Editing Substantia nigra (section)

== Structure ==
[[File:Basal-ganglia-coronal-sections-large.png|thumb|300px|right|[[Coronal plane|Coronal]] slices of human brain showing the [[basal ganglia]], [[globus pallidus]]: external segment (GPe), [[subthalamic nucleus]] (STN), [[globus pallidus]]: internal segment (GPi), and substantia  nigra (SN, red). The right section is the deeper one, closer to the back of the head]]
[[File:Basal ganglia diagram.svg|thumb|220px|right|Diagram of the main components of the basal ganglia and their interconnections]]
[[File:Basal ganglia circuits.svg|thumb|320px|Anatomical overview of the main circuits of the basal ganglia, substantia nigra is shown in black. 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.]]

{{See also|Primate basal ganglia system}}
The substantia nigra, along with four other nuclei, is part of the [[basal ganglia]]. It is the largest [[nucleus (neuroanatomy)|nucleus]] in the midbrain, lying dorsal to the [[cerebral peduncles]]. Humans have two substantiae nigrae, one on each side of the midline.

The SN is divided into two parts: the [[pars reticulata]] (SNpr) and the [[pars compacta]] (SNpc), which lies medial to the pars reticulata. Sometimes, a third region, the pars lateralis, is mentioned, though it is usually classified as part of the pars reticulata. The (SNpr) and the internal [[globus pallidus]] (GPi) are separated by the [[internal capsule]].<ref>{{cite book |doi=10.1016/B978-0-12-802206-1.00013-1 |chapter=Organization of the Globus Pallidus |title=Handbook of Basal Ganglia Structure and Function, Second Edition |volume=24 |pages=259–276 |series=Handbook of Behavioral Neuroscience |year=2016 | vauthors = Kita H, Jaeger D |isbn=9780128022061 }}</ref>

===Pars reticulata===
{{Main|Pars reticulata}}
The pars reticulata bears a strong structural and functional resemblance to the internal part of the globus pallidus. The two are sometimes considered parts of the same structure, separated by the white matter of the internal capsule. Like those of the globus pallidus, the neurons in pars reticulata are mainly [[Gamma-aminobutyric acid|GABAergic]].<ref>{{Cite journal |last1=Correa |first1=M |last2=Mingote |first2=S |last3=Betz |first3=A |last4=Wisniecki |first4=A |last5=Salamone |first5=J.D |date=July 2003 |title=Substantia nigra pars reticulata GABA is involved in the regulation of operant lever pressing: pharmacological and microdialysis studies |url=https://linkinghub.elsevier.com/retrieve/pii/S0306452203001179 |journal=Neuroscience |volume=119 |issue=3 |pages=759–766 |doi=10.1016/s0306-4522(03)00117-9 |pmid=12809696 |issn=0306-4522}}</ref><ref name=":0">{{Cite journal |last1=Zhou |first1=F.-M. |last2=Lee |first2=C.R. |date=December 2011 |title=Intrinsic and integrative properties of substantia nigra pars reticulata neurons |journal=Neuroscience |volume=198 |pages=69–94 |doi=10.1016/j.neuroscience.2011.07.061 |issn=0306-4522 |pmc=3221915 |pmid=21839148}}</ref>

====Afferent connections====
The main input to the SNpr derives from the [[striatum]]. It comes by two routes, known as the [[Direct pathway|direct]] and [[indirect pathway]]s. The direct pathway consists of axons from medium spiny cells in the striatum that project directly to pars reticulata. The indirect pathway consists of three links: a projection from striatal medium spiny cells to the external part of the [[globus pallidus]]; a [[GABAergic]] projection from the globus pallidus to the [[subthalamic nucleus]], and a [[glutamatergic]] projection from the subthalamic nucleus to the pars reticulata.<ref name=":0" /><ref>{{cite journal | vauthors = Nauta HJ, Cole M | title = Efferent projections of the subthalamic nucleus: an autoradiographic study in monkey and cat | journal = The Journal of Comparative Neurology | volume = 180 | issue = 1 | pages = 1–16 | date = July 1978 | pmid = 418083 | doi = 10.1002/cne.901800102 | s2cid = 43046462 }}</ref>{{better source needed|date=November 2012}} Thus, striatal activity via the direct pathway exerts an inhibitory effect on neurons in the (SNpr) but an excitatory effect via the indirect pathway. The direct and indirect pathways originate from different subsets of striatal medium spiny cells: They are tightly intermingled, but express different types of dopamine receptors, as well as showing other neurochemical differences.

====Efferent connections====
Significant projections occur to the thalamus (ventral lateral and ventral anterior nuclei), [[superior colliculus]], and other caudal nuclei from the pars reticulata (the nigrothalamic pathway),<ref>{{cite journal | vauthors = Carpenter MB, Nakano K, Kim R | title = Nigrothalamic projections in the monkey demonstrated by autoradiographic technics | journal = The Journal of Comparative Neurology | volume = 165 | issue = 4 | pages = 401–15 | date = February 1976 | pmid = 57125 | doi = 10.1002/cne.901650402 | s2cid = 11790266 }}</ref> which use GABA as their neurotransmitter. In addition, these neurons form up to five collaterals that branch within both the pars compacta and pars reticulata, likely modulating dopaminergic activity in the pars compacta.<ref>{{cite journal | vauthors = Deniau JM, Kitai ST, Donoghue JP, Grofova I | title = Neuronal interactions in the substantia nigra pars reticulata through axon collaterals of the projection neurons. An electrophysiological and morphological study | journal = Experimental Brain Research | volume = 47 | issue = 1 | pages = 105–13 | year = 1982 | pmid = 6288427 | doi = 10.1007/BF00235891 | s2cid = 20289802 }}</ref>