Gravity assist

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Gravity_assist

 

Animation of Voyager 1's trajectory from 5 September 1977 to 30 December 1981
  Voyager 1 ·   Earth ·   Jupiter ·   Saturn ·   Sun

Animation of Voyager 2's trajectory from 20 August 1977 to 30 December 2000
  Voyager 2 ·   Earth ·   Jupiter ·   Saturn ·   Uranus ·   Neptune ·   Sun

A gravity assist, gravity assist maneuver, swing-by, or generally a gravitational slingshot in orbital mechanics, is a type of spaceflight flyby which makes use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the path and speed of a spacecraft, typically to save propellant and reduce expense.

Gravity assistance can be used to accelerate a spacecraft, that is, to increase or decrease its speed or redirect its path. The "assist" is provided by the motion of the gravitating body as it pulls on the spacecraft. Any gain or loss of kinetic energy and linear momentum by a passing spacecraft is correspondingly lost or gained by the gravitational body, in accordance with Newton's Third Law. The gravity assist maneuver was first used in 1959 when the Soviet probe Luna 3 photographed the far side of Earth's Moon and it was used by interplanetary probes from Mariner 10 onward, including the two Voyager probes' notable flybys of Jupiter and Saturn.

Explanation

Example encounter.
In the planet's frame of reference, the space probe leaves with the exact same speed at which it had arrived. But when observed in the reference frame of the Solar System (fixed to the Sun), the benefit of this maneuver becomes apparent. Here it can be seen how the probe gains speed by tapping energy from the speed of the planet as it orbits the Sun. (If the trajectory is designed to pass in front of the planet instead of behind it, the gravity assist can be used as a braking maneuver rather than accelerating.) Because the mass of the probe is many orders of magnitude smaller than that of the planet, while the result on the probe is quite significant, the deceleration reaction experienced by the planet, according to Newton's third law, is utterly imperceptible.

Possible outcomes of a gravity assist maneuver depending on the velocity vector and flyby position of the incoming spacecraft

A gravity assist around a planet changes a spacecraft's velocity (relative to the Sun) by entering and leaving the gravitational sphere of influence of a planet. The spacecraft's speed increases as it approaches the planet and decreases as it leaves the planet. To increase speed, the spacecraft approaches the planet in the same direction the planet is orbiting the Sun, and departs in the opposite direction. To decrease speed, the spacecraft approaches the planet traveling the opposite direction from planet's orbital velocity. In both types of maneuver the energy transfer compared to the planet's total orbital energy is negligible. The sum of the kinetic energies of both bodies remains constant (see elastic collision). A slingshot maneuver can therefore be used to change the spaceship's trajectory and speed relative to the Sun.

A close terrestrial analogy is provided by a tennis ball bouncing off the front of a moving train. Imagine standing on a train platform, and throwing a ball at 30 km/h toward a train approaching at 50 km/h. The driver of the train sees the ball approaching at 80 km/h and then departing at 80 km/h after the ball bounces elastically off the front of the train. Because of the train's motion, however, that departure is at 130 km/h relative to the train platform; the ball has added twice the train's velocity to its own.

Translating this analogy into space: in the planet reference frame, the spaceship has a vertical velocity of v relative to the planet. After the slingshot occurs the spaceship is leaving on a course 90 degrees to that which it arrived on. It will still have a velocity of v, but in the horizontal direction. In the Sun reference frame, the planet has a horizontal velocity of v, and by using the Pythagorean Theorem, the spaceship initially has a total velocity of √2v. After the spaceship leaves the planet, it will have a velocity of v + v = 2v, gaining approximately 0.6v.

This oversimplified example is impossible to refine without additional details regarding the orbit, but if the spaceship travels in a path which forms a hyperbola, it can leave the planet in the opposite direction without firing its engine. This example is one of many trajectories and gains of speed the spaceship can experience.

This explanation might seem to violate the conservation of energy and momentum, apparently adding velocity to the spacecraft out of nothing, but the spacecraft's effects on the planet must also be taken into consideration to provide a complete picture of the mechanics involved. The linear momentum gained by the spaceship is equal in magnitude to that lost by the planet, so the spacecraft gains velocity and the planet loses velocity. However, the planet's enormous mass compared to the spacecraft makes the resulting change in its speed negligibly small even when compared to the orbital perturbations planets undergo due to interactions with other celestial bodies on astronomically short timescales. For example, one metric ton is a typical mass for an interplanetary space probe whereas Jupiter has a mass of almost 2 x 10²⁴ metric tons. Therefore, a one-ton spacecraft passing Jupiter will theoretically cause the planet to lose approximately 5 x 10⁻²⁵ km/s of orbital velocity for every km/s of velocity relative to the Sun gained by the spacecraft. For all practical purposes the effects on the planet can be ignored in the calculation.

Realistic portrayals of encounters in space require the consideration of three dimensions. The same principles apply as above except adding the planet's velocity to that of the spacecraft requires vector addition as shown below.

Two-dimensional schematic of gravitational slingshot. The arrows show the direction in which the spacecraft is traveling before and after the encounter. The length of the arrows shows the spacecraft's speed.

Due to the reversibility of orbits, gravitational slingshots can also be used to reduce the speed of a spacecraft. Both Mariner 10 and MESSENGER performed this maneuver to reach Mercury.

If more speed is needed than available from gravity assist alone, a rocket burn near the periapsis (closest planetary approach) uses the least fuel. A given rocket burn always provides the same change in velocity (Δv), but the change in kinetic energy is proportional to the vehicle's velocity at the time of the burn. Therefore the maximum kinetic energy is obtained when the burn occurs at the vehicle's maximum velocity (periapsis). The Oberth effect describes this technique in more detail.

