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The Caesar cipher is named after Julius Caesar, who, according to Suetonius, used it with a shift of three (A becoming D when encrypting, and D becoming A when decrypting) to protect messages of military significance. While Caesar's was the first recorded use of this scheme, other substitution ciphers are known to have been used earlier.
His nephew, Augustus, also used the cipher, but with a right shift of one, and it did not wrap around to the beginning of the alphabet:
Evidence exists that Julius Caesar also used more complicated systems, and one writer, Aulus Gellius, refers to a (now lost) treatise on his ciphers:
It is unknown how effective the Caesar cipher was at the time; there is no record at that time of any techniques for the solution of simple substitution ciphers. The earliest surviving records date to the 9th-century works of Al-Kindi in the Arab world with the discovery of frequency analysis.
A piece of text encrypted in a Hebrew version of the Caesar cipher is sometimes found on the back of Jewish mezuzah scrolls. When each letter is replaced with the letter before it in the Hebrew alphabet the text translates as "YHWH, our God, YHWH", a quotation from the main part of the scroll.
In the 19th century, the personal advertisements section in newspapers would sometimes be used to exchange messages encrypted using simple cipher schemes. David Kahn (1967) describes instances of lovers engaging in secret communications enciphered using the Caesar cipher in The Times. Even as late as 1915, the Caesar cipher was in use: the Russian army employed it as a replacement for more complicated ciphers which had proved to be too difficult for their troops to master; German and Austrian cryptanalysts had little difficulty in decrypting their messages.
Caesar ciphers can be found today in children's toys such as secret decoder rings. A Caesar shift of thirteen is also performed in the ROT13 algorithm, a simple method of obfuscating text widely found on Usenet and used to obscure text (such as joke punchlines and story spoilers), but not seriously used as a method of encryption. | Caesar cipher | Wikipedia | 455 | 48405 | https://en.wikipedia.org/wiki/Caesar%20cipher | Technology | Computer security | null |
The Vigenère cipher uses a Caesar cipher with a different shift at each position in the text; the value of the shift is defined using a repeating keyword. If the keyword is as long as the message, is chosen at random, never becomes known to anyone else, and is never reused, this is the one-time pad cipher, proven unbreakable. However the problems involved in using a random key as long as the message make the one-time pad difficult to use in practice. Keywords shorter than the message (e.g., "Complete Victory" used by the Confederacy during the American Civil War), introduce a cyclic pattern that might be detected with a statistically advanced version of frequency analysis.
In April 2006, fugitive Mafia boss Bernardo Provenzano was captured in Sicily partly because some of his messages, clumsily written in a variation of the Caesar cipher, were broken. Provenzano's cipher used numbers, so that "A" would be written as "4", "B" as "5", and so on.
In 2011, Rajib Karim was convicted in the United Kingdom of "terrorism offences" after using the Caesar cipher to communicate with Bangladeshi Islamic activists discussing plots to blow up British Airways planes or disrupt their IT networks. Although the parties had access to far better encryption techniques (Karim himself used PGP for data storage on computer disks), they chose to use their own scheme (implemented in Microsoft Excel), rejecting a more sophisticated code program called Mujahideen Secrets "because 'kaffirs', or non-believers, know about it, so it must be less secure".
Breaking the cipher
The Caesar cipher can be easily broken even in a ciphertext-only scenario. Since there are only a limited number of possible shifts (25 in English), an attacker can mount a brute force attack by deciphering the message, or part of it, using each possible shift. The correct description will be the one which makes sense as English text. An example is shown on the right for the ciphertext ""; the candidate plaintext for shift four "" is the only one which makes sense as English text. Another type of brute force attack is to write out the alphabet beneath each letter of the ciphertext, starting at that letter. Again the correct decryption is the one which makes sense as English text. This technique is sometimes known as "completing the plain component". | Caesar cipher | Wikipedia | 494 | 48405 | https://en.wikipedia.org/wiki/Caesar%20cipher | Technology | Computer security | null |
Another approach is to match up the frequency distribution of the letters. By graphing the frequencies of letters in the ciphertext, and by knowing the expected distribution of those letters in the original language of the plaintext, a human can easily spot the value of the shift by looking at the displacement of particular features of the graph. This is known as frequency analysis. For example, in the English language the plaintext frequencies of the letters , , (usually most frequent), and , (typically least frequent) are particularly distinctive. Computers can automate this process by assessing the similarity between the observed frequency distribution and the expected distribution. This can be achieved, for instance, through the utilization of the chi-squared statistic or by minimizing the sum of squared errors between the observed and known language distributions.
The unicity distance for the Caesar cipher is about 2, meaning that on average at least two characters of ciphertext are required to determine the key. In rare cases more text may be needed. For example, the words "" and "" can be converted to each other with a Caesar shift, which means they can produce the same ciphertext with different shifts. However, in practice the key can almost certainly be found with at least 6 characters of ciphertext.
With the Caesar cipher, encrypting a text multiple times provides no additional security. This is because two encryptions of, say, shift A and shift B, will be equivalent to a single encryption with shift . In mathematical terms, the set of encryption operations under each possible key forms a group under composition. | Caesar cipher | Wikipedia | 318 | 48405 | https://en.wikipedia.org/wiki/Caesar%20cipher | Technology | Computer security | null |
A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Venus, Earth and Mars. Among astronomers who use the geophysical definition of a planet, two or three planetary-mass satellites – Earth's Moon, Io, and sometimes Europa – may also be considered terrestrial planets. The large rocky asteroids Pallas and Vesta are sometimes included as well, albeit rarely. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth (Terra and Tellus), as these planets are, in terms of structure, Earth-like. Terrestrial planets are generally studied by geologists, astronomers, and geophysicists.
Terrestrial planets have a solid planetary surface, making them substantially different from larger gaseous planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.
Structure
All terrestrial planets in the Solar System have the same basic structure, such as a central metallic core (mostly iron) with a surrounding silicate mantle.
The large rocky asteroid 4 Vesta has a similar structure; possibly so does the smaller one 21 Lutetia. Another rocky asteroid 2 Pallas is about the same size as Vesta, but is significantly less dense; it appears to have never differentiated a core and a mantle. The Earth's Moon and Jupiter's moon Io have similar structures to terrestrial planets, but Earth's Moon has a much smaller iron core. Another Jovian moon Europa has a similar density but has a significant ice layer on the surface: for this reason, it is sometimes considered an icy planet instead.
Terrestrial planets can have surface structures such as canyons, craters, mountains, volcanoes, and others, depending on the presence at any time of an erosive liquid or tectonic activity or both.
Terrestrial planets have secondary atmospheres, generated by volcanic out-gassing or from comet impact debris. This contrasts with the outer, giant planets, whose atmospheres are primary; primary atmospheres were captured directly from the original solar nebula.
Terrestrial planets within the Solar System | Terrestrial planet | Wikipedia | 459 | 48510 | https://en.wikipedia.org/wiki/Terrestrial%20planet | Physical sciences | Planetary science | null |
The Solar System has four terrestrial planets under the dynamical definition: Mercury, Venus, Earth and Mars. The Earth's Moon as well as Jupiter's moons Io and Europa would also count geophysically, as well as perhaps the large protoplanet-asteroids Pallas and Vesta (though those are borderline cases). Among these bodies, only the Earth has an active surface hydrosphere. Europa is believed to have an active hydrosphere under its ice layer.
During the formation of the Solar System, there were many terrestrial planetesimals and proto-planets, but most merged with or were ejected by the four terrestrial planets, leaving only Pallas and Vesta to survive more or less intact. These two were likely both dwarf planets in the past, but have been battered out of equilibrium shapes by impacts. Some other protoplanets began to accrete and differentiate but suffered catastrophic collisions that left only a metallic or rocky core, like 16 Psyche or 8 Flora respectively. Many S-type and M-type asteroids may be such fragments.
The other round bodies from the asteroid belt outward are geophysically icy planets. They are similar to terrestrial planets in that they have a solid surface, but are composed of ice and rock rather than of rock and metal. These include the dwarf planets, such as Ceres, Pluto and Eris, which are found today only in the regions beyond the formation snow line where water ice was stable under direct sunlight in the early Solar System. It also includes the other round moons, which are ice-rock (e.g. Ganymede, Callisto, Titan, and Triton) or even almost pure (at least 99%) ice (Tethys and Iapetus). Some of these bodies are known to have subsurface hydrospheres (Ganymede, Callisto, Enceladus, and Titan), like Europa, and it is also possible for some others (e.g. Ceres, Mimas, Dione, Miranda, Ariel, Triton, and Pluto). Titan even has surface bodies of liquid, albeit liquid methane rather than water. Jupiter's Ganymede, though icy, does have a metallic core like the Moon, Io, Europa, and the terrestrial planets.
The name Terran world has been suggested to define all solid worlds (bodies assuming a rounded shape), without regard to their composition. It would thus include both terrestrial and icy planets. | Terrestrial planet | Wikipedia | 499 | 48510 | https://en.wikipedia.org/wiki/Terrestrial%20planet | Physical sciences | Planetary science | null |
Density trends
The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure. A greater uncompressed density indicates a greater metal content. Uncompressed density differs from the true average density (also often called "bulk" density) because compression within planet cores increases their density; the average density depends on planet size, temperature distribution, and material stiffness as well as composition.
Calculations to estimate uncompressed density inherently require a model of the planet's structure. Where there have been landers or multiple orbiting spacecraft, these models are constrained by seismological data and also moment of inertia data derived from the spacecraft's orbits. Where such data is not available, uncertainties are inevitably higher.
The uncompressed densities of the rounded terrestrial bodies directly orbiting the Sun trend towards lower values as the distance from the Sun increases, consistent with the temperature gradient that would have existed within the primordial solar nebula. The Galilean satellites show a similar trend going outwards from Jupiter; however, no such trend is observable for the icy satellites of Saturn or Uranus. The icy worlds typically have densities less than 2 g·cm−3. Eris is significantly denser (), and may be mostly rocky with some surface ice, like Europa. It is unknown whether extrasolar terrestrial planets in general will follow such a trend.
The data in the tables below are mostly taken from a list of gravitationally rounded objects of the Solar System and planetary-mass moon. All distances from the Sun are averages.
Extrasolar terrestrial planets
Most of the planets discovered outside the Solar System are giant planets, because they are more easily detectable. But since 2005, hundreds of potentially terrestrial extrasolar planets have also been found, with several being confirmed as terrestrial. Most of these are super-Earths, i.e. planets with masses between Earth's and Neptune's; super-Earths may be gas planets or terrestrial, depending on their mass and other parameters.
During the early 1990s, the first extrasolar planets were discovered orbiting the pulsar PSR B1257+12, with masses of 0.02, 4.3, and 3.9 times that of Earth, by pulsar timing. | Terrestrial planet | Wikipedia | 460 | 48510 | https://en.wikipedia.org/wiki/Terrestrial%20planet | Physical sciences | Planetary science | null |
When 51 Pegasi b, the first planet found around a star still undergoing fusion, was discovered, many astronomers assumed it to be a gigantic terrestrial, because it was assumed no gas giant could exist as close to its star (0.052 AU) as 51 Pegasi b did. It was later found to be a gas giant.
In 2005, the first planets orbiting a main-sequence star and which showed signs of being terrestrial planets were found: Gliese 876 d and OGLE-2005-BLG-390Lb. Gliese 876 d orbits the red dwarf Gliese 876, 15 light years from Earth, and has a mass seven to nine times that of Earth and an orbital period of just two Earth days. OGLE-2005-BLG-390Lb has about 5.5 times the mass of Earth and orbits a star about 21,000 light-years away in the constellation Scorpius.
From 2007 to 2010, three (possibly four) potential terrestrial planets were found orbiting within the Gliese 581 planetary system. The smallest, Gliese 581e, is only about 1.9 Earth masses, but orbits very close to the star. Two others, Gliese 581c and the disputed Gliese 581d, are more-massive super-Earths orbiting in or close to the habitable zone of the star, so they could potentially be habitable, with Earth-like temperatures.
Another possibly terrestrial planet, HD 85512 b, was discovered in 2011; it has at least 3.6 times the mass of Earth.
The radius and composition of all these planets are unknown.
The first confirmed terrestrial exoplanet, Kepler-10b, was found in 2011 by the Kepler space telescope, specifically designed to discover Earth-size planets around other stars using the transit method.
In the same year, the Kepler space telescope mission team released a list of 1235 extrasolar planet candidates, including six that are "Earth-size" or "super-Earth-size" (i.e. they have a radius less than twice that of the Earth) and in the habitable zone of their star.
Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range (see image). | Terrestrial planet | Wikipedia | 480 | 48510 | https://en.wikipedia.org/wiki/Terrestrial%20planet | Physical sciences | Planetary science | null |
In 2016, statistical modeling of the relationship between a planet's mass and radius using a broken power law appeared to suggest that the transition point between rocky, terrestrial worlds and mini-Neptunes without a defined surface was in fact very close to Earth and Venus's, suggesting that rocky worlds much larger than our own are in fact quite rare. This resulted in some advocating for the retirement of the term "super-earth" as being scientifically misleading. Since 2016 the catalog of known exoplanets has increased significantly, and there have been several published refinements of the mass-radius model. As of 2024, the expected transition point between rocky and intermediate-mass planets sits at roughly 4.4 earth masses, and roughly 1.6 earth radii.
In September 2020, astronomers using microlensing techniques reported the detection, for the first time, of an Earth-mass rogue planet (named OGLE-2016-BLG-1928) unbounded by any star, and free-floating in the Milky Way galaxy.
List of terrestrial exoplanets
The following exoplanets have a density of at least 5 g/cm3 and a mass below Neptune's and are thus very likely terrestrial:
Kepler-10b, Kepler-20b, Kepler-36b, Kepler-48d, Kepler 68c, Kepler-78b, Kepler-89b, Kepler-93b, Kepler-97b, Kepler-99b, Kepler-100b, Kepler-101c, Kepler-102b, Kepler-102d, Kepler-113b, Kepler-131b, Kepler-131c, Kepler-138c, Kepler-406b, Kepler-406c, Kepler-409b.
Frequency
In 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth- and super-Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way. Eleven billion of these estimated planets may be orbiting Sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. However, this does not give estimates for the number of extrasolar terrestrial planets, because there are planets as small as Earth that have been shown to be gas planets (see Kepler-138d). | Terrestrial planet | Wikipedia | 477 | 48510 | https://en.wikipedia.org/wiki/Terrestrial%20planet | Physical sciences | Planetary science | null |
Estimates show that about 80% of potentially habitable worlds are covered by land, and about 20% are ocean planets. Planets with rations more like those of Earth, which was 30% land and 70% ocean, only make up 1% of these worlds.
Types
Several possible classifications for solid planets have been proposed.
Silicate planet
A solid planet like Venus, Earth, or Mars, made primarily of a silicon-based rocky mantle with a metallic (iron) core.
Carbon planet (also called "diamond planet")
A theoretical class of planets, composed of a metal core surrounded by primarily carbon-based minerals. They may be considered a type of terrestrial planet if the metal content dominates. The Solar System contains no carbon planets but does have carbonaceous asteroids, such as Ceres and Hygiea. It is unknown if Ceres has a rocky or metallic core.
Iron planet
A theoretical type of solid planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other solid planets of comparable mass. Mercury in the Solar System has a metallic core equal to 60–70% of its planetary mass, and is sometimes called an iron planet, though its surface is made of silicates and is iron-poor. Iron planets are thought to form in the high-temperature regions close to a star, like Mercury, and if the protoplanetary disk is rich in iron.
Icy planet
A type of solid planet with an icy surface of volatiles. In the Solar System, most planetary-mass moons (such as Titan, Triton, and Enceladus) and many dwarf planets (such as Pluto and Eris) have such a composition. Europa is sometimes considered an icy planet due to its surface ice, but its higher density indicates that its interior is mostly rocky. Such planets can have internal saltwater oceans and cryovolcanoes erupting liquid water (i.e. an internal hydrosphere, like Europa or Enceladus); they can have an atmosphere and hydrosphere made from methane or nitrogen (like Titan). A metallic core is possible, as exists on Ganymede.
Coreless planet | Terrestrial planet | Wikipedia | 436 | 48510 | https://en.wikipedia.org/wiki/Terrestrial%20planet | Physical sciences | Planetary science | null |
A theoretical type of solid planet that consists of silicate rock but has no metallic core, i.e. the opposite of an iron planet. Although the Solar System contains no coreless planets, chondrite asteroids and meteorites are common in the Solar System. Ceres and Pallas have mineral compositions similar to carbonaceous chondrites, though Pallas is significantly less hydrated. Coreless planets are thought to form farther from the star where volatile oxidizing material is more common. | Terrestrial planet | Wikipedia | 100 | 48510 | https://en.wikipedia.org/wiki/Terrestrial%20planet | Physical sciences | Planetary science | null |
The term antelope refers to numerous extant or recently extinct species of the ruminant artiodactyl family Bovidae that are indigenous to most of Africa, India, the Middle East, Central Asia, and a small area of Eastern Europe. Antelopes do not form a monophyletic group, as some antelopes are more closely related to other bovid groups, like bovines, goats, and sheep, than to other antelopes.
A better definition, also known as the "true antelopes", includes only the genera Gazella, Nanger, Eudorcas, and Antilope. One North American mammal, the pronghorn or "pronghorn antelope", is colloquially referred to as the "American antelope", despite the fact that it belongs to a completely different family (Antilocapridae) than the true Old-World antelopes; pronghorn are the sole extant member of an extinct prehistoric lineage that once included many unique species.
Although antelope are sometimes referred to, and easily misidentified as, "deer" (cervids), true deer are only distant relatives of antelopes. While antelope are found in abundance in Africa, only one deer species is found on the continent—the Barbary red deer of Northern Africa. By comparison, numerous deer species are usually found in regions of the world with fewer or no antelope species present, such as throughout Southeast Asia, Europe and all of the Americas. This is likely due to competition over shared resources, as deer and antelope fill a virtually identical ecological niche in their respective habitats. Countries like India, however, have large populations of endemic deer and antelope, with the different species generally keeping to their own "niches" with minimal overlap.
Unlike deer, in which the males sport elaborate head antlers that are shed and regrown annually, antelope horns are bone and grow steadily, never falling off. If a horn is broken, it will either remain broken or take years to partially regenerate, depending on the species of the antelope.
Etymology | Antelope | Wikipedia | 444 | 48517 | https://en.wikipedia.org/wiki/Antelope | Biology and health sciences | Artiodactyla | null |
The English word "antelope" first appeared in 1417 and is derived from the Old French antelop, itself derived from Medieval Latin ant(h)alopus, which in turn comes from the Byzantine Greek word ἀνθόλοψ, anthólops, first attested in Eustathius of Antioch (), according to whom it was a fabulous animal "haunting the banks of the Euphrates, very savage, hard to catch and having long, saw-like horns capable of cutting down trees". It perhaps derives from Greek ἀνθος, anthos (flower) and ώψ, ops (eye), perhaps meaning "beautiful eye" or alluding to the animals' long eyelashes. This, however, may be a folk etymology in Greek based on some earlier root. The word talopus and calopus, from Latin, came to be used in heraldry. In 1607, it was first used for living, cervine animals .
Species
There are 91 antelope species, most of which are native to Africa, occur in about 30 genera. The classification of tribes or subfamilies within Bovoidea is still a matter of debate, with several alternative systems proposed.
Antelope are not a cladistic or taxonomically defined group. The term is used to describe all members of the family Bovidae that do not fall under the category of sheep, cattle, or goats. Usually, all species of the Antilopinae, Hippotraginae, Reduncinae, Cephalophinae, many Bovinae, the grey rhebok, and the impala are called antelope.
Distribution and habitat
More species of antelope are native to Africa than to any other continent, almost exclusively in savannahs, with 25-40 species co-occurring over much of East Africa. Because savannah habitat in Africa has expanded and contracted five times over the last three million years, and the fossil record indicates this is when most extant species evolved, it is believed that isolation in refugia during contractions was a major driver of this diversification. Other species occur in Asia: the Arabian Peninsula is home to the Arabian oryx and Dorcas gazelle. South Asia is home to the nilgai, chinkara, blackbuck, Tibetan antelope, and four-horned antelope, while Russia and Central Asia have the Tibetan antelope and saiga. | Antelope | Wikipedia | 500 | 48517 | https://en.wikipedia.org/wiki/Antelope | Biology and health sciences | Artiodactyla | null |
No antelope species is native to Australasia or Antarctica, nor do any extant species occur in the Americas, though the nominate saiga subspecies occurred in North America during the Pleistocene. North America is currently home to the native pronghorn, which taxonomists do not consider a member of the antelope group, but which is often locally referred to as such (e.g., "American antelope"). In Europe, several extinct species occur in the fossil record, and the saiga was found widely during the Pleistocene but did not persist into the later Holocene, except in Russian Kalmykia and Astrakhan Oblast.
