Cone cell
The cones are specialized as photoreceptors sensory cells of the vertebrate eye and are found mainly in central areas of the retina. In humans, there are three types of cones, the S- type ( blue receptor ), M- type ( green receptor ) and L- type ( Rotrezeptor ). Your stimulus response describes the spectral absorption curve, which forms the basis of colorimetry. There are vertebrates with two, three or four types of cones.
Cones and color vision
As a pin is referred to some type of light-sensitive cells in the retina of the eye. There are photoreceptors of vertebrates, which together with the chopsticks vision. Pins are only active when a sufficient intensity, since they are not very sensitive to light. Since at least two types of cones of different spectral sensitivities occur in most vertebrates, they usually color perception is possible. Man has three different types of cones, making the distinction of yellow, red and green tones is improved. The predominant activation of certain types of cones results in corresponding colors, balanced contrast, stimulation of all types of cones gives the impression of gray to white. If excessive illumination - both only in some places as a gloss or over the entire field - are the pins overwrought ( saturated) and there is a glare. The rods are already saturated at much lower light intensities, and therefore much more sensitive to the brightness, which is why they do not contribute in bright daylight for visual impression. Since only one type of rod is present in humans, no color impression can be given by the chopsticks. With decreasing amount of light the pins are no longer strong enough exposed, thus the colors seem to disappear, such as during dusk. In the low exposure intensity only rods are sufficiently irritated.
The term " pin " was used in different historical contexts and then referred to the staphyloma on the eye.
Cone types
There are three different types of human pin.
Short wavelength receptor or K -cones ( short wavelength ). This receptor covers the blue region of the visible color spectrum. The absorption maximum is at a wavelength of about 420 nanometers ( blue-violet ), therefore it is also called blue receptor. The S-cones of man is genetically very closely related to the UV pin other vertebrates. One speaks therefore of S2 - pin - in contrast to the S-cones in other vertebrates. A color deficiency rarely affects the blue cones.
The obsolete designation T - pins indicates the disease tritanopia, which is caused by the lack of this type PTO.
Medium wavelength receptor. The absorption maximum of the green receptor is about 534 nanometers ( emerald green), it covers an area between the blue and orange light. The human M-cones is genetically very closely related to the L cones. It is believed that he developed through a gene duplication of the L cones million only a few years ago. L and M pins are located on the X chromosome, next to each other. Although there are four to six copies of it, the M cones is often responsible for the color vision defects in humans, since it is located at a crossing over location of the X chromosome.
The obsolete designation D- pin points to the disease deuteranopia, which is caused by the lack of this power take-off types.
Long wavelength receptor. Its absorption maximum is at about 563 nanometers ( green ). Despite this color value, he is also known as Rotrezeptor, as he takes over the main power in the perception of the red range. The L cones of man is evolutionarily old and comply with all other vertebrates. It is also encoded on the X chromosome.
The obsolete designation P- pin points to the disease protanopia, which is caused by the lack of this power take-off types.
Spectral absorption curves
The spectral absorption curves describe the stimulation response of the three color receptors, depending on the wavelength of the applied light stimulus.
The absorption curve of a journal type is the construction of the visual pigment opsin his, the iodopsin, depending. The photochemical transduction, ie the conversion of light signals in neural information, works the same in rods and cones.
The sum of the three absorption curves that determine the receptors, describes the spectral luminosity curve for the day marriages (the V ( λ ) - curve), its maximum is located at 555 nanometers, the color medium green (the value is also used for definition of the candela). The weighted and averaged for the standard observer sum of the three curves describes the CIE tristimulus curve. Your maximum shifts thereby 546 nanometers ( leaf green ). The maximum for the night vision of the rods is, however, at 498 nm (cyan).
Interconnection model of color cells
The interconnection of the pins in the human eye according to the following principle:
Distribution on the retina
In the photoreceptors layer (stratum neuroepithelial ) of the retina of the people there are 6 million cones and about 120 million rods.
The proportion of blue -sensitive cones is almost constant twelve percent of all people. The ratio of the 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 cones on the retina at the center (the fovea, the area of sharpest vision ) and decreases from the periphery towards. Conversely, the density of the rod from the center to the periphery. The differentiation into rods and cones has functional reasons: the pin function only in daylight and twilight and make color vision possible, while in the dark twilight or almost complete darkness because of their much higher sensitivity to light essentially only work the chopsticks. The rods are even able to perceive individual photons in absolute darkness, where this perception can be significantly disrupted by spontaneous reactions to heat, intraocular pressure, or very strong magnetic fields.
