Internal capsule

Ascending projections from the thalamus to the cortex and descending projections from the cortex to subcortical structures (thalamus, brain stem, and spinal cord). These interconnecting axons are called the internal capsule. It incompletely separates the caudate nucleus from the putamen anteriorly, and separates the thalamus from the putamen posteriorly. In a horizontal section, the internal capsule has a > shape and has three named sections, the anterior limb, the posterior limb, and the genu.


Descending axons from layer 5 of the motor areas course in the genu (corticobulbar) and posterior limb (corticospinal) of the internal capsule with some somatotopy (FATL: Face, Arm, Trunk, Leg - ) that is continued in the basis pedunculi.

• Thalamocortical and corticofugal fibers within the internal capsule occupy a small compact area.
• Lesions in this area produces more widespread disturbances than lesions in any other region of the nervous system.
  
• Thrombosis or hemorrhage of the anterior, choroidal, striate, or capsular branches of the middle cerebral arteries are responsible for most of the lesions in the IC.
  
• Vascular lesions in the posterior limb of the IC result in contralateral hemianasthesia due to injury of thalamocortical fibers.
  
• There is also a contralateral hemiplegia due to injury of corticospinal fibers. If the genu of the internal capsule is included in the injury, corticobulbar fibers may be destroyed.
  
• Lesions in the most posterior region of the posterior limb may include the optic and auditory radiations. In such instances there may be a contralateral triad consisting of hemianesthesia, hemianopsia and hemihypacusis.

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