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.
Genu The
genu is the bend, or flexure in the V of the internal capsule. It is formed by fibers from the
corticobulbar tract. The fibers in this region are named the
geniculate fibers that carry
upper motor neurons from the
motor cortex to
cranial 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 the
cerebral peduncle with the
cerebrospinal fibers, undergo
decussation and end in the motor nuclei of the
cranial nerves of the opposite side.
Anterior 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. It contains: •
Thalamocortical fibers passing from the
lateral thalamic nuclei to the
frontal lobe •
Corticothalamic fibres passing from the
frontal lobe to the lateral thalamic nuclei • Transversely oriented fibers connecting the
caudate nucleus to the
putamen •
Corticostriatal fibers connecting the cortex with the
striatum •
Frontopontine fibers passing from the frontal lobe through the medial fifth of the base of the
cerebral peduncle to the
pontine nuclei • Thalamic pontine fibers
Posterior limb The
posterior limb of internal capsule (or
occipital part) is the portion of the internal capsule posterior to the genu. It is situated between the thalamus and the
lentiform nucleus. 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. The posterior third of the posterior limb contains: •
Third-order sensory neurons passing from the posterolateral nucleus of thalamus to the
postcentral gyrus (in an anteroposterior somatotopic arrangement) • Fibers of the
optic radiation, from the lower visual centers to the cortex of the
occipital lobe; • Acoustic fibers, from the
lateral lemniscus to the
temporal lobe • 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. 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. 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==