Internal Capsule (Posterior Limb)

The posterior limb of the internal capsule is a compact band of projection fibers located between the thalamus medially and the lentiform nucleus laterally. It extends posteriorly from the genu of the internal capsule and contains several of the most clinically important ascending and descending pathways of the cerebral white matter.

Among its major fibers are the corticospinal tract, which carries voluntary motor commands from the cerebral cortex toward the spinal cord, and somatosensory thalamocortical fibers, which carry sensory information from the thalamus toward the primary somatosensory cortex.

Because these fibers are densely concentrated within a relatively small region, even a small lesion of the posterior limb can produce marked contralateral motor or sensory deficits.

Internal Capsule

The internal capsule is a major white matter pathway containing projection fibers that connect the cerebral cortex with subcortical structures, brainstem, and spinal cord.

Superiorly, its fibers spread into the corona radiata. Inferiorly, descending fibers continue toward the cerebral peduncle of the midbrain.

Parts of the Internal Capsule

Part	General Position	Important Fiber Systems
Anterior limb	Between caudate head and lentiform nucleus	Anterior thalamic radiations, frontopontine fibers
Genu	Bend between anterior and posterior limbs	Corticobulbar fibers
Posterior limb	Between thalamus and lentiform nucleus	Corticospinal and somatosensory thalamocortical fibers
Retrolentiform part	Posterior to lentiform nucleus	Optic and posterior thalamic radiations
Sublentiform part	Inferior to lentiform nucleus	Auditory radiations and other fibers

Location

The posterior limb lies deep within the cerebral hemisphere and extends posteriorly from the genu.

On axial sections, it appears as a band of white matter separating the thalamus from the lentiform nucleus.

Medial Boundary

The thalamus forms the principal medial boundary of the posterior limb.

This differs from the anterior limb, where the head of the caudate nucleus forms the medial relationship.

Lateral Boundary

The lentiform nucleus forms the lateral boundary. It consists of the putamen laterally and globus pallidus medially.

The posterior limb therefore separates the thalamus from the lentiform nucleus.

Anterior Continuation

Anteriorly, the posterior limb is continuous with the genu of the internal capsule.

Corticobulbar fibers are concentrated around the genu, while corticospinal fibers become prominent in the posterior limb.

Posterior Continuation

Posteriorly, the internal capsule continues into the retrolentiform part, which lies behind the lentiform nucleus and contains important posterior thalamic and visual projection fibers.

Superior Continuation

Superiorly, fibers of the posterior limb spread into the corona radiata.

Descending fibers converge from broad cortical territories into the compact capsule, while ascending fibers diverge from the capsule toward their cortical destinations.

Inferior Continuation

Inferiorly, major descending motor fibers continue into the cerebral peduncle of the midbrain.

From there, corticospinal fibers descend through the pons and medulla before entering the spinal cord.

Fiber Classification

The posterior limb contains projection fibers, which connect the cerebral cortex with deeper structures of the nervous system.

Fiber Type	Connection	Example
Projection	Cortex with subcortical structures	Internal capsule
Association	Regions within one hemisphere	Arcuate fasciculus
Commissural	Between cerebral hemispheres	Corpus callosum

Corticospinal Fibers

The corticospinal tract is one of the major descending pathways passing through the posterior limb.

Its fibers arise from motor and related cortical regions, descend through the corona radiata, pass through the posterior limb, and continue into the cerebral peduncle.

They subsequently descend through the brainstem and form the medullary pyramids before most cross in the pyramidal decussation and continue within the spinal cord.

Function of the Corticospinal Tract

The corticospinal tract is essential for voluntary motor control, particularly skilled and fractionated movements of the distal limbs.

Damage to these fibers above their major decussation produces weakness on the side of the body opposite the cerebral lesion.

Somatotopic Organization

Corticospinal fibers within the posterior limb show broad somatotopic organization.

Classically, fibers related to the upper limb are positioned more anteriorly than fibers related to the lower limb, with cranial motor fibers lying still farther anteriorly around the genu. Modern anatomical studies indicate that this arrangement is more complex and overlapping than simplified diagrams suggest.

Upper Limb Fibers

Motor fibers controlling the upper limb pass through relatively anterior portions of the corticospinal territory within the posterior limb.

