Basic Neurosciences · Paper A
Cerebral arterial territories
Each major brain artery supplies a particular region, so a stroke in that artery produces a recognisable pattern of deficits. This concept maps each artery to its symptoms. A patient with leg weakness far worse than arm weakness points straight to the anterior cerebral artery before any imaging, since the pattern of weakness alone can localise a stroke before any scan is done.
The anterior cerebral artery (ACA) supplies the inner frontal lobeThe front part of the brain, involved in planning, personality and impulse control.Read more →, so its stroke causes weakness mainly in the opposite LEG, with incontinence and apathy. The middle cerebral artery (MCA) supplies the outer hemisphere, causing weakness mainly in the opposite FACE and ARM, with language problems (if on the dominant side) or neglect (if on the non-dominant side). The posterior cerebral artery (PCA) supplies the occipital (vision) cortex, causing loss of one half of the visual field. A patient with sudden face and arm weakness plus difficulty speaking fits this pattern well: the distribution of weakness, leg-predominant vs face/arm-predominant, localises the stroke before language or visual testing even starts. E.g., a patient with sudden isolated leg weakness and urinary incontinence but a normal face and arm points to an ACA territory infarct.
A stroke (a sudden loss of blood supply to part of the brain) produces a pattern of deficits that depends entirely on which artery was blocked. Recognising that pattern lets a clinician localise the stroke from the bedside examination alone, before any scan confirms it: a skill this concept tests directly.
A cerebral arterial TERRITORY is the specific patch of brain tissue that one artery supplies with blood. Each major artery has a signature deficit pattern because it supplies a particular set of structures and nothing else. Weakness on the side of the body OPPOSITE the stroke occurs because the motor pathways cross over (decussate) as they descend from brain to spinal cord.
Candidates often assume 'a stroke causes weakness', full stop, without noticing that the DISTRIBUTION of that weakness is itself the clue to which vessel is affected. Suppose a patient has profound leg weakness but only mild arm weakness: that specific, lopsided distribution is the finding, and memorising it precisely is what separates a correct answer from a vague one.
Imagine the brain's outer surface divided into three overlapping zones, each fed by one artery arising from the circle of Willis. The zone nearest the midline, running along the top of the brain, is fed by the anterior cerebral artery (ACA). The zone covering most of the outer (lateral) hemisphere is fed by the middle cerebral artery (MCA): by far the largest territory, which is why MCA strokes are the most common type seen clinically. The zone at the back, over the , is fed by the posterior cerebral artery (PCA). Because the motor and sensory strip (running from the top of the brain down its side) crosses these zones, the LEG representation sits in ACA territory while the FACE and ARM representation sits in MCA territory: this single anatomical fact explains the whole pattern below.
The ANTERIOR CEREBRAL ARTERY supplies the inner (medial) . Its stroke causes weakness mainly in the opposite LEG, often with urinary incontinence and apathy (loss of drive and motivation), because the medial frontal lobe also governs continence and initiative.
The MIDDLE CEREBRAL ARTERY supplies the outer hemisphere, including the primary motor and sensory cortex for the face and arm, and the language areas on the dominant side (usually the left). Its stroke causes weakness mainly in the opposite FACE and ARM, with the leg relatively spared. If the stroke is on the dominant side, expect language problems (aphasia); if on the non-dominant side, expect neglect (failing to attend to one side of space or body).
The POSTERIOR CEREBRAL ARTERY supplies the occipital (visual) cortex. Its stroke causes loss of one half of the visual field (homonymous hemianopia) in both eyes, typically without any weakness at all, since it does not supply the motor strip.
A fourth pattern worth knowing: a stroke of the posterior inferior cerebellar artery (PICA), a branch of the vertebral artery, causes the lateral medullary (Wallenberg) syndrome: crossed sensory loss (pain and temperature lost on one side of the face and the opposite side of the body), a droopy eyelid, (unsteady, uncoordinated movement) and swallowing difficulty with vertigo.
ACA versus MCA is the classic discriminator: leg-predominant weakness points to ACA, while face-and-arm-predominant weakness with language trouble or neglect points to MCA. PCA stroke is discriminated by visual loss with little or no weakness, unlike the motor-heavy MCA picture. PICA infarction is discriminated from a hemispheric (ACA/MCA/PCA) stroke by its CROSSED sensory pattern and brainstem signs (vertigo, swallowing difficulty), rather than a simple one-sided weakness.
Bedside examination and the distribution of weakness give an immediate working localisation. Confirmatory imaging follows: a CT scan is done urgently to rule out haemorrhage, and either CT or MRI can then show the territory of infarction directly. NICE stroke guidance recommends brain imaging within one hour for anyone potentially eligible for thrombolysis, since the treatment decision cannot safely wait for a full neuropsychological picture.
Acute management of an ischaemic stroke is time-critical: thrombolysis (clot-dissolving medication) and thrombectomy (mechanical clot removal) are considered first-line where the patient presents early enough and imaging confirms no haemorrhage, per NICE guidance. Longer-term management is broadly similar across territories, antiplatelet therapy, blood pressure control, statin therapy, and rehabilitation, but the REHABILITATION plan is built around the specific deficit: leg-weighted physiotherapy for an ACA stroke, speech and language therapy for MCA-territory aphasia, and visual field compensation strategies for a PCA stroke.
When a vignette gives you a distribution of weakness before mentioning any scan, work out the likely artery first: the exam is testing the clinical localisation skill, not just recall of anatomy.
Leg-worse weakness localises to the ACA, face-and-arm weakness with language trouble or neglect localises to the MCA, and isolated visual-field loss localises to the PCA.