Historical origins

In his paper "To those who will be reading in order to build" ("Тем, кто будет читать, чтобы строить"), published in 1938 but dated 1918–1919, Yuri Kondratyuk suggested that a spacecraft traveling between two planets could be accelerated at the beginning and end of its trajectory by using the gravity of the two planets' moons. The portion of his manuscript considering gravity-assists received no later development and was not published until the 1960s. In his 1925 paper "Problems of flight by jet propulsion: interplanetary flights" ("Проблема полета при помощи реактивных аппаратов: межпланетные полеты"), Friedrich Zander showed a deep understanding of the physics behind the concept of gravity assist and its potential for the interplanetary exploration of the solar system.

Italian engineer Gaetano Crocco was first to calculate an interplanetary journey considering multiple gravity-assists.

The gravity assist maneuver was first attempted in 1959 when the Soviet probe Luna 3 photographed the far side of the Moon. The maneuver relied on research performed under the direction of Mstislav Keldysh at the Keldysh Institute of Applied Mathematics.

In 1961, Michael Minovitch, UCLA graduate student who worked at NASA's Jet Propulsion Laboratory (JPL), developed a gravity assist technique, that would later be used for the Gary Flandro's Planetary Grand Tour idea.

During the summer of 1964 at the NASA JPL, Gary Flandro was assigned the task of studying techniques for exploring the outer planets of the solar system. In this study he discovered the rare alignment of the outer planets (Jupiter, Saturn, Uranus, and Neptune) and conceived the Planetary Grand Tour multi-planet mission utilizing gravity assist to reduce mission duration from forty years to less than ten.

Purpose

Plot of Voyager 2's heliocentric velocity against its distance from the Sun, illustrating the use of gravity assist to accelerate the spacecraft by Jupiter, Saturn and Uranus. To observe Triton, Voyager 2 passed over Neptune's north pole resulting in an acceleration out of the plane of the ecliptic and reduced velocity away from the Sun.

A spacecraft traveling from Earth to an inner planet will increase its relative speed because it is falling toward the Sun, and a spacecraft traveling from Earth to an outer planet will decrease its speed because it is leaving the vicinity of the Sun.

Although the orbital speed of an inner planet is greater than that of the Earth, a spacecraft traveling to an inner planet, even at the minimum speed needed to reach it, is still accelerated by the Sun's gravity to a speed notably greater than the orbital speed of that destination planet. If the spacecraft's purpose is only to fly by the inner planet, then there is typically no need to slow the spacecraft. However, if the spacecraft is to be inserted into orbit about that inner planet, then there must be some way to slow it down.

Similarly, while the orbital speed of an outer planet is less than that of the Earth, a spacecraft leaving the Earth at the minimum speed needed to travel to some outer planet is slowed by the Sun's gravity to a speed far less than the orbital speed of that outer planet. Therefore, there must be some way to accelerate the spacecraft when it reaches that outer planet if it is to enter orbit about it.

Rocket engines can certainly be used to increase and decrease the speed of the spacecraft. However, rocket thrust takes propellant, propellant has mass, and even a small change in velocity (known as Δv, or "delta-v", the delta symbol being used to represent a change and "v" signifying velocity) translates to a far larger requirement for propellant needed to escape Earth's gravity well. This is because not only must the primary-stage engines lift the extra propellant, they must also lift the extra propellant beyond that which is needed to lift that additional propellant. The liftoff mass requirement increases exponentially with an increase in the required delta-v of the spacecraft.

Because additional fuel is needed to lift fuel into space, space missions are designed with a tight propellant "budget", known as the "delta-v budget". The delta-v budget is in effect the total propellant that will be available after leaving the earth, for speeding up, slowing down, stabilization against external buffeting (by particles or other external effects), or direction changes, if it cannot acquire more propellant. The entire mission must be planned within that capability. Therefore, methods of speed and direction change that do not require fuel to be burned are advantageous, because they allow extra maneuvering capability and course enhancement, without spending fuel from the limited amount which has been carried into space. Gravity assist maneuvers can greatly change the speed of a spacecraft without expending propellant, and can save significant amounts of propellant, so they are a very common technique to save fuel.

Limits

The trajectories that enabled NASA's twin Voyager spacecraft to tour the four giant planets and achieve velocity to escape the Solar System

The main practical limit to the use of a gravity assist maneuver is that planets and other large masses are seldom in the right places to enable a voyage to a particular destination. For example, the Voyager missions which started in the late 1970s were made possible by the "Grand Tour" alignment of Jupiter, Saturn, Uranus and Neptune. A similar alignment will not occur again until the middle of the 22nd century. That is an extreme case, but even for less ambitious missions there are years when the planets are scattered in unsuitable parts of their orbits.

Another limitation is the atmosphere, if any, of the available planet. The closer the spacecraft can approach, the faster its periapsis speed as gravity accelerates the spacecraft, allowing for more kinetic energy to be gained from a rocket burn. However, if a spacecraft gets too deep into the atmosphere, the energy lost to drag can exceed that gained from the planet's gravity. On the other hand, the atmosphere can be used to accomplish aerobraking. There have also been theoretical proposals to use aerodynamic lift as the spacecraft flies through the atmosphere. This maneuver, called an aerogravity assist, could bend the trajectory through a larger angle than gravity alone, and hence increase the gain in energy.

Even in the case of an airless body, there is a limit to how close a spacecraft may approach. The magnitude of the achievable change in velocity depends on the spacecraft's approach velocity and the planet's escape velocity at the point of closest approach (limited by either the surface or the atmosphere.)

Interplanetary slingshots using the Sun itself are not possible because the Sun is at rest relative to the Solar System as a whole. However, thrusting when near the Sun has the same effect as the powered slingshot described as the Oberth effect. This has the potential to magnify a spacecraft's thrusting power enormously, but is limited by the spacecraft's ability to resist the heat.