Many species of antelope have been imported to other parts of the world, especially the United States, for exotic game hunting. With some species possessing spectacular leaping and evasive skills, individuals may escape. Texas in particular has many game ranches, as well as habitats and climates that are very hospitable to African and Asian plains antelope species. Accordingly, wild populations of blackbuck antelope, gemsbok, and nilgai may be found in Texas.
Antelope live in a wide range of habitats. Most live in the African savannahs. However, many species are more secluded, such as the forest antelope, as well as the extreme cold-living saiga, the desert-adapted Arabian oryx, the rocky koppie-living klipspringer, and semiaquatic sitatunga.
Species living in forests, woodland, or bush tend to be sedentary, but many of the plains species undertake long migrations. These enable grass-eating species to follow the rains and thereby their food supply. The gnus and gazelles of East Africa perform some of the most impressive mass migratory circuits of all mammals.
Morphology
Body and covering
Antelope vary greatly in size. For example, a male common eland can measure at the shoulder and weigh almost , whereas an adult royal antelope may stand only at the shoulder and weigh a mere . | Antelope | Wikipedia | 422 | 48517 | https://en.wikipedia.org/wiki/Antelope | Biology and health sciences | Artiodactyla | null |
Not surprisingly for animals with long, slender yet powerful legs, many antelope have long strides and can run fast. Some (e.g. klipspringer) are also adapted to inhabiting rock koppies and crags. Both dibatags and gerenuks habitually stand on their two hind legs to reach acacia and other tree foliage. Different antelope have different body types, which can affect movement. Duikers are short, bush-dwelling antelope that can pick through dense foliage and dive into the shadows rapidly. Gazelle and springbok are known for their speed and leaping abilities. Even larger antelope, such as nilgai, elands, and kudus, are capable of jumping or greater, although their running speed is restricted by their greater mass.
Antelope have a wide variety of coverings, though most have a dense coat of short fur. In most species, the coat (pelage) is some variation of a brown colour (or several shades of brown), often with white or pale underbodies. Exceptions include the zebra-marked zebra duiker, the grey, black, and white Jentink's duiker, and the black lechwe. Most of the "spiral-horned" antelope have pale, vertical stripes on their backs. Many desert and semidesert species are particularly pale, some almost silvery or whitish (e.g. Arabian oryx); the beisa and southern oryxes have gray and black pelages with vivid black-and-white faces. Common features of various gazelles are white rumps, which flash a warning to others when they run from danger, and dark stripes midbody (the latter feature is also shared by the springbok and beira). The springbok also has a pouch of white, brushlike hairs running along its back, which opens up when the animal senses danger, causing the dorsal hairs to stand on end. | Antelope | Wikipedia | 407 | 48517 | https://en.wikipedia.org/wiki/Antelope | Biology and health sciences | Artiodactyla | null |
Many antelope are sexually dimorphic. In most species, both sexes have horns, but those of males tend to be larger. Males tend to be larger than the females, but exceptions in which the females tend to be heavier than the males include the bush duiker, dwarf antelope, Cape grysbok, and oribi, all rather small species. A number of species have hornless females (e.g., sitatunga, red lechwe, and suni). In some species, the males and females have differently coloured pelages (e.g. blackbuck and nyala).
Sensory and digestive systems
Antelope are ruminants, so they have well-developed molar teeth, which grind cud (food balls stored in the stomach) into a pulp for further digestion. They have no upper incisors, but rather a hard upper gum pad, against which their lower incisors bite to tear grass stems and leaves.
Like many other herbivores, antelope rely on keen senses to avoid predators. Their eyes are placed on the sides of their heads, giving them a broad radius of vision with minimal binocular vision. Their horizontally elongated pupils also help in this respect. Acute senses of smell and hearing give antelope the ability to perceive danger at night out in the open (when predators are often on the prowl). These same senses play an important role in contact between individuals of the same species; markings on their heads, ears, legs, and rumps are used in such communication. Many species "flash" such markings, as well as their tails; vocal communications include loud barks, whistles, "moos", and trumpeting; many species also use scent marking to define their territories or simply to maintain contact with their relatives and neighbors.
Antelope horns | Antelope | Wikipedia | 384 | 48517 | https://en.wikipedia.org/wiki/Antelope | Biology and health sciences | Artiodactyla | null |
The size and shape of antelope horns varies greatly. Those of the duikers and dwarf antelope tend to be simple "spikes", but differ in the angle to the head from backward curved and backward pointing (e.g. yellow-backed duiker) to straight and upright (e.g. steenbok). Other groups have twisted (e.g. common eland), spiral (e.g. greater kudu), "recurved" (e.g. the reedbucks), lyrate (e.g. impala), or long, curved (e.g. the oryxes) horns. Horns are not shed and their bony cores are covered with a thick, persistent sheath of horny material, both of which distinguish them from antlers.
Antelope horns are efficient weapons, and tend to be better developed in those species where males fight over females (large herd antelope) than in solitary or lekking species. With male-male competition for mates, horns are clashed in combat. Males more commonly use their horns against each other than against another species. The boss of the horns is typically arranged in such a way that two antelope striking at each other's horns cannot crack each other's skulls, making a fight via horn more ritualized than dangerous. Many species have ridges in their horns for at least two-thirds the length of their horns, but these ridges are not a direct indicator of age.
Behavior
Mating strategies
Antelope are often classified by their reproductive behavior.
Small antelope, such as dik-diks, tend to be monogamous. They live in a forest environment with patchy resources, and a male is unable to monopolize more than one female due to this sparse distribution. Larger forest species often form very small herds of two to four females and one male.
Some species, such as lechwes, pursue a lek breeding system, where the males gather on a lekking ground and compete for a small territory, while the females appraise males and choose one with which to mate.
Large grazing antelope, such as impala or wildebeest, form large herds made up of many females and a single breeding male, which excludes all other males, often by combat.
Defense
Antelope pursue a number of defense strategies, often dictated by their morphology. | Antelope | Wikipedia | 496 | 48517 | https://en.wikipedia.org/wiki/Antelope | Biology and health sciences | Artiodactyla | null |
Large antelope that gather in large herds, such as wildebeest, rely on numbers and running speed for protection. In some species, adults will encircle the offspring, protecting them from predators when threatened. Many forest antelope rely on cryptic coloring and good hearing to avoid predators. Forest antelope often have very large ears and dark or striped colorations. Small antelope, especially duikers, evade predation by jumping into dense bush where the predator cannot pursue. Springboks use a behavior known as stotting to confuse predators.
Open grassland species have nowhere to hide from predators, so they tend to be fast runners. They are agile and have good endurance—these are advantages when pursued by sprint-dependent predators such as cheetahs, which are the fastest of land animals, but tire quickly. Reaction distances vary with predator species and behaviour. For example, gazelles may not flee from a lion until it is closer than 200 m (650 ft)—lions hunt as a pride or by surprise, usually by stalking; one that can be seen clearly is unlikely to attack. However, sprint-dependent cheetahs will cause gazelles to flee at a range of over .
If escape is not an option, antelope are capable of fighting back. Oryxes in particular have been known to stand sideways like many unrelated bovids to appear larger than they are, and may charge at a predator as a last resort.
Status
About 25 species are rated by the IUCN as endangered, such as the dama gazelle and mountain nyala. A number of subspecies are also endangered, including the giant sable antelope and the mhorr gazelle. The main causes for concern for these species are habitat loss, competition with cattle for grazing, and trophy hunting.
The chiru or Tibetan antelope is hunted for its pelt, which is used in making shahtoosh wool, used in shawls. Since the fur can only be removed from dead animals, and each animal yields very little of the downy fur, several antelope must be killed to make a single shawl. This unsustainable demand has led to enormous declines in the chiru population. | Antelope | Wikipedia | 449 | 48517 | https://en.wikipedia.org/wiki/Antelope | Biology and health sciences | Artiodactyla | null |
The saiga is hunted for its horns, which are considered an aphrodisiac by some cultures. Only the males have horns, and have been so heavily hunted that some herds contain up to 800 females to one male. The species showed a steep decline and was formerly classified as critically endangered. However, the saigas have experienced a massive regrowth and are now classified as near threatened.
Lifespan
It is difficult to determine how long antelope live in the wild. With the preference of predators towards old and infirm individuals, which can no longer sustain peak speeds, few wild prey-animals live as long as their biological potential. In captivity, wildebeest have lived beyond 20 years old, and impalas have reached their late teens.
Relationship with humans
Culture
The antelope's horn is prized for supposed medicinal and magical powers in many places. The horn of the male saiga, in Eastern practice, is ground as an aphrodisiac, for which it has been hunted nearly to extinction. In the Congo, it is thought to confine spirits. The antelope's ability to run swiftly has also led to their association with the wind, such as in the Rig Veda, as the steeds of the Maruts and the wind god Vayu. There is, however, no scientific evidence that the horns of any antelope have any change on a human's physiology or characteristics.
In Mali, antelope were believed to have brought the skills of agriculture to mankind.
Humans have also used the term "Antelope" to refer to a tradition usually found in the sport of track and field. | Antelope | Wikipedia | 335 | 48517 | https://en.wikipedia.org/wiki/Antelope | Biology and health sciences | Artiodactyla | null |
Domestication
Domestication of animals requires certain traits in the animal that antelope do not typically display. Most species are difficult to contain in any density, due to the territoriality of the males, or in the case of oryxes (which have a relatively hierarchical social structure), an aggressive disposition; they can easily kill a human. Because many have extremely good jumping abilities, providing adequate fencing is a challenge. Also, antelope will consistently display a fear response to perceived predators, such as humans, making them very difficult to herd or handle. Although antelope have diets and rapid growth rates highly suitable for domestication, this tendency to panic and their non-hierarchical social structure explains why farm-raised antelope are uncommon. Ancient Egyptians kept herds of gazelles and addax for meat, and occasionally pets. It is unknown whether they were truly domesticated, but it seems unlikely, as no domesticated gazelles exist today.
However, humans have had success taming certain species, such as the elands. These antelope sometimes jump over each other's backs when alarmed, but this incongruous talent seems to be exploited only by wild members of the species; tame elands do not take advantage of it and can be enclosed within a very low fence. Their meat, milk, and hides are all of excellent quality, and experimental eland husbandry has been going on for some years in both Ukraine and Zimbabwe. In both locations, the animal has proved wholly amenable to domestication. Similarly, European visitors to Arabia reported "tame gazelles are very common in the Asiatic countries of which the species is a native; and the poetry of these countries abounds in allusions both to the beauty and the gentleness of the gazelle." Other antelope that have been tamed successfully include the gemsbok, the kudu, and the springbok.
Hybrid antelope
A wide variety of antelope hybrids have been recorded in zoos, game parks, and wildlife ranches, due to either a lack of more appropriate mates in enclosures shared with other species or a misidentification of species. The ease of hybridization shows how closely related some antelope species are. With few exceptions, most hybrid antelope occur only in captivity.
Most hybrids occur between species within the same genus. All reported examples occur within the same subfamily. As with most mammal hybrids, the less closely related the parents, the more likely the offspring will be sterile. | Antelope | Wikipedia | 511 | 48517 | https://en.wikipedia.org/wiki/Antelope | Biology and health sciences | Artiodactyla | null |
Heraldry
Antelope are a common symbol in heraldry, though they occur in a highly distorted form from nature. The heraldic antelope has the body of a stag and the tail of a lion, with serrated horns, and a small tusk at the end of its snout. This bizarre and inaccurate form was invented by European heralds in the Middle Ages, who knew little of foreign animals and made up the rest. The antelope was mistakenly imagined to be a monstrous beast of prey; the 16th century poet Edmund Spenser referred to it as being "as fierce and fell as a wolf."
Antelope can all also occur in their natural form, in which case they are termed "natural antelope" to distinguish them from the more usual heraldic antelope. The arms previously used by the Republic of South Africa featured a natural antelope, along with an oryx. | Antelope | Wikipedia | 185 | 48517 | https://en.wikipedia.org/wiki/Antelope | Biology and health sciences | Artiodactyla | null |
A highway is any public or private road or other public way on land. It includes not just major roads, but also other public roads and rights of way. In the United States, it is also used as an equivalent term to controlled-access highway, or a translation for motorway, Autobahn, autostrada, autoroute, etc.
According to Merriam-Webster, the use of the term predates the 12th century. According to Etymonline, "high" is in the sense of "main".
In North American and Australian English, major roads such as controlled-access highways or arterial roads are often state highways (Canada: provincial highways). Other roads may be designated "county highways" in the US and Ontario. These classifications refer to the level of government (state, provincial, county) that maintains the roadway. In British English, "highway" is primarily a legal term. Everyday use normally implies roads, while the legal use covers any route or path with a public right of access, including footpaths etc.
The term has led to several related derived terms, including highway system, highway code, highway patrol and highwayman.
Overview
Major highways are often named and numbered by the governments that typically develop and maintain them. Australia's Highway 1 is the longest national highway in the world at over and runs almost the entire way around the continent. China has the world's largest network of highways, followed closely by the United States. Some highways, like the Pan-American Highway or the European routes, span multiple countries. Some major highway routes include ferry services, such as US Route 10, which crosses Lake Michigan.
Traditionally highways were used by people on foot or on horses. Later they also accommodated carriages, bicycles and eventually motor cars, facilitated by advancements in road construction. In the 1920s and 1930s, many nations began investing heavily in highway systems in an effort to spur commerce and bolster national defence.
Major highways that connect cities in populous developed and developing countries usually incorporate features intended to enhance the road's capacity, efficiency, and safety to various degrees. Such features include a reduction in the number of locations for user access, the use of dual carriageways with two or more lanes on each carriageway, and grade-separated junctions with other roads and modes of transport. These features are typically present on highways built as motorways (freeways).
Terminology | Highway | Wikipedia | 482 | 48519 | https://en.wikipedia.org/wiki/Highway | Technology | Road transport | null |
England and Wales
The general legal definition deals with right of use, not the form of construction; this is distinct from e.g. the popular use of the word in the US. A highway is defined in English common law by a number of similarly worded definitions such as "a way over which all members of the public have the right to pass and repass without hindrance" usually accompanied by "at all times"; ownership of the ground is for most purposes irrelevant, thus the term encompasses all such ways from the widest trunk roads in public ownership to the narrowest footpath providing unlimited pedestrian access over private land.
A highway might be open to all forms of lawful land traffic (e.g. vehicular, horse, pedestrian) or limited to specific modes of traffic; usually a highway available to vehicles is also available to foot or horse traffic, a highway available to horse traffic is available to cyclists and pedestrians; but there are exceptional cases in which a highway is only available to vehicles, or is subdivided into dedicated parallel sections for different users.
A highway can share ground with a private right of way for which full use is not available to the general public: for example farm roads which the owner may use for any purpose but for which the general public only has a right of use on foot or horseback. The status of highway on most older roads has been gained by established public use, while newer roads are typically dedicated as highways from the time they are adopted (taken into the care and control of a council or other public authority). In England and Wales, a public highway is also known as "The King's Highway".
The core definition of a highway is modified in various legislation for a number of purposes but only for the specific matters dealt with in each such piece of legislation. This is typically in the case of bridges, tunnels and other structures whose ownership, mode of use or availability would otherwise exclude them from the general definition of a highway. Recent examples include toll bridges and tunnels which have the definition of highway imposed upon them (in a legal order applying only to the individual structure) to allow application of most traffic laws to those using them but without causing all of the general obligations or rights of use otherwise applicable to a highway.
Limited access highways for vehicles, with their own traffic rules, are called "motorways" in the UK. | Highway | Wikipedia | 474 | 48519 | https://en.wikipedia.org/wiki/Highway | Technology | Road transport | null |
Scotland
Scots law is similar to English law with regard to highways but with differing terminology and legislation. What is defined in England as a highway will often in Scotland be what is defined by s.151 Roads (Scotland) Act 1984 (but only "in this act" although other legislation could imitate) simply as a road, that is:
"any way (other than a waterway) over which there is a public right of passage (by whatever means [and whether subject to a toll or not]) and includes the road’s verge, and any bridge (whether permanent or temporary) over which, or tunnel through which, the road passes; and any reference to a road includes a part thereof"
The word highway is itself no longer a statutory expression in Scots law but remains in common law.
United States
In American law, the word "highway" is sometimes used to denote any public way used for travel, whether a "road, street, and parkway"; however, in practical and useful meaning, a "highway" is a major and significant, well-constructed road that is capable of carrying reasonably heavy to extremely heavy traffic. Highways generally have a route number designated by the state and federal departments of transportation.
California Vehicle Code, Sections 360, 590, define a "highway" as only a way open for use by motor vehicles, but the California Supreme Court has held that "the definition of 'highway' in the Vehicle Code is used for special purposes of that act" and that canals of the Los Angeles neighborhood of Venice are "highways" that are entitled to be maintained with state highway funds.
History
Large scale highway systems developed in the 20th century as automobile usage increased. The first United States limited-access road was constructed on Long Island, New York, and known as the Long Island Motor Parkway or the Vanderbilt Motor Parkway. It was completed in 1911. It included many modern features, including banked turns, guard rails and reinforced concrete tarmac. Traffic could turn left between the parkway and connectors, crossing oncoming traffic, so it was not a controlled-access highway (or "freeway" as later defined by the federal government's Manual on Uniform Traffic Control Devices). | Highway | Wikipedia | 440 | 48519 | https://en.wikipedia.org/wiki/Highway | Technology | Road transport | null |
Italy was the first country in the world to build controlled-access highways reserved for fast traffic and for motor vehicles only. The Autostrada dei Laghi ("Lakes Highway"), the first built in the world, connecting Milan to Lake Como and Lake Maggiore, and now parts of the A8 and A9 highways, was devised by Piero Puricelli and was inaugurated in 1924. This highway, called autostrada, contained only one lane in each direction and no interchanges.
The Southern State Parkway opened in 1927, while the Long Island Motor Parkway was closed in 1937 and replaced by the Northern State Parkway (opened 1931) and the contiguous Grand Central Parkway (opened 1936). In Germany, construction of the Bonn-Cologne Autobahn began in 1929 and was opened in 1932 by Konrad Adenauer, then the mayor of Cologne. Soon the Autobahn was the first limited-access, high-speed road network in the world, with the first section from Frankfurt am Main to Darmstadt opening in 1935.
In the US, the Federal Aid Highway Act of 1921 (Phipps Act) enacted a fund to create an extensive highway system. In 1922, the first blueprint for a national highway system (the Pershing Map) was published. The Federal Aid Highway Act of 1956 allocated $25 billion for the construction of the Interstate Highway System over a 20-year period.
In Great Britain, the Special Roads Act 1949 provided the legislative basis for roads for restricted classes of vehicles and non-standard or no speed limits applied (later mostly termed motorways but now with speed limits not exceeding 70 mph); in terms of general road law this legislation overturned the usual principle that a road available to vehicular traffic was also available to horse or pedestrian traffic as is usually the only practical change when non-motorways are reclassified as special roads. The first section of motorway in the UK opened in 1958 (part of the M6 motorway) and then in 1959 the first section of the M1 motorway.
Social effects | Highway | Wikipedia | 406 | 48519 | https://en.wikipedia.org/wiki/Highway | Technology | Road transport | null |
Often reducing travel times relative to city or town streets, highways with limited access and grade separation can create increased opportunities for people to travel for business, trade or pleasure and also provide trade routes for goods. Highways can reduce commute and other travel time but additional road capacity can also release latent traffic demand. If not accurately predicted at the planning stage, this extra traffic may lead to the new road becoming congested sooner than would otherwise be anticipated by considering increases in vehicle ownership. More roads allow drivers to use their cars when otherwise alternatives may have been sought, or the journey may not have been made, which can mean that a new road brings only short-term mitigation of traffic congestion.
Where highways are created through existing communities, there can be reduced community cohesion and more difficult local access. Consequently, property values have decreased in many cutoff neighborhoods, leading to decreased housing quality over time. Mostly in the U.S., many of these effects are from racist planning practices from before the advent of civil rights. This would result in the vast majority of displacement and social effects mostly going to people like African Americans.
In recent times, the use of freeway removal or the public policy of urban planning to demolish freeways and create mixed-use urban areas, parks, residential, commercial, or other land uses is being popular in many cities to combat most of the social problems caused from highways.
