Internal Capsule — Anatomy & Blood Supply

The internal capsule is a compact, V-shaped band of white matter that funnels almost the entire projection traffic between the cerebral cortex and the brainstem/spinal cord. Because dense, somatotopically arranged fibre tracts are squeezed into a tiny space here, even a small lacunar infarct produces a disproportionately large, classic clinical deficit — which is exactly why it is one of the highest-yield topics in NEET PG neuroanatomy.

Definition & gross orientation

The internal capsule (IC) is the layer of myelinated projection fibres lying between the caudate nucleus + thalamus medially and the lentiform nucleus (putamen + globus pallidus) laterally. It is continuous superiorly with the corona radiata and inferiorly with the crus cerebri (cerebral peduncle) of the midbrain.

On a standard horizontal (axial) section of the cerebrum, the IC appears as a boomerang / "V" (or ">") shape pointing medially, with the apex (genu) directed towards the third ventricle. This is the single most-tested image in anatomy MCQs.

High-yield: On a horizontal section, the internal capsule is bounded by — head of caudate (anteromedial), thalamus (posteromedial), and lentiform nucleus (lateral). The genu points medially toward the interventricular foramen.

Parts of the internal capsule

The IC has five named parts. Memorise them in order from front to back:

Part	Located between	Major fibres carried
Anterior limb	Head of caudate (medial) & lentiform (lateral)	Frontopontine fibres; anterior thalamic radiation (thalamus ↔ prefrontal/cingulate)
Genu (bend)	Apex, at the level of the interventricular foramen	Corticobulbar (corticonuclear) fibres to motor cranial nerve nuclei + superior thalamic radiation (part)
Posterior limb	Thalamus (medial) & lentiform (lateral)	Corticospinal tract, corticorubral, superior thalamic radiation (sensory), some corticobulbar
Retrolentiform part	Behind the lentiform nucleus	Optic radiation (geniculocalcarine), posterior thalamic radiation, parieto-occipito-pontine
Sublentiform part	Below the lentiform nucleus	Auditory radiation (acoustic), part of optic radiation (Meyer loop), temporopontine fibres

High-yield: The optic radiation lies in the retrolentiform and sublentiform parts, NOT in the posterior limb proper. The auditory radiation lies only in the sublentiform part. Examiners love this distinction.

Fibre tracts — what travels where (the core exam fact)

The descending corticospinal and corticobulbar fibres, plus the ascending thalamocortical sensory fibres, are the workhorses tested.

Somatotopy of the corticospinal tract within the posterior limb (anterior → posterior order):

Face/head fibres (genu, corticobulbar) → arm → trunk → leg/lower limb (most posterior)

So along the posterior limb from genu backwards the order is arm → trunk → lower limb. The face is represented most anteriorly (at the genu).

High-yield: Classic teaching — corticospinal fibres occupy the anterior two-thirds of the posterior limb; the posterior one-third carries the sensory (thalamocortical) radiation plus auditory/visual radiations more posteriorly.

Fibre system	Direction	Part of IC	Function lost if damaged
Corticospinal	Descending	Posterior limb (ant. 2/3)	Contralateral hemiplegia
Corticobulbar	Descending	Genu	Contralateral lower-face & tongue weakness, dysarthria
Superior thalamic radiation (sensory)	Ascending	Posterior limb (post. 1/3)	Contralateral hemianaesthesia
Optic radiation	Posterior	Retro- & sublentiform	Contralateral homonymous hemianopia
Auditory radiation	Posterior	Sublentiform	Subtle/usually bilateral — clinically minimal
Anterior thalamic radiation	Ascending	Anterior limb	Cognitive/affective changes

Blood supply — the most clinically important sub-topic

The arterial supply is mixed and segmental, and "which artery supplies which part" is a perennial MCQ.

