The internal capsule is a white matter structure situated in the inferomedial
part of each cerebral hemisphere of the brain. It carries information past the
basal ganglia, separating 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 contains fibres going to and
coming from the cerebral cortex. 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 crus cerebri and below the pons it is
referred to as the corticospinal tract. Structure
The internal capsule is V-shaped when cut horizontally, in a transverse plane.
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 the optic radiation also known as the
geniculocalcarine tract the sublenticular portion is beneath the
lenticular nucleus and are tracts involved in the auditory pathway from
the medial geniculate nucleus to the primary auditory cortex
= Genu = The genu is the flexure of the internal
capsule. It is formed by fibers from the corticonuclear tracts. The fibers in
this region are named the geniculate fibers; they originate in the motor part
of the cerebral 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 nerves of the opposite side. It contains the
Corticobulbar tract. = Anterior limb =
The anterior limb of internal capsule contains:
fibers running from the thalamus to the frontal lobe;
fibers connecting the lentiform and caudate nuclei;
fibers connecting the cortex with the corpus striatum; and
fibers passing from the frontal lobe through the medial fifth of the base of
the cerebral peduncle to the nuclei pontis.
= Posterior limb = The posterior limb of internal capsule
is the portion of the internal capsule posterior to the genu.
The anterior two-thirds of the occipital part of the internal capsule contains
fibers of the corticospinal tract, which arise in the motor area of the cerebral
cortex and, passing downward through the middle three-fifths of the base of the
cerebral peduncle, are continued into the pyramids of the medulla oblongata.
The posterior third of the occipital part contains:
sensory fibers, largely derived from the thalamus, though some may be continued
upward from the medial lemniscus; the fibers of optic radiation, from the
lower visual centers to the cortex of the occipital lobe;
acoustic fibers, from the lateral lemniscus to the temporal lobe; and
fibers which pass from the occipital and temporal lobes to the nuclei pontis.
= 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. 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 and recurrent artery of Heubner off of the anterior
cerebral artery Genu: lenticulostriate branches of
middle cerebral artery Posterior limb: lenticulostriate
branches of middle cerebral artery and anterior choroidal artery branch of the
internal 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 the basilar artery, occasionally supply the
inferior half of the posterior limb. Ellis clinical anatomy book says
posterior cerebral artery also supply the posterior part of internal capsule
Function Working anterior to posterior:
The anterior limb of the internal capsule contains:
1) Frontopontine fibers project from frontal cortex to pons;
2) Thalamocortical fibers connect the medial and anterior nuclei of the
thalamus to the frontal lobes. The genu contains corticobulbar fibers,
which run between the cortex and the brainstem.
The posterior limb of the internal capsule contains corticospinal fibers,
sensory fibers 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 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. Clinical significance
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. Additional images
References External links
Stained brain slice images which include the "internal capsule" at the BrainMaps
project hier-180 at NeuroNames
Anatomy diagram: 13048.000-2 at Roche Lexicon - illustrated navigator,
Elsevier