A rotating black hole might provide additional assistance, if its spin axis is aligned the right way. General relativity predicts that a large spinning mass produces frame-dragging—close to the object, space itself is dragged around in the direction of the spin. Any ordinary rotating object produces this effect. Although attempts to measure frame dragging about the Sun have produced no clear evidence, experiments performed by Gravity Probe B have detected frame-dragging effects caused by Earth. General relativity predicts that a spinning black hole is surrounded by a region of space, called the ergosphere, within which standing still (with respect to the black hole's spin) is impossible, because space itself is dragged at the speed of light in the same direction as the black hole's spin. The Penrose process may offer a way to gain energy from the ergosphere, although it would require the spaceship to dump some "ballast" into the black hole, and the spaceship would have had to expend energy to carry the "ballast" to the black hole.

Another potential application of gravity assist is alteration of the Earth's orbital distance from the Sun to reduce increasing global temperatures.

Tisserand parameter and gravity assists

The use of gravity assists is constrained by a conserved quantity called the Tisserand parameter (or invariant). This is an approximation to the Jacobi constant of the restricted three-body problem. Considering the case of a comet orbiting the Sun and the effects a Jupiter encounter would have, Félix Tisserand showed that

${\displaystyle T_P=\frac{a_J}{a}+2\cdot \sqrt{\frac{a}{a_J}(1-e^2)}\cos i}$

will remain constant (where ${\displaystyle a}$ is the comet's semi-major axis, ${\displaystyle e}$ its eccentricity, ${\displaystyle i}$ its inclination, and ${\displaystyle a_J}$ is the semi-major axis of Jupiter).

This applies when the comet is sufficiently far from Jupiter to have well-defined orbital elements and to the extent that Jupiter is much less massive than the Sun and on a circular orbit.

This quantity is conserved for any system of three objects, one of which has negligible mass, and another of which is of intermediate mass and on a circular orbit. Examples are the Sun, Earth and a spacecraft, or Saturn, Titan and the Cassini spacecraft (using the semi-major axis of the perturbing body instead of ${\displaystyle a_J}$). This imposes a constraint on how a gravity assist may be used to alter a spacecraft's orbit.

The Tisserand parameter will change if the spacecraft makes a propulsive maneuver or a gravity assist of some fourth object, which is one reason that many spacecraft frequently combine Earth and Venus (or Mars) gravity assists or also perform large deep space maneuvers.

Notable examples of use

Luna 3

The gravity assist maneuver was first attempted in 1959 for Luna 3, to photograph the far side of the Moon. The satellite did not gain speed, but its orbit was changed that allowed successful transmission of the photos.

Pioneer 10

NASA's Pioneer 10 is a space probe launched in 1972 that completed the first mission to the planet Jupiter. Thereafter, Pioneer 10 became the first of five artificial objects to achieve the escape velocity needed to leave the Solar System. In December 1973, Pioneer 10 spacecraft was the first one to use the gravitational slingshot effect to reach escape velocity to leave Solar System.

Pioneer 11

Pioneer 11 was launched by NASA in 1973, to study the asteroid belt, the environment around Jupiter and Saturn, solar winds, and cosmic rays. It was the first probe to encounter Saturn, the second to fly through the asteroid belt, and the second to fly by Jupiter. To get to Saturn, the spacecraft got a gravity assist on Jupiter.

Mariner 10

The Mariner 10 probe was the first spacecraft to use the gravitational slingshot effect to reach another planet, passing by Venus on 5 February 1974 on its way to becoming the first spacecraft to explore Mercury.

Voyager 1

Voyager 1 was launched by NASA on September 5, 1977. It gained the energy to escape the Sun's gravity by performing slingshot maneuvers around Jupiter and Saturn. Having operated for 46 years and 22 days as of September 28, 2023 UTC, the spacecraft still communicates with the Deep Space Network to receive routine commands and to transmit data to Earth. Real-time distance and velocity data is provided by NASA and JPL. At a distance of 152.2 AU (22.8 billion km; 14.1 billion mi) from Earth as of January 12, 2020, it is the most distant human-made object from Earth.

Voyager 2

Voyager 2 was launched by NASA on August 20, 1977, to study the outer planets. Its trajectory took longer to reach Jupiter and Saturn than its twin spacecraft but enabled further encounters with Uranus and Neptune.

Galileo

The Galileo spacecraft was launched by NASA in 1989 and on its route to Jupiter get three gravity assists, one from Venus (February 10, 1990), and two from Earth (December 8, 1990 and December 8, 1992). Spacecraft reached Jupiter in December 1995. Gravity assists also allowed Galileo to flyby two asteroids, 243 Ida and 951 Gaspra.

Ulysses

In 1990, NASA launched the ESA spacecraft Ulysses to study the polar regions of the Sun. All the planets orbit approximately in a plane aligned with the equator of the Sun. Thus, to enter an orbit passing over the poles of the Sun, the spacecraft would have to eliminate the speed it inherited from the Earth's orbit around the Sun and gain the speed needed to orbit the Sun in the pole-to-pole plane. It was achieved by a gravity assist from Jupiter on February 8, 1992.

MESSENGER

The MESSENGER mission (launched in August 2004) made extensive use of gravity assists to slow its speed before orbiting Mercury. The MESSENGER mission included one flyby of Earth, two flybys of Venus, and three flybys of Mercury before finally arriving at Mercury in March 2011 with a velocity low enough to permit orbit insertion with available fuel. Although the flybys were primarily orbital maneuvers, each provided an opportunity for significant scientific observations.

Cassini

The Cassini–Huygens spacecraft was launched from Earth on 15 October 1997, followed by gravity assist flybys of Venus (26 April 1998 and 21 June 1999), Earth (18 August 1999), and Jupiter (30 December 2000). Transit to Saturn took 6.7 years, the spacecraft arrived at 1 July 2004. Its trajectory was called "the Most Complex Gravity-Assist Trajectory Flown to Date" in 2019.

