Editing Striatum (section)

==Structure==
[[File:Striatum Structural MRI.png|thumb|200px|alt=The striatum as seen on [[MRI]]. The striatum includes the [[caudate nucleus]] and the [[lentiform nucleus]] which includes the [[putamen]] and the [[globus pallidus]] |The striatum in red as seen on [[MRI]]. The striatum includes the [[caudate nucleus]] (''top''), and the [[lentiform nucleus]] (the [[putamen]] (''right'') and the [[globus pallidus]] (''lower left''))]]

The striatum is the largest structure of the [[basal ganglia]]. The striatum is divided into two subdivisions, a ventral striatum and a dorsal striatum, based upon function and connections. It is also divisible into a matrix and embedded striosomes.

===Ventral striatum===
The ventral striatum is composed of the [[nucleus accumbens]] and the [[olfactory tubercle]].<ref name=FERRE2010 /><ref name="pmid18047654">{{Cite journal |vauthors=Ubeda-Bañon I, Novejarque A, Mohedano-Moriano A, etal |title=Projections from the posterolateral olfactory amygdala to the ventral striatum: neural basis for reinforcing properties of chemical stimuli |journal=BMC Neurosci |volume=8 |pages=103 |year=2007 |pmid=18047654 |pmc=2216080 |doi=10.1186/1471-2202-8-103 |doi-access=free }}</ref> The nucleus accumbens is made up of the [[Nucleus accumbens#Core|nucleus accumbens core]] and the [[Nucleus accumbens#Shell|nucleus accumbens shell]], which differ by neural populations. The olfactory tubercle receives input from the [[olfactory bulb]] but has not been shown to play a role in [[olfaction|processing smell]].<ref name="pmid18047654" /> In non-primate species, the [[islands of Calleja]] are included.<ref name=":2">{{Cite web|title = Ventral striatum – NeuroLex|url = http://neurolex.org/wiki/Nlx_57107#tab=Basic|website = neurolex.org|access-date = 2015-12-12}}</ref> The ventral striatum is associated with the [[limbic system]] and has been implicated as a vital part of the [[neural circuit|circuitry]] for decision making and reward-related behavior.<ref>{{Cite web|title = Ventral Striatum Definition – Medical Dictionary|url = http://medicaldictionary.net/ventral-striatum.html|website = medicaldictionary.net|access-date = 2015-11-18}}</ref><ref>{{Cite web|title = Ventral Striatum – Medical Definition|url = http://www.medilexicon.com/medicaldictionary.php?t=85559|website = www.medilexicon.com|access-date = 2015-11-18}}</ref>

===Dorsal striatum===
The dorsal striatum is composed of the [[caudate nucleus]] and the [[putamen]].
Primarily it mediates cognition and involves motor and executive function. The dorsal striatum can be further subdivided into the '''dorsomedial striatum''', and the '''dorsolateral striatum'''. Both of these areas have different roles in the acquisition of learnt behaviour and skill formation.<ref name="JoN2022">{{cite journal |last1=Turner |first1=Karly M. |last2=Svegborn |first2=Anna |last3=Langguth |first3=Mia |last4=McKenzie |first4=Colin |last5=Robbins |first5=Trevor W. |title=Opposing Roles of the Dorsolateral and Dorsomedial Striatum in the Acquisition of Skilled Action Sequencing in Rats |journal=The Journal of Neuroscience |date=9 March 2022 |volume=42 |issue=10 |pages=2039–2051 |doi=10.1523/JNEUROSCI.1907-21.2022|pmid=35086903 |pmc=8916752 }}</ref> The dorsomedial region receives projections from the frontal and the parietal cortices. The dorsolateral region receives projections from the sensorimotor cortex.<ref name="Macpherson2019">{{cite journal |last1=Macpherson |first1=Tom |last2=Hikida |first2=Takatoshi |title=Role of basal ganglia neurocircuitry in the pathology of psychiatric disorders |journal=Psychiatry and Clinical Neurosciences |date=June 2019 |volume=73 |issue=6 |pages=289–301 |doi=10.1111/pcn.12830}}</ref>