Economic effects
In transport, demand can be measured in numbers of journeys made or in total distance travelled across all journeys (e.g. passenger-kilometres for public transport or vehicle-kilometres of travel (VKT) for private transport). Supply is considered to be a measure of capacity. The price of the good (travel) is measured using the generalised cost of travel, which includes both money and time expenditure.
The effect of increases in supply (capacity) are of particular interest in transport economics (see induced demand), as the potential environmental consequences are significant (see externalities below). | Highway | Wikipedia | 401 | 48519 | https://en.wikipedia.org/wiki/Highway | Technology | Road transport | null |
In addition to providing benefits to their users, transport networks impose both positive and negative externalities on non-users. The consideration of these externalities—particularly the negative ones—is a part of transport economics. Positive externalities of transport networks may include the ability to provide emergency services, increases in land value and agglomeration benefits. Negative externalities are wide-ranging and may include local air pollution, noise pollution, light pollution, safety hazards, community severance and congestion. The contribution of transport systems to potentially hazardous climate change is a significant negative externality which is difficult to evaluate quantitatively, making it difficult (but not impossible) to include in transport economics-based research and analysis. Congestion is considered a negative externality by economists.
A 2016 study found that for the United States, "a 10% increase in a region's stock of highways causes a 1.7% increase in regional patenting over a five-year period." A 2021 study found that areas that obtained access to a new highway experienced a substantial increase in top-income taxpayers and a decline in low-income taxpayers. Highways also contributed to job and residential urban sprawl.
Environmental effects
Highways are extended linear sources of pollution.
Roadway noise increases with operating speed so major highways generate more noise than arterial streets. Therefore, considerable noise health effects are expected from highway systems. Noise mitigation strategies exist to reduce sound levels at nearby sensitive receptors. The idea that highway design could be influenced by acoustical engineering considerations first arose about 1973.
Air quality issues: Highways may contribute fewer emissions than arterials carrying the same vehicle volumes. This is because high, constant-speed operation creates an emissions reduction compared to vehicular flows with stops and starts. However, concentrations of air pollutants near highways may be higher due to increased traffic volumes. Therefore, the risk of exposure to elevated levels of air pollutants from a highway may be considerable, and further magnified when highways have traffic congestion.
New highways can also cause habitat fragmentation, encourage urban sprawl and allow human intrusion into previously untouched areas, as well as (counterintuitively) increasing congestion, by increasing the number of intersections.
They can also reduce the use of public transport, indirectly leading to greater pollution. | Highway | Wikipedia | 455 | 48519 | https://en.wikipedia.org/wiki/Highway | Technology | Road transport | null |
High-occupancy vehicle lanes are being added to some newer/reconstructed highways in the United States and other countries around the world to encourage carpooling and mass transit. These lanes help reduce the number of cars on the highway and thus reduces pollution and traffic congestion by promoting the use of carpooling in order to be able to use these lanes. However, they tend to require dedicated lanes on a highway, which makes them difficult to construct in dense urban areas where they are the most effective.
To address habitat fragmentation, wildlife crossings have become increasingly popular in many countries. Wildlife crossings allow animals to safely cross human-made barriers like highways.
Road traffic safety
Road traffic safety describes the safety performance of roads and streets, and methods used to reduce the harm (deaths, injuries, and property damage) on the highway system from traffic collisions. It includes the design, construction and regulation of the roads, the vehicles used on them and the training of drivers and other road-users.
A report published by the World Health Organization in 2004 estimated that some 1.2 million people were killed and 50 million injured on the roads around the world each year and was the leading cause of death among children 10–19 years of age.
The report also noted that the problem was most severe in developing countries and that simple prevention measures could halve the number of deaths. For reasons of clear data collection, only harm involving a road vehicle is included. A person tripping with fatal consequences or dying for some unrelated reason on a public road is not included in the relevant statistics.
Statistics
The United States has the world's largest network of highways, including both the Interstate Highway System and the United States Numbered Highway System. At least one of these networks is present in every state and they interconnect most major cities. It is also the world's most expensive mega-project, as the entirety of the Interstate Highway System was estimated to cost $27 billion in 1955 (equivalent to $ in ).
China's highway network is the second most extensive in the world, with a total length of about . China's expressway network is the longest Expressway system in the world, and it is quickly expanding, stretching some at the end of 2011. In 2008 alone, expressways were added to the network. | Highway | Wikipedia | 459 | 48519 | https://en.wikipedia.org/wiki/Highway | Technology | Road transport | null |
Longest international highway The Pan-American Highway, which connects many countries in the Americas, is nearly long . The Pan-American Highway is discontinuous because there is a significant gap in it in southeastern Panama, where the rainfall is immense and the terrain is entirely unsuitable for highway construction.
Longest national highway (point to point) The Trans-Canada Highway has one main route, a northern route through the western provinces, and several branches in the central and eastern provinces. The main route is long alone, and the entire system is over long. The TCH runs east–west across southern Canada, the populated portion of the country, and it connects many of the major urban centres along its route crossing all provinces, and reaching nearly all of their capital cities. The TCH begins on the east coast in Newfoundland, traverses that island, and crosses to the mainland by ferry. It crosses the Maritime Provinces of eastern Canada with a branch route serving the province of Prince Edward Island via a ferry and bridge. After crossing the remainder of the country's mainland, the highway reaches Vancouver, British Columbia on the Pacific coast, where a ferry continues it to Vancouver Island and the provincial capital of Victoria. Numeric designation is the responsibility of the provinces, and there is no single route number across the country.
Longest national highway (circuit) Australia's Highway 1 at over . It runs almost the entire way around the country's coastline. With the exception of the Federal Capital of Canberra, which is far inland, Highway 1 links all of Australia's capital cities, although Brisbane and Darwin are not directly connected, but rather are bypassed short distances away. Also, there is a ferry connection to the island state of Tasmania, and then a stretch of Highway 1 that links the major towns and cities of Tasmania, including Launceston and Hobart (this state's capital city).
Largest national highway system The United States of America has approximately of highway within its borders .
Busiest highway Highway 401 in Ontario, Canada, has volumes surpassing an average of 500,000 vehicles per day in some sections of Toronto .
Widest highway (maximum number of lanes) The Katy Freeway (part of Interstate 10) in Houston, Texas, has a total of 26 lanes in some sections . However, they are divided up into general use/ frontage roads/ HOV lanes, restricting the traverse traffic flow. | Highway | Wikipedia | 477 | 48519 | https://en.wikipedia.org/wiki/Highway | Technology | Road transport | null |
Widest highway (maximum number of through lanes) Interstate 5 along a section between Interstate 805 and California State Route 56 in San Diego, California, which was completed in April 2007, is 22 lanes wide.
Highest international highway The Karakoram Highway, between Pakistan and China, is at an altitude of .
Highest national highway National Highway 5, in India, connecting Amritsar in Punjab with Manali in Himachal Pradesh & Leh in Ladakh, reaches an approximate altitude of . The highest motorable road passes through Umling La at an altitude of falls under the branch highway connecting National Highway 5 in India. | Highway | Wikipedia | 124 | 48519 | https://en.wikipedia.org/wiki/Highway | Technology | Road transport | null |
Bus lane
South Korea
In South Korea, in February 1995 a bus lane (essentially an HOV-9) was established between the northern terminus and Sintanjin for important holidays and on 1 July 2008 bus lane enforcement between Seoul and Osan (Sintanjin on weekends) became daily between 6 a.m. and 10 p.m. On 1 October this was adjusted to 7 a.m. to 9 p.m. weekdays, and 9 a.m. to 9 p.m. weekends.
On the dotted line, vehicles except buses can make a right turn and temporarily pass for joining. However, when the lanes are not open, it is treated as white dotted lines.
On the double-dotted line, bus-only is implemented even during hours other than commuting. Vehicles except for buses can temporarily pass for right turns and joining. However, when the dedicated vehicle is not operating hours, it is treated as a white dotted line.
On the solid line, vehicles except buses are prohibited from driving, but it is operated flexibly according to the time and day of the week. When it is not operating hours for exclusive vehicles, it is treated as a white solid line.
On the double line, buses will be operated even during hours other than commuting hours. However, if the cars are not operated, it will be treated as solid white lines.
Hong Kong
In Hong Kong, some highways are set up with bus lanes to solve the traffic congestion.
Philippines
Traffic congestion was a principal problem in major roads and highways in the Philippines, especially in Metro Manila and other major cities. The government decided to set up some bus lanes in Metro Manila like in the Epifanio delos Santos Avenue.
Gallery
Highways by country
The following is a list of highways by country in alphabetical order. | Highway | Wikipedia | 365 | 48519 | https://en.wikipedia.org/wiki/Highway | Technology | Road transport | null |
Algeria East–West Highway
Autobahns of Austria
Autoput and Autocesta
Rodovia
Avtomagistrala
Highways in Canada
Expressway
Autocesta
Dálnice
Autostrada
Autoroute
Autobahns of Germany
Aftokinitodromos
Autópálya
National Highways and Expressways
Motorway
List of highways in Israel
Autostrade of Italy
Kōsokudōro
Lebuhraya
Autopista de Carretera Federal
Autoroute
Avtopat
Motorvei
Motorways and National Highways of Pakistan
Philippine highway network
Autoestrada
Russian federal highways
Autoput
Avtocesta
Expressways in South Korea
Autopista
Motorväg
Autobahns of Switzerland
Freeways in Taiwan
Thai highway network
State Highways (Ukraine)
Highways in the United Kingdom
Autofamba | Highway | Wikipedia | 154 | 48519 | https://en.wikipedia.org/wiki/Highway | Technology | Road transport | null |
In modern usage, a sloop is a sailboat with a single mast generally having only one headsail in front of the mast and one mainsail abaft (behind) the mast. It is a type of fore-and-aft rig. The mainsail may be of any type, most often Bermuda rig, but also others, such as gaff or gunter.
In naval terminology, "sloop-of-war" refers to the purpose of the craft, rather than to the specific size or sail-plan, and thus a sloop should not be confused with a sloop-of-war. As with many rig definitions, it was some time before the term sloop referred to the type of rig.
Regionally, the definition also takes into account the position of the mast. A forward mast placement and a fixed (as opposed to ) bowsprit, but with two headsails may give categorisation as a sloop. An example is the Friendship Sloop.
Origins
The name originates from the Dutch sloep, which is related to the Old English slūpan, to glide. The original Dutch term applied to an open rowing boat. A sloop is usually regarded as a single-masted rig with a single headsail and a fore-and-aft mainsail. In this form, the sloop is the commonest of all sailing rigswith the Bermuda sloop being the default rig for leisure craft, being used on types that range from simple cruising dinghies to large racing yachts with high-tech sail fabrics and large powerful winches.If the vessel has two or more headsails, the term cutter is usually applied, though there are regional and historic variations on this. A boat with a forward mast placement and a fixed bowsprit, but more than one headsail, may be called a sloop. The Friendship sloop is an example of this. Particularly with historic craft, categorisation as a cutter may rely on having a running bowsprit.
Variations
Before the Bermuda rig became popular outside of Bermuda in the early 20th century, a (non-Bermudian) sloop might carry one or more square-rigged topsails which will be hung from a topsail yard and be supported from below by a crossjack. | Sloop | Wikipedia | 459 | 48520 | https://en.wikipedia.org/wiki/Sloop | Technology | Naval transport | null |
A sloop's headsail may be masthead-rigged or fractional-rigged. On a masthead-rigged sloop, the forestay (on which the headsail is carried) attaches at the top of the mast. On a fractional-rigged sloop, the forestay attaches to the mast at a point below the top. A sloop may use a bowsprit, a spar that projects forward from the bow.
Gallery | Sloop | Wikipedia | 90 | 48520 | https://en.wikipedia.org/wiki/Sloop | Technology | Naval transport | null |
Blood vessels are the tubular structures of a circulatory system that transport blood throughout a vertebrate's body. Blood vessels transport blood cells, nutrients, and oxygen to most of the tissues of a body. They also take waste and carbon dioxide away from the tissues. Some tissues such as cartilage, epithelium, and the lens and cornea of the eye are not supplied with blood vessels and are termed avascular.
There are five types of blood vessels: the arteries, which carry the blood away from the heart; the arterioles; the capillaries, where the exchange of water and chemicals between the blood and the tissues occurs; the venules; and the veins, which carry blood from the capillaries back towards the heart.
The word vascular, is derived from the Latin vas, meaning vessel, and is mostly used in relation to blood vessels.
Etymology
artery – late Middle English; from Latin arteria, from Greek artēria, probably from airein ("raise").
vein – Middle English; from Old French veine, from Latin vena.
capillary – mid-17th century; from Latin capillaris, from capillus ("hair"), influenced by Old French capillaire. | Blood vessel | Wikipedia | 259 | 48530 | https://en.wikipedia.org/wiki/Blood%20vessel | Biology and health sciences | Circulatory system | Biology |
Structure
The arteries and veins have three layers. The middle layer is thicker in the arteries than it is in the veins:
The inner layer, tunica intima, is the thinnest layer. It is a single layer of flat cells (simple squamous epithelium) glued by a polysaccharide intercellular matrix, surrounded by a thin layer of subendothelial connective tissue interlaced with a number of circularly arranged elastic bands called the internal elastic lamina. A thin membrane of elastic fibers in the tunica intima run parallel to the vessel.
The middle layer of tunica media is the thickest layer in arteries. It consists of circularly arranged elastic fiber, connective tissue and polysaccharide substances; the second and third layer are separated by another thick elastic band called external elastic lamina. The tunica media may (especially in arteries) be rich in vascular smooth muscle, which controls the caliber of the vessel. Veins do not have the external elastic lamina, but only an internal one. The tunica media is thicker in the arteries rather than the veins.
The outer layer is the tunica adventitia and the thickest layer in veins. It is entirely made of connective tissue. It also contains nerves that supply the vessel as well as nutrient capillaries (vasa vasorum) in the larger blood vessels.
Capillaries consist of a single layer of endothelial cells with a supporting subendothelium consisting of a basement membrane and connective tissue. When blood vessels connect to form a region of diffuse vascular supply, it is called an anastomosis. Anastomoses provide alternative routes for blood to flow through in case of blockages. Veins can have valves that prevent the backflow of the blood that was being pumped against gravity by the surrounding muscles. In humans, arteries do not have valves except for the two 'arteries' that originate from the heart's ventricles.
Early estimates by Danish physiologist August Krogh suggested that the total length of capillaries in human muscles could reach approximately (assuming a high muscle mass human body, like that of a bodybuilder). However, later studies suggest a more conservative figure of taking into account updated capillary density and average muscle mass in adults. Despite these later studies, many textbooks and other types of media include Krogh's estimates as a fun fact as opposed to the more recent studies. | Blood vessel | Wikipedia | 504 | 48530 | https://en.wikipedia.org/wiki/Blood%20vessel | Biology and health sciences | Circulatory system | Biology |
Types
There are various kinds of blood vessels:
Arteries
Elastic arteries
Distributing arteries
Arterioles
Capillaries (smallest type of blood vessels)
Venules
Veins
Large collecting vessels, such as the subclavian vein, the jugular vein, the renal vein and the iliac vein.
Venae cavae (the two largest veins, carry blood into the heart).
Sinusoids
Extremely small vessels located within bone marrow, the spleen and the liver.
They are roughly grouped as "arterial" and "venous", determined by whether the blood in it is flowing away from (arterial) or toward (venous) the heart. The term "arterial blood" is nevertheless used to indicate blood high in oxygen, although the pulmonary artery carries "venous blood" and blood flowing in the pulmonary vein is rich in oxygen. This is because they are carrying the blood to and from the lungs, respectively, to be oxygenated.
Function
Blood vessels function to transport blood to an animal's body tissues. In general, arteries and arterioles transport oxygenated blood from the lungs to the body and its organs, and veins and venules transport deoxygenated blood from the body to the lungs. Blood vessels also circulate blood throughout the circulatory system. Oxygen (bound to hemoglobin in red blood cells) is the most critical nutrient carried by the blood. In all arteries apart from the pulmonary artery, hemoglobin is highly saturated (95–100%) with oxygen. In all veins, apart from the pulmonary vein, the saturation of hemoglobin is about 75%. (The values are reversed in the pulmonary circulation.) In addition to carrying oxygen, blood also carries hormones, and nutrients to the cells of a body and removes waste products.
Blood vessels do not actively engage in the transport of blood (they have no appreciable peristalsis). Blood is propelled through arteries and arterioles through pressure generated by the heartbeat. Blood vessels also transport red blood cells. Hematocrit tests can be performed to calculate the proportion of red blood cells in the blood. Higher proportions result in conditions such as dehydration or heart disease, while lower proportions could lead to anemia and long-term blood loss. | Blood vessel | Wikipedia | 466 | 48530 | https://en.wikipedia.org/wiki/Blood%20vessel | Biology and health sciences | Circulatory system | Biology |
Permeability of the endothelium is pivotal in the release of nutrients to the tissue. It is also increased in inflammation in response to histamine, prostaglandins and interleukins, which leads to most of the symptoms of inflammation (swelling, redness, warmth and pain).
Constriction
Arteries—and veins to a degree—can regulate their inner diameter by contraction of the muscular layer. This changes the blood flow to downstream organs and is determined by the autonomic nervous system. Vasodilation and vasoconstriction are also used antagonistically as methods of thermoregulation.
The size of blood vessels is different for each of them. It ranges from a diameter of about 30–25 millimeters for the aorta to only about 5 micrometers (0,005mm) for the capillaries. Vasoconstriction is the constriction of blood vessels (narrowing, becoming smaller in cross-sectional area) by contracting the vascular smooth muscle in the vessel walls. It is regulated by vasoconstrictors (agents that cause vasoconstriction). These can include paracrine factors (e.g., prostaglandins), a number of hormones (e.g., vasopressin and angiotensin) and neurotransmitters (e.g., epinephrine) from the nervous system.
Vasodilation is a similar process mediated by antagonistically acting mediators. The most prominent vasodilator is nitric oxide (termed endothelium-derived relaxing factor for this reason).
Flow
The circulatory system uses the channel of blood vessels to deliver blood to all parts of the body. This is a result of the left and right sides of the heart working together to allow blood to flow continuously to the lungs and other parts of the body. Oxygen-poor blood enters the right side of the heart through two large veins. Oxygen-rich blood from the lungs enters through the pulmonary veins on the left side of the heart into the aorta and then reaches the rest of the body. The capillaries are responsible for allowing the blood to receive oxygen through tiny air sacs in the lungs. This is also the site where carbon dioxide exits the blood. This all occurs in the lungs where blood is oxygenated. | Blood vessel | Wikipedia | 493 | 48530 | https://en.wikipedia.org/wiki/Blood%20vessel | Biology and health sciences | Circulatory system | Biology |
The blood pressure in blood vessels is traditionally expressed in millimetres of mercury (1 mmHg = 133 Pa). In the arterial system, this is usually around 120 mmHg systolic (high pressure wave due to contraction of the heart) and 80 mmHg diastolic (low pressure wave). In contrast, pressures in the venous system are constant and rarely exceed 10 mmHg.
Vascular resistance occurs when the vessels away from the heart oppose the flow of blood. Resistance is an accumulation of three different factors: blood viscosity, blood vessel length and vessel radius. Blood viscosity is the thickness of the blood and its resistance to flow as a result of the different components of the blood. Blood is 92% water by weight and the rest of blood is composed of protein, nutrients, electrolytes, wastes, and dissolved gases. Depending on the health of an individual, the blood viscosity can vary (i.e., anemia causing relatively lower concentrations of protein, high blood pressure an increase in dissolved salts or lipids, etc.).
Vessel length is the total length of the vessel measured as the distance away from the heart. As the total length of the vessel increases, the total resistance as a result of friction will increase. Vessel radius also affects the total resistance as a result of contact with the vessel wall. As the radius of the wall gets smaller, the proportion of the blood making contact with the wall will increase. The greater amount of contact with the wall will increase the total resistance against the blood flow.