Region of IC	Principal arterial supply
Anterior limb	Medial striate artery of Heubner (recurrent artery of Heubner, branch of ACA) + lateral striate (lenticulostriate of MCA)
Genu	Direct branches of internal carotid artery; some from MCA lenticulostriate
Posterior limb	Lenticulostriate (striate) branches of the middle cerebral artery (MCA) — chiefly; plus anterior choroidal artery
Retrolentiform / posterior part	Anterior choroidal artery (branch of ICA) + lateral striate

High-yield: The posterior limb (and hence the corticospinal tract) is supplied mainly by the lenticulostriate branches of the MCA — these are end arteries ("arteries of cerebral haemorrhage" of Charcot). Their rupture/occlusion is the commonest cause of capsular stroke.

High-yield: The anterior choroidal artery supplies the genu and posterior limb (especially its posterior/retrolentiform part), the optic tract and parts of the optic radiation. It is notoriously prone to occlusion → classic anterior choroidal artery syndrome (contralateral hemiplegia + hemianaesthesia + homonymous hemianopia).

Mnemonic — lenticulostriate vessels: "Charcot's artery of cerebral haemorrhage = the lateral lenticulostriate branch of MCA → posterior limb → capsular bleed."

Clinical features — capsular stroke syndromes

Because motor, sensory and visual fibres are tightly packed, lesions cause contralateral deficits that may combine. The lacunar syndromes (small-vessel, ≤15 mm infarcts) are exam favourites.

Approach to localising a pure capsular lesion:

1. Deficit is contralateral to the lesion (fibres have not yet decussated for corticospinal, but motor cortex is contralateral to body).
2. Dense, equal involvement of face, arm AND leg (because fibres are compacted) → suggests capsule rather than cortex (where deficit is patchy).
3. No cortical signs (no aphasia, no neglect, no seizures, no visual field defect in pure motor type).

Key lacunar / capsular syndromes

Syndrome	Lesion site	Clinical picture
Pure motor hemiplegia (commonest lacunar syndrome)	Posterior limb of IC (or basis pontis)	Contralateral face + arm + leg weakness; NO sensory, visual or cognitive loss
Pure sensory stroke	Ventral posterolateral (VPL) thalamus (not capsule itself)	Contralateral hemisensory loss only
Sensorimotor stroke	Posterior limb + adjacent thalamus	Combined hemiplegia + hemianaesthesia
Dysarthria–clumsy hand syndrome	Genu of IC (or basis pontis)	Dysarthria, facial weakness, clumsiness/mild weakness of one hand
Ataxic hemiparesis	Posterior limb / corona radiata (or pons)	Ipsilateral-to-weakness cerebellar-type ataxia + pyramidal weakness (leg > arm)

High-yield: Pure motor hemiplegia from a posterior-limb lacune is the single commonest lacunar syndrome and a top NEET PG one-liner. There is no sensory loss and no aphasia — this distinguishes it from a large MCA cortical stroke.

High-yield: Dysarthria–clumsy hand syndrome localises to the genu of the internal capsule (corticobulbar fibres) — or to the basis pontis. Remember genu = corticobulbar = face/speech.

A large capsular haemorrhage (e.g., hypertensive bleed from a lenticulostriate vessel) classically presents with dense contralateral hemiplegia, hemianaesthesia, hemianopia and conjugate gaze deviation towards the side of the lesion ("the patient looks at the lesion"). With supratentorial mass effect, gaze deviates towards the lesion; in a destructive pontine lesion the eyes deviate away.

Diagnosis & investigation of choice

• Investigation of choice for acute stroke triage: Non-contrast CT (NCCT) head — first-line to exclude haemorrhage before thrombolysis.
• Most sensitive for acute ischaemic infarct (including small lacunes): MRI with diffusion-weighted imaging (DWI) — shows restricted diffusion within minutes, far superior to CT for tiny capsular lacunes.
• Vascular imaging: CT/MR angiography to assess MCA, ICA and lenticulostriate territory.
• Work up small-vessel disease risk factors: blood pressure, HbA1c, lipid profile, ECG (atrial fibrillation), carotid Doppler.

High-yield: DWI–MRI is the investigation of choice to detect an acute capsular lacunar infarct; NCCT is done first only to rule out bleed before considering thrombolysis.

