The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus . It consists of axonal fibres that run between the cerebral cortex and the pyramids of the medulla.

The internal capsule is V-shaped when cut both coronally (on the same plane as the face) and horizontally (the same plane as the brim of a top hat).

When cut horizontally the bend in the V is called the genu, the part in front of the genu is the anterior limb, and the part behind the genu is called the posterior limb. There is also a retrolenticular and a sublenticular part to the internal capsule.

Fibres in the internal capsule

The posterior limb of the internal capsule contains corticospinal fibres and sensory fibres from the body. The genu contains corticobulbar fibres, which run between the cortex and the brainstem.

The retrolenticular part contains fibres from the optic system, coming from the lateral geniculate nucleus of the thalamus. More posteriorly, this becomes the optic radiation.

Some fibres from the medial geniculate nucleus (which carry auditory information) also pass in the retrolenticular internal capsule, but most are in the sublenticular part.

Blood supply

Blood supply is similar to the other structures of the region. Striate arteries, which come off the middle cerebral artery , enter through the anterior perforated substance in the base of the brain.

Infarctions to the internal capsule tend to be small, punctate lesions. They can affect sensory and motor systems on the opposite side of the body, and possibly eyesight (to the contralateral visual field). Hearing should not be affected in a single capsule lesion, as this information crosses over to both sides of the brain while in the brainstem.