The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus. The internal capsule contains both ascending and descending axons.

It consists of axonal fibres that run between the cerebral cortex and the pyramids of the medulla.

Components

The internal capsule is V-shaped when cut both coronally (on the same plane as the face) and horizontally/transversely (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". or crus anterius.
• the part behind the genu is called the "posterior limb" or crus posterius.

There is also a retrolenticular and a sublenticular part to the internal capsule.

Fibres

• The posterior limb of the internal capsule contains corticospinal fibers and sensory fibers from the body.
• The genu contains corticobulbar fibers, which run between the cortex and the brainstem.
• The anterior limb of the internal capsule contains:
• 1) frontopontine (corticofugal) fibers project from frontal cortex to pons;
• 2) thalamocortico fibers connect the medial and anterior nuclei of the thalamus to the frontal lobes (these are severed during a prefrontal lobotomy).

• 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.

Blood supply

The anterior limb of the internal capsule is supplied by lenticulostriate arteries coming off the middle cerebral artery and by Heubner's artery, which comes from the anterior cerebral artery.

The posterior limb of the internal capsule is supplied by the thalamogeniculate artery, which is a branch of the posterior cerebral artery.

Pathology

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.

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