Disease
Blood vessels play a huge role in virtually every medical condition. Cancer, for example, cannot progress unless the tumor causes angiogenesis (formation of new blood vessels) to supply the malignant cells' metabolic demand. Atherosclerosis represents around 85% of all deaths from cardiovascular diseases due to the buildup of plaque. Coronary artery disease that often follows after atherosclerosis can cause heart attacks or cardiac arrest, resulting in 370,000 worldwide deaths in 2022. In 2019, around 17.9 million people died from cardiovascular diseases. Of these deaths, around 85% of them were due to heart attack and stroke. | Blood vessel | Wikipedia | 443 | 48530 | https://en.wikipedia.org/wiki/Blood%20vessel | Biology and health sciences | Circulatory system | Biology |
Blood vessel permeability is increased in inflammation. Damage, due to trauma or spontaneously, may lead to hemorrhage due to mechanical damage to the vessel endothelium. In contrast, occlusion of the blood vessel by atherosclerotic plaque, an embolised blood clot or a foreign body leads to downstream ischemia (insufficient blood supply) and possibly infarction (necrosis due to lack of blood supply). Vessel occlusion tends to be a positive feedback system; an occluded vessel creates eddies in the normally laminar flow or plug flow blood currents. These eddies create abnormal fluid velocity gradients which push blood elements, such as cholesterol or chylomicron bodies, to the endothelium. These deposit onto the arterial walls which are already partially occluded and build upon the blockage.
The most common disease of the blood vessels is hypertension or high blood pressure. This is caused by an increase in the pressure of the blood flowing through the vessels. Hypertension can lead to heart failure and stroke. Aspirin helps prevent blood clots and can also help limit inflammation. Vasculitis is inflammation of the vessel wall due to autoimmune disease or infection. | Blood vessel | Wikipedia | 260 | 48530 | https://en.wikipedia.org/wiki/Blood%20vessel | Biology and health sciences | Circulatory system | Biology |
Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic chemical of the catecholamine and phenethylamine families. Dopamine constitutes about 80% of the catecholamine content in the brain. It is an amine synthesized by removing a carboxyl group from a molecule of its precursor chemical, L-DOPA, which is synthesized in the brain and kidneys. Dopamine is also synthesized in plants and most animals. In the brain, dopamine functions as a neurotransmitter—a chemical released by neurons (nerve cells) to send signals to other nerve cells. Neurotransmitters are synthesized in specific regions of the brain but affect many regions systemically. The brain includes several distinct dopamine pathways, one of which plays a major role in the motivational component of reward-motivated behavior. The anticipation of most types of rewards increases the level of dopamine in the brain, and many addictive drugs increase dopamine release or block its reuptake into neurons following release. Other brain dopamine pathways are involved in motor control and in controlling the release of various hormones. These pathways and cell groups form a dopamine system which is neuromodulatory.
In popular culture and media, dopamine is often portrayed as the main chemical of pleasure, but the current opinion in pharmacology is that dopamine instead confers motivational salience; in other words, dopamine signals the perceived motivational prominence (i.e., the desirability or aversiveness) of an outcome, which in turn propels the organism's behavior toward or away from achieving that outcome. | Dopamine | Wikipedia | 379 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Outside the central nervous system, dopamine functions primarily as a local paracrine messenger. In blood vessels, it inhibits norepinephrine release and acts as a vasodilator; in the kidneys, it increases sodium excretion and urine output; in the pancreas, it reduces insulin production; in the digestive system, it reduces gastrointestinal motility and protects intestinal mucosa; and in the immune system, it reduces the activity of lymphocytes. With the exception of the blood vessels, dopamine in each of these peripheral systems is synthesized locally and exerts its effects near the cells that release it.
Several important diseases of the nervous system are associated with dysfunctions of the dopamine system, and some of the key medications used to treat them work by altering the effects of dopamine. Parkinson's disease, a degenerative condition causing tremor and motor impairment, is caused by a loss of dopamine-secreting neurons in an area of the midbrain called the substantia nigra. Its metabolic precursor L-DOPA can be manufactured; Levodopa, a pure form of L-DOPA, is the most widely used treatment for Parkinson's. There is evidence that schizophrenia involves altered levels of dopamine activity, and most antipsychotic drugs used to treat this are dopamine antagonists which reduce dopamine activity. Similar dopamine antagonist drugs are also some of the most effective anti-nausea agents. Restless legs syndrome and attention deficit hyperactivity disorder (ADHD) are associated with decreased dopamine activity. Dopaminergic stimulants can be addictive in high doses, but some are used at lower doses to treat ADHD. Dopamine itself is available as a manufactured medication for intravenous injection. It is useful in the treatment of severe heart failure or cardiogenic shock. In newborn babies it may be used for hypotension and septic shock. | Dopamine | Wikipedia | 416 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Structure
A dopamine molecule consists of a catechol structure (a benzene ring with two hydroxyl side groups) with one amine group attached via an ethyl chain. As such, dopamine is the simplest possible catecholamine, a family that also includes the neurotransmitters norepinephrine and epinephrine. The presence of a benzene ring with this amine attachment makes it a substituted phenethylamine, a family that includes numerous psychoactive drugs.
Like most amines, dopamine is an organic base. As a base, it is generally protonated in acidic environments (in an acid-base reaction). The protonated form is highly water-soluble and relatively stable, but can become oxidized if exposed to oxygen or other oxidants. In basic environments, dopamine is not protonated. In this free base form, it is less water-soluble and also more highly reactive. Because of the increased stability and water-solubility of the protonated form, dopamine is supplied for chemical or pharmaceutical use as dopamine hydrochloride—that is, the hydrochloride salt that is created when dopamine is combined with hydrochloric acid. In dry form, dopamine hydrochloride is a fine powder which is white to yellow in color.
Biochemistry
Synthesis
Dopamine is synthesized in a restricted set of cell types, mainly neurons and cells in the medulla of the adrenal glands. The primary and minor metabolic pathways respectively are:
Primary: L-Phenylalanine → L-Tyrosine → L-DOPA → Dopamine
Minor: L-Phenylalanine → L-Tyrosine → p-Tyramine → Dopamine
Minor: L-Phenylalanine → m-Tyrosine → m-Tyramine → Dopamine
The direct precursor of dopamine, L-DOPA, can be synthesized indirectly from the essential amino acid phenylalanine or directly from the non-essential amino acid tyrosine. These amino acids are found in nearly every protein and so are readily available in food, with tyrosine being the most common. Although dopamine is also found in many types of food, it is incapable of crossing the blood–brain barrier that surrounds and protects the brain. It must therefore be synthesized inside the brain to perform its neuronal activity. | Dopamine | Wikipedia | 509 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
L-Phenylalanine is converted into L-tyrosine by the enzyme phenylalanine hydroxylase, with molecular oxygen (O2) and tetrahydrobiopterin as cofactors. L-Tyrosine is converted into L-DOPA by the enzyme tyrosine hydroxylase, with tetrahydrobiopterin, O2, and iron (Fe2+) as cofactors. L-DOPA is converted into dopamine by the enzyme aromatic L-amino acid decarboxylase (also known as DOPA decarboxylase), with pyridoxal phosphate as the cofactor.
Dopamine itself is used as precursor in the synthesis of the neurotransmitters norepinephrine and epinephrine. Dopamine is converted into norepinephrine by the enzyme dopamine β-hydroxylase, with O2 and L-ascorbic acid as cofactors. Norepinephrine is converted into epinephrine by the enzyme phenylethanolamine N-methyltransferase with S-adenosyl-L-methionine as the cofactor.
Some of the cofactors also require their own synthesis. Deficiency in any required amino acid or cofactor can impair the synthesis of dopamine, norepinephrine, and epinephrine.
Degradation
Dopamine is broken down into inactive metabolites by a set of enzymes—monoamine oxidase (MAO), catechol-O-methyl transferase (COMT), and aldehyde dehydrogenase (ALDH), acting in sequence. Both isoforms of monoamine oxidase, MAO-A and MAO-B, effectively metabolize dopamine. Different breakdown pathways exist but the main end-product is homovanillic acid (HVA), which has no known biological activity. From the bloodstream, homovanillic acid is filtered out by the kidneys and then excreted in the urine. The two primary metabolic routes that convert dopamine into HVA are:
Dopamine → DOPAL → DOPAC → HVA – catalyzed by MAO, ALDH, and COMT respectively
Dopamine → 3-Methoxytyramine → HVA – catalyzed by COMT and MAO+ALDH respectively | Dopamine | Wikipedia | 508 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
In clinical research on schizophrenia, measurements of homovanillic acid in plasma have been used to estimate levels of dopamine activity in the brain. A difficulty in this approach however, is separating the high level of plasma homovanillic acid contributed by the metabolism of norepinephrine.
Although dopamine is normally broken down by an oxidoreductase enzyme, it is also susceptible to oxidation by direct reaction with oxygen, yielding quinones plus various free radicals as products. The rate of oxidation can be increased by the presence of ferric iron or other factors. Quinones and free radicals produced by autoxidation of dopamine can poison cells, and there is evidence that this mechanism may contribute to the cell loss that occurs in Parkinson's disease and other conditions.
Functions
Cellular effects
Dopamine exerts its effects by binding to and activating cell surface receptors. In humans, dopamine has a high binding affinity at dopamine receptors and human trace amine-associated receptor 1 (hTAAR1). In mammals, five subtypes of dopamine receptors have been identified, labeled from D1 to D5. All of them function as metabotropic, G protein-coupled receptors, meaning that they exert their effects via a complex second messenger system. These receptors can be divided into two families, known as D1-like and D2-like. For receptors located on neurons in the nervous system, the ultimate effect of D1-like activation (D1 and D5) can be excitation (via opening of sodium channels) or inhibition (via opening of potassium channels); the ultimate effect of D2-like activation (D2, D3, and D4) is usually inhibition of the target neuron. Consequently, it is incorrect to describe dopamine itself as either excitatory or inhibitory: its effect on a target neuron depends on which types of receptors are present on the membrane of that neuron and on the internal responses of that neuron to the second messenger cAMP. D1 receptors are the most numerous dopamine receptors in the human nervous system; D2 receptors are next; D3, D4, and D5 receptors are present at significantly lower levels.
Storage, release, and reuptake | Dopamine | Wikipedia | 472 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Inside the brain, dopamine functions as a neurotransmitter and neuromodulator, and is controlled by a set of mechanisms common to all monoamine neurotransmitters. After synthesis, dopamine is transported from the cytosol into secretory vesicles, including synaptic vesicles, small and large dense core vesicles by a solute carrier—a vesicular monoamine transporter, VMAT2. Dopamine is stored in these vesicles until it is ejected into the synaptic cleft. In most cases, the release of dopamine occurs through a process called exocytosis which is caused by action potentials, but it can also be caused by the activity of an intracellular trace amine-associated receptor, TAAR1. TAAR1 is a high-affinity receptor for dopamine, trace amines, and certain substituted amphetamines that is located along membranes in the intracellular milieu of the presynaptic cell; activation of the receptor can regulate dopamine signaling by inducing dopamine reuptake inhibition and efflux as well as by inhibiting neuronal firing through a diverse set of mechanisms.
Once in the synapse, dopamine binds to and activates dopamine receptors. These can be postsynaptic dopamine receptors, which are located on dendrites (the postsynaptic neuron), or presynaptic autoreceptors (e.g., the D2sh and presynaptic D3 receptors), which are located on the membrane of an axon terminal (the presynaptic neuron). After the postsynaptic neuron elicits an action potential, dopamine molecules quickly become unbound from their receptors. They are then absorbed back into the presynaptic cell, via reuptake mediated either by the dopamine transporter or by the plasma membrane monoamine transporter. Once back in the cytosol, dopamine can either be broken down by a monoamine oxidase or repackaged into vesicles by VMAT2, making it available for future release. | Dopamine | Wikipedia | 459 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
In the brain the level of extracellular dopamine is modulated by two mechanisms: phasic and tonic transmission. Phasic dopamine release, like most neurotransmitter release in the nervous system, is driven directly by action potentials in the dopamine-containing cells. Tonic dopamine transmission occurs when small amounts of dopamine are released without being preceded by presynaptic action potentials. Tonic transmission is regulated by a variety of factors, including the activity of other neurons and neurotransmitter reuptake.
Central nervous system
Inside the brain, dopamine plays important roles in executive functions, motor control, motivation, arousal, reinforcement, and reward, as well as lower-level functions including lactation, sexual gratification, and nausea. The dopaminergic cell groups and pathways make up the dopamine system which is neuromodulatory.
Dopaminergic neurons (dopamine-producing nerve cells) are comparatively few in number—a total of around 400,000 in the human brain—and their cell bodies are confined in groups to a few relatively small brain areas. However their axons project to many other brain areas, and they exert powerful effects on their targets. These dopaminergic cell groups were first mapped in 1964 by Annica Dahlström and Kjell Fuxe, who assigned them labels starting with the letter "A" (for "aminergic"). In their scheme, areas A1 through A7 contain the neurotransmitter norepinephrine, whereas A8 through A14 contain dopamine. The dopaminergic areas they identified are the substantia nigra (groups 8 and 9); the ventral tegmental area (group 10); the posterior hypothalamus (group 11); the arcuate nucleus (group 12); the zona incerta (group 13) and the periventricular nucleus (group 14). | Dopamine | Wikipedia | 419 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
The substantia nigra is a small midbrain area that forms a component of the basal ganglia. This has two parts—an input area called the pars reticulata and an output area called the pars compacta. The dopaminergic neurons are found mainly in the pars compacta (cell group A8) and nearby (group A9). In humans, the projection of dopaminergic neurons from the substantia nigra pars compacta to the dorsal striatum, termed the nigrostriatal pathway, plays a significant role in the control of motor function and in learning new motor skills. These neurons are especially vulnerable to damage, and when a large number of them die, the result is a parkinsonian syndrome.
The ventral tegmental area (VTA) is another midbrain area. The most prominent group of VTA dopaminergic neurons projects to the prefrontal cortex via the mesocortical pathway and another smaller group projects to the nucleus accumbens via the mesolimbic pathway. Together, these two pathways are collectively termed the mesocorticolimbic projection. The VTA also sends dopaminergic projections to the amygdala, cingulate gyrus, hippocampus, and olfactory bulb. Mesocorticolimbic neurons play a central role in reward and other aspects of motivation. Accumulating literature shows that dopamine also plays a crucial role in aversive learning through its effects on a number of brain regions.
The posterior hypothalamus has dopamine neurons that project to the spinal cord, but their function is not well established. There is some evidence that pathology in this area plays a role in restless legs syndrome, a condition in which people have difficulty sleeping due to an overwhelming compulsion to constantly move parts of the body, especially the legs. | Dopamine | Wikipedia | 394 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
The arcuate nucleus and the periventricular nucleus of the hypothalamus have dopamine neurons that form an important projection—the tuberoinfundibular pathway which goes to the pituitary gland, where it influences the secretion of the hormone prolactin. Dopamine is the primary neuroendocrine inhibitor of the secretion of prolactin from the anterior pituitary gland. Dopamine produced by neurons in the arcuate nucleus is secreted into the hypophyseal portal system of the median eminence, which supplies the pituitary gland. The prolactin cells that produce prolactin, in the absence of dopamine, secrete prolactin continuously; dopamine inhibits this secretion.
The zona incerta, grouped between the arcuate and periventricular nuclei, projects to several areas of the hypothalamus, and participates in the control of gonadotropin-releasing hormone, which is necessary to activate the development of the male and female reproductive systems, following puberty.
An additional group of dopamine-secreting neurons is found in the retina of the eye. These neurons are amacrine cells, meaning that they have no axons. They release dopamine into the extracellular medium, and are specifically active during daylight hours, becoming silent at night. This retinal dopamine acts to enhance the activity of cone cells in the retina while suppressing rod cells—the result is to increase sensitivity to color and contrast during bright light conditions, at the cost of reduced sensitivity when the light is dim.
Basal ganglia
The largest and most important sources of dopamine in the vertebrate brain are the substantia nigra and ventral tegmental area. Both structures are components of the midbrain, closely related to each other and functionally similar in many respects. The largest component of the basal ganglia is the striatum. The substantia nigra sends a dopaminergic projection to the dorsal striatum, while the ventral tegmental area sends a similar type of dopaminergic projection to the ventral striatum. | Dopamine | Wikipedia | 462 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Progress in understanding the functions of the basal ganglia has been slow. The most popular hypotheses, broadly stated, propose that the basal ganglia play a central role in action selection. The action selection theory in its simplest form proposes that when a person or animal is in a situation where several behaviors are possible, activity in the basal ganglia determines which of them is executed, by releasing that response from inhibition while continuing to inhibit other motor systems that if activated would generate competing behaviors. Thus the basal ganglia, in this concept, are responsible for initiating behaviors, but not for determining the details of how they are carried out. In other words, they essentially form a decision-making system.
The basal ganglia can be divided into several sectors, and each is involved in controlling particular types of actions. The ventral sector of the basal ganglia (containing the ventral striatum and ventral tegmental area) operates at the highest level of the hierarchy, selecting actions at the whole-organism level. The dorsal sectors (containing the dorsal striatum and substantia nigra) operate at lower levels, selecting the specific muscles and movements that are used to implement a given behavior pattern.
Dopamine contributes to the action selection process in at least two important ways. First, it sets the "threshold" for initiating actions. The higher the level of dopamine activity, the lower the impetus required to evoke a given behavior. As a consequence, high levels of dopamine lead to high levels of motor activity and impulsive behavior; low levels of dopamine lead to torpor and slowed reactions. Parkinson's disease, in which dopamine levels in the substantia nigra circuit are greatly reduced, is characterized by stiffness and difficulty initiating movement—however, when people with the disease are confronted with strong stimuli such as a serious threat, their reactions can be as vigorous as those of a healthy person. In the opposite direction, drugs that increase dopamine release, such as cocaine or amphetamine, can produce heightened levels of activity, including, at the extreme, psychomotor agitation and stereotyped movements. | Dopamine | Wikipedia | 434 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
The second important effect of dopamine is as a "teaching" signal. When an action is followed by an increase in dopamine activity, the basal ganglia circuit is altered in a way that makes the same response easier to evoke when similar situations arise in the future. This is a form of operant conditioning, in which dopamine plays the role of a reward signal.
Reward
In the language used to discuss the reward system, reward is the attractive and motivational property of a stimulus that induces appetitive behavior (also known as approach behavior) and consummatory behavior. A rewarding stimulus is one that can induce the organism to approach it and choose to consume it. Pleasure, learning (e.g., classical and operant conditioning), and approach behavior are the three main functions of reward. As an aspect of reward, pleasure provides a definition of reward; however, while all pleasurable stimuli are rewarding, not all rewarding stimuli are pleasurable (e.g., extrinsic rewards like money). The motivational or desirable aspect of rewarding stimuli is reflected by the approach behavior that they induce, whereas the pleasure from intrinsic rewards results from consuming them after acquiring them. A neuropsychological model which distinguishes these two components of an intrinsically rewarding stimulus is the incentive salience model, where "wanting" or desire (less commonly, "seeking") corresponds to appetitive or approach behavior while "liking" or pleasure corresponds to consummatory behavior. In human drug addicts, "wanting" becomes dissociated with "liking" as the desire to use an addictive drug increases, while the pleasure obtained from consuming it decreases due to drug tolerance. | Dopamine | Wikipedia | 354 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Within the brain, dopamine functions partly as a global reward signal. An initial dopamine response to a rewarding stimulus encodes information about the salience, value, and context of a reward. In the context of reward-related learning, dopamine also functions as a reward prediction error signal, that is, the degree to which the value of a reward is unexpected. According to this hypothesis proposed by Montague, Dayan, and Sejnowski, rewards that are expected do not produce a second phasic dopamine response in certain dopaminergic cells, but rewards that are unexpected, or greater than expected, produce a short-lasting increase in synaptic dopamine, whereas the omission of an expected reward actually causes dopamine release to drop below its background level. The "prediction error" hypothesis has drawn particular interest from computational neuroscientists, because an influential computational-learning method known as temporal difference learning makes heavy use of a signal that encodes prediction error. This confluence of theory and data has led to a fertile interaction between neuroscientists and computer scientists interested in machine learning.
