Internal capsule

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Brain: Internal capsule
Horizontal section of right cerebral hemisphere. (Capsula interna labeled at upper left.)
The motor tract.
Latin	capsula interna
Gray's	subject #189 836
NeuroNames	hier-180
NeuroLex ID	birnlex_1659

The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus. The internal capsule contains both ascending and descending axons.

It consists of axonal fibres that run between the cerebral cortex and the pyramids of the medulla.

Components

The internal capsule is V-shaped when cut both coronally (on the same plane as the face) and horizontally/transversely (the same plane as the brim of a top hat).

When cut horizontally:

• the bend in the V is called the "genu".

• the part in front of the genu is the "anterior limb". or crus anterius.

• the part behind the genu is called the "posterior limb" or crus posterius.

There is also a retrolenticular and a sublenticular part to the internal capsule.

Fibers

• The posterior limb of the internal capsule contains corticospinal fibers, sensory fibers from the body and a few corticobulbar fibers.

• The genu contains corticobulbar fibers, which run between the cortex and the brainstem.

• The anterior limb of the internal capsule contains:
  • 1) frontopontine (corticofugal) fibers project from frontal cortex to pons;
  • 2) thalamocortico fibers connect the medial and anterior nuclei of the thalamus to the frontal lobes (these are severed during a prefrontal lobotomy).

• 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.

Blood supply

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 off 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.

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 (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.

Pathology

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 hemorrhage 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.

Additional images

Deep dissection of brain-stem. Lateral view.	Superficial dissection of brain-stem. Ventral view.	Coronal section of brain immediately in front of pons.	Coronal section of brain through intermediate mass of third ventricle.
Section of brain showing upper surface of temporal lobe.	Coronal section through anterior cornua of lateral ventricles.	Coronal section of brain through anterior commissure.	Dissection showing the course of the cerebrospinal fibers.
Diagram of the tracts in the internal capsule.	Human brain right dissected lateral view

References

External links

• BrainMaps at UCDavis internal capsule

• NeuroNames hier-180
• Roche Lexicon - illustrated navigator, at Elsevier 13048.000-2