====Matrix and striosomes====
[[Neurochemistry]] studies have used [[staining|staining techniques]] on the striatum that have identified two distinct striatal compartments, the matrix, and the [[striosome]] (or patch). The matrix is seen to be rich in [[acetylcholinesterase]], while the embedded striosomes are acetylcholinesterase-poor.<ref name="Brimblecombe"/> The matrix forms the bulk of the striatum, and receives input from most areas of the cerebral cortex.<ref name="Squire">{{cite book |last1=Squire |first1=Larry |title=Fundamental neuroscience |date=2013 |publisher=Elsevier Academic Press |location=Amsterdam Heidelberg |isbn=9780123858702 |page=658 |edition=4.}}</ref> Clusters of neurons in the matrix, called matrisomes receive a similar input. Their output goes to both regions of the globus pallidus and to the substantia nigra pars reticulata.<ref name="Squire"/>

The striosomes receive input from the prefrontal cortex and give outputs to the substantia nigra pars compacta.<ref name="Squire"/> There are more striosomes present in the dorsal striatum making up 10-15% of the striatal volume, than in the ventral striatum.<ref name="Brimblecombe">{{Cite journal|pmid=27977131|year=2017|last1=Brimblecombe|first1=K. R.|title=The Striosome and Matrix Compartments of the Striatum: A Path through the Labyrinth from Neurochemistry toward Function|journal=ACS Chemical Neuroscience|volume=8|issue=2|pages=235–242|last2=Cragg|first2=S. J.|doi=10.1021/acschemneuro.6b00333|doi-access=free}}</ref>

