Internal Capsule: Connections and Functions

Overview

• Medial relations: Caudate nucleus (anteriorly) and thalamus (posteriorly)
• Lateral relation: Lenticular nucleus (globus pallidus + putamen)

Parts and Their Connections

1. Anterior Limb (Lenticulocaudate Segment)

• Boundaries: Separates the lenticular nucleus (laterally) from the head of the caudate nucleus (medially)
• Length: ~2 cm
• Fibers carried:
  • Frontopontine fibers (Arnold's bundle) - from frontal cortex to pontine nuclei
  • Anterior thalamic radiation - connecting frontal lobe and anterior/medial thalamic nuclei (mediodorsal nucleus)
  • Corticothalamic and thalamocortical fibers (frontal lobe <-> thalamus)
  • Caudate-putamen fibers
• Function: Frontal lobe relay to pons and thalamic connections involved in prefrontal/limbic activity

2. Genu ("Knee")

• Position: At the apex of the pallidal part of the lentiform nucleus, at the level of the foramen of Monro, between the anterior and posterior limbs
• Fibers carried:
  • Corticobulbar (corticonuclear) fibers - from motor cortex to cranial nerve nuclei in the brainstem
  • Corticoreticular fibers - to both sides of the brainstem reticular formation
  • Motor corticopontine fibers (possibly)
• Function: Voluntary control of head, face, and neck musculature via cranial nerves; the genu syndrome (lesion here) causes facial and lingual hemiparesis with relatively mild limb weakness

3. Posterior Limb (Lenticulothalamic Segment)

• Boundaries: Separates the lenticular nucleus (laterally) from the thalamus (medially)
• Length: ~3-4 cm
• Fibers carried (anterior to posterior):
  • Corticospinal tract (pyramidal tract) - occupies the posterior half; somatotopically organized: face (F) most anteriorly, then arm (A), trunk (T), leg (L) most posteriorly
  • Superior thalamic radiation - includes somatosensory fibers from VPL/VPM nuclei of thalamus to postcentral gyrus (parietal lobe)
  • Corticorubral fibers - cortex to red nucleus
  • Corticothalamic fibers - cortex to thalamus
  • Corticopontine fibers (parietal, temporal, occipital portions)
• Function: Voluntary motor control of limbs and trunk; relay of somatosensory information to cortex

4. Retrolenticular Segment

• Position: Behind the lenticular nucleus
• Fibers carried:
  • Visual (optic) radiations (Gratiolet's radiation / geniculo-calcarine tract) - from lateral geniculate nucleus (LGN) to primary visual cortex (calcarine cortex)
  • Corticotectal fibers (to superior colliculus)
  • Corticonigral and corticotegmental fibers
• Function: Transmission of visual information from thalamus to occipital cortex

5. Sublenticular Segment

• Position: Below the lenticular nucleus
• Fibers carried:
  • Auditory radiations (inferior thalamic peduncle) - from medial geniculate nucleus (MGN) to primary auditory cortex (Heschl's gyri, temporal lobe)
  • Temporopontine and parietopontine fibers
• Function: Relay of auditory information from thalamus to temporal cortex

Summary Table

Blood Supply

Clinical Significance

• Posterior limb lesion (e.g., hypertensive hemorrhage or lacunar infarct): Contralateral hemiplegia (face, arm, and leg - "pure motor stroke") due to corticospinal tract involvement. If sensory radiations are also affected, hemianesthesia is added.
• Genu lesion: "Capsular genu syndrome" - contralateral facial and lingual weakness with mild limb involvement, sometimes with dysarthria.
• Retrolenticular lesion: Contralateral homonymous hemianopia (optic radiations).
• Sublenticular lesion: Contralateral superior quadrantanopia or cortical deafness (auditory radiation).
• Because all motor and sensory fibers are tightly compacted, even small capsular lesions produce large deficits - a key principle distinguishing internal capsule strokes from cortical strokes.

Ascending Tracts of the Spinal Cord

General Principles

• Almost all sensory input enters the spinal cord via dorsal nerve roots
• The central end of the dorsal root splits into:
  • Lateral bundle - finely myelinated/unmyelinated fibers (pain, temperature) - enter the zone of Lissauer
  • Medial bundle - heavily myelinated fibers (touch, proprioception) - enter dorsal funiculus directly
• All ascending pathways use a 3-neuron relay: 1st order (periphery to spinal cord/medulla), 2nd order (to thalamus), 3rd order (thalamus to cortex)

I. CONSCIOUS SENSORY PATHWAYS

A. Posterior Column - Medial Lemniscal (DCML) Pathway

• Fasciculus gracilis (Goll's column): fibers from sacral, lumbar, and lower 6 thoracic levels - located medially in dorsal column
• Fasciculus cuneatus (Burdach's column): fibers from upper 6 thoracic and all cervical levels - located laterally in dorsal column
• Somatotopy in dorsal columns: sacral most medial, cervical most lateral (opposite of anterolateral system)
• The medial lemniscus rotates as it ascends: in the medulla it is vertical (leg medial), in the pons it rotates to horizontal (leg lateral), at VPL it terminates with leg lateral and arm medial

B. Anterolateral System (Spinothalamic + related tracts)

1. Lateral Spinothalamic Tract (Neospinothalamic)

2. Anterior (Ventral) Spinothalamic Tract

• Pathway: similar to lateral spinothalamic - 1st order synapses in dorsal horn → 2nd order crosses ventral commissure → ascends in anterior funiculus → VPL → somatosensory cortex
• Because light touch is also carried by dorsal columns, anterior spinothalamic lesions alone rarely cause detectable loss

3. Spinoreticular Tract (Paleospinothalamic System)

• 2nd order axons ascend bilaterally → synapse in brainstem reticular formation (medullary-pontine) → project to intralaminar thalamic nuclei (centromedian nucleus) → wide cortical projections
• This is the phylogenetically older pain pathway
• Explains why spinothalamic cordotomy (cutting lateral spinothalamic) may relieve sharp pain but not the emotional suffering of chronic pain

4. Spinomesencephalic Tract

• Projects to periaqueductal gray (PAG) and superior colliculus
• Involved in pain modulation (descending inhibitory pathways originate here), orientation responses, and reflexive responses to pain

II. UNCONSCIOUS SENSORY PATHWAYS (Spinocerebellar)

A. Dorsal Spinocerebellar Tract (DSCT)

B. Ventral Spinocerebellar Tract (VSCT)

C. Cuneocerebellar Tract

• 1st order fibers ascend in fasciculus cuneatus → synapse in external (accessory) cuneate nucleus in medulla
• 2nd order fibers enter cerebellum via inferior cerebellar peduncle ipsilaterally
• Functional equivalent of DSCT for upper limbs

D. Rostral Spinocerebellar Tract

• Travels via superior and inferior cerebellar peduncles

Summary Table

Key Clinical Correlations