The internal capsule is a paired white matter structure, as a two-way tract, carrying ascending and descending fibers, to and from the cerebral cortex. The internal capsule is situated in the inferomedial part of each cerebral hemisphere of the brain. It carries information past the subcortical basal ganglia. As it courses it separates the caudate nucleus and the thalamus from the putamen and the globus pallidus. It also separates the caudate nucleus and the putamen in the dorsal striatum, a brain region involved in motor and reward pathways.
Structure
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 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.
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.