===Cell types===
[[File:Dendritic spines.jpg|thumb|180px|[[Dendritic spine]]s on [[medium spiny neuron]] of striatum]]
Types of cells in the striatum include:
* [[Medium spiny neurons]] (MSNs), which are the principal neurons of the striatum.<ref name=YAGER2015 /> They are [[GABAergic]] and, thus, are classified as inhibitory neurons. Medium spiny projection neurons comprise 95% of the total neuronal population of the human striatum.<ref name=YAGER2015 />  Medium spiny neurons have two [[phenotype|characteristic types]]: [[D1-type]] MSNs and [[D2-type]] MSNs.<ref name=YAGER2015 /><ref name=FERRE2010 /><ref name="MSN 40% mixed-type with DRD1 and DRD2" /> A subpopulation of MSNs contain both D1-type and D2-type receptors, with approximately&nbsp;40% of striatal MSNs expressing both [[DRD1]] and [[DRD2]] [[Messenger RNA|mRNA]].<ref name=YAGER2015 /><ref name=FERRE2010 /><ref name="MSN 40% mixed-type with DRD1 and DRD2">{{cite journal |last1=Nishi |first1=Akinori |last2=Kuroiwa |first2=Mahomi |last3=Shuto |first3=Takahide |title=Mechanisms for the Modulation of Dopamine D1 Receptor Signaling in Striatal Neurons |journal=Frontiers in Neuroanatomy |date=2011 |volume=5 |pages=43 |doi=10.3389/fnana.2011.00043 |pmid=21811441 |pmc=3140648 |doi-access=free }}</ref>
* [[acetylcholine|Cholinergic]] [[interneurons]] release acetylcholine, which has a variety of important effects in the striatum. In humans, other primates, and rodents, these interneurons respond to salient environmental stimuli with stereotyped responses that are temporally aligned with the responses of dopaminergic neurons of the [[substantia nigra]].<ref>{{cite journal |last1=Goldberg |first1=J.A. |last2=Reynolds |first2=J.N.J. |title=Spontaneous firing and evoked pauses in the tonically active cholinergic interneurons of the striatum |journal=Neuroscience |date=December 2011 |volume=198 |pages=27–43 |doi=10.1016/j.neuroscience.2011.08.067 |pmid=21925242 |s2cid=21908514 }}</ref><ref>{{cite journal |last1=Morris |first1=Genela |last2=Arkadir |first2=David |last3=Nevet |first3=Alon |last4=Vaadia |first4=Eilon |last5=Bergman |first5=Hagai |title=Coincident but Distinct Messages of Midbrain Dopamine and Striatal Tonically Active Neurons |journal=Neuron |date=July 2004 |volume=43 |issue=1 |pages=133–143 |doi=10.1016/j.neuron.2004.06.012 |pmid=15233923 |doi-access=free }}</ref> The large aspiny cholinergic interneurons themselves are affected by dopamine through [[dopamine receptor D5|D5 dopamine receptors]].<ref>{{cite journal |last1=Bergson |first1=C |last2=Mrzljak |first2=L |last3=Smiley |first3=JF |last4=Pappy |first4=M |last5=Levenson |first5=R |last6=Goldman-Rakic |first6=PS |title=Regional, cellular, and subcellular variations in the distribution of D1 and D5 dopamine receptors in primate brain |journal=The Journal of Neuroscience |date=1 December 1995 |volume=15 |issue=12 |pages=7821–7836 |doi=10.1523/JNEUROSCI.15-12-07821.1995 |pmid=8613722 |pmc=6577925 }}</ref> Dopamine also directly controls communication between cholinergic interneurons.<ref>{{cite journal |last1=Raz |first1=Aeyal |title=Neuronal synchronization of tonically active neurons in the striatum of normal and parkinsonian primates |journal=Journal of Neurophysiology |date=1996 |volume=76 |issue=3 |pages=2083–2088 |doi=10.1152/jn.1996.76.3.2083 |pmid=8890317}}</ref><ref>{{cite journal |last1=Dorst |first1=Matthijs |title=Polysynaptic inhibition between striatal cholinergic interneurons shapes their network activity patterns in a dopamine-dependent manner |journal=Nature Communications |date=2020 |volume=11 |issue=1 |page=5113 |doi=10.1038/s41467-020-18882-y |pmid=33037215 |pmc=7547109 |bibcode=2020NatCo..11.5113D }}</ref>
* There are many types of GABAergic interneurons.<ref name="Ibáñez-Sandoval O. Front Neuroanat 2010"/> The best known are [[parvalbumin]] expressing interneurons, also known as [[Action potential|fast-spiking]] interneurons, which participate in powerful [[feed forward (control)|feedforward]] inhibition of principal neurons.<ref>{{cite journal |last1=Koós |first1=Tibor |last2=Tepper |first2=James M. |title=Inhibitory control of neostriatal projection neurons by GABAergic interneurons |journal=Nature Neuroscience |date=May 1999 |volume=2 |issue=5 |pages=467–472 |doi=10.1038/8138 |pmid=10321252 |s2cid=16088859 }}</ref> Also, there are GABAergic interneurons that express [[tyrosine hydroxylase]],<ref>{{cite journal |last1=Ibanez-Sandoval |first1=O. |last2=Tecuapetla |first2=F. |last3=Unal |first3=B. |last4=Shah |first4=F. |last5=Koos |first5=T. |last6=Tepper |first6=J. M. |title=Electrophysiological and Morphological Characteristics and Synaptic Connectivity of Tyrosine Hydroxylase-Expressing Neurons in Adult Mouse Striatum |journal=Journal of Neuroscience |date=19 May 2010 |volume=30 |issue=20 |pages=6999–7016 |doi=10.1523/JNEUROSCI.5996-09.2010 |pmid=20484642 |pmc=4447206 }}</ref> [[somatostatin]], [[nitric oxide synthase]] and [[Neuropeptide Y|neuropeptide-y]]. Recently, two types of neuropeptide-y expressing GABAergic interneurons have been described in detail,<ref>{{cite journal |last1=Ibanez-Sandoval |first1=O. |last2=Tecuapetla |first2=F. |last3=Unal |first3=B. |last4=Shah |first4=F. |last5=Koos |first5=T. |last6=Tepper |first6=J. M. |title=A Novel Functionally Distinct Subtype of Striatal Neuropeptide Y Interneuron |journal=Journal of Neuroscience |date=16 November 2011 |volume=31 |issue=46 |pages=16757–16769 |doi=10.1523/JNEUROSCI.2628-11.2011 |pmid=22090502 |pmc=3236391 }}</ref> one of which translates synchronous activity of cholinergic interneurons into inhibition of principal neurons.<ref>{{cite journal |last1=English |first1=Daniel F |last2=Ibanez-Sandoval |first2=Osvaldo |last3=Stark |first3=Eran |last4=Tecuapetla |first4=Fatuel |last5=Buzsáki |first5=György |last6=Deisseroth |first6=Karl |last7=Tepper |first7=James M |last8=Koos |first8=Tibor |title=GABAergic circuits mediate the reinforcement-related signals of striatal cholinergic interneurons |journal=Nature Neuroscience |date=11 December 2011 |volume=15 |issue=1 |pages=123–130 |doi=10.1038/nn.2984 |pmid=22158514 |pmc=3245803 }}</ref> These [[neuron]]s of the striatum are not distributed evenly.<ref name="Ibáñez-Sandoval O. Front Neuroanat 2010">{{cite journal |last1=Tepper |first1=James M. |last2=Tecuapetla |first2=Fatuel |last3=Koós |first3=Tibor |last4=Ibáñez-Sandoval |first4=Osvaldo |title=Heterogeneity and Diversity of Striatal GABAergic Interneurons |journal=Frontiers in Neuroanatomy |date=2010 |volume=4 |pages=150 |doi=10.3389/fnana.2010.00150 |pmid=21228905 |pmc=3016690 |doi-access=free }}</ref>

