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.
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.[4] It contains:
The posterior limb of internal capsule (or occipital part) is the portion of the internal capsule posterior to the genu.[citation needed] It is situated between the thalamus and the lentiform nucleus.[4]
The anterior half of the posterior limb contains fibers of the corticospinal tract, and corticobulbar tract (in an anteroposterior somatotopic arrangement), as well as corticorubral fibres (passing from the frontal lobe to the red nucleus) that accompany the corticospinal tract.[4]
The posterior third of the posterior limb contains:
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 radiations 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 arteries, which are branches of the M1 segment of the middle cerebral 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 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.[1] Fibers include frontopontine fibers from the frontal lobe to the pontine nuclei; thalamocortical radiations; corticobulbar fibers from the cortex to the medulla oblongata, and corticospinal fibers.
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 infarctions or intraparenchymal haemorrhage due to vessel rupture.[citation needed]
Focal lesions
Due to the orderly somatotopic arrangement of elements of the posterior limb of the internal capsule, small lesions can produce selective functional deficits.[4]
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.[1]
