Cerebral Artery Anatomy and Territories: The Map Behind Every Stroke Syndrome
Every stroke syndrome is geography. One working map of the territories, the perforators, the borderzones, and the variants that make the exam lie to you — fetal PCA above all.
Every stroke syndrome is geography — learn the map once and the exam starts reading like coordinates.
- →Two systems meet at the circle of Willis: carotid (anterior) feeding ACA and MCA, and vertebrobasilar (posterior) feeding brainstem, cerebellum, and PCA. The circle is a pressure-equalizing roundabout — and it is complete in only a minority of people.
- →Cortical territories carry signatures: MCA = face-and-arm weakness with aphasia or neglect; ACA = leg-predominant weakness with abulia; PCA = hemianopia and memory, not weakness.
- →The perforators — lenticulostriates, thalamoperforators, basilar paramedians — are end-arteries into the deep structures: their occlusion makes lacunes, their rupture makes deep hemorrhages. Same streets, both disasters.
- →Borderzones between territories fail when pressure falls or a carotid chokes: cortical watershed strips and the internal borderzone's "string of pearls" — an infarct pattern that says hemodynamics, and demands a look at the proximal vessel.
- →Variants rewrite the rules: a fetal PCA makes an occipital infarct carotid disease; an artery of Percheron makes one occlusion a bilateral thalamic catastrophe; a hypoplastic vertebral or A1 changes what one occlusion can take.
Localization is often taught as a memory feat, but it is really cartography. The brain's arterial map has a handful of countries, predictable border disputes, and a short list of surveying errors — and the exam findings clinicians memorize as "syndromes" are just what each country looks like when its power fails.1
The plumbing, in one paragraph
Four vessels climb the neck: two internal carotids (the anterior circulation, roughly two-thirds of flow) and two vertebrals, which merge into the basilar (the posterior circulation). They interconnect at the circle of Willis — anterior communicating artery bridging the ACAs, posterior communicating arteries bridging carotid to PCA. The circle's job is collateral insurance; its punchline is that a textbook-complete circle is the exception, not the rule, which is why the same carotid occlusion is a TIA in one patient and a hemispheric catastrophe in another. (The other insurance policy — leptomeningeal collaterals between cortical territories — is the one that sets infarct pace, as covered in core and penumbra.)
The territories and their signatures
| Artery | Feeds | Signature when it fails |
|---|---|---|
| MCA | Lateral hemisphere convexity; deep territory via lenticulostriates | Contralateral face-and-arm-predominant weakness and sensory loss; aphasia (dominant) or neglect (nondominant); gaze preference toward the lesion. Superior vs inferior division splits the motor/Broca picture from Wernicke/field findings. |
| ACA | Medial frontal and parietal lobes | Contralateral leg-predominant weakness, abulia, urinary incontinence; bilateral ACA (azygos or ACom-dependent) infarcts produce profound abulia. |
| PCA | Occipital lobe, inferior temporal, thalamus (via perforators), midbrain | Contralateral homonymous hemianopia, memory and behavioral change, alexia without agraphia (dominant + splenium) — the syndromes of the PCA chapter. Weakness is the exception. |
| Vertebrobasilar | Brainstem (paramedian + circumferential), cerebellum (PICA/AICA/SCA) | The crossed-signs world of the brainstem syndromes chapter; cerebellar infarcts with vertigo and ataxia that masquerade as benign dizziness. |
Radiology reads these countries on axial slices; the ASPECTS regions are simply the MCA country subdivided into ten provinces for scoring early ischemic change.
The perforators: small streets, big politics
Off the great vessels branch families of small end-arteries with no collateral backup: the lenticulostriates (MCA stem → basal ganglia and internal capsule), the thalamoperforators and thalamogeniculates (PCA/PCom → thalamus), Heubner's artery (ACA → caudate), and the basilar paramedians (→ pons).12 This is the vasculature of small vessel disease: occlude a perforator and you get a lacune — pure motor hemiparesis from the capsule, pure sensory stroke from the thalamus, and their siblings. Rupture the same vessel under decades of hypertension and you get the classic deep hemorrhage map: putamen, thalamus, pons, cerebellum. One street grid, both catastrophes — and the same grid CADASIL degrades early, as its chapter describes.
Borderzones: where the map runs out of pressure
Between territories lie the borderzones — last-field-of-irrigation strips that fail first when perfusion pressure drops. The cortical (external) watersheds run ACA/MCA (high frontal parasagittal) and MCA/PCA (parieto-occipital); bilateral cortical watershed infarcts after an arrest or profound hypotension produce the classic syndromes of the "man in a barrel." The internal borderzone — between the deep perforators and the pial supply, in the corona radiata and centrum semiovale — infarcts as a rosary of small lesions: the "string of pearls" that should always prompt one question — what is wrong with the proximal vessel? Unilateral borderzone infarction is a carotid stenosis or occlusion finding until proven otherwise, which routes straight back to the vessel imaging step of the stroke workup.
