Internal capsule

Last revised by Daniel J Bell on 30 Aug 2023

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

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)

  • genu

    • lies medial to the apex of the lentiform nucleus 

    • contains corticonuclear fibers (previously called corticobulbar fibers)

  • posterior limb (posterior crus)

    • lies between the thalamus medially and the lentiform nucleus laterally

    • contains

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

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

      • the middle thalamic radiation which contains somatosensory fibers from the ventral posterior thalamic nucleus  

  • retrolentiform part

    • lies behind the lentiform nucleus 

    • contains the

  • sublentiform part

    • lies below the lentiform nucleus 

    • contains the

      • auditory radiations (from the medial geniculate nucleus)

      • temporopontine fibers

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

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

  • appears relatively hypodense to surrounding basal ganglia structures

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

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Cases and figures

  • Figure 1: human brain - lateral view
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  • Figure 2
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  • Figure 3: basal ganglia (Gray's illustration)
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  • Figure 4: internal capsule fibers (Gray's illustration)
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