Evidence from microelectrode recordings from the brains of animals shows that dopamine neurons in the ventral tegmental area (VTA) and substantia nigra are strongly activated by a wide variety of rewarding events. These reward-responsive dopamine neurons in the VTA and substantia nigra are crucial for reward-related cognition and serve as the central component of the reward system. The function of dopamine varies in each axonal projection from the VTA and substantia nigra; for example, the VTA–nucleus accumbens shell projection assigns incentive salience ("want") to rewarding stimuli and its associated cues, the VTA–prefrontal cortex projection updates the value of different goals in accordance with their incentive salience, the VTA–amygdala and VTA–hippocampus projections mediate the consolidation of reward-related memories, and both the VTA–nucleus accumbens core and substantia nigra–dorsal striatum pathways are involved in learning motor responses that facilitate the acquisition of rewarding stimuli. Some activity within the VTA dopaminergic projections appears to be associated with reward prediction as well. | Dopamine | Wikipedia | 483 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Pleasure
While dopamine has a central role in causing "wanting," associated with the appetitive or approach behavioral responses to rewarding stimuli, detailed studies have shown that dopamine cannot simply be equated with hedonic "liking" or pleasure, as reflected in the consummatory behavioral response. Dopamine neurotransmission is involved in some but not all aspects of pleasure-related cognition, since pleasure centers have been identified both within the dopamine system (i.e., nucleus accumbens shell) and outside the dopamine system (i.e., ventral pallidum and parabrachial nucleus). For example, direct electrical stimulation of dopamine pathways, using electrodes implanted in the brain, is experienced as pleasurable, and many types of animals are willing to work to obtain it. Antipsychotic drugs reduce dopamine levels and tend to cause anhedonia, a diminished ability to experience pleasure. Many types of pleasurable experiences—such as sexual intercourse, eating, and playing video games—increase dopamine release. All addictive drugs directly or indirectly affect dopamine neurotransmission in the nucleus accumbens; these drugs increase drug "wanting", leading to compulsive drug use, when repeatedly taken in high doses, presumably through the sensitization of incentive-salience. Drugs that increase synaptic dopamine concentrations include psychostimulants such as methamphetamine and cocaine. These produce increases in "wanting" behaviors, but do not greatly alter expressions of pleasure or change levels of satiation. However, opiate drugs such as heroin and morphine produce increases in expressions of "liking" and "wanting" behaviors. Moreover, animals in which the ventral tegmental dopamine system has been rendered inactive do not seek food, and will starve to death if left to themselves, but if food is placed in their mouths they will consume it and show expressions indicative of pleasure. | Dopamine | Wikipedia | 414 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
A clinical study from January 2019 that assessed the effect of a dopamine precursor (levodopa), dopamine antagonist (risperidone), and a placebo on reward responses to music – including the degree of pleasure experienced during musical chills, as measured by changes in electrodermal activity as well as subjective ratings – found that the manipulation of dopamine neurotransmission bidirectionally regulates pleasure cognition (specifically, the hedonic impact of music) in human subjects. This research demonstrated that increased dopamine neurotransmission acts as a sine qua non condition for pleasurable hedonic reactions to music in humans.
A study published in Nature in 1998 found evidence that playing video games releases dopamine in the human striatum. This dopamine is associated with learning, behavior reinforcement, attention, and sensorimotor integration. Researchers used positron emission tomography scans and 11C-labelled raclopride to track dopamine levels in the brain during goal-directed motor tasks and found that dopamine release was positively correlated with task performance and was greatest in the ventral striatum. This was the first study to demonstrate the behavioral conditions under which dopamine is released in humans. It highlights the ability of positron emission tomography to detect neurotransmitter fluxes during changes in behavior. According to research, potentially problematic video game use is related to personality traits such as low self-esteem and low self-efficacy, anxiety, aggression, and clinical symptoms of depression and anxiety disorders. Additionally, the reasons individuals play video games vary and may include coping, socialization, and personal satisfaction. The DSM-5 defines Internet Gaming Disorder as a mental disorder closely related to Gambling Disorder. This has been supported by some researchers but has also caused controversy.
Outside the central nervous system | Dopamine | Wikipedia | 381 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Dopamine does not cross the blood–brain barrier, so its synthesis and functions in peripheral areas are to a large degree independent of its synthesis and functions in the brain. A substantial amount of dopamine circulates in the bloodstream, but its functions there are not entirely clear. Dopamine is found in blood plasma at levels comparable to those of epinephrine, but in humans, over 95% of the dopamine in the plasma is in the form of dopamine sulfate, a conjugate produced by the enzyme sulfotransferase 1A3/1A4 acting on free dopamine. The bulk of this dopamine sulfate is produced in the mesenteric organs. The production of dopamine sulfate is thought to be a mechanism for detoxifying dopamine that is ingested as food or produced by the digestive process—levels in the plasma typically rise more than fifty-fold after a meal. Dopamine sulfate has no known biological functions and is excreted in urine.
The relatively small quantity of unconjugated dopamine in the bloodstream may be produced by the sympathetic nervous system, the digestive system, or possibly other organs. It may act on dopamine receptors in peripheral tissues, or be metabolized, or be converted to norepinephrine by the enzyme dopamine beta hydroxylase, which is released into the bloodstream by the adrenal medulla. Some dopamine receptors are located in the walls of arteries, where they act as a vasodilator and an inhibitor of norepinephrine release from postganglionic sympathetic nerves terminals (dopamine can inhibit norepinephrine release by acting on presynaptic dopamine receptors, and also on presynaptic α-1 receptors, like norepinephrine itself). These responses might be activated by dopamine released from the carotid body under conditions of low oxygen, but whether arterial dopamine receptors perform other biologically useful functions is not known.
Beyond its role in modulating blood flow, there are several peripheral systems in which dopamine circulates within a limited area and performs an exocrine or paracrine function. The peripheral systems in which dopamine plays an important role include the immune system, the kidneys and the pancreas.
Immune system | Dopamine | Wikipedia | 492 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
In the immune system dopamine acts upon receptors present on immune cells, especially lymphocytes. Dopamine can also affect immune cells in the spleen, bone marrow, and circulatory system. In addition, dopamine can be synthesized and released by immune cells themselves. The main effect of dopamine on lymphocytes is to reduce their activation level. The functional significance of this system is unclear, but it affords a possible route for interactions between the nervous system and immune system, and may be relevant to some autoimmune disorders.
Kidneys
The renal dopaminergic system is located in the cells of the nephron in the kidney, where all subtypes of dopamine receptors are present. Dopamine is also synthesized there, by tubule cells, and discharged into the tubular fluid. Its actions include increasing the blood supply to the kidneys, increasing the glomerular filtration rate, and increasing the excretion of sodium in the urine. Hence, defects in renal dopamine function can lead to reduced sodium excretion and consequently result in the development of high blood pressure. There is strong evidence that faults in the production of dopamine or in the receptors can result in a number of pathologies including oxidative stress, edema, and either genetic or essential hypertension. Oxidative stress can itself cause hypertension. Defects in the system can also be caused by genetic factors or high blood pressure.
Pancreas
In the pancreas the role of dopamine is somewhat complex. The pancreas consists of two parts, an exocrine and an endocrine component. The exocrine part synthesizes and secretes digestive enzymes and other substances, including dopamine, into the small intestine. The function of this secreted dopamine after it enters the small intestine is not clearly established—the possibilities include protecting the intestinal mucosa from damage and reducing gastrointestinal motility (the rate at which content moves through the digestive system). | Dopamine | Wikipedia | 429 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
The pancreatic islets make up the endocrine part of the pancreas, and synthesize and secrete hormones including insulin into the bloodstream. There is evidence that the beta cells in the islets that synthesize insulin contain dopamine receptors, and that dopamine acts to reduce the amount of insulin they release. The source of their dopamine input is not clearly established—it may come from dopamine that circulates in the bloodstream and derives from the sympathetic nervous system, or it may be synthesized locally by other types of pancreatic cells.
Medical uses
Dopamine as a manufactured medication is sold under the trade names Intropin, Dopastat, and Revimine, among others. It is on the World Health Organization's List of Essential Medicines. It is most commonly used as a stimulant drug in the treatment of severe low blood pressure, slow heart rate, and cardiac arrest. It is especially important in treating these in newborn infants. It is given intravenously. Since the half-life of dopamine in plasma is very short—approximately one minute in adults, two minutes in newborn infants and up to five minutes in preterm infants—it is usually given in a continuous intravenous drip rather than a single injection.
Its effects, depending on dosage, include an increase in sodium excretion by the kidneys, an increase in urine output, an increase in heart rate, and an increase in blood pressure. At low doses it acts through the sympathetic nervous system to increase heart muscle contraction force and heart rate, thereby increasing cardiac output and blood pressure. Higher doses also cause vasoconstriction that further increases blood pressure. Older literature also describes very low doses thought to improve kidney function without other consequences, but recent reviews have concluded that doses at such low levels are not effective and may sometimes be harmful. While some effects result from stimulation of dopamine receptors, the prominent cardiovascular effects result from dopamine acting at α1, β1, and β2 adrenergic receptors. | Dopamine | Wikipedia | 425 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Side effects of dopamine include negative effects on kidney function and irregular heartbeats. The LD50, or lethal dose which is expected to prove fatal in 50% of the population, has been found to be: 59 mg/kg (mouse; administered intravenously); 95 mg/kg (mouse; administered intraperitoneally); 163 mg/kg (rat; administered intraperitoneally); 79 mg/kg (dog; administered intravenously).
Disease, disorders, and pharmacology
The dopamine system plays a central role in several significant medical conditions, including Parkinson's disease, attention deficit hyperactivity disorder, Tourette syndrome, schizophrenia, bipolar disorder, and addiction. Aside from dopamine itself, there are many other important drugs that act on dopamine systems in various parts of the brain or body. Some are used for medical or recreational purposes, but neurochemists have also developed a variety of research drugs, some of which bind with high affinity to specific types of dopamine receptors and either agonize or antagonize their effects, and many that affect other aspects of dopamine physiology, including dopamine transporter inhibitors, VMAT inhibitors, and enzyme inhibitors.
Aging brain
A number of studies have reported an age-related decline in dopamine synthesis and dopamine receptor density (i.e., the number of receptors) in the brain. This decline has been shown to occur in the striatum and extrastriatal regions. Decreases in the D1, D2, and D3 receptors are well documented. The reduction of dopamine with aging is thought to be responsible for many neurological symptoms that increase in frequency with age, such as decreased arm swing and increased rigidity. Changes in dopamine levels may also cause age-related changes in cognitive flexibility.
Multiple sclerosis
Studies reported that dopamine imbalance influences the fatigue in multiple sclerosis. In patients with multiple sclerosis, dopamine inhibits production of IL-17 and IFN-γ by peripheral blood mononuclear cells.
Parkinson's disease | Dopamine | Wikipedia | 438 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Parkinson's disease is an age-related disorder characterized by movement disorders such as stiffness of the body, slowing of movement, and trembling of limbs when they are not in use. In advanced stages it progresses to dementia and eventually death. The main symptoms are caused by the loss of dopamine-secreting cells in the substantia nigra. These dopamine cells are especially vulnerable to damage, and a variety of insults, including encephalitis (as depicted in the book and movie Awakenings), repeated sports-related concussions, and some forms of chemical poisoning such as MPTP, can lead to substantial cell loss, producing a parkinsonian syndrome that is similar in its main features to Parkinson's disease. Most cases of Parkinson's disease, however, are idiopathic, meaning that the cause of cell death cannot be identified.
The most widely used treatment for parkinsonism is administration of L-DOPA, the metabolic precursor for dopamine. L-DOPA is converted to dopamine in the brain and various parts of the body by the enzyme DOPA decarboxylase. L-DOPA is used rather than dopamine itself because, unlike dopamine, it is capable of crossing the blood–brain barrier. It is often co-administered with an enzyme inhibitor of peripheral decarboxylation such as carbidopa or benserazide, to reduce the amount converted to dopamine in the periphery and thereby increase the amount of L-DOPA that enters the brain. When L-DOPA is administered regularly over a long time period, a variety of unpleasant side effects such as dyskinesia often begin to appear; even so, it is considered the best available long-term treatment option for most cases of Parkinson's disease.
L-DOPA treatment cannot restore the dopamine cells that have been lost, but it causes the remaining cells to produce more dopamine, thereby compensating for the loss to at least some degree. In advanced stages the treatment begins to fail because the cell loss is so severe that the remaining ones cannot produce enough dopamine regardless of L-DOPA levels. Other drugs that enhance dopamine function, such as bromocriptine and pergolide, are also sometimes used to treat Parkinsonism, but in most cases L-DOPA appears to give the best trade-off between positive effects and negative side-effects. | Dopamine | Wikipedia | 501 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Dopaminergic medications that are used to treat Parkinson's disease are sometimes associated with the development of a dopamine dysregulation syndrome, which involves the overuse of dopaminergic medication and medication-induced compulsive engagement in natural rewards like gambling and sexual activity. The latter behaviors are similar to those observed in individuals with a behavioral addiction.
Drug addiction and psychostimulants
Cocaine, substituted amphetamines (including methamphetamine), Adderall, methylphenidate (marketed as Ritalin or Concerta), and other psychostimulants exert their effects primarily or partly by increasing dopamine levels in the brain by a variety of mechanisms. Cocaine and methylphenidate are dopamine transporter blockers or reuptake inhibitors; they non-competitively inhibit dopamine reuptake, resulting in increased dopamine concentrations in the synaptic cleft. Like cocaine, substituted amphetamines and amphetamine also increase the concentration of dopamine in the synaptic cleft, but by different mechanisms.
The effects of psychostimulants include increases in heart rate, body temperature, and sweating; improvements in alertness, attention, and endurance; increases in pleasure produced by rewarding events; but at higher doses agitation, anxiety, or even loss of contact with reality. Drugs in this group can have a high addiction potential, due to their activating effects on the dopamine-mediated reward system in the brain. However some can also be useful, at lower doses, for treating attention deficit hyperactivity disorder (ADHD) and narcolepsy. An important differentiating factor is the onset and duration of action. Cocaine can take effect in seconds if it is injected or inhaled in free base form; the effects last from 5 to 90 minutes. This rapid and brief action makes its effects easily perceived and consequently gives it high addiction potential. Methylphenidate taken in pill form, in contrast, can take two hours to reach peak levels in the bloodstream, and depending on formulation the effects can last for up to 12 hours. These longer acting formulations have the benefit of reducing the potential for abuse, and improving adherence for treatment by using more convenient dosage regimens. | Dopamine | Wikipedia | 469 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
A variety of addictive drugs produce an increase in reward-related dopamine activity. Stimulants such as nicotine, cocaine and methamphetamine promote increased levels of dopamine which appear to be the primary factor in causing addiction. For other addictive drugs such as the opioid heroin, the increased levels of dopamine in the reward system may play only a minor role in addiction. When people addicted to stimulants go through withdrawal, they do not experience the physical suffering associated with alcohol withdrawal or withdrawal from opiates; instead they experience craving, an intense desire for the drug characterized by irritability, restlessness, and other arousal symptoms, brought about by psychological dependence.
The dopamine system plays a crucial role in several aspects of addiction. At the earliest stage, genetic differences that alter the expression of dopamine receptors in the brain can predict whether a person will find stimulants appealing or aversive. Consumption of stimulants produces increases in brain dopamine levels that last from minutes to hours. Finally, the chronic elevation in dopamine that comes with repetitive high-dose stimulant consumption triggers a wide-ranging set of structural changes in the brain that are responsible for the behavioral abnormalities which characterize an addiction. Treatment of stimulant addiction is very difficult, because even if consumption ceases, the craving that comes with psychological withdrawal does not. Even when the craving seems to be extinct, it may re-emerge when faced with stimuli that are associated with the drug, such as friends, locations and situations. Association networks in the brain are greatly interlinked.
Psychosis and antipsychotic drugs | Dopamine | Wikipedia | 344 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Psychiatrists in the early 1950s discovered that a class of drugs known as typical antipsychotics (also known as major tranquilizers), were often effective at reducing the psychotic symptoms of schizophrenia. The introduction of the first widely used antipsychotic, chlorpromazine (Thorazine), in the 1950s, led to the release of many patients with schizophrenia from institutions in the years that followed. By the 1970s researchers understood that these typical antipsychotics worked as antagonists on the D2 receptors. This realization led to the so-called dopamine hypothesis of schizophrenia, which postulates that schizophrenia is largely caused by hyperactivity of brain dopamine systems. The dopamine hypothesis drew additional support from the observation that psychotic symptoms were often intensified by dopamine-enhancing stimulants such as methamphetamine, and that these drugs could also produce psychosis in healthy people if taken in large enough doses. In the following decades other atypical antipsychotics that had fewer serious side effects were developed. Many of these newer drugs do not act directly on dopamine receptors, but instead produce alterations in dopamine activity indirectly. These drugs were also used to treat other psychoses. Antipsychotic drugs have a broadly suppressive effect on most types of active behavior, and particularly reduce the delusional and agitated behavior characteristic of overt psychosis.
Later observations, however, have caused the dopamine hypothesis to lose popularity, at least in its simple original form. For one thing, patients with schizophrenia do not typically show measurably increased levels of brain dopamine activity. Even so, many psychiatrists and neuroscientists continue to believe that schizophrenia involves some sort of dopamine system dysfunction. As the "dopamine hypothesis" has evolved over time, however, the sorts of dysfunctions it postulates have tended to become increasingly subtle and complex.
Psychopharmacologist Stephen M. Stahl suggested in a review of 2018 that in many cases of psychosis, including schizophrenia, three interconnected networks based on dopamine, serotonin, and glutamate – each on its own or in various combinations – contributed to an overexcitation of dopamine D2 receptors in the ventral striatum. | Dopamine | Wikipedia | 472 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Attention deficit hyperactivity disorder
Altered dopamine neurotransmission is implicated in attention deficit hyperactivity disorder (ADHD), a condition associated with impaired cognitive control, in turn leading to problems with regulating attention (attentional control), inhibiting behaviors (inhibitory control), and forgetting things or missing details (working memory), among other problems. There are genetic links between dopamine receptors, the dopamine transporter, and ADHD, in addition to links to other neurotransmitter receptors and transporters. The most important relationship between dopamine and ADHD involves the drugs that are used to treat ADHD. Some of the most effective therapeutic agents for ADHD are psychostimulants such as methylphenidate (Ritalin, Concerta) and amphetamine (Evekeo, Adderall, Dexedrine), drugs that increase both dopamine and norepinephrine levels in the brain. The clinical effects of these psychostimulants in treating ADHD are mediated through the indirect activation of dopamine and norepinephrine receptors, specifically dopamine receptor D1 and adrenoceptor α2, in the prefrontal cortex.
Pain
Dopamine plays a role in pain processing in multiple levels of the central nervous system including the spinal cord, periaqueductal gray, thalamus, basal ganglia, and cingulate cortex. Decreased levels of dopamine have been associated with painful symptoms that frequently occur in Parkinson's disease. Abnormalities in dopaminergic neurotransmission also occur in several painful clinical conditions, including burning mouth syndrome, fibromyalgia, and restless legs syndrome.
Nausea
Nausea and vomiting are largely determined by activity in the area postrema in the medulla of the brainstem, in a region known as the chemoreceptor trigger zone. This area contains a large population of type D2 dopamine receptors. Consequently, drugs that activate D2 receptors have a high potential to cause nausea. This group includes some medications that are administered for Parkinson's disease, as well as other dopamine agonists such as apomorphine. In some cases, D2-receptor antagonists such as metoclopramide are useful as anti-nausea drugs.
Fear and Anxiety | Dopamine | Wikipedia | 486 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Simultaneous positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), have shown that the amount of dopamine release is dependent on the strength of conditioned fear response and is linearly coupled to learning-induced activity in the amygdala. Dopamine is generally linked to reward learning, but it also plays a key role in fear learning and extinction by helping to form, store and update fear memories through its interaction with other brain regions like amygdala, ventromedial prefrontal cortex and striatum.
Comparative biology and evolution
Microorganisms
There are no reports of dopamine in archaea, but it has been detected in some types of bacteria and in the protozoan called Tetrahymena. Perhaps more importantly, there are types of bacteria that contain homologs of all the enzymes that animals use to synthesize dopamine. It has been proposed that animals derived their dopamine-synthesizing machinery from bacteria, via horizontal gene transfer that may have occurred relatively late in evolutionary time, perhaps as a result of the symbiotic incorporation of bacteria into eukaryotic cells that gave rise to mitochondria.
Animals
Dopamine is used as a neurotransmitter in most multicellular animals. In sponges there is only a single report of the presence of dopamine, with no indication of its function; however, dopamine has been reported in the nervous systems of many other radially symmetric species, including the cnidarian jellyfish, hydra and some corals. This dates the emergence of dopamine as a neurotransmitter back to the earliest appearance of the nervous system, over 500 million years ago in the Cambrian Period. Dopamine functions as a neurotransmitter in vertebrates, echinoderms, arthropods, molluscs, and several types of worm.
In every type of animal that has been examined, dopamine has been seen to modify motor behavior. In the model organism, nematode Caenorhabditis elegans, it reduces locomotion and increases food-exploratory movements; in flatworms it produces "screw-like" movements; in leeches it inhibits swimming and promotes crawling. Across a wide range of vertebrates, dopamine has an "activating" effect on behavior-switching and response selection, comparable to its effect in mammals. | Dopamine | Wikipedia | 510 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Dopamine has also consistently been shown to play a role in reward learning, in all animal groups. As in all vertebrates – invertebrates such as roundworms, flatworms, molluscs and common fruit flies can all be trained to repeat an action if it is consistently followed by an increase in dopamine levels. In fruit flies, distinct elements for reward learning suggest a modular structure to the insect reward processing system that broadly parallels that in the mammalian one. For example, dopamine regulates short- and long-term learning in monkeys; in fruit flies, different groups of dopamine neurons mediate reward signals for short- and long-term memories.