There are two regions of [[neurogenesis]] in the brain – the [[subventricular zone]] (SVZ) in the [[lateral ventricle]]s, and the [[dentate gyrus]] in the [[hippocampal formation]].  [[Neuroblast]]s that form in the lateral ventricle adjacent to the striatum, integrate in the striatum.<ref name="Ernst">{{cite journal|last1=Ernst|first1=Aurélie|last2=Alkass|first2=Kanar|last3=Bernard|first3=Samuel|last4=Salehpour|first4=Mehran|last5=Perl|first5=Shira|last6=Tisdale|first6=John|last7=Possnert|first7=Göran|last8=Druid|first8=Henrik|last9=Frisén|first9=Jonas|title=Neurogenesis in the Striatum of the Adult Human Brain|journal=Cell|date=February 2014|volume=156|issue=5|pages=1072–1083|doi=10.1016/j.cell.2014.01.044|pmid=24561062|doi-access=free}}</ref><ref>{{cite journal|last1=Inta|first1=D|last2=Lang|first2=U E|last3=Borgwardt|first3=S|last4=Meyer-Lindenberg|first4=A|last5=Gass|first5=P|title=Adult neurogenesis in the human striatum: possible implications for psychiatric disorders|journal=Molecular Psychiatry|date=16 February 2016|volume=21|issue=4|pages=446–447|doi=10.1038/mp.2016.8|pmid=26878892|doi-access=free}}</ref> This has been noted in the human striatum following an [[Stroke#Ischemic|ischemic stroke]]. Injury caused to the striatum stimulates the migration of neuroblasts from the SVZ, to the striatum, where they differentiate into adult neurons.<ref name="Kernie">{{cite journal|last1=Kernie|first1=SG|last2=Parent|first2=JM|title=Forebrain neurogenesis after focal Ischemic and traumatic brain injury.|journal=Neurobiology of Disease|date=February 2010|volume=37|issue=2|pages=267–74|pmid=19909815|doi=10.1016/j.nbd.2009.11.002|pmc=2864918}}</ref> The normal passage of SVZ neuroblasts is to the [[olfactory bulb]] but this traffic is diverted to the striatum after an ischemic stroke. However, few of the new developed neurons survive.<ref name="SVZ">{{cite journal|last1=Yamashita|first1=T|last2=Ninomiya|first2=M|last3=Hernández Acosta|first3=P|last4=García-Verdugo|first4=JM|last5=Sunabori|first5=T|last6=Sakaguchi|first6=M|last7=Adachi|first7=K|last8=Kojima|first8=T|last9=Hirota|first9=Y|last10=Kawase|first10=T|last11=Araki|first11=N|last12=Abe|first12=K|last13=Okano|first13=H|last14=Sawamoto|first14=K|title=Subventricular zone-derived neuroblasts migrate and differentiate into mature neurons in the post-stroke adult striatum.|journal=The Journal of Neuroscience|date=14 June 2006|volume=26|issue=24|pages=6627–36|pmid=16775151|doi=10.1523/jneurosci.0149-06.2006|pmc=6674034|url=http://ousar.lib.okayama-u.ac.jp/files/public/1/11746/20160527190854209485/K003322.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://ousar.lib.okayama-u.ac.jp/files/public/1/11746/20160527190854209485/K003322.pdf |archive-date=2022-10-09 |url-status=live}}</ref>

