Internal Capsule Simplified

Internal Capsule is a “boomerang” shaped (on horizontal section) and “funnel” shaped, i.e. tapering from superior to inferior (on sagittal section) white matter structure sandwiched between:

  • Medially: Head of Caudate nucleus and Thalamus
  • Laterally: Lenticular nucleus (Globus pallidus and Putamen)

internal capsule

Parts of Internal Capsule

1. Anterior limb:

  • Carries fibers to and from the prefrontal cortex and cingulate gyrus.
  • Fronto-pontine fibers descend through it.

2. Posterior limb: Carries fibers to and from the parietal lobe

  • Thorugh it, corticospinal tract descend from cerebral cortex to the anterior horn of spinal cord.
  • The somatotropy from anterior to posterior is Arm, Trunk and Leg.

3. Intervening genu (knee):

  • Through it, corticonuclear fibers descend from cerebral cortex to the motor nuclei of cranial nerves, i.e. to the muscles of head and neck.

4. Retrolenticular part (behind lenticular nucleus):

  • Carries visual fibers to and from the visual cortex of the occipital lobe.

5. Sublenticular part (below lenticular nucleus):

  • Carries auditory system fibers to and from the auditory cortex of  the temporal lobe.

Thalamo-cortical fibers:

  • Run from anterior to posterior of internal capsule.
  • Anterior limb: Carries fibers from anterior and dorsomedial thalamus to prefrontal cortex and cingulate gyrus.
  • Posterior limb: Carries fibers from ventral (sensory) thalamus to somatosensory cortex.

Blood Supply of Internal Capsule

Superior part: Lateral striate (lenticulostriate) branch of MCA (Site of Charcot-Bouchard Aneurysm)

  • Lenticulostriate branches also supply the basal ganglia.

Inferior part:

  • Anterior to posterior limb: Medial striate (recurrent branch of Heubner) of ACA
  • Posterior limb: Anterior choroidal artery from ICA

Lesions of Internal Capsule

  • Upper Motor Neuron Lesions (UMNL) with contralateral hemiparesis (lesion is above the crossing of corticospinal tract which occurs in medulla) and contralateral lower facial palsy.
    • Arms and legs are equally affected.
  • Sensory loss is contralateral as the fibers cross below the internal capsule.

Charcot Bouchard Microaneurysm of Lenticulostriate branches of Middle Cerebral Artery:

  • “Lacunar” syndromes of:
    • Pure motor hemiparesis (Hemiparesis): Corticospinal tract invovlement in posterior limb of internal capsule
    • Mixed sensorimotor stroke (Hemiparesis + Hemisensory loss): Additional involvement of spinothalamic tract in posterior limb of internal capsule

Thrombosis of Reccurent branch (Heubner’s) of Anterior Cerebral Artery:

  • Affects the genu
  • Paralysis of contralateral lower face, tongue and upper limb (UMNL).
  • If the lesion is on left: also motor dysphasia.

Obstruction of Anterior choroidal artery:

  • Affects the posterior limb of internal capsule.
  • Depends upon the severity of infarction: Produces syndrome involving one of the many features listed below.
    • May be symptomless due to collateral circulation.
    • Contralateral hemiparesis – UMNL (Corticospinal tract)
    • Contralateral homonymous hemianopia – Involvement of retrolenticular part which carries visual fibers
    • Contralateral hemianesthesia – Involvement of thalamic radiation

Pure sensory stroke:

  • It is a rare form of lacunar stroke.
  • Can occur with exclusive involvement of thalamic radiations in internal capsule or ventral thalamus.

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