Cassini interplanetary trajectory

 

Animation of Cassini's trajectory from 15 October 1997 to 4 May 2008
  Cassini–Huygens ·   Jupiter ·   Saturn ·   Earth ·   Venus ·    Mars

Cassini's speed relative to the Sun. Gravity assists form peaks to the left, while periodic variations on the right are caused by the spacecraft's orbit around Saturn.

After entering orbit around Saturn, the Cassini spacecraft used multiple Titan gravity assists to achieve significant changes in the inclination of its orbit as well so that instead of staying nearly in the equatorial plane, the spacecraft's flight path was inclined well out of the plane of the rings. A typical Titan encounter changed the spacecraft's velocity by 0.75 km/s, and the spacecraft made 127 Titan encounters. These encounters enabled an orbital tour with a wide range of periapsis and apoapsis distances, various alignments of the orbit with respect to the Sun, and orbital inclinations from 0° to 74°. The multiple flybys of Titan also allowed Cassini to flyby other moons, such as Rhea and Enceladus.

Rosetta

Animation of Rosetta's trajectory from 2 March 2004 to 9 September 2016
  Rosetta ·   67P/C-G ·   Earth ·   Mars ·   21 Lutetia  ·   2867 Šteins

The Rosetta probe, launched in March 2004, used four gravity assist maneuvers (including one just 250 km from the surface of Mars, and three assists from Earth) to accelerate throughout the inner Solar System. That enabled it to flyby the asteroids 21 Lutetia and 2867 Šteins as well as eventually match the velocity of the 67P/Churyumov–Gerasimenko comet at the rendezvous point in August 2014.

New Horizons

New Horizons was launched by NASA in 2006, and reached Pluto in 2015. In 2007 it performed a gravity assist on Jupiter.

Juno

The Juno spacecraft was launched on August 5, 2011 (UTC). The trajectory used a gravity assist speed boost from Earth, accomplished by an Earth flyby in October 2013, two years after its launch on August 5, 2011. In that way Juno changed its orbit (and speed) toward its final goal, Jupiter, after only five years.

Parker Solar Probe

Parker Solar Probe, launched by NASA in 2018, has seven planned Venus gravity assists. Each gravity assist bring Parker Solar Probe progressively closer to the Sun. As of 2022, the spacecraft performed five of its seven assists. Parker Solar Probe's mission will make the closest approach to the Sun by any space mission.

Solar Orbiter

Solar Orbiter was launched by ESA in 2020. In its initial cruise phase, which lasts until November 2021, Solar Orbiter performed two gravity-assist manoeuvres around Venus and one around Earth to alter the spacecraft's trajectory, guiding it towards the innermost regions of the Solar System. The first close solar pass will take place on 26 March 2022 at around a third of Earth's distance from the Sun.

BepiColombo

BepiColombo is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to the planet Mercury. It was launched on 20 October 2018. It will use the gravity assist technique with Earth once, with Venus twice, and six times with Mercury. It will arrive in 2025. BepiColombo is named after Giuseppe (Bepi) Colombo who was a pioneer thinker with this way of maneuvers.

Lucy

Lucy was launched by NASA on 16 October 2021. It gained one gravity assist from Earth on the 16th of October, 2022, and after a flyby of the main-belt asteroid 152830 Dinkinesh it will gain another in 2024. In 2025, it will fly by the inner main-belt asteroid 52246 Donaldjohanson. In 2027, it will arrive at the L₄ Trojan cloud (the Greek camp of asteroids that orbits about 60° ahead of Jupiter), where it will fly by four Trojans, 3548 Eurybates (with its satellite), 15094 Polymele, 11351 Leucus, and 21900 Orus. After these flybys, Lucy will return to Earth in 2031 for another gravity assist toward the L₅ Trojan cloud (the Trojan camp which trails about 60° behind Jupiter), where it will visit the binary Trojan 617 Patroclus with its satellite Menoetius in 2033.

Basal ganglia

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Basal_ganglia

 

Basal ganglia
Basal ganglia (red) and related structures (blue) shown within the brain
Basal ganglia from anterior view of brain
Details
Part of	Cerebrum
Identifiers
Latin	nuclei basales
Acronym(s)	BG
MeSH	D001479
NeuroNames	224, 2677
NeuroLex ID	birnlex_826
TA98	A14.1.09.501
TA2	5559
FMA	84013

The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei found in the brains of vertebrates. In humans and some primates, differences exist, primarily in the division of the globus pallidus into external and internal regions, and in the division of the striatum. Positioned at the base of the forebrain and the top of the midbrain, they have strong connections with the cerebral cortex, thalamus, brainstem and other brain areas. The basal ganglia are associated with a variety of functions, including regulating voluntary motor movements, procedural learning, habit formation, conditional learning, eye movements, cognition, and emotion.

The main functional components of the basal ganglia include the striatum, consisting of both the dorsal striatum (caudate nucleus and putamen) and the ventral striatum (nucleus accumbens and olfactory tubercle), the globus pallidus, the ventral pallidum, the substantia nigra, and the subthalamic nucleus. Each of these components has complex internal anatomical and neurochemical structures. The largest component, the striatum (dorsal and ventral), receives input from various brain areas but only sends output to other components of the basal ganglia. The globus pallidus receives input from the striatum and sends inhibitory output to a number of motor-related areas. The substantia nigra is the source of the striatal input of the neurotransmitter dopamine, which plays an important role in basal ganglia function. The subthalamic nucleus mainly receives input from the striatum and cerebral cortex and projects to the globus pallidus.

The basal ganglia are thought to play a key role in action selection, aiding in the choice of behaviors to execute. More specifically, they regulate motor and premotor cortical areas, facilitating smooth voluntary movements.  Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active. The "behavior switching" that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which plays a key role in executive functions. It has also been hypothesized that the basal ganglia are not only responsible for motor action selection, but also for the selection of more cognitive actions. Computational models of action selection in the basal ganglia incorporate this.