It had long been believed that arthropods were an exception to this with dopamine being seen as having an adverse effect. Reward was seen to be mediated instead by octopamine, a neurotransmitter closely related to norepinephrine. More recent studies, however, have shown that dopamine does play a part in reward learning in fruit flies. It has also been found that the rewarding effect of octopamine is due to its activating a set of dopaminergic neurons not previously accessed in the research.
Plants
Many plants, including a variety of food plants, synthesize dopamine to varying degrees. The highest concentrations have been observed in bananas—the fruit pulp of red and yellow bananas contains dopamine at levels of 40 to 50 parts per million by weight. Potatoes, avocados, broccoli, and Brussels sprouts may also contain dopamine at levels of 1 part per million or more; oranges, tomatoes, spinach, beans, and other plants contain measurable concentrations less than 1 part per million. The dopamine in plants is synthesized from the amino acid tyrosine, by biochemical mechanisms similar to those that animals use. It can be metabolized in a variety of ways, producing melanin and a variety of alkaloids as byproducts. The functions of plant catecholamines have not been clearly established, but there is evidence that they play a role in the response to stressors such as bacterial infection, act as growth-promoting factors in some situations, and modify the way that sugars are metabolized. The receptors that mediate these actions have not yet been identified, nor have the intracellular mechanisms that they activate. | Dopamine | Wikipedia | 496 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Dopamine consumed in food cannot act on the brain, because it cannot cross the blood–brain barrier. However, there are also a variety of plants that contain L-DOPA, the metabolic precursor of dopamine. The highest concentrations are found in the leaves and bean pods of plants of the genus Mucuna, especially in Mucuna pruriens (velvet beans), which have been used as a source for L-DOPA as a drug. Another plant containing substantial amounts of L-DOPA is Vicia faba, the plant that produces fava beans (also known as "broad beans"). The level of L-DOPA in the beans, however, is much lower than in the pod shells and other parts of the plant. The seeds of Cassia and Bauhinia trees also contain substantial amounts of L-DOPA.
In a species of marine green algae Ulvaria obscura, a major component of some algal blooms, dopamine is present in very high concentrations, estimated at 4.4% of dry weight. There is evidence that this dopamine functions as an anti-herbivore defense, reducing consumption by snails and isopods.
As a precursor for melanin
Melanins are a family of dark-pigmented substances found in a wide range of organisms. Chemically they are closely related to dopamine, and there is a type of melanin, known as dopamine-melanin, that can be synthesized by oxidation of dopamine via the enzyme tyrosinase. The melanin that darkens human skin is not of this type: it is synthesized by a pathway that uses L-DOPA as a precursor but not dopamine. However, there is substantial evidence that the neuromelanin that gives a dark color to the brain's substantia nigra is at least in part dopamine-melanin.
Dopamine-derived melanin probably appears in at least some other biological systems as well. Some of the dopamine in plants is likely to be used as a precursor for dopamine-melanin. The complex patterns that appear on butterfly wings, as well as black-and-white stripes on the bodies of insect larvae, are also thought to be caused by spatially structured accumulations of dopamine-melanin.
History and development | Dopamine | Wikipedia | 492 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
Dopamine was first synthesized in 1910 by George Barger and James Ewens at Wellcome Laboratories in London, England and first identified in the human brain by Katharine Montagu in 1957. It was named dopamine because it is a monoamine whose precursor in the Barger-Ewens synthesis is 3,4-dihydroxyphenylalanine (levodopa or L-DOPA). Dopamine's function as a neurotransmitter was first recognized in 1958 by Arvid Carlsson and Nils-Åke Hillarp at the Laboratory for Chemical Pharmacology of the National Heart Institute of Sweden. Carlsson was awarded the 2000 Nobel Prize in Physiology or Medicine for showing that dopamine is not only a precursor of norepinephrine (noradrenaline) and epinephrine (adrenaline), but is also itself a neurotransmitter.
Polydopamine
Research motivated by adhesive polyphenolic proteins in mussels led to the discovery in 2007 that a wide variety of materials, if placed in a solution of dopamine at slightly basic pH, will become coated with a layer of polymerized dopamine, often referred to as polydopamine. This polymerized dopamine forms by a spontaneous oxidation reaction, and is formally a type of melanin. Furthermore, dopamine self-polymerization can be used to modulate the mechanical properties of peptide-based gels. Synthesis of polydopamine usually involves reaction of dopamine hydrochloride with Tris as a base in water. The structure of polydopamine is unknown.
Polydopamine coatings can form on objects ranging in size from nanoparticles to large surfaces. Polydopamine layers have chemical properties that have the potential to be extremely useful, and numerous studies have examined their possible applications. At the simplest level, they can be used for protection against damage by light, or to form capsules for drug delivery. At a more sophisticated level, their adhesive properties may make them useful as substrates for biosensors or other biologically active macromolecules. | Dopamine | Wikipedia | 448 | 48548 | https://en.wikipedia.org/wiki/Dopamine | Biology and health sciences | Biochemistry and molecular biology | null |
In graph theory, a tree is an undirected graph in which any two vertices are connected by path, or equivalently a connected acyclic undirected graph. A forest is an undirected graph in which any two vertices are connected by path, or equivalently an acyclic undirected graph, or equivalently a disjoint union of trees.
A directed tree, oriented tree, polytree, or singly connected network is a directed acyclic graph (DAG) whose underlying undirected graph is a tree. A polyforest (or directed forest or oriented forest) is a directed acyclic graph whose underlying undirected graph is a forest.
The various kinds of data structures referred to as trees in computer science have underlying graphs that are trees in graph theory, although such data structures are generally rooted trees. A rooted tree may be directed, called a directed rooted tree, either making all its edges point away from the root—in which case it is called an arborescence or out-tree—or making all its edges point towards the root—in which case it is called an anti-arborescence or in-tree. A rooted tree itself has been defined by some authors as a directed graph. A rooted forest is a disjoint union of rooted trees. A rooted forest may be directed, called a directed rooted forest, either making all its edges point away from the root in each rooted tree—in which case it is called a branching or out-forest—or making all its edges point towards the root in each rooted tree—in which case it is called an anti-branching or in-forest.
The term was coined in 1857 by the British mathematician Arthur Cayley.
Definitions | Tree (graph theory) | Wikipedia | 357 | 48560 | https://en.wikipedia.org/wiki/Tree%20%28graph%20theory%29 | Mathematics | Graph theory | null |
Tree
A tree is an undirected graph that satisfies any of the following equivalent conditions:
is connected and acyclic (contains no cycles).
is acyclic, and a simple cycle is formed if any edge is added to .
is connected, but would become disconnected if any single edge is removed from .
is connected and the complete graph is not a minor of .
Any two vertices in can be connected by a unique simple path.
If has finitely many vertices, say of them, then the above statements are also equivalent to any of the following conditions:
is connected and has edges.
is connected, and every subgraph of includes at least one vertex with zero or one incident edges. (That is, is connected and 1-degenerate.)
has no simple cycles and has edges.
As elsewhere in graph theory, the order-zero graph (graph with no vertices) is generally not considered to be a tree: while it is vacuously connected as a graph (any two vertices can be connected by a path), it is not 0-connected (or even (−1)-connected) in algebraic topology, unlike non-empty trees, and violates the "one more vertex than edges" relation. It may, however, be considered as a forest consisting of zero trees.
An (or inner vertex) is a vertex of degree at least 2. Similarly, an (or outer vertex, terminal vertex or leaf) is a vertex of degree 1. A branch vertex in a tree is a vertex of degree at least 3.
An (or series-reduced tree) is a tree in which there is no vertex of degree 2 (enumerated at sequence in the OEIS).
Forest
A is an undirected acyclic graph or equivalently a disjoint union of trees. Trivially so, each connected component of a forest is a tree. As special cases, the order-zero graph (a forest consisting of zero trees), a single tree, and an edgeless graph, are examples of forests.
Since for every tree , we can easily count the number of trees that are within a forest by subtracting the difference between total vertices and total edges. number of trees in a forest.
Polytree | Tree (graph theory) | Wikipedia | 460 | 48560 | https://en.wikipedia.org/wiki/Tree%20%28graph%20theory%29 | Mathematics | Graph theory | null |
A (or directed tree or oriented tree or singly connected network) is a directed acyclic graph (DAG) whose underlying undirected graph is a tree. In other words, if we replace its directed edges with undirected edges, we obtain an undirected graph that is both connected and acyclic.
Some authors restrict the phrase "directed tree" to the case where the edges are all directed towards a particular vertex, or all directed away from a particular vertex (see arborescence).
Polyforest
A (or directed forest or oriented forest) is a directed acyclic graph whose underlying undirected graph is a forest. In other words, if we replace its directed edges with undirected edges, we obtain an undirected graph that is acyclic.
As with directed trees, some authors restrict the phrase "directed forest" to the case where the edges of each connected component are all directed towards a particular vertex, or all directed away from a particular vertex (see branching).
Rooted tree
A is a tree in which one vertex has been designated the root. The edges of a rooted tree can be assigned a natural orientation, either away from or towards the root, in which case the structure becomes a directed rooted tree. When a directed rooted tree has an orientation away from the root, it is called an arborescence or out-tree; when it has an orientation towards the root, it is called an anti-arborescence or in-tree. The tree-order is the partial ordering on the vertices of a tree with if and only if the unique path from the root to passes through . A rooted tree that is a subgraph of some graph is a normal tree if the ends of every -path in are comparable in this tree-order . Rooted trees, often with an additional structure such as an ordering of the neighbors at each vertex, are a key data structure in computer science; see tree data structure.
In a context where trees typically have a root, a tree without any designated root is called a free tree.
A labeled tree is a tree in which each vertex is given a unique label. The vertices of a labeled tree on vertices (for nonnegative integers ) are typically given the labels . A recursive tree is a labeled rooted tree where the vertex labels respect the tree order (i.e., if for two vertices and , then the label of is smaller than the label of ). | Tree (graph theory) | Wikipedia | 499 | 48560 | https://en.wikipedia.org/wiki/Tree%20%28graph%20theory%29 | Mathematics | Graph theory | null |
In a rooted tree, the parent of a vertex is the vertex connected to on the path to the root; every vertex has a unique parent, except the root has no parent. A child of a vertex is a vertex of which is the parent. An ascendant of a vertex is any vertex that is either the parent of or is (recursively) an ascendant of a parent of . A descendant of a vertex is any vertex that is either a child of or is (recursively) a descendant of a child of . A sibling to a vertex is any other vertex on the tree that shares a parent with . A leaf is a vertex with no children. An internal vertex is a vertex that is not a leaf.
The height of a vertex in a rooted tree is the length of the longest downward path to a leaf from that vertex. The height of the tree is the height of the root. The depth of a vertex is the length of the path to its root (root path). The depth of a tree is the maximum depth of any vertex. Depth is commonly needed in the manipulation of the various self-balancing trees, AVL trees in particular. The root has depth zero, leaves have height zero, and a tree with only a single vertex (hence both a root and leaf) has depth and height zero. Conventionally, an empty tree (a tree with no vertices, if such are allowed) has depth and height −1.
A -ary tree (for nonnegative integers ) is a rooted tree in which each vertex has at most children. 2-ary trees are often called binary trees, while 3-ary trees are sometimes called ternary trees.
Ordered tree
An ordered tree (alternatively, plane tree or positional tree) is a rooted tree in which an ordering is specified for the children of each vertex. This is called a "plane tree" because an ordering of the children is equivalent to an embedding of the tree in the plane, with the root at the top and the children of each vertex lower than that vertex. Given an embedding of a rooted tree in the plane, if one fixes a direction of children, say left to right, then an embedding gives an ordering of the children. Conversely, given an ordered tree, and conventionally drawing the root at the top, then the child vertices in an ordered tree can be drawn left-to-right, yielding an essentially unique planar embedding. | Tree (graph theory) | Wikipedia | 505 | 48560 | https://en.wikipedia.org/wiki/Tree%20%28graph%20theory%29 | Mathematics | Graph theory | null |
Properties
Every tree is a bipartite graph. A graph is bipartite if and only if it contains no cycles of odd length. Since a tree contains no cycles at all, it is bipartite.
Every tree with only countably many vertices is a planar graph.
Every connected graph G admits a spanning tree, which is a tree that contains every vertex of G and whose edges are edges of G. More specific types spanning trees, existing in every connected finite graph, include depth-first search trees and breadth-first search trees. Generalizing the existence of depth-first-search trees, every connected graph with only countably many vertices has a Trémaux tree. However, some uncountable-order graphs do not have such a tree.
Every finite tree with n vertices, with , has at least two terminal vertices (leaves). This minimal number of leaves is characteristic of path graphs; the maximal number, , is attained only by star graphs. The number of leaves is at least the maximum vertex degree.
For any three vertices in a tree, the three paths between them have exactly one vertex in common. More generally, a vertex in a graph that belongs to three shortest paths among three vertices is called a median of these vertices. Because every three vertices in a tree have a unique median, every tree is a median graph.
Every tree has a center consisting of one vertex or two adjacent vertices. The center is the middle vertex or middle two vertices in every longest path. Similarly, every n-vertex tree has a centroid consisting of one vertex or two adjacent vertices. In the first case removal of the vertex splits the tree into subtrees of fewer than n/2 vertices. In the second case, removal of the edge between the two centroidal vertices splits the tree into two subtrees of exactly n/2 vertices.
The maximal cliques of a tree are precisely its edges, implying that the class of trees has few cliques.
Enumeration
Labeled trees
Cayley's formula states that there are trees on labeled vertices. A classic proof uses Prüfer sequences, which naturally show a stronger result: the number of trees with vertices of degrees respectively, is the multinomial coefficient | Tree (graph theory) | Wikipedia | 451 | 48560 | https://en.wikipedia.org/wiki/Tree%20%28graph%20theory%29 | Mathematics | Graph theory | null |
A more general problem is to count spanning trees in an undirected graph, which is addressed by the matrix tree theorem. (Cayley's formula is the special case of spanning trees in a complete graph.) The similar problem of counting all the subtrees regardless of size is #P-complete in the general case ().
Unlabeled trees
Counting the number of unlabeled free trees is a harder problem. No closed formula for the number of trees with vertices up to graph isomorphism is known. The first few values of are
1, 1, 1, 1, 2, 3, 6, 11, 23, 47, 106, 235, 551, 1301, 3159, … .
proved the asymptotic estimate
with and . Here, the symbol means that
This is a consequence of his asymptotic estimate for the number of unlabeled rooted trees with vertices:
with and the same as above (cf. , chap. 2.3.4.4 and , chap. VII.5, p. 475).
The first few values of are
1, 1, 2, 4, 9, 20, 48, 115, 286, 719, 1842, 4766, 12486, 32973, … .
Types of trees
A path graph (or linear graph) consists of vertices arranged in a line, so that vertices and are connected by an edge for .
A starlike tree consists of a central vertex called root and several path graphs attached to it. More formally, a tree is starlike if it has exactly one vertex of degree greater than 2.
A star tree is a tree which consists of a single internal vertex (and leaves). In other words, a star tree of order is a tree of order with as many leaves as possible.
A caterpillar tree is a tree in which all vertices are within distance 1 of a central path subgraph.
A lobster tree is a tree in which all vertices are within distance 2 of a central path subgraph.
A regular tree of degree is the infinite tree with edges at each vertex. These arise as the Cayley graphs of free groups, and in the theory of Tits buildings. In statistical mechanics they are known as Bethe lattices. | Tree (graph theory) | Wikipedia | 463 | 48560 | https://en.wikipedia.org/wiki/Tree%20%28graph%20theory%29 | Mathematics | Graph theory | null |
Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airspace. The primary purpose of ATC is to prevent collisions, organize and expedite the flow of traffic in the air, and provide information and other support for pilots.
Personnel of air traffic control monitor aircraft location in their assigned airspace by radar, and communicate with the pilots by radio. To prevent collisions, ATC enforces traffic separation rules, which ensure each aircraft maintains a minimum amount of 'empty space' around it at all times. It is also common for ATC to provide services to all private, military, and commercial aircraft operating within its airspace; not just civilian aircraft. Depending on the type of flight and the class of airspace, ATC may issue instructions that pilots are required to obey, or advisories (known as flight information in some countries) that pilots may, at their discretion, disregard. The pilot in command of an aircraft always retains final authority for its safe operation, and may, in an emergency, deviate from ATC instructions to the extent required to maintain safe operation of the aircraft.
Language
Pursuant to requirements of the International Civil Aviation Organization (ICAO), ATC operations are conducted either in the English language, or the local language used by the station on the ground. In practice, the native language for a region is used; however, English must be used upon request.
History
In 1920, Croydon Airport near London, England, was the first airport in the world to introduce air traffic control. The 'aerodrome control tower' was a wooden hut high with windows on all four sides. It was commissioned on 25 February 1920, and provided basic traffic, weather, and location information to pilots. | Air traffic control | Wikipedia | 370 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
In the United States, air traffic control developed three divisions. The first of several air mail radio stations (AMRS) was created in 1922, after World War I, when the U.S. Post Office began using techniques developed by the U.S. Army to direct and track the movements of reconnaissance aircraft. Over time, the AMRS morphed into flight service stations. Today's flight service stations do not issue control instructions, but provide pilots with many other flight related informational services. They do relay control instructions from ATC in areas where flight service is the only facility with radio or phone coverage. The first airport traffic control tower, regulating arrivals, departures, and surface movement of aircraft in the US at a specific airport, opened in Cleveland in 1930. Approach / departure control facilities were created after adoption of radar in the 1950s to monitor and control the busy airspace around larger airports. The first air route traffic control center (ARTCC), which directs the movement of aircraft between departure and destination, was opened in Newark in 1935, followed in 1936 by Chicago and Cleveland. Currently in the US, the Federal Aviation Administration (FAA) operates 22 Air Route Traffic Control Centers.
After the 1956 Grand Canyon mid-air collision, killing all 128 on board, the FAA was given the air-traffic responsibility in the United States in 1958, and this was followed by other countries. In 1960, Britain, France, Germany, and the Benelux countries set up Eurocontrol, intending to merge their airspaces. The first and only attempt to pool controllers between countries is the Maastricht Upper Area Control Centre (MUAC), founded in 1972 by Eurocontrol, and covering Belgium, Luxembourg, the Netherlands, and north-western Germany. In 2001, the European Union (EU) aimed to create a 'Single European Sky', hoping to boost efficiency and gain economies of scale.
Airport traffic control tower | Air traffic control | Wikipedia | 390 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
The primary method of controlling the immediate airport environment is visual observation from the airport control tower. The tower is typically a tall, windowed structure, located within the airport grounds. The air traffic controllers, usually abbreviated 'controller', are responsible for separation and efficient movement of aircraft and vehicles operating on the taxiways and runways of the airport itself, and aircraft in the air near the airport, generally , depending on the airport procedures. A controller must carry out the job using the precise and effective application of rules and procedures; however, they need flexible adjustments according to differing circumstances, often under time pressure. In a study that compared stress in the general population and this kind of system markedly showed more stress level for controllers. This variation can be explained, at least in part, by the characteristics of the job.
Surveillance displays are also available to controllers at larger airports to assist with controlling air traffic. Controllers may use a radar system called secondary surveillance radar for airborne traffic approaching and departing. These displays include a map of the area, the position of various aircraft, and data tags that include aircraft identification, speed, altitude, and other information described in local procedures. In adverse weather conditions, the tower controllers may also use surface movement radar (SMR), surface movement guidance and control system (SMGCS), or advanced surface movement guidance and control system (ASMGCS) to control traffic on the manoeuvring area (taxiways and runways).
The areas of responsibility for tower controllers fall into three general operational disciplines: local control or air control, ground control, and flight data / clearance delivery. Other categories, such as airport apron control, or ground movement planner, may also exist at extremely busy airports. While each tower may have unique airport-specific procedures, such as multiple teams of controllers () at major or complex airports with multiple runways, the following provides a general concept of the delegation of responsibilities within the air traffic control tower environment.
Remote and virtual tower (RVT) is a system based on air traffic controllers being located somewhere other than at the local airport tower, and still able to provide air traffic control services. Displays for the air traffic controllers may be live video, synthetic images based on surveillance sensor data, or both.
