Internal capsule

The internal capsule is a compact, V-shaped structure of subcortical white matter composed of densely packed myelinated ascending and descending projection fibres. It lies in the inferomedial part of each cerebral hemisphere and acts as a major two-way pathway between the cerebral cortex and the thalamus, brainstem, and spinal cord.

In horizontal section, the apex of the V points medially; the lentiform nucleus lies laterally, while the caudate nucleus and thalamus lie medially.

Superior to the lentiform nucleus, internal-capsule fibres spread out to form the corona radiata. Inferiorly, they become increasingly compact as they descend through the basal ganglia region and continue into the crura cerebri of the midbrain.

Sub-parts of internal capsule

The internal capsule is divided into five parts:

• Anterior limb – lies between the head of the caudate nucleus medially and the lentiform nucleus laterally. It contains the anterior thalamic radiation and frontopontine fibres, as well as fibres passing between the caudate and putamen.

• Genu – the medial bend where the anterior and posterior limbs meet. It contains important corticobulbar (corticonuclear) fibres, which connect motor cortical areas with cranial nerve motor nuclei and participate in movements of the face, mastication, and swallowing. Superior thalamic radiation fibres also traverse this region.

• Posterior limb – lies between the thalamus medially and lentiform nucleus laterally. Its anterior portion contains corticospinal, corticorubral, and corticopontine fibres, whereas its posterior portion contains major somatosensory thalamocortical fibres projecting toward the postcentral gyrus. The corticospinal fibres have an organised somatotopic arrangement.

• Retrolenticular segment – lies around the posterior aspect of the lentiform nucleus and contains the optic radiation (geniculocalcarine fibres) connecting the lateral geniculate nucleus with the visual cortex.

• Sublenticular segment – lies inferior to the lentiform nucleus and contains the auditory radiation, which extends from the medial geniculate body to the primary auditory cortex in the transverse temporal gyri. Some fibres of the optic radiation also traverse this region.

Clinical correlate

The internal capsule receives blood from small perforating arteries arising from the middle cerebral, anterior cerebral, anterior choroidal, and internal carotid arteries. These vessels are susceptible to occlusion, making the internal capsule an important site of lacunar infarction. The dense concentration of important pathways means that a relatively small internal-capsule lesion can cause substantial neurological deficits.