===Inputs===
[[File:Basalganglien.png|thumb|240px|Simplified diagram of frontal cortex to striatum to thalamus pathways – [[frontostriatal circuit]]]]
[[File:Basal ganglia circuits.svg|thumb|upright=1.2|Overview of the main circuits of the basal ganglia. The striatum is shown in blue. Picture shows 2 coronal slices that have been superimposed to include the involved basal ganglia structures. '''+''' and '''–''' signs at the point of the arrows indicate respectively whether the pathway is excitatory or inhibitory in effect. {{color|green|Green arrows}} refer to excitatory [[:en:Glutamic acid|glutamatergic]] pathways, {{color|red|red arrows}} refer to inhibitory [[:en:gamma-Aminobutyric acid|GABAergic]] pathways and {{color|turquoise|turquoise arrows}} refer to [[:en:dopamine|dopaminergic]] pathways that are excitatory on the [[direct pathway]] and inhibitory on the [[indirect pathway]].]]
The largest connection is from the [[cerebral cortex|cortex]], in terms of cell axons. Many parts of the [[neocortex]] [[wikt:innervate|innervate]] the dorsal striatum. The cortical [[pyramidal neurons]]  projecting to the striatum are located in layers II-VI, with the most dense projections come from layer V.<ref>{{cite journal |last1=Rosell |first1=Antonio |last2=Giménez-Amaya |first2=José Manuel |title=Anatomical re-evaluation of the corticostriatal projections to the caudate nucleus: a retrograde labeling study in the cat |journal=Neuroscience Research |date=September 1999 |volume=34 |issue=4 |pages=257–269 |doi=10.1016/S0168-0102(99)00060-7 |pmid=10576548 |s2cid=31392396 }}</ref>  They end mainly on the [[dendritic spine]]s of the spiny neurons. They are [[glutamatergic]], exciting striatal neurons.

The striatum is seen as having its own internal microcircuitry.<ref name="Stocco">{{cite journal |last1=Stocco |first1=Andrea |last2=Lebiere |first2=Christian |last3=Anderson |first3=John R. |title=Conditional Routing of Information to the Cortex: A Model of the Basal Ganglia's Role in Cognitive Coordination |journal=Psychological Review |volume=117 |issue=2 |pages=541–74 |year=2010 |pmid=20438237 |doi=10.1037/a0019077 |pmc=3064519}}</ref> The ventral striatum receives direct input from multiple regions in the [[cerebral cortex]] and limbic structures such as the [[amygdala]], [[thalamus]], and [[hippocampus]], as well as the [[entorhinal cortex]] and the [[inferior temporal gyrus]].<ref name=":1">{{Cite web|title = Ventral striatum – NeuroLex|url = http://neurolex.org/wiki/Nlx_57107#tab=Factbox|website = neurolex.org|access-date = 2015-12-12}}</ref> Its primary input is to the [[basal ganglia]] system. Additionally, the [[mesolimbic pathway]] projects from the [[ventral tegmental area]] to the [[nucleus accumbens]] of the ventral striatum.<ref>{{Cite web|title = Icahn School of Medicine {{!}} Neuroscience Department {{!}} Nestler Lab {{!}} Brain Reward Pathways|url = http://neuroscience.mssm.edu/nestler/brainRewardpathways.html|website = neuroscience.mssm.edu|access-date = 2015-12-12|archive-date = 5 June 2019|archive-url = https://web.archive.org/web/20190605073910/https://neuroscience.mssm.edu/nestler/brainRewardpathways.html|url-status = dead}}</ref>

