||It has been suggested that Anterior limb of internal capsule be merged into this article. (Discuss) Proposed since February 2014.|
|Brain: Internal capsule|
Horizontal section of right cerebral hemisphere. (Capsula interna labeled at upper left.)
The motor tract.
The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the putamen and the globus pallidus. The internal capsule contains both ascending and descending axons.
The internal capsule is V-shaped when cut transversely (horizontally).
When cut horizontally:
- the bend in the V is called the genu
- the anterior limb or crus anterius is the part in front of the genu, between the head of the caudate nucleus and the lenticular nucleus
- the posterior limb or crus posterius is the part behind the genu, between the thalamus and lenticular nucleus
- the retrolenticular portion is caudal to the lenticular nucleus and carries optic tracts including the geniculocalcarine radiations.
- the sublenticular portion is beneath the lenticular nucleus and are tracts involved in the auditory pathway from medial geniculate nucleus to the primary auditory cortex (Brodmann Area 41)
The superior parts of both the anterior and posterior limbs and the genu of the internal capsule are supplied by the lenticulostriate arteries, which are branches of the M1 segment of the middle cerebral artery.
In summary, the blood supply of the internal capsule is
Anterior limb: lenticulostriate branches of middle cerebral artery (superior half) & recurrent artery of Heubner off of the anterior cerebral artery (inferior half)
Genu: lenticulostriate branches of middle cerebral artery
Posterior limb: lenticulostriate branches of middle cerebral artery (superior half) & anterior choroidal artery off of the internal carotid artery (inferior half)
As in many parts of the body, some degree of variation in the blood supply exists. For example, thalamoperforator arteries, which are branches of the basilar artery, occasionally supply the inferior half of the posterior limb.
Working anterior to posterior:
- The anterior limb of the internal capsule contains:
- The posterior limb of the internal capsule contains corticospinal fibers, sensory fibers (including the medial lemniscus and the anterolateral system) from the body and a few corticobulbar fibers.
Other fibers within the internal capsule
- 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 contains fibers connecting with the temporal lobe. These include the auditory radiations and temporopontine fibers.
The lenticulostriate arteries supply a substantial amount of the internal capsule. These small vessels are particularly vulnerable to narrowing in the setting of chronic hypertension and can result in small, punctate infarctions or intraparenchymal haemorrhage due to vessel rupture.
Lesions of the genu of the internal capsule affect fibers of the corticobulbar tract.
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.
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