Blood Supply & Ventricular System of the CNS – Part 2

Circle of Willis & High-Level Scheme

Internal carotid + vertebral–basilar systems form an arterial polygon → collateral flow possible when one limb is obstructed.
Pre-communicating (A1/P1) segments lie between the parent vessel and the intracircle anastomoses; obstruction here can be partly compensated by contralateral flow.

Origin: terminal branch of internal carotid.
Course
- A1 (pre-communicating): from carotid bifurcation → anterior communicating artery (ACoA).
- A2 (post-communicating) then divides into:
- Pericallosal a. (hugs corpus callosum; supplies cingulate gyrus, medial parietal lobe).
- Callosomarginal a. (follows cingulate sulcus; supplies medial frontal).
Cortical territory
- Medial aspects of frontal & parietal lobes.
- Small strip of superolateral hemisphere around superior edge ("sparing" of motor/sensory for leg that wraps over).
- Homunculus: trunk + lower limb.
Deep perforators (anterior medial group)
- Supply head of caudate, anterior putamen, anterior internal capsule, anterior thalamus.
Clinical
- Infarcts ≈ <!4\% of strokes (collateral via ACoA).
- Deficits: contralateral leg/foot weakness & sensory loss, abulia, urinary incontinence.

Largest & most common site of embolic stroke.
Course/Segments
- M1: horizontal in depth of Sylvian fissure; gives lenticulostriate (anterolateral) perforators.
- M2: insular segment.
- M3/M4: opercular & cortical branches radiating over lateral surface: frontal, parietal, temporal.
Cortical territory
- Entire lateral frontal, parietal, superior/lateral temporal, part of occipital pole.
- Primary motor & sensory cortex for face, hand, arm.
- Dominant hemisphere: Broca/Wernicke areas → aphasia.
Deep supply
- Lenticulostriate aa. → putamen, globus pallidus, posterior limb of internal capsule.
Clinical
- FAST acronym (Face, Arm, Speech, Time) maps to MCA symptoms.
- Proximal M1 occlusion → dense hemiplegia (arm > leg), gaze deviation, aphasia/neglect.

Terminal branch of basilar.
Segments
- P1: between basilar bifurcation & posterior communicating a.
- P2: around cerebral peduncle (ambient cistern) → gives calcarine & parieto-occipital branches.
- Posterior choroidal aa.
Cortical territory
- Entire occipital lobe (primary visual cortex), inferior/medial temporal lobe.
- Small lateral occipital strip shared with MCA.
Deep perforators (posterior medial & lateral groups)
- Supply thalamus (except very anterior tip), posterior internal capsule, midbrain.
Clinical
- Homonymous hemianopia with macular sparing, visual agnosias, thalamic syndrome.

ACA–MCA watershed: superior frontal/parietal convexity.
MCA–PCA watershed: lateral occipital / posterior temporal region.
Gradual hypoperfusion may spare tissue due to anastomoses; sudden drop → watershed infarcts ("man-in-the-barrel" bilateral proximal arm weakness).

Groups (named by parent vessel & position on transverse section)
- Anterior medial (ACA): head of caudate.
- Anterior lateral (MCA lenticulostriate + Ant Choroidal): putamen, GP, body of caudate, posterior limb IC.
- Posterior medial & lateral (PCA): thalamus.
Anterior Choroidal Artery
- Origin: proximal MCA (occasionally ICA).
- Course: enters inferior horn of lateral ventricle; supplies choroid plexus, tail of caudate, hippocampus, amygdala, optic tract, posterior IC.
- Small caliber → lacunar infarcts with dense contralateral hemiplegia + hemianesthesia but sparing cortical signs.

Superior Cerebellar Artery (SCA)
- Origin: top of basilar (below PCA).
- Territory: superior vermis, superior hemispheres, deep nuclei (dentate).
Anterior Inferior Cerebellar Artery (AICA)
- Origin: proximal basilar.
- Territory: flocculus, anterior inferior surface, middle cerebellar peduncle.
Posterior Inferior Cerebellar Artery (PICA)
- Origin: vertebral a.
- Territory: inferior vermis, posterolateral hemispheres, inferior cerebellar peduncle, dorsolateral medulla.
Clinical correlations
- SCA stroke → gait ataxia, dysmetria, sparing of brainstem.
- AICA stroke → "lateral pontine syndrome": ipsilateral facial weakness, deafness (labyrinthine branch), ataxia.
- PICA stroke → Wallenberg (lateral medullary) syndrome.

Medulla
- Anterior spinal a.: midline pyramids, medial lemniscus.
- Vertebral perforators: anteromedial regions (caudal medulla).
- PICA: dorsolateral medulla (nucleus ambiguus, vestibular nuclei, spinothalamic).
- Posterior spinal a.: dorsal columns.
Pons
- Basilar perforators (paramedian): corticospinal/bulbar tracts, medial lemniscus.
- Long circumferential branches
- AICA (caudal/lateral pons, facial & abducens nuclei).
- SCA (rostral/lateral pons, superior cerebellar peduncle).
Midbrain
- Basilar tip perforators (paramedian).
- PCA short & long circumferentials: tectum, tegmentum, cerebral peduncle.
- SCA contributes to inferior colliculus level.

Anterior & posterior choroidal aa. run within choroid plexus → access to deep temporal & diencephalic structures.
Deep nuclei sit immediately lateral to lateral ventricles; hence small penetrating arteries enter through ventricular walls.

ACA territory → contralateral leg motor/sensory loss ± behavioral changes.
MCA superior division (dominant) → Broca aphasia; inferior division → Wernicke aphasia.
MCA deep (lenticulostriate) → pure motor hemiparesis from IC involvement.
PCA → visual field defects; if thalamic, contralateral sensory loss + painful dysesthesia (Dejerine-Roussy).
Lacunar (small vessel) syndromes from perforator occlusion (pure sensory, ataxic hemiparesis, dysarthria-clumsy hand).
Brainstem stroke localization guided by “crossed findings”: ipsilateral cranial‐nerve + contralateral long-tract deficits, reflecting vascular territories above.

Homuncular mapping links vascular territory to clinical examination.
Circle of Willis redundancy—clinically relevant in gradual carotid stenosis vs. acute embolic occlusion.
Watershed zones illustrate interaction between systemic perfusion pressure & focal vascular disease.
Choroidal supply underscores proximity of ventricular system to limbic (hippocampus, amygdala) and motor relay (IC) structures.
Cerebellar vascular patterns parallel afferent systems: SCA ↔ cerebral cortex input, PICA/AICA ↔ spinal & vestibular inputs.

Rapid stroke recognition (FAST) & reperfusion (tPA, thrombectomy) rely on anatomical prediction of deficits.
Knowledge of low-incidence ACA strokes prevents under-diagnosis when presentation is leg‐predominant.
Brainstem/cerebellar strokes often missed on initial CT; vascular anatomy directs MRI sequencing & bedside cranial-nerve testing.