Another well-known afferent is the [[nigrostriatal]] connection arising from the neurons of the [[substantia nigra]] pars compacta. While cortical axons synapse mainly on spine heads of spiny neurons, nigral axons synapse mainly on spine shafts.
In primates, the thalamostriatal afferent comes from the central median-parafascicular complex of the [[thalamus]] (see [[primate basal ganglia system]]). This afferent is glutamatergic. The participation of truly intralaminar neurons is much more limited.
The striatum also receives afferents from other elements of the basal ganglia such as the [[subthalamic nucleus]] (glutamatergic) or the [[external globus pallidus]] ([[GABAergic]]).

===Targets===
{{further|Medium spiny neuron}}
The primary outputs of the ventral striatum project to the [[ventral pallidum]], then the [[medial dorsal nucleus]] of the [[thalamus]], which is part of the [[frontostriatal circuit]]. Additionally, the ventral striatum projects to the [[globus pallidus]], and substantia nigra pars reticulata. Some of its other outputs include projections to the [[extended amygdala]], [[lateral hypothalamus]], and [[pedunculopontine nucleus]].<ref name=":3">{{cite journal |last1=Robbins |first1=Trevor W. |last2=Everitt |first2=Barry J. |title=Functions of dopamine in the dorsal and ventral striatum |journal=Seminars in Neuroscience |date=April 1992 |volume=4 |issue=2 |pages=119–127 |doi=10.1016/1044-5765(92)90010-Y }}</ref>

Striatal outputs from both the dorsal and ventral components are primarily composed of [[medium spiny neuron]]s (MSNs), a type of [[projection neuron]], which have two primary [[phenotype]]s: "indirect" MSNs that express [[D2-like receptor]]s and "direct" MSNs that express [[D1-like receptor]]s.<ref name=YAGER2015>{{cite journal |vauthors=Yager LM, Garcia AF, Wunsch AM, Ferguson SM | title = The ins and outs of the striatum: Role in drug addiction | journal = Neuroscience | volume = 301 | pages = 529–541 | date = August 2015 | pmid = 26116518 | doi = 10.1016/j.neuroscience.2015.06.033 | quote = [The striatum] receives dopaminergic inputs from the ventral tegmental area (VTA) and the substantia nigra (SNr) and glutamatergic inputs from several areas, including the cortex, hippocampus, amygdala, and thalamus (Swanson, 1982; Phillipson and Griffiths, 1985; Finch, 1996; Groenewegen et al., 1999; Britt et al., 2012). These glutamatergic inputs make contact on the heads of dendritic spines of the striatal GABAergic medium spiny projection neurons (MSNs) whereas dopaminergic inputs synapse onto the spine neck, allowing for an important and complex interaction between these two inputs in modulation of MSN activity&nbsp;... It should also be noted that there is a small population of neurons in the NAc that coexpress both D1 and D2 receptors, though this is largely restricted to the NAc shell (Bertran- Gonzalez et al., 2008).&nbsp;... Neurons in the NAc core and NAc shell subdivisions also differ functionally. The NAc core is involved in the processing of conditioned stimuli whereas the NAc shell is more important in the processing of unconditioned stimuli; Classically, these two striatal MSN populations are thought to have opposing effects on basal ganglia output. Activation of the dMSNs causes a net excitation of the thalamus resulting in a positive cortical feedback loop; thereby acting as a ‘go’ signal to initiate behavior. Activation of the iMSNs, however, causes a net inhibition of thalamic activity resulting in a negative cortical feedback loop and therefore serves as a ‘brake’ to inhibit behavior&nbsp;... there is also mounting evidence that iMSNs play a role in motivation and addiction (Lobo and Nestler, 2011; Grueter et al., 2013).&nbsp;... Together these data suggest that iMSNs normally act to restrain drug-taking behavior and recruitment of these neurons may in fact be protective against the development of compulsive drug use. | pmc=4523218}}</ref><ref name=FERRE2010>{{cite journal |vauthors=Ferré S, Lluís C, Justinova Z, Quiroz C, Orru M, Navarro G, Canela EI, Franco R, Goldberg SR | title = Adenosine-cannabinoid receptor interactions. Implications for striatal function | journal = Br. J. Pharmacol. | volume = 160 | issue = 3 | pages = 443–453 | date = June 2010 | pmid = 20590556 | pmc = 2931547 | doi = 10.1111/j.1476-5381.2010.00723.x | quote = Two classes of MSNs, which are homogeneously distributed in the striatum, can be differentiated by their output connectivity and their expression of dopamine and adenosine receptors and neuropeptides. In the dorsal striatum (mostly represented by the nucleus caudate-putamen), enkephalinergic MSNs connect the striatum with the external globus pallidus and express the peptide enkephalin and a high density of dopamine D2 and adenosine A2A receptors (they also express adenosine A1 receptors), while dynorphinergic MSNs connect the striatum with the substantia nigra (pars compacta and reticulata) and the entopeduncular nucleus ([[internal globus pallidus]]) and express the peptides dynorphin and substance P and dopamine D1 and adenosine A1 but not A2A receptors&nbsp;... These two different phenotypes of MSN are also present in the ventral striatum (mostly represented by the nucleus accumbens and the olfactory tubercle). However, although they are phenotypically equal to their dorsal counterparts, they have some differences in terms of connectivity. First, not only enkephalinergic but also dynorphinergic MSNs project to the ventral counterpart of the external globus pallidus, the ventral pallidum, which, in fact, has characteristics of both the external and internal globus pallidus in its afferent and efferent connectivity. In addition to the ventral pallidum, the internal globus pallidus and the substantia nigra-VTA, the ventral striatum sends projections to the extended amygdala, the lateral hypothalamus and the pedunculopontine tegmental nucleus.&nbsp;... It is also important to mention that a small percentage of MSNs have a mixed phenotype and express both D1 and D2 receptors (Surmeier et al., 1996).}}</ref>