The basal ganglia are of major importance for normal brain function and behaviour. Their dysfunction results in a wide range of neurological conditions including disorders of behaviour control and movement, as well as cognitive deficits that are similar to those that result from damage to the prefrontal cortex. Those of behaviour include Tourette syndrome, obsessive–compulsive disorder, and addiction. Movement disorders include, most notably Parkinson's disease, which involves degeneration of the dopamine-producing cells in the substantia nigra; Huntington's disease, which primarily involves damage to the striatum; dystonia; and more rarely hemiballismus. The basal ganglia have a limbic sector whose components are assigned distinct names: the nucleus accumbens, ventral pallidum, and ventral tegmental area (VTA). There is considerable evidence that this limbic part plays a central role in reward learning as well as cognition and frontal lobe functioning, via the mesolimbic pathway from the VTA to the nucleus accumbens that uses the neurotransmitter dopamine, and the mesocortical pathway. A number of highly addictive drugs, including cocaine, amphetamine, and nicotine, are thought to work by increasing the efficacy of this dopamine signal. There is also evidence implicating overactivity of the VTA dopaminergic projection in schizophrenia.

Structure

In terms of development, the human central nervous system is often classified based on the original three primitive vesicles from which it develops: These primary vesicles form in the normal development of the neural tube of the embryo and initially include the prosencephalon, mesencephalon, and rhombencephalon, in rostral to caudal (from head to tail) orientation. Later in development of the nervous system each section itself turns into smaller components. During development, the cells that migrate tangentially to form the basal ganglia are directed by the lateral and medial ganglionic eminences. The following table demonstrates this developmental classification and traces it to the anatomic structures found in the basal ganglia. The structures relevant to the basal ganglia are shown in bold.

Primary division of the neural tube	Secondary subdivision	Final segments in a human adult
Prosencephalon	Telencephalon Diencephalon	On each side of the brain: the cerebral cortices, caudate, putamen, Globus pallidus, ventral pallidum Thalamus, subthalamus, epithalamus, hypothalamus, subthalamic nucleus
Mesencephalon	Mesencephalon	Mesencephalon (midbrain): substantia nigra pars compacta (SNc), substantia nigra pars reticulata (SNr)
Rhombencephalon	Metencephalon Myelencephalon	Pons and cerebellum Medulla

Coronal slices of human brain showing the basal ganglia. White matter is shown in dark gray, gray matter is shown in light gray.
Anterior: striatum, globus pallidus (GPe and GPi)
Posterior: subthalamic nucleus (STN), substantia nigra (SN)

The basal ganglia form a fundamental component of the cerebrum. In contrast to the cortical layer that lines the surface of the forebrain, the basal ganglia are a collection of distinct masses of gray matter lying deep in the brain not far from the junction of the thalamus. They lie to the side of and surround the thalamus. Like most parts of the brain, the basal ganglia consist of left and right sides that are virtual mirror images of each other.

In terms of anatomy, the basal ganglia are divided into four distinct structures, depending on how superior or rostral they are (in other words depending on how close to the top of the head they are): Two of them, the striatum and the pallidum, are relatively large; the other two, the substantia nigra and the subthalamic nucleus, are smaller. In the illustration to the right, two coronal sections of the human brain show the location of the basal ganglia components. Of note, and not seen in this section, the subthalamic nucleus and substantia nigra lie farther back (posteriorly) in the brain than the striatum and pallidum.

Striatum

Main article: Striatum

Basal ganglia

The striatum is a subcortical structure generally divided into the dorsal striatum and ventral striatum, although a medial lateral classification has been suggested to be more relevant behaviorally and is being more widely used.

The striatum is composed mostly of medium spiny neurons. These GABAergic neurons project to the external (lateral) globus pallidus and internal (medial) globus pallidus as well as the substantia nigra pars reticulata. The projections into the globus pallidus and substantia nigra are primarily dopaminergic, although enkephalin, dynorphin and substance P are expressed. The striatum also contains interneurons that are classified into nitrergic neurons (due to use of nitric oxide as a neurotransmitter), tonically active (i.e. constantly releasing neurotransmitter unless inhibited) cholinergic interneurons, parvalbumin-expressing neurons and calretinin-expressing neurons. The dorsal striatum receives significant glutamatergic inputs from the cortex, as well as dopaminergic inputs from the substantia nigra pars compacta. The dorsal striatum is generally considered to be involved in sensorimotor activities. The ventral striatum receives glutamatergic inputs from the limbic areas as well as dopaminergic inputs from the VTA, via the mesolimbic pathway. The ventral striatum is believed to play a role in reward and other limbic functions. The dorsal striatum is divided into the caudate and putamen by the internal capsule while the ventral striatum is composed of the nucleus accumbens and olfactory tubercle. The caudate has three primary regions of connectivity, with the head of the caudate demonstrating connectivity to the prefrontal cortex, cingulate cortex and amygdala. The body and tail show differentiation between the dorsolateral rim and ventral caudate, projecting to the sensorimotor and limbic regions of the striatum respectively. Striatopallidal fibres connect the striatum to the pallidus.

Pallidum

The pallidum consists of a large structure called the globus pallidus ("pale globe") together with a smaller ventral extension called the ventral pallidum. The globus pallidus appears as a single neural mass, but can be divided into two functionally distinct parts, called the internal (or medial) and external (lateral) segments, abbreviated GPi and GPe. Both segments contain primarily GABAergic neurons, which therefore have inhibitory effects on their targets. The two segments participate in distinct neural circuits. The GPe receives input mainly from the striatum, and projects to the subthalamic nucleus. The GPi receives signals from the striatum via the "direct" and "indirect" pathways. Pallidal neurons operate using a disinhibition principle. These neurons fire at steady high rates in the absence of input, and signals from the striatum cause them to pause or reduce their rate of firing. Because pallidal neurons themselves have inhibitory effects on their targets, the net effect of striatal input to the pallidum is a reduction of the tonic inhibition exerted by pallidal cells on their targets (disinhibition) with an increased rate of firing in the targets.

