Internal Capsule

Overview

The internal capsule is a compact lamina of white matter representing the largest projection fiber tract in the brain. It contains both afferent and efferent nerve fibers passing to and from the brainstem and cerebral hemispheres. Superiorly it continues as the corona radiata; inferiorly it continues as the cerebral peduncles (basis pedunculi) of the midbrain.

Shape and Location

In a horizontal section, the internal capsule appears V-shaped (like two letter Vs with their points facing inward), with its convexity directed inward.

Medial neighbors: Caudate nucleus + Thalamus Lateral neighbor: Lenticular nucleus (Putamen + Globus pallidus)

A useful memory rule: The caudate and thalamus are ALWAYS medial to the internal capsule; the lentiform nucleus is ALWAYS lateral.

Parts (5 Segments)

Internal Capsule Fiber Diagram - 3D representation showing anterior limb, genu, posterior limb, with labeled fiber tracts

Segment	Location	Fibers Contained
Anterior limb	Between caudate head (medial) and lentiform nucleus (lateral)	Frontopontine fibers; anterior thalamic radiation (thalamocortical/corticothalamic fibers connecting frontal lobe to thalamus); caudate-putaminal fibers
Genu	At the "knee" (level of foramen of Monro), between caudate and thalamus	Corticobulbar tract (motor fibers to cranial nerve nuclei in brainstem); motor corticopontine fibers; corticoreticular pathway
Posterior limb	Between lentiform nucleus (lateral) and thalamus (medial)	Corticospinal tract (somatotopically organized: face most anterior, then arm, trunk, leg most posterior); superior thalamic radiation (somatosensory); corticorubral, corticothalamic fibers
Retrolenticular segment	Behind the lentiform nucleus	Optic radiations (geniculo-calcarine tract / radiation of Gratiolet); corticotectal, corticonigral fibers
Sublenticular segment	Below the lentiform nucleus	Auditory radiation; temporopontine fibers

Somatotopic Organization of Corticospinal Fibers in the Posterior Limb

Moving from anterior to posterior: Face (F) → Arm (A) → Trunk (T) → Leg (L)

Because these fibers are so compact, even a small lesion in the posterior limb can cause complete contralateral hemiplegia (face + arm + leg).

Blood Supply

Segment	Arterial Supply
Anterior limb	Recurrent artery of Heubner (branch of anterior cerebral artery)
Genu + middle/inferior posterior limb	Anterior choroidal artery (branch of ICA)
Superior anterior + posterior limb	Lenticulostriate arteries (branches of middle cerebral artery)

These are all small perforating arteries - particularly prone to hypertensive damage and lacunar infarction.

Clinical Syndromes

Posterior Limb Lesion (most common capsular stroke)

Contralateral pure motor hemiplegia (face, arm, leg equally affected) - classic "capsular" pattern
Because fibers are so compact, a tiny lacunar infarct causes complete hemiplegia

Genu Lesion - "Capsular Genu Syndrome"

Facial and lingual hemiparesis (corticobulbar involvement)
Dysarthria
Mild limb involvement

Lateral and Posterior Capsule (e.g., anterior choroidal artery territory)

Contralateral hemiparesis
Contralateral hemisensory loss (diminished pain and temperature)
Dysarthria
Homonymous hemianopia (usually with a tongue of sparing along the horizontal meridian, due to optic radiation involvement)
Memory impairment
With right-sided lesions: visuoperceptual abnormalities

Internal Capsular Hemorrhage

Small hemorrhages in the genu or posterior limb cause pure motor hemiplegia (PMH) or a mixed sensorimotor syndrome
Bilateral posterior limb hemorrhages can cause paraparesis (rare)
Hypertensive hemorrhage classically ruptures the lenticulostriate arteries in this region

Decoricate Rigidity

Seen on the hemiplegic side after hemorrhages or thromboses in the internal capsule; the small arteries here are especially prone to rupture or thrombotic occlusion in hypertension

Key Relationships Summary

Lateral ventricle
      |
Caudate nucleus  ←  ANTERIOR LIMB  →  Lentiform nucleus
      |                                      |
    GENU (corticobulbar)
      |
  Thalamus  ←  POSTERIOR LIMB  →  Lentiform nucleus
      |
Cerebral peduncle (below)

Sources:

Neuroanatomy through Clinical Cases, 3rd Edition, pp. 257-258, 590+
Localization in Clinical Neurology, 8e, pp. 31-32
Bradley and Daroff's Neurology in Clinical Practice, p. 1395

cortical and sup cortical area

Finding Sources

Your credits are exhausted. You can purchase a top-up or subscribe to a plan to continue using Orris. If you're already subscribed, your daily credits will reset tomorrow.

Create a cortical and subcortical areas reference guide

Your credits are exhausted. You can purchase a top-up or subscribe to a plan to continue using Orris. If you're already subscribed, your daily credits will reset tomorrow.

This is a shared conversation. Sign in to Orris to start your own chat.