Ground control | Air traffic control | Wikipedia | 452 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Ground control (sometimes known as , GMC) is responsible for the airport areas, as well as areas not released to the airlines or other users. This generally includes all taxiways, inactive runways, holding areas, and some transitional aprons or intersections where aircraft arrive, having vacated the runway or departure gate. Exact areas and control responsibilities are clearly defined in local documents and agreements at each airport. Any aircraft, vehicle, or person walking or working in these areas is required to have clearance from ground control. This is normally done via VHF / UHF radio, but there may be special cases where other procedures are used. Aircraft or vehicles without radios must respond to ATC instructions via aviation light signals, or else be led by official airport vehicles with radios. People working on the airport surface normally have a communications link through which they can communicate with ground control, commonly either by handheld radio or even cell phone. Ground control is vital to the smooth operation of the airport because this position impacts the sequencing of departure aircraft, affecting the safety and efficiency of the airport's operation.
Some busier airports have surface movement radar (SMR), such as ASDE-3, AMASS, or ASDE-X, designed to display aircraft and vehicles on the ground. These are used by ground control as an additional tool to control ground traffic, particularly at night or in poor visibility. There is a wide range of capabilities on these systems as they are being modernised. Older systems will display a map of the airport and the target. Newer systems include the capability to display higher-quality mapping, radar targets, data blocks, and safety alerts, and to interface with other systems, such as digital flight strips.
Air control or local control
Air control (known to pilots as or ) is responsible for the active runway surfaces. Air control gives clearance for aircraft takeoff or landing, whilst ensuring that prescribed runway separation will exist at all times. If the air controller detects any unsafe conditions, a landing aircraft may be instructed to 'go-around', and be re-sequenced into the landing pattern. This re-sequencing will depend on the type of flight, and may be handled by the air controller, approach, or terminal area controller. | Air traffic control | Wikipedia | 447 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Within the tower, a highly disciplined communications process between the air control and ground control is an absolute necessity. Air control must ensure that ground control is aware of any operations that will impact the taxiways, and work with the approach radar controllers to create in the arrival traffic; to allow taxiing traffic to cross runways, and to allow departing aircraft to take off. Ground control needs to keep the air controllers aware of the traffic flow towards their runways to maximise runway utilisation through effective approach spacing. Crew resource management (CRM) procedures are often used to ensure this communication process is efficient and clear. Within ATC, it is usually known as 'team resource management' (TRM), and the level of focus on TRM varies within different ATC organisations.
Flight data and clearance delivery
Clearance delivery is the position that issues route clearances to aircraft, typically before they commence taxiing. These clearances contain details of the route that the aircraft is expected to fly after departure. Clearance delivery, or, at busy airports, (GMP) or (TMC) will, if necessary, coordinate with the relevant radar centre or flow control unit to obtain releases for aircraft. At busy airports, these releases are often automatic, and are controlled by local agreements allowing 'free-flow' departures. When weather or extremely high demand for a certain airport or airspace becomes a factor, there may be ground 'stops' (or 'slot delays'), or re-routes may be necessary to ensure the system does not get overloaded. The primary responsibility of clearance delivery is to ensure that the aircraft has the correct aerodrome information, such as weather and airport conditions, the correct route after departure, and time restrictions relating to that flight. This information is also coordinated with the relevant radar centre or flow control unit and ground control, to ensure that the aircraft reaches the runway in time to meet the time restriction provided by the relevant unit. At some airports, clearance delivery also plans aircraft push-backs and engine starts, in which case it is known as the (GMP): this position is particularly important at heavily congested airports to prevent taxiway and aircraft parking area gridlock. | Air traffic control | Wikipedia | 439 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Flight data (which is routinely combined with clearance delivery) is the position that is responsible for ensuring that both controllers and pilots have the most current information: pertinent weather changes, outages, airport ground delays / ground stops, runway closures, etc. Flight data may inform the pilots using a recorded continuous loop on a specific frequency known as the (ATIS).
Approach and terminal control
Many airports have a radar control facility that is associated with that specific airport. In most countries, this is referred to as terminal control and abbreviated to TMC; in the U.S., it is referred to as a 'terminal radar approach control' or TRACON. While every airport varies, terminal controllers usually handle traffic in a radius from the airport. Where there are many busy airports close together, one consolidated terminal control centre may service all the airports. The airspace boundaries and altitudes assigned to a terminal control centre, which vary widely from airport to airport, are based on factors such as traffic flows, neighbouring airports, and terrain. A large and complex example was the London Terminal Control Centre (LTCC), which controlled traffic for five main London airports up to an altitude of and out to a distance of .
Terminal controllers are responsible for providing all ATC services within their airspace. Traffic flow is broadly divided into departures, arrivals, and overflights. As aircraft move in and out of the terminal airspace, they are 'handed off' to the next appropriate control facility (a control tower, an en-route control facility, or a bordering terminal or approach control). Terminal control is responsible for ensuring that aircraft are at an appropriate altitude when they are handed off, and that aircraft arrive at a suitable rate for landing.
Not all airports have a radar approach or terminal control available. In this case, the en-route centre or a neighbouring terminal or approach control may co-ordinate directly with the tower on the airport and vector inbound aircraft to a position from where they can land visually. At some of these airports, the tower may provide a non-radar procedural approach service to arriving aircraft handed over from a radar unit before they are visual to land. Some units also have a dedicated approach unit, which can provide the procedural approach service either all the time, or for any periods of radar outage for any reason.
In the U.S., TRACONs are additionally designated by a three-digit alphanumeric code. For example, the Chicago TRACON is designated C90. | Air traffic control | Wikipedia | 505 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Area control centre / en-route centre
Air traffic control also provides services to aircraft in flight between airports. Pilots fly under one of two sets of rules for separation: visual flight rules (VFR), or instrument flight rules (IFR). Air traffic controllers have different responsibilities to aircraft operating under the different sets of rules. While IFR flights are under positive control, in the US and Canada, VFR pilots can request 'flight following' (radar advisories), which provides traffic advisory services on a time permitting basis, and may also provide assistance in avoiding areas of weather and flight restrictions, as well as allowing pilots into the air traffic control system prior to the need to a clearance into certain airspace. Throughout Europe, pilots may request a 'Flight Information Service', which is similar to flight following. In the United Kingdom, it is known as a 'basic service'.
En-route air traffic controllers issue clearances and instructions for airborne aircraft, and pilots are required to comply with these instructions. En-route controllers also provide air traffic control services to many smaller airports around the country, including clearance off the ground and clearance for approach to an airport. Controllers adhere to a set of separation standards that define the minimum distance allowed between aircraft. These distances vary depending on the equipment and procedures used in providing ATC services.
General characteristics
En-route air traffic controllers work in facilities called air traffic control centres, each of which is commonly referred to as a 'centre'. The United States uses the equivalent term air route traffic control center. Each centre is responsible for a given flight information region (FIR). Each flight information region typically covers many thousands of square miles of airspace, and the airports within that airspace. Centres control IFR aircraft from the time they depart from an airport or terminal area's airspace, to the time they arrive at another airport or terminal area's airspace. Centres may also 'pick up' VFR aircraft that are already airborne, and integrate them into their system. These aircraft must continue under VFR flight rules until the centre provides a clearance. | Air traffic control | Wikipedia | 418 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Centre controllers are responsible for issuing instructions to pilots to climb their aircraft to their assigned altitude, while, at the same time, ensuring that the aircraft is properly separated from all other aircraft in its immediate area. Additionally, the aircraft must be placed in a flow consistent with the aircraft's route of flight. This effort is complicated by crossing traffic, severe weather, special missions that require large airspace allocations, and traffic density. When the aircraft approaches its destination, the centre is responsible for issuing instructions to pilots so that they will meet altitude restrictions by specific points, as well as providing many destination airports with a traffic flow, which prohibits all of the arrivals being 'bunched together'. These 'flow restrictions' often begin in the middle of the route, as controllers will position aircraft landing in the same destination so that when the aircraft are close to their destination they are sequenced.
As an aircraft reaches the boundary of a centre's control area, it is 'handed off' or 'handed over' to the next area control centre. In some cases, this 'hand-off' process involves a transfer of identification and details between controllers so that air traffic control services can be provided in a seamless manner; in other cases, local agreements may allow 'silent handovers', such that the receiving centre does not require any co-ordination if traffic is presented in an agreed manner. After the hand-off, the aircraft is given a frequency change, and its pilot begins talking to the next controller. This process continues until the aircraft is handed off to a terminal controller ('approach').
Radar coverage
Since centres control a large airspace area, they will typically use long-range radar, that has the capability, at higher altitudes, to see aircraft within of the radar antenna. They may also use radar data to control when it provides a better 'picture' of the traffic, or when it can fill in a portion of the area not covered by the long range radar. | Air traffic control | Wikipedia | 397 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
In the U.S. system, at higher altitudes, over 90% of the U.S. airspace is covered by radar, and often by multiple radar systems; however, coverage may be inconsistent at lower altitudes used by aircraft, due to high terrain or distance from radar facilities. A centre may require numerous radar systems to cover the airspace assigned to them, and may also rely on pilot position reports from aircraft flying below the floor of radar coverage. This results in a large amount of data being available to the controller. To address this, automation systems have been designed that consolidate the radar data for the controller. This consolidation includes eliminating duplicate radar returns, ensuring the best radar for each geographical area is providing the data, and displaying the data in an effective format.
Centres also exercise control over traffic travelling over the world's ocean areas. These areas are also flight information regions (FIRs). Because there are no radar systems available for oceanic control, oceanic controllers provide ATC services using procedural control. These procedures use aircraft position reports, time, altitude, distance, and speed, to ensure separation. Controllers record information on flight progress strips, and in specially developed oceanic computer systems, as aircraft report positions. This process requires that aircraft be separated by greater distances, which reduces the overall capacity for any given route. The North Atlantic Track system is a notable example of this method. | Air traffic control | Wikipedia | 275 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Some air navigation service providers (e.g., Airservices Australia, the U.S. Federal Aviation Administration, Nav Canada, etc.) have implemented automatic dependent surveillance – broadcast (ADS-B) as part of their surveillance capability. This newer technology reverses the radar concept. Instead of radar 'finding' a target by interrogating the transponder, the ADS-B equipped aircraft 'broadcasts' a position report as determined by the navigation equipment on board the aircraft. ADS-C is another mode of automatic dependent surveillance, however ADS-C operates in the 'contract' mode, where the aircraft reports a position, automatically or initiated by the pilot, based on a predetermined time interval. It is also possible for controllers to request more frequent reports to more quickly establish aircraft position for specific reasons. However, since the cost for each report is charged by the ADS service providers to the company operating the aircraft, more frequent reports are not commonly requested, except in emergency situations. ADS-C is significant, because it can be used where it is not possible to locate the infrastructure for a radar system (e.g., over water). Computerised radar displays are now being designed to accept ADS-C inputs as part of their display. This technology is currently used in portions of the North Atlantic and the Pacific by a variety of states who share responsibility for the control of this airspace.
'Precision approach radars' (PAR) are commonly used by military controllers of air forces of several countries, to assist the pilot in final phases of landing in places where instrument landing system and other sophisticated airborne equipment are unavailable to assist the pilots in marginal or near zero visibility conditions. This procedure is also called a 'talk-down'.
A radar archive system (RAS) keeps an electronic record of all radar information, preserving it for a few weeks. This information can be useful for search and rescue. When an aircraft has 'disappeared' from radar screens, a controller can review the last radar returns from the aircraft to determine its likely position. For an example, see the crash report in the following citation. RAS is also useful to technicians who are maintaining radar systems. | Air traffic control | Wikipedia | 441 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Flight traffic mapping
The mapping of flights in real-time is based on the air traffic control system, and volunteer ADS-B receivers. In 1991, data on the location of aircraft was made available by the Federal Aviation Administration to the airline industry. The National Business Aviation Association (NBAA), the General Aviation Manufacturers Association, the Aircraft Owners and Pilots Association, the Helicopter Association International, and the National Air Transportation Association, petitioned the FAA to make ASDI information available on a 'need-to-know' basis. Subsequently, NBAA advocated the broad-scale dissemination of air traffic data. The Aircraft Situational Display to Industry (ASDI) system now conveys up-to-date flight information to the airline industry and the public. Some companies that distribute ASDI information are Flightradar24, FlightExplorer, FlightView, and FlyteComm. Each company maintains a website that provides free updated information to the public on flight status. Stand-alone programmes are also available for displaying the geographic location of airborne instrument flight rules (IFR) air traffic anywhere in the FAA air traffic system. Positions are reported for both commercial and general aviation traffic. The programmes can overlay air traffic with a wide selection of maps such as, geo-political boundaries, air traffic control centre boundaries, high altitude jet routes, satellite cloud and radar imagery.
Problems
Traffic | Air traffic control | Wikipedia | 276 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
The day-to-day problems faced by the air traffic control system are primarily related to the volume of air traffic demand placed on the system, and weather. Several factors dictate the amount of traffic that can land at an airport in a given amount of time. Each landing aircraft must touch down, slow, and exit the runway, before the next aircraft crosses the approach end of the runway. This process requires at least one, and up to four minutes for each aircraft. Allowing for departures between arrivals, each runway can thus handle about 30 aircraft arrivals per hour. A large airport with two arrival runways can handle about 60 arrivals per hour in good weather. Problems arise when airlines schedule more arrivals into an airport than can be physically handled, or when delays elsewhere cause groups of aircraft – that would otherwise be separated in time – to arrive simultaneously. Aircraft must then be delayed in the air by holding over specified locations until they may be safely sequenced to the runway. Up until the 1990s, holding, which has significant environmental and cost implications, was a routine occurrence at many airports. Advances in computers now allow the sequencing of aircraft hours in advance. Thus, aircraft may be delayed before they even take off (by being given a 'slot'), or may reduce speed in flight and proceed more slowly thus significantly reducing the amount of holding.
Air traffic control errors occur when the separation (either vertical or horizontal) between airborne aircraft falls below the minimum prescribed separation set (for the domestic United States) by the US Federal Aviation Administration. Separation minimums for terminal control areas (TCAs) around airports are lower than en-route standards. Errors generally occur during periods following times of intense activity, when controllers tend to relax and overlook the presence of traffic and conditions that lead to loss of minimum separation.
Weather
Beyond runway capacity issues, the weather is a major factor in traffic capacity. Rain, ice, snow, or hail on the runway cause landing aircraft to take longer to slow and exit, thus reducing the safe arrival rate, and requiring more space between landing aircraft. Fog also requires a decrease in the landing rate. These, in turn, increase airborne delay for holding aircraft. If more aircraft are scheduled than can be safely and efficiently held in the air, a ground delay programme may be established, delaying aircraft on the ground before departure due to conditions at the arrival airport. | Air traffic control | Wikipedia | 474 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
In Area Control Centres, a major weather problem is thunderstorms, which present a variety of hazards to aircraft. Airborne aircraft will deviate around storms, reducing the capacity of the en-route system, by requiring more space per aircraft, or causing congestion, as many aircraft try to move through a single hole in a line of thunderstorms. Occasionally, weather considerations cause delays to aircraft prior to their departure as routes are closed by thunderstorms.
Much money has been spent on creating software to streamline this process. However, at some ACCs, air traffic controllers still record data for each flight on strips of paper and personally coordinate their paths. In newer sites, these flight progress strips have been replaced by electronic data presented on computer screens. As new equipment is brought in, more and more sites are upgrading away from paper flight strips.
Congestion
Constrained control capacity and growing traffic lead to flight cancellation and delays:
In America, delays caused by ATC grew by 69% between 2012 and 2017. ATC staffing issues were a major factor in congestion.
In China, the average delay per domestic flight spiked by 50% in 2017 to 15 minutes per flight.
In Europe, en route delays grew by 105% in 2018, due to a lack of capacity or staff (60%), weather (25%) or strikes (14%), costing the European economy €17.6bn ($20.8bn), up by 28% on 2017.
By then the market for air-traffic services was worth $14bn. More efficient ATC could save 5-10% of aviation fuel by avoiding holding patterns and indirect airways.
The military takes 80% of Chinese airspace, congesting the thin corridors open to airliners. The United Kingdom closes its military airspace only during military exercises. | Air traffic control | Wikipedia | 363 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Call signs
A prerequisite to safe air traffic separation is the assignment and use of distinctive call signs. These are permanently allocated by ICAO on request, usually to scheduled flights, and some air forces and other military services for military flights. There are written call signs with a two or three letter combination followed by the flight number such as AAL872 or VLG1011. As such, they appear on flight plans and ATC radar labels. There are also the audio or radio-telephony call signs used on the radio contact between pilots and air traffic control. These are not always identical to their written counterparts. An example of an audio call sign would be 'Speedbird 832', instead of the written 'BAW832'. This is used to reduce the chance of confusion between ATC and the aircraft. By default, the call sign for any other flight is the registration number (or tail number in US parlance) of the aircraft, such as 'N12345', 'C-GABC', or 'EC-IZD'. The short radio-telephony call signs for these tail numbers is the last three letters using the NATO phonetic alphabet (e.g. ABC, spoken alpha-bravo-charlie for C-GABC), or the last three numbers (e.g. three-four-five for N12345). In the United States, the prefix may be an aircraft type, model, or manufacturer in place of the first registration character, for example, 'N11842' could become 'Cessna 842'. This abbreviation is only allowed after communications have been established in each sector. | Air traffic control | Wikipedia | 342 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Before around 1980, International Air Transport Association (IATA) and ICAO were using the same two-letter call signs. Due to the larger number of new airlines after deregulation, the ICAO established the three-letter call signs as mentioned above. The IATA call signs are currently used in aerodromes on the announcement tables, but are no longer used in air traffic control. For example, AA is the IATA call sign for American Airlines; the ATC equivalent is AAL. Flight numbers in regular commercial flights are designated by the aircraft operator, and identical call sign might be used for the same scheduled journey each day it is operated, even if the departure time varies a little across different days of the week. The call sign of the return flight often differs only by the final digit from the outbound flight. Generally, airline flight numbers are even if east-bound, and odd if west-bound. In order to reduce the possibility of two call signs on one frequency at any time sounding too similar, a number of airlines, particularly in Europe, have started using alphanumeric call signs that are not based on flight numbers (e.g. DLH23LG, spoken as Lufthansa-two-three-lima-golf, to prevent confusion between incoming DLH23 and outgoing DLH24 in the same frequency). Additionally, it is the right of the air traffic controller to change the 'audio' call sign for the period the flight is in his sector if there is a risk of confusion, usually choosing the aircraft registration identifier instead.
Technology
Many technologies are used in air traffic control systems. Primary and secondary radars are used to enhance a controller's situational awareness within their assigned airspace; all types of aircraft send back primary echoes of varying sizes to controllers' screens as radar energy is bounced off their skins, and transponder-equipped aircraft reply to secondary radar interrogations by giving an ID (Mode A), an altitude (Mode C), and / or a unique callsign (Mode S). Certain types of weather may also register on the radar screen. These inputs, added to data from other radars, are correlated to build the air situation. Some basic processing occurs on the radar tracks, such as calculating ground speed and magnetic headings. | Air traffic control | Wikipedia | 462 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Usually, a flight data processing system manages all the flight plan related data, incorporating, in a low or high degree, the information of the track once the correlation between them (flight plan and track) is established. All this information is distributed to modern operational display systems, making it available to controllers.
The Federal Aviation Administration (FAA) has spent over US$3 billion on software, but a fully automated system is still yet to be achieved. In 2002, the United Kingdom commissioned a new area control centre into service at the London Area Control Centre (LACC) at Swanwick in Hampshire, relieving a busy suburban centre at West Drayton in Middlesex, north of London Heathrow Airport. Software from Lockheed-Martin predominates at the London Area Control Centre. However, the centre was initially troubled by software and communications problems causing delays and occasional shutdowns. | Air traffic control | Wikipedia | 175 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Some tools are available in different domains to help the controller further:
Flight data processing systems: this is the system (usually one per centre) that processes all the information related to the flight (the flight plan), typically in the time horizon from gate to gate (airport departure / arrival gates). It uses such processed information to invoke other flight plan related tools (such as e.g. Medium Term Conflict Detection (MTCD), and distributes such processed information to all the stakeholders (air traffic controllers, collateral centres, airports, etc).
Short-term conflict alert (STCA) that checks possible conflicting trajectories in a time horizon of about two or three minutes (or even less in approach context; 35 seconds in the French Roissy & Orly approach centres) and alerts the controller prior to the loss of separation. The algorithms used may also provide in some systems a possible vectoring solution, that is, the manner in which to turn, descend, increase / decrease speed, or climb the aircraft in order to avoid infringing the minimum safety distance or altitude clearance.