Lesions in this region may therefore produce prominent contralateral arm and hand weakness.

Lower Limb Fibers

Fibers associated with the lower limb generally occupy more posterior portions of the corticospinal territory.

Damage extending posteriorly within the posterior limb may therefore produce substantial contralateral leg weakness.

Somatosensory Thalamocortical Fibers

The posterior limb also carries important thalamocortical sensory fibers.

These fibers arise primarily from the ventral posterior nuclei of the thalamus and travel toward the primary somatosensory cortex.

Body Sensory Fibers

Somatic sensory information from the body reaches the ventral posterolateral nucleus of the thalamus.

Third-order thalamocortical neurons then project through the posterior limb and corona radiata to the postcentral gyrus.

Facial Sensory Fibers

Somatic sensory information from the face is relayed primarily through the ventral posteromedial nucleus of the thalamus.

Thalamocortical projections then travel toward the facial representation of the somatosensory cortex through deep cerebral projection pathways.

Sensory Modalities

Ascending thalamocortical fibers within the posterior internal capsule carry information that has already been processed through major ascending sensory pathways.

These include information related to:

• Fine touch
• Vibration
• Conscious proprioception
• Pain
• Temperature
• Crude touch and pressure

Motor and Sensory Fibers Together

The close proximity of motor and sensory fibers explains why lesions of the posterior limb may produce combined contralateral hemiparesis and hemisensory loss.

A smaller lesion may preferentially affect one fiber population and produce a more restricted syndrome.

Other Projection Fibers

In addition to corticospinal and somatosensory pathways, the posterior limb contains other corticofugal and thalamocortical fibers.

The exact distribution of individual fiber systems varies along the superior-inferior and anterior-posterior extent of the capsule.

Posterior Limb Compared with Anterior Limb

Feature	Posterior Limb	Anterior Limb
Medial boundary	Thalamus	Head of caudate nucleus
Lateral boundary	Lentiform nucleus	Lentiform nucleus
Major descending fibers	Corticospinal	Frontopontine
Major ascending fibers	Somatosensory thalamocortical	Anterior thalamic radiations
Major functional association	Motor and sensory	Frontal-subcortical connectivity

Posterior Limb Compared with Genu

Feature	Posterior Limb	Genu
Major descending pathway	Corticospinal	Corticobulbar
Principal motor distribution	Trunk and limbs	Cranial musculature
Sensory fibers	Prominent	Less prominent
Position	Posterior to genu	Bend of capsule

Posterior Limb and Corona Radiata

The posterior limb and corona radiata are continuous parts of the same projection fiber system.

Within the corona radiata, motor and sensory fibers are relatively dispersed. They become much more compact as they converge toward the internal capsule.

This concentration helps explain why a small capsular lesion can produce a dense neurological deficit.

Posterior Limb and Cerebral Peduncle

Descending fibers leave the internal capsule inferiorly and enter the crus cerebri of the cerebral peduncle.

Corticospinal and corticobulbar pathways remain organized as they descend through the midbrain toward the pons and medulla.

Blood Supply

The posterior limb receives blood from several deep penetrating arterial systems.

Important contributors include lenticulostriate branches of the middle cerebral artery and the anterior choroidal artery. Their territories can vary and overlap.

Lenticulostriate Arteries

The lenticulostriate arteries arise from the middle cerebral artery and penetrate the anterior perforated substance to supply portions of the basal ganglia and internal capsule.

Occlusion of these small vessels can produce lacunar infarction involving the posterior limb.

Anterior Choroidal Artery

The anterior choroidal artery usually arises from the internal carotid artery and supplies important structures in the deep cerebral region.

Its territory commonly includes a substantial portion of the posterior limb of the internal capsule, although vascular anatomy varies among individuals.

MRI Appearance

The posterior limb is readily identified on axial MRI as white matter lying between the thalamus medially and lentiform nucleus laterally.

Diffusion-weighted imaging is particularly important for detecting acute ischemic lesions affecting this region.

Diffusion MRI and Tractography

Diffusion MRI and tractography can demonstrate the orientation of corticospinal and other projection fibers passing through the posterior limb.

These techniques can help estimate the relationship between lesions and major motor pathways, particularly during neurosurgical planning and evaluation of white matter injury.