Cell biological structure
In construction, the two types of photoreceptors - rods and cones - similarly organized and consist of a cell body, a synapse and a cell specialization: the inner and outer segment.
However, there are also differences:
- The pins are substantially wider than the rod.
- In both cell types in the outer segment is ( " outer " segment OS) by means of the phototransduction coupled seven-transmembrane protein retinal opsin instead. The visual pigment ( opsin plus retinal) is, in the journal iodopsin and is incorporated in many Membraneinfaltungen. When the rods is analogous to the the iodopsin similar rhodopsin ( " visual purple " ) in so-called " disks".
- The outer segments of the cones are shorter and need to be contacted by the retinal pigment epithelium ( RPE) by Extended overlap -like extensions, as opposed to the rod outer segments.
An outer segment is a modified cilium in a decentralized location, Verbindungszilium ( "Connecting cilium ", CC), connected to the inner segment. Nine microtubule doublets in nonagonaler arrangement, the internal structure of such immovable cilium.
At this, the metabolically active inner segment ( "Inner segment", IS) joins. This can be divided into the mitochondria -rich ellipsoid and in the myoid containing the endoplasmic reticulum (ER), in turn. Here, among other things, the protein biosynthesis.
The next layer is the outer nuclear layer ( " Outer nuclear layer", ONL ), which include the nucleus to the cell body. Then follows the outer plexiform layer ( " Outer plexiform layer", OPL ) with a synaptic region. The synapse at the proximal end of the photoreceptors are partly flat and partly indented membrane sites. The latter being so-called " ribbon synapses ," in reference to a strip-or plate -like structure directly on the active zone of the presynaptic. On the ribbon structure many synaptic vesicles are coupled and it can have a far higher number of vesicles per unit time will be paid as compared to "normal" synapses.
Signal transduction
In the dark, is continuously distributed the neurotransmitter glutamate from the presynaptic membrane of the journal ( or even of the stick ). When light hits the pin, sodium ion channels are closed in the journal - cell membrane via a signal transduction cascade. Since the pin on its inner segment, due to the prevailing high potassium concentration and potassium ion channels expressed there potassium ion loses, he developed a negative receptor potential, ie he hyperpolarized and pours thus less or no further glutamate from. The neurotransmitter glutamate is excitatory or inhibitory effect on the downstream bipolar cells, because there are two different types of bipolar cells, so-called ON- and OFF Bipolar Bipolar. A pin connected to an ON- bipolar, the reduced release of glutamate at exposure causes a depolarization of the ON- bipolar. Glutamate thus has an inhibiting effect on the ON- bipolar, upon exposure eliminates the inhibition. This effect is due to the fact that in the post-synaptic membrane of the ON- bipolar named metabotropic glutamate receptors mGluR6 are incorporated. In the dark, studded with mGluR6 glutamate receptors activate the ON- Bipolar a signaling cascade that includes cation channels, ie the cell is unexcitable. Missing glutamate remain, the mGluR6 receptors unoccupied, open the cation channels of ON- Bipolar, depolarized them and forwards the excitement. This mechanism resulted in the exposure in the journal hyperpolarization is converted on the side of ON- bipolar to a depolarization that is ON- bipolar excited upon exposure to light and inhibited by dimming.
The second type, the OFF bipolar react when exposed to light and thus reduced glutamate release of the cones with a hyperpolarization. Ionotropic glutamate receptors you have, the close unoccupied cation channels. That is, OFF - bipolar be inhibited by light and stimulated by blackout.
The separation into ON- and OFF - bipolar interconnection is retained until the brain throughout the subsequent visual pathway. Glutamate actually applies as a typical excitatory neurotransmitter. This system can recognize that ultimately the postsynaptic glutamate receptors decide on excitation or inhibition.
Development of color vision in primates
The color vision of mammals, and thus also of the people is less developed compared to the other vertebrates:
- The tetrachromatische than originally suspected blueprint of vertebrates contain four different cone types, UV -, S-, M - and L- cones, where the wavelength of the respective absorption maximum ( in that order ) apart is about 90 to 100 nanometers.
- Most mammals have only two cone types ( dichromate vision), the S- and M-cones are not present. The usual diurnal vertebrates oil droplets as well as the most existing double cones come in monotremes ( Monotremata ) and marsupials before ( Marsupialiern ), absent in placental animals, however.
- However, people Old World monkeys and New World monkeys have three types of cones ( trichromatic ). Seals and whales are most Monochromats.
Achromat | monochromate | dichromate | trichromatic | Tetrachromat | Pentachromat