Minimum safe altitude warning (MSAW): a tool that alerts the controller if an aircraft appears to be flying too low to the ground or will impact terrain based on its current altitude and heading.
System coordination (SYSCO) to enable controller to negotiate the release of flights from one sector to another.
Area penetration warning (APW) to inform a controller that a flight will penetrate a restricted area.
Arrival and departure manager to help sequence the takeoff and landing of aircraft.
The departure manager (DMAN): a system aid for the ATC at airports, that calculates a planned departure flow with the goal to maintain an optimal throughput at the runway, reduce queuing at holding point, and distribute the information to various stakeholders at the airport (i.e. the airline, ground handling and air traffic control (ATC)).
The arrival manager (AMAN): a system aid for the ATC at airports, that calculates a planned arrival flow with the goal to maintain an optimal throughput at the runway, reduce arrival queuing and distribute the information to various stakeholders. | Air traffic control | Wikipedia | 445 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Passive final approach spacing tool (pFAST): a CTAS tool, provides runway assignment and sequence number advisories to terminal controllers to improve the arrival rate at congested airports. pFAST was deployed and operational at five US TRACONs before being cancelled. NASA research included an active FAST capability that also provided vector and speed advisories to implement the runway and sequence advisories.
Converging runway display aid (CRDA): enables approach controllers to run two final approaches that intersect, and make sure that go arounds are minimised.
Center TRACON automation system (CTAS): a suite of human centred decision support tools developed by NASA Ames Research Center. Several of the CTAS tools have been field tested and transitioned to the FAA for operational evaluation and use. Some of the CTAS tools are: traffic management advisor (TMA), passive final approach spacing tool (pFAST), collaborative arrival planning (CAP), direct-to (D2), en route descent advisor (EDA), and multi-center TMA. The software is running on Linux.
Traffic management advisor (TMA): a CTAS tool, is an en-route decision support tool that automates time based metering solutions to provide an upper limit of aircraft to a TRACON from the centre over a set period of time. Schedules are determined that will not exceed the specified arrival rate, and controllers use the scheduled times to provide the appropriate delay to arrivals, while in the en-route domain. This results in an overall reduction in en-route delays, and also moves the delays to more efficient airspace (higher altitudes) than occur if holding near the TRACON boundary, which is required in order to prevent overloading the TRACON controllers. TMA is operational at most en-route air route traffic control centres (ARTCCs), and continues to be enhanced to address more complex traffic situations (e.g. adjacent centre metering (ACM) and en route departure capability (EDC))
MTCD and URET
In the US, user request evaluation tool (URET) takes paper strips out of the equation for en-route controllers at ARTCCs by providing a display that shows all aircraft that are either in, or currently routed into the sector. | Air traffic control | Wikipedia | 466 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
In Europe, several MTCD tools are available: (National Air Traffic Services), VAFORIT (Deutsche Flugsicherung), new FDPS (Maastricht Upper Area Control). The Single European Sky ATM Research (SESAR) programme should soon launch new MTCD concepts.
URET and MTCD provide conflict advisories up to 30 minutes in advance, and have a suite of assistance tools that assist in evaluating resolution options and pilot requests.
Mode S: provides a data downlink of flight parameters via secondary surveillance radars allowing radar processing systems and therefore controllers to see various data on a flight, including airframe unique id (24-bits encoded), indicated airspeed, and flight director selected level, amongst others.
Controller–pilot data link communications (CPDLC): allows digital messages to be sent between controllers and pilots, avoiding the need to use radiotelephony. It is especially useful in areas where difficult-to-use HF radiotelephony was previously used for communication with aircraft, e.g. oceans. This is currently in use in various parts of the world including the Atlantic and Pacific oceans.
ADS-B: automatic dependent surveillance broadcast; provides a data downlink of various flight parameters to air traffic control systems via the transponder (1090 MHz), and reception of those data by other aircraft in the vicinity. The most important is the aircraft's latitude, longitude and level: such data can be utilised to create a radar-like display of aircraft for controllers, and thus allows a form of pseudo-radar control to be done in areas where the installation of radar is either prohibitive on the grounds of low traffic levels, or technically not feasible (e.g. oceans). This is currently in use in Australia, Canada, and parts of the Pacific Ocean, and Alaska. | Air traffic control | Wikipedia | 376 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
The electronic flight strip system (e-strip): a system of electronic flight strips replacing the existing paper strips is being used by several service providers, such as Nav Canada, MASUAC, DFS, DECEA. E-strips allows controllers to manage electronic flight data online without paper strips, reducing the need for manual functions, creating new tools, and reducing the ATCO's workload. The firsts electronic flight strips systems were independently and simultaneously invented and implemented by Nav Canada and Saipher ATC in 1999. The Nav Canada system known as EXCDS and rebranded in 2011 to NAVCANstrips, and Saipher's first generation system known as SGTC, which is now being updated by its 2nd generation system, the TATIC TWR. DECEA in Brazil is the world's largest user of tower e-strips system, ranging from very small airports up to the busiest ones, taking the advantage of real time information and data collection from each of more than 150 sites for use in air traffic flow management (ATFM), billing, and statistics.
Screen content recording: hardware or software based recording function which is part of most modern automation system, and that captures the screen content shown to the ATCO. Such recordings are used for a later replay together with audio recording for investigations and post event analysis.
Communication navigation surveillance / air traffic management (CNS / ATM) systems are communications, navigation, and surveillance systems, employing digital technologies, including satellite systems, together with various levels of automation, applied in support of a seamless global air traffic management system.
Air navigation service providers (ANSPs) and air traffic service providers (ATSPs) | Air traffic control | Wikipedia | 342 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Azerbaijan – AzərAeroNaviqasiya
Albania – Albcontrol
Algeria – Etablissement National de la Navigation Aérienne (ENNA)
Argentina – Empresa Argentina de Navegación Aérea (EANA)
Armenia – Armenian Air Traffic Services (ARMATS)
Australia – Airservices Australia (government owned corporation) and Royal Australian Air Force
Austria – Austro Control
Bangladesh – Civil Aviation Authority, Bangladesh
Belarus – Republican Unitary Enterprise Белаэронавигация (Belarusian Air Navigation)
Belgium – Skeyes - Authority of Airways
Bosnia and Herzegovina – Agencija za pružanje usluga u zračnoj plovidbi (Bosnia and Herzegovina Air Navigation Services Agency)
Brazil – Departamento de Controle do Espaço Aéreo (ATC/ATM Authority) and ANAC – Agência Nacional de Aviação Civil (Civil Aviation Authority)
Bulgaria – Air Traffic Services Authority
Cambodia – Cambodia Air Traffic Services (CATS)
Canada – Nav Canada, formerly provided by Transport Canada and Canadian Forces
Cayman Islands – CIAA Cayman Islands Airports Authority
Central America – Corporación Centroamericana de Servicios de Navegación Aérea
Guatemala – Dirección General de Aeronáutica Civil (DGAC)
El Salvador
Honduras
Nicaragua – Empresa Administradora Aeropuertos Internacionales (EAAI)
Costa Rica – Dirección General de Aviación Civil
Belize
Chile – Dirección General de Aeronáutica Civil (DGAC)
Colombia – Aeronáutica Civil Colombiana (UAEAC)
Croatia – Hrvatska kontrola zračne plovidbe (Croatia Control Ltd.)
Cuba – Instituto de Aeronáutica Civil de Cuba (IACC)
Czech Republic – Řízení letového provozu ČR
Cyprus – Department of Civil Aviation
Denmark – Naviair (Danish ATC)
Dominican Republic – Instituto Dominicano de Aviación Civil (IDAC) 'Dominican Institute of Civil Aviation'
Eastern Caribbean – Eastern Caribbean Civil Aviation Authority (ECCAA)
Anguilla
Antigua and Barbuda
British Virgin Islands
Dominica
Grenada
Saint Kitts and Nevis
Saint Lucia
Saint Vincent and the Grenadines
Ecuador – Dirección General de Aviación Civil (DGAC) 'General Direction of Civil Aviation' government body
Estonia – Estonian Air Navigation Services
Europe – Eurocontrol (European organisation for the safety of air navigation) | Air traffic control | Wikipedia | 503 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Fiji – Fiji Airports (fully owned government commercial company)
Finland – Finavia
France – Direction Générale de l'Aviation Civile (DGAC): Direction des Services de la Navigation Aérienne (DSNA) (government body)
Georgia – SAKAERONAVIGATSIA, Ltd. (Georgian Air Navigation)
Germany – Deutsche Flugsicherung (German ATC – state-owned company)
Greece – Hellenic Civil Aviation Authority (HCAA)
Hong Kong – Civil Aviation Department (CAD)
Hungary – HungaroControl Magyar Légiforgalmi Szolgálat Zrt. (HungaroControl Hungarian Air Navigation Services Pte. Ltd. Co.)
Iceland – ISAVIA
India – Airports Authority of India (AAI) (under Ministry of Civil Aviation, Government of India and Indian Air Force)
Indonesia – AirNav Indonesia
Iran – Iran Civil Aviation Organization (ICAO)
Ireland – Irish Aviation Authority (IAA)
Iraq – Iraqi Air Navigation – ICAA
Israel – Israeli Airports Authority (IIA)
Italy – ENAV SpA and Italian Air Force
Jamaica – Jamaica Civil Aviation Authority (JCAA)
Japan – Japan Civil Aviation Bureau (JCAB)
Kenya – Kenya Civil Aviation Authority (KCAA)
Latvia – LGS (Latvian ATC)
Lithuania – ANS (Lithuanian ATC)
Luxembourg – Administration de la navigation aérienne (ANA – government administration)
Macedonia – DGCA (Macedonian ATC)
Malaysia – Civil Aviation Authority of Malaysia (CAAM)
Malta – Malta Air Traffic Services Ltd
Mexico – Servicios a la Navegación en el Espacio Aéreo Mexicano
Morocco – Office National Des Aeroports (ONDA)
Nepal – Civil Aviation Authority of Nepal
Netherlands – Luchtverkeersleiding Nederland (LVNL) (Dutch ATC) Eurocontrol (Maastricht Upper Area Control Centre)
New Zealand – Airways New Zealand (state owned enterprise)
Nigeria – Nigeria Civil Aviation Authority (NCAA)
Norway – Avinor (state-owned private company)
Oman – Directorate General of Meteorology & Air Navigation (Government of Oman)
Pakistan – Civil Aviation Authority (under Government of Pakistan)
Peru – Centro de Instrucción de Aviación Civil (CIAC)
Philippines – Civil Aviation Authority of the Philippines (CAAP) (under the Philippine Government)
Poland – Polish Air Navigation Services Agency (PANSA)
Portugal – NAV (Portuguese ATC)
Puerto Rico – Administracion Federal de Aviacion | Air traffic control | Wikipedia | 509 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Romania – Romanian Air Traffic Services Administration (ROMATSA)
Russia – Federal State Unitary Enterprise (State ATM Corporation)
Saudi Arabia – Saudi Air Navigation Services (SANS)
Seychelles – Seychelles Civil Aviation Authority (SCAA)
Singapore – Civil Aviation Authority of Singapore (CAAS)
Serbia – Serbia and Montenegro Air Traffic Services Agency Ltd. (SMATSA)
Slovakia – Letové prevádzkové služby Slovenskej republiky
Slovenia – Slovenia Control
South Africa – Air Traffic and Navigation Services (ATNS)
South Korea – Korea Office of Civil Aviation
Spain – AENA now AENA S.A. (Spanish Airports) and ENAIRE (ATC & ATSP)
Sri Lanka – Airport & Aviation Services (Sri Lanka) Limited (government owned company)
Sweden – LFV (government body)
Switzerland – Skyguide
Taiwan – ANWS (Civil Aeronautical Administration)
Thailand – AEROTHAI (Aeronautical Radio of Thailand)
Trinidad and Tobago – Trinidad and Tobago Civil Aviation Authority (TTCAA)
Turkey – General Directorate of State Airports Authority (DHMI)
United Arab Emirates – General Civil Aviation Authority (GCAA)
United Kingdom – National Air Traffic Services (NATS) (49% state-owned public-private partnership, civilian and military)
United States – Federal Aviation Administration (FAA) (government body)
Ukraine – Ukrainian State Air Traffic Service Enterprise (UkSATSE)
Venezuela – Instituto Nacional de Aeronautica Civil (INAC)
Vietnam – Vietnam Air Traffic Management Corporation (VATM)
Zambia – Zambia Civil Aviation Authority (ZCAA)
Zimbabwe – Zimbabwe Civil Aviation Authority | Air traffic control | Wikipedia | 321 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Proposed changes
In the United States, some alterations to traffic control procedures are being examined:
The Next Generation Air Transportation System examines how to overhaul the United States national airspace system.
Free flight is a developing air traffic control method that uses no centralised control (e.g. air traffic controllers). Instead, parts of airspace are reserved dynamically and automatically in a distributed way using computer communication to ensure the required separation between aircraft.
In Europe, the Single European Sky ATM Research (SESAR) programme plans to develop new methods, technologies, procedures, and systems to accommodate future (2020 and beyond) air traffic needs. In October 2018, European controller unions dismissed setting targets to improve ATC as "a waste of time and effort", as new technology could cut costs for users but threaten their jobs. In April 2019, the EU called for a 'Digital European Sky', focusing on cutting costs by including a common digitisation standard, and allowing controllers to move to where they are needed instead of merging national ATCs, as it would not solve all problems. Single air-traffic control services in continent-sized America and China does not alleviate congestion. Eurocontrol tries to reduce delays by diverting flights to less busy routes: flight paths across Europe were redesigned to accommodate the new airport in Istanbul, which opened in April, but the extra capacity will be absorbed by rising demand for air travel.
Well-paid jobs in western Europe could move east with cheaper labour. The average Spanish controller earn over €200,000 a year, over seven times the country average salary, more than pilots, and at least ten controllers were paid over €810,000 ($1.1m) a year in 2010. French controllers spent a cumulative nine months on strike between 2004 and 2016. | Air traffic control | Wikipedia | 358 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
Privatisation
Many countries have also privatised or corporatised their air navigation service providers. There are several models that can be used for ATC service providers. The first is to have the ATC services be part of a government agency as is currently the case in the United States. The problem with this model is that funding can be inconsistent, and can disrupt the development and operation of services. Sometimes funding can disappear when lawmakers cannot approve budgets in time. Both proponents and opponents of privatisation recognise that stable funding is one of the major factors for successful upgrades of ATC infrastructure. Some of the funding issues include sequestration and politicisation of projects. Proponents argue that moving ATC services to a private corporation could stabilise funding over the long term which will result in more predictable planning and rollout of new technology as well as training of personnel. As of November 2024, The United States had 265 contractor towers that are staffed by private companies but administered by FAA through its FAA Contract Tower Program, which was established in 1982. These contract control towers cover 51% of all the Federal air traffic control towers in the U.S..
Another model is to have ATC services provided by a government corporation. This model is used in Germany, where funding is obtained through user fees. Yet another model is to have a for-profit corporation operate ATC services. This is the model used in the United Kingdom, but there have been several issues with the system there, including a large-scale failure in December 2014 which caused delays and cancellations and has been attributed to cost-cutting measures put in place by this corporation. In fact, earlier that year, the corporation owned by the German government won the bid to provide ATC services for Gatwick Airport in the United Kingdom. The last model, which is often the suggested model for the United States to transition to is to have a non-profit organisation that would handle ATC services as is used in Canada. | Air traffic control | Wikipedia | 401 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
The Canadian system is the one most often used as a model by proponents of privatisation. Air traffic control privatisation has been successful in Canada with the creation of Nav Canada, a private non-profit organisation which has reduced costs, and has allowed new technologies to be deployed faster due to the elimination of much of the bureaucratic red tape. This has resulted in shorter flights and less fuel usage. It has also resulted in flights being safer due to new technology. Nav Canada is funded from fees that are collected from the airlines based on the weight of the aircraft and the distance flown.
Air traffic control is operated by national governments with few exceptions: in the European Union, only Italy has private shareholders. Privatisation does not guarantee lower prices: the profit margin of MUAC was 70% in 2017, as there is no competition, but governments could offer fixed terms concessions. Australia, Fiji, and New Zealand run the upper-airspace for the Pacific islands' governments. HungaroControl offers remote airport tower services from Budapest, and since 2014 provides upper airspace management for Kosovo.
ATC regulations in the United States
The United States airspace is divided into 21 zones (centres), and each zone is divided into sectors. Also within each zone are portions of airspace, about in diameter, called TRACON (Terminal Radar Approach Control) airspaces. Within each TRACON airspace are a number of airports, each of which has its own airspace with a radius. FAA control tower operators (CTO) / air traffic controllers use FAA Order 7110.65 as the authority for all procedures regarding air traffic. | Air traffic control | Wikipedia | 326 | 48563 | https://en.wikipedia.org/wiki/Air%20traffic%20control | Technology | Aviation | null |
In number theory, a Gaussian integer is a complex number whose real and imaginary parts are both integers. The Gaussian integers, with ordinary addition and multiplication of complex numbers, form an integral domain, usually written as or
Gaussian integers share many properties with integers: they form a Euclidean domain, and thus have a Euclidean division and a Euclidean algorithm; this implies unique factorization and many related properties. However, Gaussian integers do not have a total ordering that respects arithmetic.
Gaussian integers are algebraic integers and form the simplest ring of quadratic integers.
Gaussian integers are named after the German mathematician Carl Friedrich Gauss.
Basic definitions
The Gaussian integers are the set
In other words, a Gaussian integer is a complex number such that its real and imaginary parts are both integers.
Since the Gaussian integers are closed under addition and multiplication, they form a commutative ring, which is a subring of the field of complex numbers. It is thus an integral domain.
When considered within the complex plane, the Gaussian integers constitute the -dimensional integer lattice.
The conjugate of a Gaussian integer is the Gaussian integer .
The norm of a Gaussian integer is its product with its conjugate.
The norm of a Gaussian integer is thus the square of its absolute value as a complex number. The norm of a Gaussian integer is a nonnegative integer, which is a sum of two squares. Thus a norm cannot be of the form , with integer.
The norm is multiplicative, that is, one has
for every pair of Gaussian integers . This can be shown directly, or by using the multiplicative property of the modulus of complex numbers.
The units of the ring of Gaussian integers (that is the Gaussian integers whose multiplicative inverse is also a Gaussian integer) are precisely the Gaussian integers with norm 1, that is, 1, –1, and .
Euclidean division | Gaussian integer | Wikipedia | 411 | 48628 | https://en.wikipedia.org/wiki/Gaussian%20integer | Mathematics | Basics | null |
Gaussian integers have a Euclidean division (division with remainder) similar to that of integers and polynomials. This makes the Gaussian integers a Euclidean domain, and implies that Gaussian integers share with integers and polynomials many important properties such as the existence of a Euclidean algorithm for computing greatest common divisors, Bézout's identity, the principal ideal property, Euclid's lemma, the unique factorization theorem, and the Chinese remainder theorem, all of which can be proved using only Euclidean division.
A Euclidean division algorithm takes, in the ring of Gaussian integers, a dividend and divisor , and produces a quotient and remainder such that
In fact, one may make the remainder smaller:
Even with this better inequality, the quotient and the remainder are not necessarily unique, but one may refine the choice to ensure uniqueness.
To prove this, one may consider the complex number quotient . There are unique integers and such that and , and thus . Taking , one has
with
and
The choice of and in a semi-open interval is required for uniqueness.
This definition of Euclidean division may be interpreted geometrically in the complex plane (see the figure), by remarking that the distance from a complex number to the closest Gaussian integer is at most .
Principal ideals
Since the ring of Gaussian integers is a Euclidean domain, is a principal ideal domain, which means that every ideal of is principal. Explicitly, an ideal is a subset of a ring such that every sum of elements of and every product of an element of by an element of belong to . An ideal is principal if it consists of all multiples of a single element , that is, it has the form
In this case, one says that the ideal is generated by or that is a generator of the ideal.
Every ideal in the ring of the Gaussian integers is principal, because, if one chooses in a nonzero element of minimal norm, for every element of , the remainder of Euclidean division of by belongs also to and has a norm that is smaller than that of ; because of the choice of , this norm is zero, and thus the remainder is also zero. That is, one has , where is the quotient. | Gaussian integer | Wikipedia | 460 | 48628 | https://en.wikipedia.org/wiki/Gaussian%20integer | Mathematics | Basics | null |
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