Editing Dopamine (section)

====Basal ganglia====
[[File:Basal ganglia circuits.svg|thumb|right|300px|Main circuits of the [[basal ganglia]]. The dopaminergic pathway from the [[substantia nigra pars compacta]] to the [[striatum]] is shown in light blue.|alt=At the top, a line drawing of a side view of the human brain, with a cross section pulled out showing the basal ganglia structures in color near the center.  At the bottom an expanded line drawing of the basal ganglia structures, showing outlines of each structure and broad arrows for their connection pathways.]]

The largest and most important sources of dopamine in the vertebrate brain are the substantia nigra and ventral tegmental area.<ref name=Bjorklund/> Both structures are components of the midbrain, closely related to each other and functionally similar in many respects.<ref name=Bjorklund/> The largest component of the basal ganglia is the striatum.<ref name=brs>{{cite book |vauthors=Fix JD| title = Neuroanatomy (Board Review Series) |edition=4th |location=Baltimore |publisher=Wulters Kluwer & Lippincott Williams & Wilkins |chapter=Basal Ganglia and the Striatal Motor System |year=2008 |pages=274–81 |isbn=978-0-7817-7245-7}}</ref> 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]].<ref name=Bjorklund/>

Progress in understanding the functions of the basal ganglia has been slow.<ref name="brs"/> The most popular hypotheses, broadly stated, propose that the basal ganglia play a central role in [[action selection]].<ref name=chakravarthy>{{cite journal | vauthors = Chakravarthy VS, Joseph D, Bapi RS | s2cid = 853119 | title = What do the basal ganglia do? A modeling perspective | journal = Biological Cybernetics | volume = 103 | issue = 3 | pages = 237–53 | date = September 2010 | pmid = 20644953 | doi = 10.1007/s00422-010-0401-y | url = https://www.researchgate.net/publication/45276082 }}</ref> 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.<ref name=Floresco>{{cite journal | vauthors = Floresco SB | title = The nucleus accumbens: an interface between cognition, emotion, and action | journal = Annual Review of Psychology | volume = 66 | pages = 25–52 | date = January 2015 | pmid = 25251489 | doi = 10.1146/annurev-psych-010213-115159 | s2cid = 28268183 | url = https://www.researchgate.net/publication/266085689 }}</ref> 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.<ref name=Floresco/>

The basal ganglia can be divided into several sectors, and each is involved in controlling particular types of actions.<ref name=Balleine>{{cite journal |vauthors=Balleine BW, Dezfouli A, Ito M, Doya K |s2cid=53148662 |year=2015 |title=Hierarchical control of goal-directed action in the cortical–basal ganglia network |journal=Current Opinion in Behavioral Sciences |volume=5 |pages=1–7 |doi=10.1016/j.cobeha.2015.06.001}}</ref> 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.<ref name=Floresco/> 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.<ref name=Balleine/>

Dopamine contributes to the action selection process in at least two important ways. First, it sets the "threshold" for initiating actions.<ref name=chakravarthy/> The higher the level of dopamine activity, the lower the impetus required to evoke a given behavior.<ref name=chakravarthy/> As a consequence, high levels of dopamine lead to high levels of motor activity and [[impulsivity|impulsive behavior]]; low levels of dopamine lead to [[torpor]] and slowed reactions.<ref name=chakravarthy/> 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.<ref name=Jankovic/> 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 [[stereotypy|stereotyped movements]].<ref name=Patti>{{cite journal | vauthors = Pattij T, Vanderschuren LJ | title = The neuropharmacology of impulsive behaviour | journal = Trends in Pharmacological Sciences | volume = 29 | issue = 4 | pages = 192–99 | date = April 2008 | pmid = 18304658 | doi = 10.1016/j.tips.2008.01.002 | url = https://www.researchgate.net/publication/5547125 }}</ref>

The second important effect of dopamine is as a "teaching" signal.<ref name=chakravarthy/> 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.<ref name=chakravarthy/> This is a form of [[operant conditioning]], in which dopamine plays the role of a reward signal.<ref name=Floresco/>