Editing Striatum (section)

===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.