Embolism paints territories. Hypotension paints borders. The infarct's shape is telling you its mechanism.
The variants that rewrite localization
- Fetal PCA — in a sizeable minority of hemispheres the PCA fills from the carotid via a large PCom rather than from the basilar. Consequence with teeth: an occipital infarct in that patient is anterior-circulation disease — the carotid workup applies, and "posterior stroke, so the carotid doesn't matter" becomes a trap.
- Artery of Percheron — a single trunk supplying both paramedian thalami (and rostral midbrain): one tiny occlusion, bilateral thalamic infarction, and a comatose-appearing patient with a nearly normal CT — the artery behind the hypersomnolent presentations in the PCA chapter.
- Vertebral asymmetry — a hypoplastic or PICA-terminating vertebral is common; it changes how much territory one dissected or occluded vertebral can claim, and matters when reading the vessel study after a dissection.
- A1 hypoplasia / azygos ACA — both ACAs riding one trunk turns a single occlusion into bilateral medial-frontal infarction.
- Persistent carotid-basilar connections (trigeminal artery and cousins) — rare, but they let anterior-circulation disease produce posterior symptoms and vice versa.
The bottom line
Hold the map at three scales: countries (territories with their signature syndromes), streets (perforators, whose failure is the lacune and whose rupture is the deep bleed), and borders (watersheds, whose infarcts accuse the proximal vessel). Then respect the surveying errors — fetal PCA first among them — because variants are where confident localization goes wrong. The exam names the country; the variant list keeps you honest about which pipeline owns it; and the vessel imaging settles the dispute.
Frequently asked questions.
What are the main arterial territories of the brain?
Three paired cortical territories — middle cerebral (lateral hemisphere), anterior cerebral (medial frontal/parietal), and posterior cerebral (occipital, inferior temporal, thalamus) — plus the vertebrobasilar system supplying brainstem and cerebellum. Deep structures are fed by small perforating end-arteries off these trunks.
How do MCA, ACA, and PCA strokes differ clinically?
MCA: face-and-arm-predominant weakness with aphasia (dominant) or neglect (nondominant). ACA: leg-predominant weakness with abulia. PCA: homonymous hemianopia and memory or behavioral change with little or no weakness — the stroke the motor exam misses.
What is a watershed (borderzone) infarct?
An infarct in the boundary strips between arterial territories, where perfusion pressure is lowest. Bilateral watershed infarcts follow global hypotension; unilateral ones — including the internal borderzone's "string of pearls" pattern — point to severe stenosis or occlusion of the proximal carotid and warrant urgent vessel imaging.
What is a fetal PCA and why does it matter?
A common variant in which the posterior cerebral artery fills from the internal carotid through a large posterior communicating artery instead of from the basilar. In that hemisphere an occipital stroke is anterior-circulation disease — carotid sources matter, and assuming "posterior stroke = vertebrobasilar problem" misses the culprit.
What is the artery of Percheron?
An anatomic variant in which one trunk from a single PCA supplies both paramedian thalami and part of the rostral midbrain. Its occlusion causes bilateral thalamic infarction — presenting as hypersomnolence or apparent coma with vertical gaze problems and memory loss, often with a deceptively bland early CT.
Which arteries cause lacunar strokes?
The deep perforators: lenticulostriate branches of the MCA (internal capsule, basal ganglia), thalamoperforators from the PCA/PCom (thalamus), and paramedian basilar branches (pons). They are end-arteries without collateral backup, so small vessel disease occludes them cleanly — and chronic hypertension ruptures the same vessels to cause deep hemorrhages.
References.
- Tatu L, Moulin T, Bogousslavsky J, Duvernoy H. Arterial territories of the human brain: cerebral hemispheres. Neurology. 1998;50(6):1699-1708. PubMed
- Tatu L, Moulin T, Vuillier F, Bogousslavsky J. Arterial territories of the human brain. Front Neurol Neurosci. 2012;30:99-110. PubMed
- Caplan LR. "Top of the basilar" syndrome. Neurology. 1980;30(1):72-79. PubMed
Related guides
Keep building the picture.
- ASPECTS calculator The MCA country subdivided into ten scored provinces.
- Brainstem stroke syndromes The posterior map's crossed-signs district.
- PCA stroke guide The occipital-thalamic country and its variants.
- Stroke workup checklist Where the vessel imaging settles the border disputes.
Article written by