Editing Subthalamic nucleus (section)

==Anatomy==
[[File:Ultra-High-Field-MRI-Post-Mortem-Structural-Connectivity-of-the-Human-Subthalamic-Nucleus-Video1.ogv|thumb|Structural connectivity of the human subthalamic nucleus as visualized through [[diffusion-weighted MRI]].]]

=== Structure ===
The principal type of [[neuron]] found in the subthalamic nucleus has rather long, sparsely spiny [[dendrite]]s.<ref name=Afsharpour1985>{{cite journal | vauthors = Afsharpour S | title = Light microscopic analysis of Golgi-impregnated rat subthalamic neurons | journal = The Journal of Comparative Neurology | volume = 236 | issue = 1 | pages = 1–13 | date = June 1985 | pmid = 4056088 | doi = 10.1002/cne.902360102 | s2cid = 12482772 }}</ref><ref>{{cite journal | vauthors = Rafols JA, Fox CA | title = The neurons in the primate subthalamic nucleus: a Golgi and electron microscopic study | journal = The Journal of Comparative Neurology | volume = 168 | issue = 1 | pages = 75–111 | date = July 1976 | pmid = 819471 | doi = 10.1002/cne.901680105 | s2cid = 11962279 }}</ref> In the more centrally located neurons, the dendritic arbors have a more [[ellipsoid]]al shape.<ref>{{cite journal | vauthors = Yelnik J, Percheron G | title = Subthalamic neurons in primates: a quantitative and comparative analysis | journal = Neuroscience | volume = 4 | issue = 11 | pages = 1717–1743 | year = 1979 | pmid = 117397 | doi = 10.1016/0306-4522(79)90030-7 | s2cid = 40909863 }}</ref> The dimensions of these arbors (1200&nbsp;μm, 600&nbsp;μm, and 300&nbsp;μm) are similar across many species—including rat, cat, monkey and human—which is unusual. However, the number of neurons increases with brain size as well as the external dimensions of the nucleus. The principal neurons are [[glutamatergic]], which give them a particular functional position in the basal ganglia system. In humans there are also a small number (about 7.5%) of [[GABA]]ergic [[interneuron]]s that participate in the local circuitry; however, the dendritic arbors of subthalamic neurons shy away from the border and primarily interact with one another.<ref>{{cite journal | vauthors = Lévesque JC, Parent A | title = GABAergic interneurons in human subthalamic nucleus | journal = Movement Disorders | volume = 20 | issue = 5 | pages = 574–584 | date = May 2005 | pmid = 15645534 | doi = 10.1002/mds.20374 | s2cid = 9551517 }}</ref>

The structure of the subthalamic nucleus has not yet been fully explored and understood, but it is likely composed of several internal domains. The primate subthalamic nucleus is often divided in three internal anatomical-functional domains. However, this so-called ''tripartite model'' has been debated because it does not fully explain the complexity of the subthalamic nucleus in brain function.<ref>{{cite journal | vauthors = Alkemade A, Forstmann BU | title = Do we need to revise the tripartite subdivision hypothesis of the human subthalamic nucleus (STN)? | journal = NeuroImage | volume = 95 | pages = 326–329 | date = July 2014 | pmid = 24642281 | doi = 10.1016/j.neuroimage.2014.03.010 | s2cid = 11010595 }}</ref><ref>{{cite journal | vauthors = Lambert C, Zrinzo L, Nagy Z, Lutti A, Hariz M, Foltynie T, Draganski B, Ashburner J, Frackowiak R | title = Confirmation of functional zones within the human subthalamic nucleus: patterns of connectivity and sub-parcellation using diffusion weighted imaging | journal = NeuroImage | volume = 60 | issue = 1 | pages = 83–94 | date = March 2012 | pmid = 22173294 | pmc = 3315017 | doi = 10.1016/j.neuroimage.2011.11.082 }}</ref>

