Editing Internal capsule

{{Short description|White matter structure of the brain}}
{{Infobox brain
| Name            = Internal capsule
| Latin           = capsula interna
| Image           = File:Telencephalon-Horiconatal.jpg
| Caption         = Horizontal section of right cerebral hemisphere. (Capsula interna labeled at upper left.)
| Image2          = File:Gray764.png
| Caption2        = The motor tract.
| IsPartOf        =
| Components      =
| Artery          =
| Vein            =
}}
The '''internal capsule''' is a paired [[white matter]] structure, as a two-way [[nerve tract|tract]], carrying [[afferent nerve fiber|ascending]] and [[efferent nerve fiber|descending]] [[axon|fibers]], to and from the [[cerebral cortex]].  The internal capsule is situated in the [[Anatomical terms of location#Medial and lateral|inferomedial]] part of each [[cerebral hemisphere]] of the [[brain]]. It carries information past the subcortical [[basal ganglia]]. As it courses it separates the [[caudate nucleus]] and the [[thalamus]] from the [[putamen]] and the [[globus pallidus]]. It also separates the caudate nucleus and the putamen in the [[dorsal striatum]], a brain region involved in motor and reward pathways.<ref name="Emos-2022"/>

The internal capsule is V-shaped in transection forming an anterior and posterior limb, with the angle between them called the genu.

The [[corticospinal tract]] constitutes a large part of the internal capsule, carrying motor information from the [[primary motor cortex]] to the [[lower motor neurons]] in the [[spinal cord]]. Above the basal ganglia the corticospinal tract is a part of the [[corona radiata]]. Below the basal ganglia the tract is called [[cerebral crus]] (a part of the [[cerebral peduncle]]) and below the [[pons]] it is referred to as the corticospinal tract.<ref>{{Cite book |last1=Schmahmann |first1=Jeremy D. |url=https://books.google.com/books?id=QIZ668QEH0wC&q=%22Internal+capsule%22+-wikipedia |title=Fiber Pathways of the Brain |last2=Schmahmann |first2=Jeremy |last3=Pandya |first3=Deepak |date=2009-02-11 |publisher=Oxford University Press, USA |isbn=978-0-19-538826-8 |language=en}}</ref><ref>{{Cite book |last=Standring |first=Susan |url=https://books.google.com/books?id=b7FVCgAAQBAJ&dq=%22Internal+capsule%22+-wikipedia&pg=PA395-IA1 |title=Gray's Anatomy E-Book: The Anatomical Basis of Clinical Practice |date=2015-09-25 |publisher=Elsevier Health Sciences |isbn=978-0-7020-6851-5 |language=en}}</ref>

==Structure==
The internal capsule is V-shaped when cut horizontally in a [[transverse plane]], and consists of three parts: the genu, anterior limb, and posterior limb.

===Genu===
The '''genu''' is the bend, or flexure in the V of the internal capsule. It is formed by fibers from the [[corticobulbar tract]]. The fibers in this region are named the [[geniculate fibers]] that carry [[upper motor neuron]]s from the [[motor cortex]] to [[cranial nerve nuclei]] that mainly govern muscle motion of the head and face. The geniculate fibers originate in the motor cortex, and after passing downward through the base of the [[cerebral peduncle]] with the [[cerebrospinal fibers]], undergo [[decussation]] and end in the motor nuclei of the [[cranial nerve]]s of the opposite side. 

=== Anterior limb ===
The '''anterior limb''' of the internal capsule (or '''crus anterius''') is situated in front of the genu, between the head of the [[caudate nucleus]] and the [[lentiform nucleus]].<ref name="Waxman-2009">{{Cite book |last=Waxman |first=Stephen G. |title=Clinical Neuroanatomy |date= |publisher=McGraw-Hill Medical |year=2009 |isbn=978-0-07-160399-7 |edition=26th |series= |location=New York |pages=145}}</ref> It contains: 
# [[Thalamocortical radiations|Thalamocortical fibers]] passing from the [[lateral thalamic nuclei]] to the [[frontal lobe]]<ref name="Waxman-2009" />
# [[Thalamocortical radiations|Corticothalamic fibres]] passing from the  [[frontal lobe]] to the lateral thalamic nuclei<ref name="Waxman-2009" />
# Transversely oriented fibers connecting the [[caudate nuclei|caudate nucleus]] to the [[putamen]]<ref name="Waxman-2009" />
# [[Corticostriatal]] fibers connecting the cortex with the [[striatum]] 
# [[Frontopontine fibers]]<ref name="Waxman-2009" /> passing from the frontal lobe through the medial fifth of the base of the [[cerebral peduncle]] to the [[pontine nuclei]]
# Thalamic pontine fibers 