Substantia nigra

Main article: Substantia nigra

Location of the substantia nigra within the basal ganglia

The substantia nigra is a midbrain gray matter portion of the basal ganglia that has two parts – the pars compacta (SNc) and the pars reticulata (SNr). SNr often works in unison with GPi, and the SNr-GPi complex inhibits the thalamus. Substantia nigra pars compacta (SNc) however, produces the neurotransmitter dopamine, which is very significant in maintaining balance in the striatal pathway. The circuit portion below explains the role and circuit connections of each of the components of the basal ganglia.

Subthalamic nucleus

Main article: Subthalamic nucleus

The subthalamic nucleus is a diencephalic gray matter portion of the basal ganglia, and the only portion of the ganglia that produces an excitatory neurotransmitter, glutamate. The role of the subthalamic nucleus is to stimulate the SNr-GPi complex and it is part of the indirect pathway. The subthalamic nucleus receives inhibitory input from the external part of the globus pallidus and sends excitatory input to the GPi.

Circuit connections

Further information: Cortico-basal ganglia-thalamo-cortical loop

Connectivity diagram showing excitatory glutamatergic pathways as red, inhibitory GABAergic pathways as blue, and modulatory dopaminergic pathways as magenta. (Abbreviations: GPe: globus pallidus external; GPi: globus pallidus internal; STN: subthalamic nucleus; SNc: substantia nigra pars compacta; SNr: substantia nigra pars reticulata)

Connectivity of the basal ganglia as revealed by diffusion spectrum imaging based on thirty subjects from the Human Connectome Project. Direct, indirect and hyperdirect pathways are visualized in different colors (see legend). Subcortical structures are rendered based on the Harvard-Oxford subcortical thalamus as well as the Basal Ganglia atlas (other structures). Rendering was generated using TrackVis software.

The left side of Fig.1 shows a region of the prefrontal cortex receiving multiple inputs from other regions, as cortico-cortical activity. The input from B is the strongest of these. The right side of Fig. 1 shows the input signals also being fed to the basal ganglia circuitry. The output from here, back to the same region, is shown to modify the strength of the input from B, by adding strength to the input from C thereby modifying the strongest signal from B to C. (Thalamic involvement is implicit but not shown).

Multiple models of basal ganglia circuits and function have been proposed, however there have been questions raised about the strict divisions of the direct and indirect pathways, their possible overlap and regulation. The circuitry model has evolved since the first proposed model in the 1990s by DeLong in the parallel processing model, in which the cortex and substantia nigra pars compacta project into the dorsal striatum giving rise to an inhibitory indirect and excitatory direct pathway.

• The inhibitory indirect pathway involved the inhibition of the globus pallidus externus, allowing for the disinhibition of the globus pallidus internus (through STN) allowing it to inhibit the thalamus.
• The direct or excitatory pathway involved the disinhibition of the thalamus through the inhibition of the GPi/SNr. However the speed of the direct pathway would not be concordant with the indirect pathway in this model leading to problems with it. To get over this, a hyperdirect pathway where the cortex sends glutamatergic projections through the subthalamic nucleus exciting the inhibitory GPe under the center surround model, as well as a shorter indirect pathway have been proposed.

Generally, the basal ganglia circuitry is divided into five pathways: one limbic, two associative (prefrontal), one oculomotor, and one motor pathway. (The motor and oculomotor pathways are sometimes grouped into one motor pathway.) The five general pathways are organized as follows:

• The motor loop involving projections from the supplementary motor area, arcuate premotor area, motor cortex and somatosensory cortex into the putamen, which projects into the ventrolateral GPi and caudolateral SNr which projects into the cortex through the ventralis lateralis pars medialis and ventralis lateralis pars oralis.
• The oculomotor loop involved projections from the frontal eye fields, the dorsolateral prefrontal cortex (DLPFC), and the posterior parietal cortex into the caudate, into the caudal dorsomedial GPi and ventrolateral SNr, finally looping back into the cortex through the lateral ventralis anterior pars magnocellularis(VAmc).
• The first cognitive/associative pathway proposes a pathway from the DLPFC, into the dorsolateral caudate, followed by a projection into the lateral dorsomedial GPi, and rostral SNr before projecting into the lateral VAmc and medial pars magnocellularis.
• The second cognitive/associative pathway proposed is a circuit projecting from the lateral orbitofrontal cortex, the temporal gyrus, and anterior cingulate cortex into the ventromedial caudate, followed by a projection into the lateromedial GPi, and rostrolateral SNr before looping into the cortex via the medial VAmc and medial magnocellularis.
• The limbic circuit involving the projections from the ACC, hippocampus, entorhinal cortex, and insula into the ventral striatum, then into the rostrodorsal GPi, ventral pallidum and rostrodorsal SNr, followed by a loop back into the cortex through the posteromedial part of the medial dorsal nucleus. However, more subdivisions of loops have been proposed, up to 20,000.

The direct pathway, originating in the dorsal striatum inhibits the GPi and SNr, resulting in a net disinhibition or excitation of the thalamus. This pathway consists of medium spiny neurons (MSNs) that express dopamine receptor D1, muscarinic acetylcholine receptor M4, and adenosine receptor A1. The direct pathway has been proposed to facilitate motor actions, timing of motor actions, gating of working memory, and motor responses to specific stimuli.

