Cerebral Blood Supply

The internal carotid artery enters the cranial cavity and, after giving off the ophthalmic artery, terminates by dividing into its major cerebral branches.

Anterior cerebral artery (ACA):

• Supplies the medial surface of the frontal and parietal lobes (approximately the anterior three-quarters), including the medial extent of the motor/sensory homunculi, corresponding to the leg and foot representation, and the anterior corpus callosum; the two ACAs are connected by the small anterior communicating artery, completing the anterior part of the circle of Willis.

Middle cerebral artery (MCA):

• The largest and most clinically significant cerebral artery, supplying the majority of the lateral hemisphere surface (most of the frontal, parietal, and temporal lobes' lateral convexity), including the lateral extent of the motor/sensory homunculi (face and arm representation), Broca's and Wernicke's areas (dominant hemisphere), and, via its small, deep lenticulostriate perforating branches: the basal ganglia and internal capsule, the most commonly affected vessel in ischaemic stroke, given both its large calibre/territory and its being the most direct continuation of internal carotid flow.

Anterior choroidal artery:

• A smaller branch, arising from the internal carotid artery (occasionally from the MCA), supplying part of the optic tract, posterior limb of the internal capsule, and the choroid plexus of the lateral ventricle's temporal horn, a relatively small territory, but clinically notable given its contribution to internal capsule blood supply.

Posterior communicating artery:

• A small connecting vessel linking the internal carotid artery to the posterior cerebral artery, completing the posterior part of the circle of Willis on each side, closely related anatomically to the oculomotor nerve (CN III), which runs just below/alongside it, explaining why a posterior communicating artery aneurysm classically compresses CN III, producing a pupil-involving third nerve palsy.

Vertebral arteries (paired):

• Ascend through the cervical transverse foramina, entering the skull via the foramen magnum, giving off branches before uniting to form the basilar artery.

Posterior inferior cerebellar artery (PICA):

• A branch of the vertebral artery (the largest branch of the vertebral artery itself), supplying the posteroinferior cerebellum and the lateral medulla: occlusion produces the classic lateral medullary (Wallenberg) syndrome (see Applied Anatomy), one of the most frequently tested brainstem vascular syndromes.

Anterior spinal artery:

• Formed by the union of small branches from each vertebral artery (just before or after their union to form the basilar artery), descending along the anterior spinal cord midline, supplying the anterior two-thirds of the spinal cord (see Applied Anatomy for anterior spinal artery syndrome).

Basilar artery:

• Formed by the union of the two vertebral arteries at the pontomedullary junction, ascending along the ventral pons, giving off multiple paired branches to the pons and cerebellum before terminating by dividing into the two posterior cerebral arteries.

Anterior inferior cerebellar artery (AICA):

• A branch of the basilar artery, supplying the anteroinferior cerebellum and part of the lateral pons (including the region near the internal acoustic meatus, explaining AICA territory infarction's frequent association with sudden sensorineural hearing loss, given the close proximity to the labyrinthine artery's origin/territory).

Superior cerebellar artery (SCA):

• A branch of the basilar artery (arising near its terminal bifurcation), supplying the superior cerebellum and part of the lateral midbrain/upper pons.

Posterior cerebral artery (PCA):

• The basilar artery's terminal bifurcation branches, supplying the occipital lobe (including the primary visual cortex along the calcarine sulcus) and the medial/inferior temporal lobe, along with (via small perforating branches) the thalamus and midbrain, connected to the internal carotid system via the posterior communicating artery, completing the circle of Willis.

Components:

• A roughly heptagonal anastomotic ring at the base of the brain, formed by: the two anterior cerebral arteries (connected by the anterior communicating artery), the two internal carotid arteries (terminal bifurcations), the two posterior communicating arteries (connecting each internal carotid to the corresponding posterior cerebral artery), and the two posterior cerebral arteries (themselves connected proximally via the basilar artery bifurcation), together forming a complete ring joining the anterior (carotid) and posterior (vertebrobasilar) circulations.

Anatomical variation:

• The "textbook" complete, symmetric circle of Willis is present in only a minority of individuals (estimates vary, but often cited around 25-30%), common variations include a hypoplastic or absent communicating artery (anterior or posterior, on one or both sides), a "fetal-type" posterior cerebral artery (where the PCA is supplied predominantly by the posterior communicating artery/internal carotid rather than the basilar artery), and duplicated or fenestrated segments, this high prevalence of anatomical variation is clinically significant, since it means the collateral protection the circle theoretically provides is often incomplete or asymmetric in any given individual.

Collateral circulation:

• The circle of Willis's primary functional purpose is to provide potential collateral flow if one of the major contributing arteries (internal carotid or vertebral) becomes occluded proximally, in an individual with a complete, well-developed circle, flow can redistribute around the ring to maintain perfusion to the affected territory; however, given the anatomical variation, this protective mechanism is unreliable in a substantial proportion of the population, and occlusion of a major vessel can still produce a significant infarct despite the circle's theoretical protective anatomy.

ACA territory:

• Infarction produces contralateral leg/foot-predominant weakness and sensory loss (given the medial hemisphere surface's leg representation), often with relative sparing of the face/arm (MCA territory), may also cause behavioural changes (apathy, abulia) if the medial frontal lobe/cingulate region is involved, and urinary incontinence in bilateral ACA territory involvement.

MCA territory:

• Infarction produces contralateral face/arm-predominant weakness and sensory loss (with relative leg sparing), and (if the dominant hemisphere is involved) aphasia (Broca's, Wernicke's, or global, depending on the specific branches affected), or (if the non-dominant hemisphere is involved) contralateral neglect (a characteristic non-dominant parietal lobe sign); deep MCA branch (lenticulostriate) involvement alone can produce a pure motor or mixed motor/sensory lacunar-type deficit via internal capsule infarction.

PCA territory:

• Infarction produces contralateral homonymous hemianopia (given primary visual cortex involvement, often with macular sparing due to the occipital pole's dual MCA/PCA collateral supply), and (if the medial temporal lobe/thalamus is involved) memory disturbance (hippocampal involvement) and sensory disturbance (thalamic involvement).

Brainstem vascular syndromes (basic anatomical level):

• Brainstem vascular territories closely follow the specific branch anatomy, producing the classic "crossed" syndromes, most notably:
• Lateral medullary (Wallenberg) syndrome: PICA territory, producing ipsilateral facial sensory loss (spinal trigeminal nucleus), ipsilateral limb ataxia (inferior cerebellar peduncle), ipsilateral Horner syndrome (descending sympathetic fibres), and contralateral body pain/temperature loss (spinothalamic tract), a classic, frequently tested combination directly illustrating the crossed brainstem syndrome principle.
• Lateral pontine syndrome: AICA territory, with a broadly similar but more rostral pattern, often additionally including ipsilateral facial weakness (CN VII nucleus involvement) and hearing loss (labyrinthine artery territory, typically a branch of AICA).
• Medial medullary syndrome: anterior spinal artery/vertebral artery territory, producing contralateral limb weakness (pyramid/corticospinal tract), contralateral loss of proprioception/vibration (medial lemniscus), and ipsilateral tongue weakness (hypoglossal nucleus/nerve), a further classic crossed pattern, this time affecting the medial rather than lateral medulla.