Management / "drug of choice" pointers

• Acute ischaemic capsular infarct within window: IV alteplase (rt-PA) thrombolysis if within 4.5 hours and no contraindication (large-vessel proximal occlusion → consider mechanical thrombectomy, though pure small-vessel lacunes are usually not thrombectomy targets).
• Antiplatelet: Aspirin is the long-term secondary-prevention drug of choice for non-cardioembolic lacunar stroke (clopidogrel if intolerant; short-term dual antiplatelet for minor stroke/TIA).
• Risk-factor control: aggressive blood pressure control is the single most effective measure to prevent recurrent lacunar/small-vessel strokes; statins and glycaemic control.
• Hypertensive capsular haemorrhage: control BP, manage raised intracranial pressure, neurosurgical consult; reverse anticoagulation if present.

Complications

• Permanent contralateral spastic hemiplegia with upper-motor-neuron signs (hypertonia, hyperreflexia, extensor plantar).
• Capsular warning syndrome — stuttering, recurrent transient hemiparesis from a critically stenosed single perforator, often heralding completed infarct.
• Post-stroke spasticity, contractures, central post-stroke (thalamic-type) pain if sensory radiation involved.
• Large haemorrhage → mass effect, transtentorial herniation, coma, death.

Key differentials (where else does the lesion sit?)

• Cortical MCA stroke: patchy weakness (face-arm OR arm predominant), plus cortical signs — aphasia (dominant), neglect (non-dominant), gaze deviation, cortical sensory loss. Distinguishes from the dense, equal capsular pattern.
• Crus cerebri (midbrain) lesion → Weber syndrome: ipsilateral CN III palsy + contralateral hemiplegia (corticospinal fibres at the peduncle). IC carries the same fibres just higher up.
• Basis pontis lacune: can also produce pure motor hemiplegia, dysarthria–clumsy hand, or ataxic hemiparesis — the two main "look-alikes" for capsular lacunes.
• VPL thalamic lacune: pure sensory stroke (vs capsule = pure motor).
• Spinal cord lesion: bilateral/level-dependent signs, no facial involvement.

Recently asked / exam angle

• "Boomerang/V-shaped white matter between caudate, thalamus and lentiform on horizontal section" → identify the internal capsule (image-based question).
• Corticospinal fibres lie in the anterior two-thirds of the posterior limb — true/false and matching questions.
• Genu carries corticobulbar fibres; dysarthria–clumsy hand syndrome localises to the genu.
• Posterior limb is supplied by lenticulostriate branches of MCA; anterior choroidal artery supplies genu + posterior/retrolentiform part (anterior choroidal syndrome triad).
• Optic radiation is in the retrolentiform & sublentiform parts; auditory radiation only in the sublentiform part.
• Pure motor hemiplegia = commonest lacunar syndrome, posterior limb of IC — repeated NEET PG one-liner.
• Recurrent artery of Heubner (branch of ACA) supplies the anterior limb.
• Somatotopy order in posterior limb: arm → trunk → leg (leg most posterior).
• Charcot's "artery of cerebral haemorrhage" = lateral lenticulostriate branch of MCA.

Rapid revision

1. Internal capsule = V-shaped white matter between caudate/thalamus (medial) and lentiform (lateral); continuous with corona radiata above, crus cerebri below.
2. Five parts: anterior limb, genu, posterior limb, retrolentiform, sublentiform.
3. Genu = corticobulbar fibres (face/tongue, speech).
4. Posterior limb = corticospinal tract in its anterior two-thirds; sensory radiation in posterior one-third.
5. Posterior-limb somatotopy: arm → trunk → leg (leg most posterior).
6. Optic radiation = retrolentiform + sublentiform; auditory radiation = sublentiform only.
7. Anterior limb supplied by recurrent artery of Heubner (ACA) + lenticulostriate.
8. Posterior limb supplied by lenticulostriate branches of MCA (end arteries — capsular haemorrhage).
9. Anterior choroidal artery (from ICA) → genu + posterior/retrolentiform part → triad: hemiplegia + hemianaesthesia + hemianopia.
10. Pure motor hemiplegia (posterior limb) = commonest lacunar syndrome; no sensory/cortical signs.
11. Dysarthria–clumsy hand syndrome → genu of IC (or basis pontis).
12. Acute capsular lacune: DWI-MRI is most sensitive; NCCT first to exclude bleed; aspirin + BP control for secondary prevention.