The internal capsule (TA: capsula interna) is a deep subcortical structure that contains a concentration of afferent and efferent white matter projection fibres. Anatomically, this is an important area because of the high concentration of both motor and sensory projection fibres ^1,2. Afferent fibres pass from cell bodies of the thalamus to the cortex, and efferent fibres pass from cell bodies of the cortex to the cerebral peduncle of the midbrain ². Fibres from the internal capsule contribute to the corona radiata.

Gross anatomy

The internal capsule is made up of five parts. These are the anterior limb, genu, posterior limb, retrolentiform and sublentiform parts of the internal capsule ^1,2: 

• anterior limb (anterior crus)

  • lies between the head of the caudate nucleus medially and the lentiform nucleus laterally

  • contains the anterior thalamic radiation and frontopontine fibres

• genu

  • lies medial to the apex of the lentiform nucleus 

  • contains corticonuclear fibres (previously called corticobulbar fibres)

• posterior limb (posterior crus)

  • lies between the thalamus medially and the lentiform nucleus laterally

  • contains the

    • corticospinal fibres lying in the anterior two-thirds of the posterior limb

      • fibres from anterior to posterior: head, arm, hand, trunk, leg, perineum ²

    • middle (superior) thalamic radiation which contains somatosensory fibres from the ventral posterior thalamic nucleus  

• retrolentiform part

  • lies posterior to the lentiform nucleus 

  • contains the

    • geniculocalcarine or optic radiation (posterior thalamic radiation) from the lateral geniculate nucleus

    • corticopontine fibres (parietopontine and occiptopontine fibres) ²

• sublentiform part

  • lies inferior to the lentiform nucleus 

  • contains the auditory radiations (inferior thalamic radiation) from the medial geniculate nucleus)and temporopontine fibres

Arterial supply

The blood supply of the internal capsule is variable but is commonly from small perforating branches of the middle cerebral artery and anterior cerebral artery. These include the lateral lenticulostriate arteries and the recurrent artery of Heubner respectively ³. In addition, the anterior choroidal artery from the internal carotid artery supplies the posterior limb and retrolentiform part of the internal capsule ^3,4. 

Radiographic features

CT

• best appreciated on axial images at the level of the insular cortex

• appears relatively hypodense to surrounding basal ganglia structures

MRI

• in term neonates, internal capsule appears as higher T1-weighted and lower T2-weighted intensity when compared to basal ganglia and thalamus ⁶

Related pathology

• lacunar infarct