Internal Capsule - Applied Anatomy

The internal capsule is a compact band of projection fibers (both ascending and descending) that separates the thalamus and caudate nucleus medially from the lentiform nucleus laterally. It is the primary conduit for sensory and motor information between the cerebral cortex and the brainstem/spinal cord.

ANATOMICAL DIVISIONS AND CONTENTS

The internal capsule is V-shaped in horizontal section, consisting of five main parts:

1. Anterior Limb: Situated between the head of the caudate nucleus and the lentiform nucleus.
   • Contents: Anterior thalamic radiation (connections to prefrontal cortex) and Frontopontine fibers.
2. Genu: The "knee" or bend between the anterior and posterior limbs.
   • Contents: Corticobulbar (corticonuclear) tract fibers, which control cranial nerve nuclei.
3. Posterior Limb: Situated between the thalamus and the lentiform nucleus.
   • Contents: Corticospinal tract (arranged somatotopically: neck, arm, trunk, leg), Superior thalamic radiation (sensory fibers from VPL/VPM of thalamus to postcentral gyrus).
4. Retrolentiform Part: Behind the lentiform nucleus.
   • Contents: Posterior thalamic radiation (including optic radiation/geniculocalcarine tract from lateral geniculate body).
5. Sublentiform Part: Below the lentiform nucleus.
   • Contents: Inferior thalamic radiation (including auditory radiation from medial geniculate body to Heschl’s gyrus).
     

ARTERIAL SUPPLY

• Anterior Limb: Recurrent artery of Heubner (branch of ACA) and branches of MCA.
• Genu: Direct branches from ICA or branches from MCA (Lateral Striate arteries).
• Posterior Limb: Charcot’s artery (Lenticulostriate branch of MCA) and Anterior Choroidal artery.
• Retro/Sublentiform parts: Anterior Choroidal artery.

APPLIED ANATOMY (CLINICAL CORRELATION)

1. Pure Motor Hemiplegia:

   • Site: Posterior limb or Genu.
   • Presentation: Contralateral hemiparesis involving the face, arm, and leg. Because fibers are so densely packed, even a small lacunar infarct (e.g., Lenticulostriate artery occlusion) causes significant, dense deficits.

2. Sensory Loss:

   • Site: Posterior part of the posterior limb.
   • Presentation: Contralateral hemianesthesia due to involvement of superior thalamic radiations.

3. Visual and Auditory Deficits:

   • Site: Retrolentiform and Sublentiform parts.
   • Presentation: Contralateral homonymous hemianopia (optic radiation) and diminished hearing (auditory radiation), though the latter is rarely clinically prominent due to bilateral cortical representation.

4. "Charcot’s Artery of Cerebral Hemorrhage":

   • The lenticulostriate branches of the MCA are frequently involved in hypertensive bleeds. Rupture leads to rapid onset of dense contralateral hemiplegia and potential herniation.

5. Somatotopic Organization:

   • Within the posterior limb, fibers for the head are most anterior (at the genu), followed by the upper limb, trunk, and lower limb. This explains why partial lesions can sometimes spare the leg or the face.