| Internal capsule | |
|---|---|
 Horizontal section of right cerebral hemisphere. (Capsula interna labeled at upper left.) | |
 The motor tract. | |
| Details | |
| Identifiers | |
| Latin | capsula interna |
| MeSH | D020772 |
| NeuroNames | 198 |
| NeuroLex ID | birnlex_1659 |
| TA98 | A14.1.09.524 |
| TA2 | 5576 |
| FMA | 61950 |
| Anatomical terms of neuroanatomy | |
Theinternal capsule is a pairedwhite matter structure, as a two-waytract, carryingascending anddescendingfibers, to and from thecerebral cortex. The internal capsule is situated in theinferomedial part of eachcerebral hemisphere of thebrain. It carries information past the subcorticalbasal ganglia. As it courses it separates thecaudate nucleus and thethalamus from theputamen and theglobus pallidus. It also separates the caudate nucleus and the putamen in thedorsal striatum, a brain region involved in motor and reward pathways.[1]
The internal capsule is V-shaped in transection forming an anterior and posterior limb, with the angle between them called the genu.
Thecorticospinal tract constitutes a large part of the internal capsule, carrying motor information from theprimary motor cortex to thelower motor neurons in thespinal cord. Above the basal ganglia the corticospinal tract is a part of thecorona radiata. Below the basal ganglia the tract is calledcerebral crus (a part of thecerebral peduncle) and below thepons it is referred to as the corticospinal tract.[2][3]
The internal capsule is V-shaped when cut horizontally in atransverse plane, and consists of three parts: the genu, anterior limb, and posterior limb.
Thegenu is the bend, or flexure in the V of the internal capsule. It is formed by fibers from thecorticobulbar tract. The fibers in this region are named thegeniculate fibers that carryupper motor neurons from themotor cortex tocranial 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 thecerebral peduncle with thecerebrospinal fibers, undergodecussation and end in the motor nuclei of thecranial nerves of the opposite side.
Theanterior limb of the internal capsule (orcrus anterius) is situated in front of the genu, between the head of thecaudate nucleus and thelentiform nucleus.[4] It contains:
Theposterior limb of internal capsule (oroccipital part) is the portion of the internal capsule posterior to the genu.[citation needed] It is situated between the thalamus and thelentiform nucleus.[4]
The anterior half of the posterior limb contains fibers of thecorticospinal tract, andcorticobulbar tract (in an anteroposterior somatotropic arrangement), as well as corticorubral fibres (passing from the frontal lobe to thered nucleus) that accompany the corticospinal tract.[4]
The posterior third of the posterior limb contains:
Theretrolenticular part contains fibers from the optic system, coming from thelateral geniculate nucleus of thethalamus. More posteriorly, this becomes theoptic radiation. Some fibers from themedial 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 theauditory radiations andtemporopontine fibers.
The superior parts of both the anterior and posterior limbs, and the genu of the internal capsule are supplied by thelenticulostriate arteries, which are branches of the M1 segment of themiddle cerebral artery.
The inferior half of the anterior limb is supplied via therecurrent artery of Heubner, which is a branch of theanterior cerebral artery.
The inferior half of the posterior limb is supplied by theanterior choroidal artery, which is a branch of theinternal carotid artery.
As in many parts of the body, some degree of variation in the blood supply exists. For example,thalamoperforator arteries, which are branches of thebasilar artery, occasionally supply the inferior half of the posterior limb.
The internal capsule provides passage to ascending and descending fibres running to and from the cerebral cortex.[1] Fibers include frontopontine fibers from the frontal lobe to the pontine nuclei;thalamocortical radiations;corticobulbar fibers from the cortex to the medulla oblongata, andcorticospinal fibers.
Thelenticulostriate arteries supply a large part of the internal capsule. These small vessels are particularly vulnerable to narrowing in the setting of chronichypertension and can result in small, punctateinfarctions or intraparenchymal haemorrhage due to vessel rupture.[citation needed]
Due to the orderly somatotopic arrangement of elements of the posterior limb of the internal capsule, small lesions can produce selective functional deficits.[4]
Lesions of the genu of the internal capsule affect fibers of thecorticobulbar tract.[citation needed]
The primary motor cortex sends its axons through the posterior limb of the internal capsule. Lesions, therefore, result in a contralateralhemiparesis 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.[1]
