-pin ( k urze wavelengths ). This receptor covers the blue area of the visible color spectrum . The absorption maximum is at a wavelength of around 455 nm ( blue-violet ). Older sources give an absorption maximum of 420 nm . Both specifications are in the range of blue light, therefore it is also called blue receptor . The human S cone is genetically closely related to the UV cones of other vertebrates. One speaks therefore of the S2 cone - in contrast to the S cone in other vertebrates.
The blue receptor is rarelyaffectedby color ametropia, but if it is not present, the ametropia tritanopia develops. Hence the outdated designation T-cones .
- M spigot
M edium wavelength receptor . The absorption maximum of the green receptor is around 534 nm ( emerald green ), it covers a range between blue and orange light. The human M-cone is genetically closely related to the L-cone. It is believed that it developedfrom the L-conethrough gene duplication only a few million years ago. L and M cones arelocated next to each otheron the X chromosome . Although there are four to six copies of its gene, the M cone is often responsible for color ametropia in humans because it is located at a crossing-over point on the X chromosome.
If there are no green receptors, the ametropia, deuteranopia, develops . This is where the outdated designation D-spigot comes from.
L ong wavelength receptor . Its absorption maximum is around 563 nm ( yellow-green ). Nevertheless, it is also referred to as a red receptor because it takes on the main task for the perception of red light. The human L-cone is phylogenetically old and corresponds to that of all other vertebrates. It is coded on the X chromosome.
If there are no red receptors, the ametropia protanopia develops. Hence the outdated designation P-cones .
Number of cone types of different animals
- 0 tenon types
- At least two nocturnal primate species have no functioning cones; they see no colors, only light and dark.
- 2 types of cones
- Most mammals (such as cats and dogs) have only two types of cones ( dichromatic color vision); the M cone is absent, which is comparable to red-green blindness . The usual diurnal vertebrates oil droplets as well as the most existing double cones are still in monotremes (monotremes) and marsupials before (marsupials), absent in placental animals , however.
- 4 types of cones
- The originally assumed tetrachromatic blueprint of vertebrates contains four different types of cones: UV, S, M and L cones, in which the wavelengths of the respective absorption maximum (in the order mentioned) are 90 to 100 nm apart. Likewise, many arthropods such as insects and jumping spiders as well as numerous birds see tetrachromatic. About twelve percent of all European women are tetrachromats with an additional abnormal L or M cone, but only a few can use this to more precisely distinguish between colors.
- 12 tenon types
- The Fangschreckenkrebs Neogondodactylus oerstedii eight different types of receptors in the visible and four in the UV range.
Spectral absorption curves
The absorption curve of a type of cone depends on the structure of the opsin of its visual pigment, iodopsin . Photochemical transduction, i.e. the conversion of light signals into neural information, works very similarly in rods and cones and the same in cone types. The reaction of a cone type to a certain spectral component of the light is therefore determined by its opsin type.
A weighted sum of the absorption curves that determine the receptors describes the spectral light sensitivity curve for daytime vision (the V (λ) curve ), its maximum is 555 nm, which corresponds to the color medium green for monochromatic light (the value also serves as a definition des candela and derived units such as lux ). The weighted sum of the three curves averaged for the standard observer describes the CIE tristimulus curve .
Interconnection model of the color cells
The connection of the cones in the human eye is illustrated by the following graphic:
Distribution on the retina
There are 6 million cones and around 120 million rods in the photoreceptor layer ( stratum neuroepitheliale ) of the human retina.
The proportion of blue-sensitive cones in all people is almost constant at twelve percent. The ratio of red and green cones on the retina varies greatly within a family. The density of the cones varies between species. In humans , the density of the cones on the retina is greatest in the center, the fovea centralis or “pit of vision”, the area of sharpest vision, and decreases towards the periphery . Conversely, the density of the rods increases from the center to the periphery. The differentiation between rods and cones has functional reasons: The cones only work in lightness and twilight and make color vision possible, while in the dark twilight or in almost complete darkness, mainly only the rods function due to their much higher light sensitivity. The rods are even able to perceive individual photons in absolute darkness , whereby the perception can be considerably disturbed by spontaneous reactions to warmth, intraocular pressure or very strong magnetic fields .
Cell biological structure
However, there are differences:
- The cones are much wider than the rods.
- In both cell types, the phototransduction takes place in the outer segment (OS) by means of the retinal-coupled seven transmembrane protein opsin . The visual pigment (opsin plus retinal) is called iodopsin in the cones and is embedded in many membrane folds. In the case of the rods, the rhodopsin (“visual purple”) similar to iodopsin is located in so-called “disks”.
