What is the anatomy of the internal capsule?

Anatomy of the Internal Capsule

The internal capsule is a critical white matter pathway in the brain that carries major motor and sensory fibers connecting the cerebral cortex with the brainstem and spinal cord, serving as an essential conduit for neural transmission between these structures.

Anatomical Location and Boundaries

The internal capsule is located deep within the cerebral hemisphere with the following boundaries:

• Medially: Bounded by the caudate nucleus and thalamus
• Laterally: Bounded by the lentiform nucleus (putamen and globus pallidus)
• Superiorly: Continues as the corona radiata
• Inferiorly: Continues into the midbrain as the cerebral peduncle

Parts of the Internal Capsule

The internal capsule is divided into five distinct parts:

1. Anterior Limb

   • Located between the head of caudate nucleus and the lentiform nucleus
   • Contains:
     • Frontopontine fibers (from frontal lobe to pons)
     • Thalamocortical fibers (connecting thalamus to frontal cortex)

2. Genu (Knee)

   • The bend in the internal capsule
   • Contains:
     • Corticobulbar fibers (controlling cranial nerve motor nuclei)
     • Majority of motor corticopontine fibers 1
     • Damage to this area can cause the "capsular genu syndrome" with contralateral facial and lingual hemiparesis with dysarthria 1

3. Posterior Limb

   • Located between the thalamus and lentiform nucleus
   • Contains:
     • Corticospinal fibers (upper motor neurons to spinal cord)
     • Thalamocortical sensory fibers
     • Organized somatotopically with face fibers anteriorly, arm fibers in the middle, and leg fibers posteriorly

4. Retrolentiform Part

   • Located behind the lentiform nucleus
   • Contains:
     • Optic radiations (from lateral geniculate body to visual cortex)
     • Corticotectal fibers (to superior colliculus)

5. Sublentiform Part

   • Located below the lentiform nucleus
   • Contains:
     • Auditory radiations (from medial geniculate body to auditory cortex)
     • Temporopontine fibers

Vascular Supply

The internal capsule receives blood supply from several perforating arteries 2:

• Anterior Limb: Primarily supplied by medial lenticulostriate arteries (MLA) from the anterior cerebral artery
• Genu and Posterior Limb: Primarily supplied by lateral lenticulostriate arteries (LLA) from the middle cerebral artery and branches from the anterior choroidal artery
• Additional Supply: Perforators from the internal carotid artery also contribute

Key vascular anatomy measurements 2:

• Distance from ICA bifurcation to first LLA on M1: 9.55 ± 2.32 mm
• Distance from ICA bifurcation to first MLA on A1: 5.35 ± 1.84 mm
• MLA branching from A1 and proximal A2: 5-9 branches
• LLA originating from the MCA: 7-12 branches

Clinical Significance

1. Stroke Impact:

   • Lesions in the posterior limb of internal capsule or thalamus tend to have poorer outcomes compared to other locations 3
   • Capsular genu infarcts can produce distinctive faciolingual syndromes with contralateral facial weakness, lingual weakness, and dysarthria 1

2. Imaging Characteristics:

   • The internal capsule is highly visible on conventional MRI 4
   • On CT, the anterior limb has slightly higher absorption values (mean 32 Hounsfield units) compared to the posterior limb (mean 28 Hounsfield units) 5
   • Diffusion-weighted and diffusion tensor imaging allow sophisticated assessment of internal capsule maturation and connectivity 4

3. Developmental Importance:

   • The internal capsule is myelinating rapidly at term in neonates 4
   • Its normal appearance on conventional MRI at term age is usually associated with relatively normal motor outcome 4
   • Abnormal appearance on MRI is a good predictor of abnormal motor outcomes in various perinatal conditions 4

4. Surgical Considerations:

   • Knowledge of the internal capsule's microsurgical anatomy is essential for neurosurgeons to avoid damage during procedures 6
   • Understanding the vascular supply helps prevent potential risks and complications during surgical interventions 2

Functional Organization

The internal capsule demonstrates somatotopic organization, particularly in the posterior limb:

• Face fibers are located anteriorly
• Arm fibers are in the middle
• Leg fibers are positioned posteriorly

This organization explains why specific deficits occur based on the location of lesions within the internal capsule.