The (long) indirect pathway originates in the dorsal striatum and inhibits the GPe, resulting in disinhibition of the GPi which is then free to inhibit the thalamus. This pathway consists of MSNs that express dopamine receptor D2, muscarinic acetylcholine receptor M1, and adenosine receptor A2a. This pathway has been proposed to result in global motor inhibition(inhibition of all motor activity), and termination of responses. Another shorter indirect pathway has been proposed, which involves cortical excitation of the subthalamic nucleus resulting in direct excitation of the GPe, and inhibition of the thalamus. This pathway is proposed to result in inhibition of specific motor programs based on associative learning.

A combination of these indirect pathways resulting in a hyperdirect pathway that results in inhibition of basal ganglia inputs besides one specific focus has been proposed as part of the center surround theory. This hyperdirect pathway is proposed to inhibit premature responses, or globally inhibit the basal ganglia to allow for more specific top down control by the cortex.

The interactions of these pathways are currently under debate. Some say that all pathways directly antagonize each other in a "push pull" fashion, while others support the center surround theory, in which one focused input into the cortex is protected by inhibition of competing inputs by the rest of the indirect pathways.

Diagram shows two coronal slices that have been superimposed to include the involved basal ganglia structures. Green arrows (+) refer to excitatory glutamatergic pathways, red arrows (–) refer to inhibitory GABAergic pathways and turquoise arrows refer to dopaminergic pathways that are excitatory on the direct pathway and inhibitory on the indirect pathway.

Neurotransmitters

The basal ganglia contains many afferent glutamatergic inputs, with predominantly GABAergic efferent fibers, modulatory cholinergic pathways, significant dopamine in the pathways originating in the ventral tegmental area and substantia nigra, as well as various neuropeptides. Neuropeptides found in the basal ganglia include substance P, neurokinin A, cholecystokinin, neurotensin, neurokinin B, neuropeptide Y, somatostatin, dynorphin, enkephaline. Other neuromodulators found in the basal ganglia include nitric oxide, carbon monoxide, and phenylethylamine.

Functional connectivity

The functional connectivity, measured by regional co-activation during functional neuroimaging studies, is broadly consistent with the parallel processing models of basal ganglia function. The putamen was generally coactivated with motor areas such as the supplementary motor area, caudal anterior cingulate cortex and primary motor cortex, while the caudate and rostral putamen were more frequently coactivated with the rostral ACC and DLPFC. The ventral striatum was significantly associated with the amygdala and hippocampus, which although was not included in the first formulations of basal ganglia models, has been an addition to more recent models.

Function

Eye movements

One intensively studied function of the basal ganglia is its role in controlling eye movements. Eye movement is influenced by an extensive network of brain regions that converges on a midbrain area called the superior colliculus (SC). The SC is a layered structure whose layers form two-dimensional retinotopic maps of visual space. A "bump" of neural activity in the deep layers of the SC drives an eye movement directed toward the corresponding point in space.

The SC receives a strong inhibitory projection from the basal ganglia, originating in the substantia nigra pars reticulata (SNr). Neurons in the SNr usually fire continuously at high rates, but at the onset of an eye movement they "pause", thereby releasing the SC from inhibition. Eye movements of all types are associated with "pausing" in the SNr; however, individual SNr neurons may be more strongly associated with some types of movements than others. Neurons in some parts of the caudate nucleus also show activity related to eye movements. Since the great majority of caudate cells fire at very low rates, this activity almost always shows up as an increase in firing rate. Thus, eye movements begin with activation in the caudate nucleus, which inhibits the SNr via the direct GABAergic projections, which in turn disinhibits the SC.

Role in motivation

Extracellular dopamine in the basal ganglia has been linked to motivational states in rodents, with high levels being linked to satiated state, medium levels with seeking, and low with aversion. The limbic basal ganglia circuits are influenced heavily by extracellular dopamine. Increased dopamine results in inhibition of the Ventral pallidum, entopeduncular nucleus, and substantia nigra pars reticulata, resulting in disinhibition of the thalamus. This model of direct D1, and indirect D2 pathways explain why selective agonists of each receptor are not rewarding, as activity at both pathways is required for disinhibition. The disinhibition of the thalamus leads to activation of the prefrontal cortex and ventral striatum, selective for increased D1 activity leading to reward. There is also evidence from non-human primate and human electrophysiology studies that other basal ganglia structures including the globus pallidus internus and subthalamic nucleus are involved in reward processing.

Decision making

Two models have been proposed for the basal ganglia, one being that actions are generated by a "critic" in the ventral striatum and estimates value, and the actions are carried out by an "actor" in the dorsal striatum. Another model proposes the basal ganglia acts as a selection mechanism, where actions are generated in the cortex and are selected based on context by the basal ganglia. The CBGTC loop is also involved in reward discounting, with firing increasing with an unexpected or greater than expected reward. One review supported the idea that the cortex was involved in learning actions regardless of their outcome, while the basal ganglia was involved in selecting appropriate actions based on associative reward based trial and error learning.

Working memory

The basal ganglia has been proposed to gate what enters and what doesn't enter working memory. One hypothesis proposes that the direct pathway (Go, or excitatory) allows information into the PFC, where it stays independent of the pathway, however another theory proposes that in order for information to stay in the PFC the direct pathway needs to continue reverberating. The short indirect pathway has been proposed to, in a direct push pull antagonism with the direct pathway, close the gate to the PFC. Together these mechanisms regulate working memory focus.

Clinical significance

Main article: Basal ganglia disease

Basal ganglia disease is a group of movement disorders that result from either excessive output from the basal ganglia to the thalamus – hypokinetic disorders, or from insufficient output – hyperkinetic disorders. Hypokinetic disorders arise from an excessive output from the basal ganglia, which inhibits the output from the thalamus to the cortex, and thus limits voluntary movement. Hyperkinetic disorders result from a low output from the basal ganglia to the thalamus which gives not enough inhibition to the thalamic projections to the cortex and thus gives uncontrolled/involuntary movements. Dysfunction of the basal ganglia circuitry can also lead to other disorders.