- The outer segments of the cones are shorter and have to be contacted by the retinal pigment epithelium (RPE) through elongated flap-like extensions, in contrast to the rod outer segments.
An outer segment is connected to the inner segment via a modified cilium in a decentralized position, the connecting cilium ("Connecting cilium", CC). Nine microtubule doublets in a nonagonal arrangement form the inner structure of this immobile cilium.
This is followed by the metabolically active inner segment ("inner segment", IS). This in turn can be divided into the mitochondria- rich ellipsoid and the myoid, which contains the endoplasmic reticulum (ER). Among other things, protein biosynthesis takes place here.
The next layer is the outer nuclear layer (ONL), which contains the cell nucleus with the cell body. This is followed by the outer plexiform layer ("Outer plexiform layer", OPL) with a synaptic region. The synapses at the proximal end of the photoreceptors are partly flat, partly indented membrane sites. The latter are so-called “ribbon synapses”, referring to a ribbon or plate-like structure directly on the active zone of the presynapse. Many synaptic vesicles are coupled to the ribbon structure and a much higher number of vesicles can be released per unit of time compared to “normal” synapses.
In the dark, the presynaptic membrane of the cone (or rod) continuously releases the neurotransmitter glutamate . When light hits the cone, a signal transduction cascade closes sodium ion channels in the cone cell membrane . Since the cone loses potassium ions through its inner segment, due to the high potassium concentration there and the potassium ion channels expressed there, it develops a negative receptor potential, ie it hyperpolarizes and thus releases less or no more glutamate.
The neurotransmitter glutamate can have an exciting or inhibiting effect on the downstream bipolar cells , because there are two different types of bipolar cells, so-called ON-bipolar and OFF-bipolar. If a cone is connected to an ON bipolar, the reduced release of glutamate upon exposure causes a depolarization of the ON bipolar. Glutamate has an inhibitory effect on the ON bipolar, so there is no inhibition when exposed to light. This effect is based on the fact that the ON bipolar metabotropic glutamate receptors called mGluR6 are stored in the postsynaptic membrane . In the dark, mGluR6 receptors occupied with glutamate activate a signal cascade in the ON bipolar that closes cation channels , ie the cell becomes inexcitable. If glutamate is missing, the mGluR6 receptors remain unoccupied, the cation channels of the ON bipolar open, they depolarize and pass on the excitation. This mechanism converts the hyperpolarization caused in the cones on the side of the ON bipolar into a depolarization during exposure, i.e. ON bipolar cells are excited when exposed to light and inhibited by darkening.
The second type, the OFF bipolar, react to light and thus reduced glutamate release of the cones with hyperpolarization. They have ionotropic glutamate receptors that close unoccupied cation channels. That is, OFF bipolar are inhibited by light and aroused by darkening.
The separation of the ON and OFF bipolar connections is retained in the entire subsequent visual pathway up to the brain. Glutamate is actually considered a typical excitatory neurotransmitter. This system shows that in the end the postsynaptic glutamate receptors decide on arousal or inhibition.
Development of color vision in primates
For color vision to be possible, there must be cone cells in the retina of the eyes that contain certain types of photo pigments (light-sensitive molecules). There are three types of photo pigment in most people's eyes . Together with an appropriately structured nervous system, they enable trichromatic (ie based on the combination of three primary colors) color vision. The result is the amazing ability of humans to distinguish around two million color nuances.
The color perception of mammals and thus also of humans, great apes and primates differs in some cases significantly with regard to the number of visual pigments and thus the perceptible color nuances.
Only the primates of Asia and Africa (old world monkeys), to which humans belong, as well as some of the South American new world monkeys have acquired the ability of improved color vision in the course of their evolution. The ability of many primates to distinguish red from green can have vital benefits. For example, colored ripe fruits can be recognized more quickly between the green leaves or the young, more easily digestible leaves can be distinguished from older, more difficult to digest. The nutrient-rich leaves are often slightly red in color, as are many ripe fruits, which can even contain toxins when they are still green. Therefore this ability prevailed in evolution. A duplication of a gene has been demonstrated in Old World monkeys, which slightly changed the longer-wave visual pigment. As a result, in addition to the information for the blue pigment, the genetic material also contained a red- and a green-sensitive visual pigment. In many primates, this resulted in the three types of cones with their different absorption maxima .
Research has shown that the development of color vision is closely related to a decrease in the sense of smell, so some of this ability was lost in favor of color vision in these primates. Primates with better odor perception, on the other hand, have a less well-developed ability to distinguish colors.
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