=== Posterior limb ===
The '''posterior limb''' of internal capsule (or '''occipital part''') is the portion of the internal capsule posterior to the genu.{{Citation needed|date=June 2023}} It is situated between the thalamus and the [[lentiform nucleus]].<ref name="Waxman-2009" />

The anterior half of the posterior limb contains fibers of the [[corticospinal tract]], and [[corticobulbar tract]] (in an anteroposterior somatotropic arrangement), as well as corticorubral fibres (passing from the frontal lobe to the [[red nucleus]]) that accompany the corticospinal tract.<ref name="Waxman-2009" />

The posterior third of the posterior limb contains: 
# [[Third-order neuron|Third-order sensory neurons]] passing from the posterolateral nucleus of thalamus to the [[postcentral gyrus]] (in an anteroposterior somatotropic arrangement)<ref name="Waxman-2009" />
# Fibers of the [[optic radiation]], from the lower visual centers to the cortex of the [[occipital lobe]];
# Acoustic fibers, from the [[lateral lemniscus]] to the [[temporal lobe]]
# Fibers that pass from the occipital and temporal lobes to the [[pontine nuclei]]

The '''retrolenticular part''' contains fibers from the optic system, coming from the [[lateral geniculate nucleus]] of the [[thalamus]]. More posteriorly, this becomes the [[optic radiation]]. Some fibers from the [[medial geniculate nucleus]] (which carry auditory information) also pass in the retrolenticular internal capsule, but most are in the sublenticular part.

The sublenticular part is beneath the lentiform nucleus, and contains fibers connecting with the temporal lobe. These include the [[auditory radiation]]s and [[temporopontine fibers]].
 
=== Blood supply ===
The superior parts of both the anterior and posterior limbs, and the genu of the internal capsule are supplied by the [[lenticulostriate artery|lenticulostriate arteries]], which are branches of the M1 segment of the [[middle cerebral artery]].

The inferior half of the anterior limb is supplied via the [[recurrent artery of Heubner]], which is a branch of the [[anterior cerebral artery]].

The inferior half of the posterior limb is supplied by the [[anterior choroidal artery]], which is a branch of the [[internal carotid artery]].

As in many parts of the body, some degree of variation in the blood supply exists. For example, [[Posterior cerebral artery#Central branches|thalamoperforator arteries]], which are branches of the [[basilar artery]], occasionally supply the inferior half of the posterior limb.

==Function==
The internal capsule provides passage to ascending and descending fibres running to and from the cerebral cortex.<ref name="Emos-2022">{{Cite journal |last1=Emos |first1=Marc Christopher |last2=Suheb |first2=Mahammed Khan |last3=Agarwal |first3=Sanjeev |date=2022-06-13 |title=Neuroanatomy, Internal Capsule |url=https://www.statpearls.com/ArticleLibrary/viewarticle/886 |journal=StatPearls |pmid=31194338 |language=en}}</ref> Fibers include frontopontine fibers from the frontal lobe to the pontine nuclei; [[thalamocortical radiations]]; [[corticobulbar fiber]]s from the cortex to the medulla oblongata, and [[corticospinal fiber]]s.

==Clinical significance==

The [[lenticulostriate arteries]] supply a large part of the internal capsule. These small vessels are particularly vulnerable to narrowing in the setting of chronic [[hypertension]] and can result in small, punctate [[infarction]]s or intraparenchymal haemorrhage due to vessel rupture.{{Citation needed|date=June 2023}}

=== Focal lesions ===
Due to the orderly somatotropic arrangement of elements of the posterior limb of the internal capsule, small lesions can produce selective functional deficits.<ref name="Waxman-2009" />

Lesions of the genu of the internal capsule affect fibers of the [[corticobulbar tract]].{{Citation needed|date=June 2023}}

The primary motor cortex sends its axons through the posterior limb of the internal capsule. Lesions, therefore, result in a contralateral [[hemiparesis]] or hemiplegia. While symptoms of weakness due to an isolated lesion of the posterior limb can initially be severe, recovery of motor function is sometimes possible due to spinal projections of premotor cortical regions that are contained more rostrally in the internal capsule.<ref name="Emos-2022" />

== See also ==
* [[External capsule]]
* [[Extreme capsule]]

== References ==
{{Reflist}}

== External links ==
{{Commons category|Internal capsule}}
* {{BrainMaps|internal%20capsule|internal capsule}}
* {{BrainInfo|hier|180}}
* {{cite web|url=http://www.tk.de/rochelexikon/pics/s13048.000-2.html |title=Anatomy diagram: 13048.000-2 |work=Roche Lexicon - illustrated navigator |publisher=Elsevier |archive-url=https://web.archive.org/web/20140101000000/http://www.tk.de/rochelexikon/pics/s13048.000-2.html |archive-date=2014-01-01 |url-status=dead }}

{{Basal ganglia}}
{{Authority control}}

[[Category:Rostral basal ganglia and associated structures]]