Internal capsule

The internal capsule is a deep subcortical structure that contains a concentration of white matter projection fibers. These fibers form the corona radiata. Anatomically, this is an important area because of the high concentration of both motor and sensory projection fibers ^1,2. 

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
  • lies between the head of the caudate nucleus medially and the lentiform nucleus laterally
  • contains the anterior thalamic radiation and frontopontine fibers
• genu
  • lies medial to the apex of the lentiform nucleus 
  • contains corticonuclear fibers (previously called corticobulbar fibers)
• posterior limb (PLIC)
  • lies between the thalamus medially and the lentiform nucleus laterally
  • contains
    • corticospinal fibers lying in the anterior two-thirds of the posterior limb
    • the middle thalamic radiation which contains somatosensory fibers from the ventral posterior thalamic nucleus  
• retrolentiform part
  • lies behind the lentiform nucleus 
  • contains the geniculocalcarine or optic radiation
• sublentiform part
  • lies below the lentiform nucleus 
  • contains the auditory radiations

Blood 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

Related pathology

• lacunar infarct