The main nucleus of the basal ganglia is the striatum which projects directly to the globus pallidus via a pathway of [[striatopallidal fibers]].<ref>{{Cite journal|pmid=28154527|pmc=5243825|year=2016|last1=Pujol|first1=S.|title=''In vivo'' Exploration of the Connectivity between the Subthalamic Nucleus and the Globus Pallidus in the Human Brain Using Multi-Fiber Tractography|journal=Frontiers in Neuroanatomy|volume=10|pages=119|last2=Cabeen|first2=R.|last3=Sébille|first3=S. B.|last4=Yelnik|first4=J.|last5=François|first5=C.|last6=Fernandez Vidal|first6=S.|last7=Karachi|first7=C.|last8=Zhao|first8=Y.|last9=Cosgrove|first9=G. R.|last10=Jannin|first10=P.|last11=Kikinis|first11=R.|last12=Bardinet|first12=E.|doi=10.3389/fnana.2016.00119|doi-access=free}}</ref> The striato-pallidal pathway has a whitish appearance due to the myelinated fibers. This projection comprises successively the external globus pallidus ('''GPe'''), the internal globus pallidus ('''GPi'''), the [[pars compacta]] of the [[substantia nigra]] ('''SNc'''), and the [[pars reticulata]] of substantia nigra ('''SNr'''). The neurons of this projection are inhibited by GABAergic synapses from the dorsal striatum. Among these targets, the GPe does not send axons outside the system. Others send axons to the [[superior colliculus]]. Two others comprise the output to the thalamus, forming two separate channels: one through the internal segment of the globus pallidus to the ventral oralis nuclei of the thalamus and from there to the cortical [[supplementary motor area]] and another through the substantia nigra to the ventral anterior nuclei of the thalamus and from there to the [[frontal cortex]] and the occulomotor cortex.

===Blood supply===
Deep penetrating [[striate arteries]] supply blood to the striatum. These arteries include the [[recurrent artery of Heubner]] arising from the [[anterior cerebral artery]], and the [[lenticulostriate arteries]] arising from the [[middle cerebral artery]].<ref name="Purves">{{cite book |last1=Purves |first1=Dale |title=Neuroscience |date=2012 |location=Sunderland, Mass. |isbn=9780878936953 |page=739 |edition=5th}}</ref>