Internal Capsule

Definition and General Description

The internal capsule is a compact, V-shaped (or arrowhead-shaped) lamina of white matter located deep within each cerebral hemisphere. It represents the largest projection fiber tract of the brain, containing both afferent and efferent nerve fibers traveling between the cerebral cortex, brainstem, and spinal cord. It continues rostrally with the corona radiata and caudally with the cerebral peduncles.

Localization in Clinical Neurology, 8e, p. 31

Relations

In a horizontal (axial) section, the internal capsule is bounded:

Side	Structure
Medially	Head of caudate nucleus (anteriorly) and thalamus (posteriorly)
Laterally	Lentiform nucleus (putamen + globus pallidus)

Mnemonic: Caudate and Thalamus are always Medial; Lentiform (globus pallidus + putamen) is always Lateral to the internal capsule.

Internal Capsule - Horizontal section showing anterior limb, genu, and posterior limb with their fiber contents and relations to adjacent structures

Parts / Segments

The internal capsule has five segments:

1. Anterior Limb (Lenticulocaudate Segment)

~2 cm long
Separates the lenticular nucleus (lateral) from the head of the caudate nucleus (medial)

Contents:

Frontopontine fibers (prefrontal cortex → pontine nuclei)
Anterior thalamic radiation (thalamocortical fibers connecting anterior thalamic nucleus to frontal lobe)
Caudate-putaminal fibers
Corticothalamic fibers (reciprocally connecting frontal lobe to thalamus)

2. Genu ("Knee")

Located at the transition between anterior and posterior limbs, at the level of the foramen of Monro
Concavity faces the lentiform nucleus

Contents:

Corticobulbar (corticonuclear) fibers - most prominent content
Corticoreticular fibers (to brainstem reticular formation, bilaterally)
Motor corticopontine fibers

Clinical note: Genu lesions produce the "capsular genu syndrome" - facial and lingual hemiparesis with mild limb weakness, because corticobulbar fibers to face and tongue predominate here.

3. Posterior Limb (Lenticulothalamic Segment)

~3-4 cm long
Separates the lenticular nucleus (lateral) from the thalamus (medial)

Contents (anterior to posterior):

Corticospinal tract (in caudal half) - somatotopically arranged:
- Most anterior: face (F)
- Then: arm (A)
- Then: trunk (T)
- Most posterior: leg (L)
Superior thalamic radiation (somatosensory fibers from thalamus to parietal cortex)
Corticorubral fibers
Corticothalamic fibers

4. Retrolenticular Segment

Located posterior to the lentiform nucleus

Contents:

Optic radiation (geniculo-calcarine tract / radiation of Gratiolet) - from lateral geniculate nucleus to primary visual cortex (calcarine cortex)
Corticotectal fibers
Corticonigral and corticotegmental fibers
Parietopontine and occipito-pontine fibers

5. Sublenticular Segment

Located inferior to the lentiform nucleus

Contents:

Auditory radiation (inferior thalamic peduncle) - from medial geniculate nucleus to auditory cortex
Temporopontine fibers
Some visual radiation fibers

Blood Supply

Segment	Artery	Source
Anterior limb	Recurrent artery of Heubner	Anterior cerebral artery (ACA)
Genu + middle/inferior posterior limb	Anterior choroidal artery	Internal carotid artery (ICA)
Superior anterior and posterior limb	Lenticulostriate arteries	Middle cerebral artery (MCA)

These are all end-arteries with poor collateral supply, making this region highly vulnerable to ischaemic and haemorrhagic stroke.

Localization in Clinical Neurology, 8e, p. 31-32

Clinical Significance

Lesions of the Internal Capsule

Because the fibers are extremely compact at this level, even a small lesion (lacunar infarct or haemorrhage) can cause deficits involving the entire contralateral body (face + arm + leg):

MRI brain (axial and coronal) showing a lesion in the posterior limb of the right internal capsule

Site of Lesion	Clinical Syndrome
Posterior limb	Contralateral pure motor hemiplegia (face, arm, leg) - "Pure Motor Hemiplegia (PMH)" - the most common lacunar syndrome
Genu	Capsular genu syndrome - facial/lingual weakness, mild limb weakness
Posterior limb (extended)	Mixed sensorimotor hemiplegia (if superior thalamic radiation also involved)
Retrolenticular	Contralateral homonymous hemianopia (optic radiation)
Sublenticular	Contralateral deafness / auditory radiation involvement
Bilateral posterior limb	Paraparesis (rare)

Internal Capsular Haemorrhage

Small hemorrhages in the genu or posterior limb cause PMH or mixed sensorimotor syndrome
The small lenticulostriate arteries are especially prone to rupture in hypertension ("Charcot's artery of cerebral haemorrhage")
Haemorrhage into the capsule with extension causes decerebrate/decorticate rigidity on the hemiplegic side
Neuroanatomy through Clinical Cases 3rd Edition; Localization in Clinical Neurology, 8e; Ganong's Review of Medical Physiology

Summary Table

Segment	Key Fibers
Anterior limb	Frontopontine; anterior thalamic radiation
Genu	Corticobulbar (face, tongue)
Posterior limb	Corticospinal (F-A-T-L); superior thalamic (somatosensory) radiation
Retrolenticular	Optic radiation
Sublenticular	Auditory radiation

The internal capsule is one of the most clinically important white matter structures in the brain. Its dense packing of motor, sensory, and other fibers means that lesions here - typically due to hypertensive lacunar infarcts or haemorrhage - produce dramatic deficits from a small anatomical insult.

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