===Afferent axons===
The subthalamic nucleus receives its main input from the [[external globus pallidus]] (GPe),<ref name="pmid2350684">{{cite journal | vauthors = Canteras NS, Shammah-Lagnado SJ, Silva BA, Ricardo JA | title = Afferent connections of the subthalamic nucleus: a combined retrograde and anterograde horseradish peroxidase study in the rat | journal = Brain Research | volume = 513 | issue = 1 | pages = 43–59 | date = April 1990 | pmid = 2350684 | doi = 10.1016/0006-8993(90)91087-W | s2cid = 22996045 }}</ref> not so much through the [[ansa lenticularis]] as often said but by radiating 'comb' fibers crossing the medial pallidum first and the internal capsule (forming part of [[Ludwig Edinger|Edinger]]'s comb system, see figure), as well as the ansa subthalamica.<ref>{{cite journal | vauthors = Alho EJ, Alho AT, Horn A, Martin MD, Edlow BL, Fischl B, Nagy J, Fonoff ET, Hamani C, Heinsen H | title = The Ansa Subthalamica: A Neglected Fiber Tract | journal = Movement Disorders | volume = 35 | issue = 1 | pages = 75–80 | date = January 2020 | pmid = 31758733 | doi = 10.1002/mds.27901 }}</ref> These [[afferent nerve fiber|afferents]] are GABAergic, inhibiting neurons in the subthalamic nucleus. Excitatory, glutamatergic inputs come from the [[cerebral cortex]] (entire frontal cortex with a predominance for motor, premotor and oculomotor input to the posterolateral part of the nucleus), and from the pars [[parafascicularis]] of the [[central complex]]. The subthalamic nucleus also receives [[neuromodulator]]y inputs, notably [[dopaminergic]] axons from the [[substantia nigra]] pars compacta.<ref>{{cite journal | vauthors = Cragg SJ, Baufreton J, Xue Y, Bolam JP, Bevan MD | title = Synaptic release of dopamine in the subthalamic nucleus | journal = The European Journal of Neuroscience | volume = 20 | issue = 7 | pages = 1788–1802 | date = October 2004 | pmid = 15380000 | doi = 10.1111/j.1460-9568.2004.03629.x | s2cid = 14698708 | doi-access = free }}</ref>
It also receives inputs from the [[pedunculopontine nucleus]].

===Efferent targets===
The axons of subthalamic nucleus neurons leave the nucleus dorsally. The efferent axons are glutamatergic (excitatory). Except for the connection to the striatum (17.3% in macaques), most of the subthalamic principal neurons are multitargets and directed to the other elements of the core of the basal ganglia.<ref name="pmid418083">{{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> Some send axons to the substantia nigra medially and to the medial and lateral nuclei of the pallidum laterally (3-target, 21.3%). Some are 2-target with the lateral pallidum and the substantia nigra (2.7%) or the lateral pallidum and the medial (48%). Less are single target for the lateral pallidum. In the pallidum, subthalamic terminals end in bands parallel to the pallidal border.<ref name="pmid418083"/><ref name=Smith1990>{{cite journal | vauthors = Smith Y, Hazrati LN, Parent A | title = Efferent projections of the subthalamic nucleus in the squirrel monkey as studied by the PHA-L anterograde tracing method | journal = The Journal of Comparative Neurology | volume = 294 | issue = 2 | pages = 306–323 | date = April 1990 | pmid = 2332533 | doi = 10.1002/cne.902940213 | s2cid = 9667393 }}</ref> When all axons reaching this target are added, the main efference of the subthalamic nucleus is, in 82.7% of the cases, clearly the [[internal globus pallidus]] (GPi).

Some researchers have reported internal [[axon]] collaterals.<ref>{{cite journal | vauthors = Kita H, Chang HT, Kitai ST | title = The morphology of intracellularly labeled rat subthalamic neurons: a light microscopic analysis | journal = The Journal of Comparative Neurology | volume = 215 | issue = 3 | pages = 245–257 | date = April 1983 | pmid = 6304154 | doi = 10.1002/cne.902150302 | s2cid = 32152785 }}</ref> However, there is little functional evidence for this.