Clinical Significance

Posterior Limb Infarction

Infarction of the posterior limb can produce major contralateral motor or sensory deficits because important pathways are concentrated within a small region.

The exact syndrome depends on the location and extent of the infarct.

Pure Motor Stroke

A small infarct involving corticospinal fibers in the posterior limb can produce a pure motor stroke.

The patient may develop weakness of the contralateral face, arm, and leg if the lesion also involves corticobulbar fibers near the genu, without the cortical signs expected from a large cortical infarction.

Contralateral Hemiparesis

Damage to corticospinal fibers above the pyramidal decussation produces contralateral weakness.

After the acute phase, upper motor neuron findings may include increased tone, hyperreflexia, reduced fine motor control, and an extensor plantar response.

Pure Sensory Stroke

Lesions affecting sensory projection fibers can contribute to contralateral sensory loss, although classic pure sensory lacunar syndromes are particularly associated with lesions of the thalamus.

Capsular sensory pathway involvement should therefore be interpreted according to imaging and the complete neurological examination.

Sensorimotor Stroke

A lesion affecting both corticospinal and somatosensory thalamocortical fibers can produce a sensorimotor syndrome with contralateral weakness and sensory impairment.

Lacunar Infarction

The posterior limb is an important location for lacunar infarcts caused by disease of small penetrating arteries.

Despite their small size, these infarcts can cause substantial deficits because the internal capsule contains densely packed motor and sensory pathways.

Capsular Hemorrhage

Deep cerebral hemorrhage involving the basal ganglia can extend into the posterior limb of the internal capsule.

Interruption or compression of corticospinal fibers may produce severe contralateral hemiparesis or hemiplegia.

Demyelinating Disease

Demyelinating lesions involving the posterior limb can interrupt corticospinal or sensory projection pathways.

Symptoms depend on lesion size, location, and additional involvement elsewhere in the central nervous system.

Brain Tumors

Tumors in the deep cerebral hemisphere may infiltrate or compress the posterior limb.

Preservation of corticospinal fibers is an important consideration during neurosurgical planning because injury can result in major permanent motor deficits.

Traumatic Injury

Traumatic lesions can damage deep white matter pathways, including fibers of the internal capsule. Motor deficits may occur when corticospinal fibers are disrupted.

Upper Motor Neuron Signs

Because the corticospinal tract is an upper motor neuron pathway, lesions of the posterior limb may produce characteristic upper motor neuron findings below the level of the lesion.

Finding	Typical Effect
Strength	Reduced contralaterally
Muscle tone	May become increased
Deep tendon reflexes	May become brisk
Plantar response	May become extensor
Fine movements	Impaired

Clinical Localization

A dense contralateral motor deficit involving multiple body regions without prominent aphasia, neglect, visual field loss, or other cortical signs can suggest a lesion of compact subcortical motor pathways such as the internal capsule.

When sensory loss accompanies weakness, involvement of neighboring sensory projection fibers within the posterior limb or nearby thalamic structures should also be considered.

Key Anatomical Features

Feature	Key Point
Structure	Posterior portion of internal capsule
Fiber classification	Projection fibers
Medial boundary	Thalamus
Lateral boundary	Lentiform nucleus
Anterior continuation	Genu
Posterior continuation	Retrolentiform part
Superior continuation	Corona radiata
Major descending fibers	Corticospinal tract
Major ascending fibers	Somatosensory thalamocortical fibers
Important arterial supply	Lenticulostriate and anterior choroidal arteries

Anatomical Importance

The posterior limb of the internal capsule is one of the most clinically important regions of cerebral white matter. Its position between the thalamus and lentiform nucleus places it along the compact route used by major ascending and descending projection pathways.

The corticospinal tract carries voluntary motor commands through the posterior limb, while thalamocortical fibers carry somatic sensory information toward the cerebral cortex. These pathways become tightly concentrated within the internal capsule after being more widely distributed in the corona radiata.

This compact organization explains why relatively small lesions can produce extensive contralateral weakness, sensory impairment, or combined sensorimotor deficits. Knowledge of the posterior limb is therefore essential for understanding motor and sensory pathways and for localizing deep cerebral lesions.