The following is a list of disorders, conditions, and symptoms that have been linked to the basal ganglia:

• Addiction
• Athetosis
• Athymhormic syndrome (PAP syndrome)
• Attention-deficit hyperactivity disorder (ADHD)
• Blepharospasm
• Bruxism
• Cerebral palsy: basal ganglia damage during second and third trimester of pregnancy
• Chorea
• Dystonia
• Epilepsy
• Fahr's disease
• Foreign accent syndrome (FAS)
• Huntington's disease
• Kernicterus
• Lesch–Nyhan syndrome
• Major depressive disorder
• Obsessive-compulsive disorder
• Other anxiety disorders
• PANDAS (disorder)
• Parkinson's disease
• Restless legs syndrome
• Schizophrenia
• Spasmodic dysphonia
• Stuttering
• Sydenham's chorea
• Tardive dyskinesia, caused by chronic antipsychotic treatment
• Tourette's syndrome
• Wilson's disease

History

The acceptance that the basal ganglia system constitutes one major cerebral system took time to arise. The first anatomical identification of distinct subcortical structures was published by Thomas Willis in 1664. For many years, the term corpus striatum was used to describe a large group of subcortical elements, some of which were later discovered to be functionally unrelated. For many years, the putamen and the caudate nucleus were not associated with each other. Instead, the putamen was associated with the pallidum in what was called the nucleus lenticularis or nucleus lentiformis.

A thorough reconsideration by Cécile and Oskar Vogt (1941) simplified the description of the basal ganglia by proposing the term striatum to describe the group of structures consisting of the caudate nucleus, the putamen, and the mass linking them ventrally, the nucleus accumbens. The striatum was named on the basis of the striated (striped) appearance created by radiating dense bundles of striato-pallido-nigral axons, described by anatomist Samuel Alexander Kinnier Wilson (1912) as "pencil-like".

The anatomical link of the striatum with its primary targets, the pallidum and the substantia nigra, was discovered later. The name globus pallidus was attributed by Déjerine to Burdach (1822). For this, the Vogts proposed the simpler "pallidum". The term "locus niger" was introduced by Félix Vicq-d'Azyr as tache noire in (1786), though that structure has since become known as the substantia nigra, due to contributions by Von Sömmering in 1788. The structural similarity between the substantia nigra and globus pallidus was noted by Mirto in 1896. Together, the two are known as the pallidonigral ensemble, which represents the core of the basal ganglia. Altogether, the main structures of the basal ganglia are linked to each other by the striato-pallido-nigral bundle, which passes through the pallidum, crosses the internal capsule as the "comb bundle of Edinger", and finally reaches the substantia nigra.

Additional structures that later became associated with the basal ganglia are the "body of Luys" (1865) (nucleus of Luys on the figure) or subthalamic nucleus, whose lesion was known to produce movement disorders. More recently, other areas such as the centromedian nucleus and the pedunculopontine complex have been thought to be regulators of the basal ganglia.

Near the beginning of the 20th century, the basal ganglia system was first associated with motor functions, as lesions of these areas would often result in disordered movement in humans (chorea, athetosis, Parkinson's disease).

Terminology

The nomenclature of the basal ganglia system and its components has always been problematic. Early anatomists, seeing the macroscopic anatomical structure but knowing nothing of the cellular architecture or neurochemistry, grouped together components that are now believed to have distinct functions (such as the internal and external segments of the globus pallidus), and gave distinct names to components that are now thought to be functionally parts of a single structure (such as the caudate nucleus and putamen).

The term "basal" comes from the fact that most of its elements are located in the basal part of the forebrain. The term ganglia is a misnomer: In modern usage, neural clusters are called "ganglia" only in the peripheral nervous system; in the central nervous system they are called "nuclei". For this reason, the basal ganglia are also occasionally known as the "basal nuclei". Terminologia anatomica (1998), the international authority for anatomical naming, retained "nuclei basales", but this is not commonly used.

The International Basal Ganglia Society (IBAGS) informally considers the basal ganglia to be made up of the striatum, the pallidum (with two nuclei), the substantia nigra (with its two distinct parts), and the subthalamic nucleus, whereas Terminologia anatomica excludes the last two. Some neurologists have included the centromedian nucleus of the thalamus as part of the basal ganglia, and some have also included the pedunculopontine nucleus.

Other animals

See also: Primate basal ganglia system

The basal ganglia form one of the basic components of the forebrain, and can be recognized in all species of vertebrates. Even in the lamprey (generally considered one of the most primitive of vertebrates), striatal, pallidal, and nigral elements can be identified on the basis of anatomy and histochemistry.

The names given to the various nuclei of the basal ganglia are different in different species. In cats and rodents the internal globus pallidus is known as the entopeduncular nucleus. In birds the striatum is called the paleostriatum augmentatum and the external globus pallidus is called the paleostriatum primitivum.

A clear emergent issue in comparative anatomy of the basal ganglia is the development of this system through phylogeny as a convergent cortically re-entrant loop in conjunction with the development and expansion of the cortical mantle. There is controversy, however, regarding the extent to which convergent selective processing occurs versus segregated parallel processing within re-entrant closed loops of the basal ganglia. Regardless, the transformation of the basal ganglia into a cortically re-entrant system in mammalian evolution occurs through a re-direction of pallidal (or "paleostriatum primitivum") output from midbrain targets such as the superior colliculus, as occurs in sauropsid brain, to specific regions of the ventral thalamus and from there back to specified regions of the cerebral cortex that form a subset of those cortical regions projecting into the striatum. The abrupt rostral re-direction of the pathway from the internal segment of the globus pallidus into the ventral thalamus—via the path of the ansa lenticularis—could be viewed as a footprint of this evolutionary transformation of basal ganglia outflow and targeted influence.