By MRCSA in MRCS Part A — 04 Jul 2026

15 CEREBRAL & CRANIAL VASCULATURE

# 16 CEREBRAL & CRANIAL VASCULATURE

👩‍⚕️ Cerebral and cranial vasculature is one of the highest-yield neuroanatomy topics for MRCS Part A. Examiners come back to the same handful of facts again and again: the order of internal carotid artery branches, the cortical territory supplied by each cerebral artery, and the clinical stroke syndrome that follows occlusion of each. Master those three lists and you will pick up easy marks across the paper.

Detailed notes

The two inflow systems

The brain is supplied by two paired arterial systems that converge on the Circle of Willis at the base of the brain:

➡ Anterior circulation — paired internal carotid arteries (ICA). Supplies the anterior two-thirds of the cerebrum (frontal, parietal and most of the temporal lobes).

➡ Posterior circulation — paired vertebral arteries that fuse to form the basilar artery. Supplies the brainstem, cerebellum, occipital lobes and inferomedial temporal lobes.

The internal carotid arises from the bifurcation of the common carotid at roughly C4 (upper border of the thyroid cartilage). It ascends through the carotid canal of the petrous temporal, traverses the cavernous sinus, then pierces the dura to enter the subarachnoid space.

The vertebral artery is the first branch of the first part of the subclavian artery. It enters the transverse foramen of C6 (NOT C7), ascends through C6 to C1, hooks around the lateral mass of the atlas, then enters the cranial cavity via the foramen magnum. The two vertebrals unite at the pontomedullary junction to form the basilar artery.

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Branches of the internal carotid artery (in order)

A favourite SBA trap. The ICA has no branches in the neck. All its branches arise intracranially. The classic order, from proximal to distal:

➡ Caroticotympanic

➡ Vidian (artery of the pterygoid canal)

➡ Meningohypophyseal trunk

➡ Inferolateral trunk

➡ Ophthalmic — first branch after the ICA exits the cavernous sinus

➡ Superior hypophyseal

➡ Posterior communicating

➡ Anterior choroidal

➡ Terminal branches: Anterior cerebral (ACA) and Middle cerebral (MCA)

The ophthalmic artery is the answer to "first branch after the cavernous sinus." The posterior communicating then completes the Circle of Willis posteriorly by joining the PCA.

Branches of the subclavian artery

Mnemonic: VIT C+D to become A

➡ Vertebral (1st part)

➡ Internal thoracic (1st part)

➡ Thyrocervical trunk (1st part)

➡ Costocervical trunk (2nd part)

➡ Dorsal scapular (3rd part)

➡ Continues as the Axillary artery at the lateral border of the first rib

The Circle of Willis

A polygonal anastomosis at the base of the brain that allows collateral flow between the anterior and posterior circulations. Components, working anteriorly to posteriorly:

Vessel	Role
ACAs (×2)	Anterior limb
Anterior communicating	Joins the two ACAs
ICAs (×2)	Lateral limb
Posterior communicating (×2)	Joins each ICA to the ipsilateral PCA
PCAs (×2)	Posterior limb
Basilar	Apex; gives rise to both PCAs

The circle is complete in fewer than half of the population — variants are common, which is why collateral flow cannot always rescue an occlusion.

Cerebral artery territories and stroke syndromes

This is the single highest-yield table in the lesson.

Artery	Cortical territory	Classical deficit
ACA	Medial frontal and parietal lobes (motor/sensory strip for the leg)	Contralateral leg weakness and sensory loss; urinary incontinence; abulia
MCA	Lateral cortex — face and arm motor/sensory areas, Broca's and Wernicke's	Contralateral face + arm weakness (forehead spared), aphasia if dominant hemisphere, contralateral homonymous hemianopia, gaze deviation toward the lesion
PCA	Occipital lobe, medial temporal lobe, thalamus	Contralateral homonymous hemianopia with macular sparing; alexia without agraphia if dominant
Lacunar	Penetrating branches (lenticulostriate, thalamoperforators) supplying internal capsule, basal ganglia, thalamus, pons	Pure motor, pure sensory, ataxic hemiparesis or dysarthria-clumsy hand — no cortical signs
Basilar	Pons, midbrain	Locked-in syndrome (quadriplegia with preserved consciousness and vertical eye movement)

Memory hook: ACA = legs, MCA = face and arms, PCA = vision.

Why does UMN facial palsy spare the forehead?

A classic MCA stroke question. The frontalis muscle (forehead) receives bilateral corticobulbar innervation from both motor cortices. The lower face receives only contralateral input. So a one-sided cortical stroke wipes out the contralateral lower face but the forehead still works because the intact hemisphere covers it. A Bell's palsy (LMN facial nerve lesion) paralyses the entire ipsilateral half of the face including the forehead — that is the differentiator.

Cerebellar arteries and the lateral medullary syndrome

Three paired arteries supply the cerebellum:

➡ SCA (Superior Cerebellar Artery) — from the upper basilar

➡ AICA (Anterior Inferior Cerebellar Artery) — from the lower basilar; supplies the inner ear via the labyrinthine artery

➡ PICA (Posterior Inferior Cerebellar Artery) — from the vertebral artery

PICA occlusion = lateral medullary (Wallenberg) syndrome. Examiners love the crossed-signs pattern:

➡ Contralateral loss of pain and temperature on the body (spinothalamic — has already decussated)

➡ Ipsilateral loss of pain and temperature on the face (spinal trigeminal nucleus — not yet decussated)

➡ Ipsilateral cerebellar signs: ataxia, dysmetria, nystagmus, vertigo

➡ Ipsilateral Horner syndrome (descending sympathetic fibres)

➡ Dysphagia, dysarthria, hoarseness (nucleus ambiguus → CN IX, X)

The mnemonic for cerebellar signs is DANISH: Dysdiadochokinesia, Ataxia, Nystagmus, Intention tremor, Slurred speech, Hypotonia/Heel-shin test.

Berry (saccular) aneurysms

Roughly 85% of non-traumatic subarachnoid haemorrhages arise from a ruptured berry aneurysm at a Circle of Willis branch point.

Common sites in order of frequency:

➡ Anterior communicating artery (~35%) — most common

➡ Posterior communicating artery (~30%) — classic cause of a painful CN III palsy with a dilated pupil because parasympathetic fibres run on the surface of the nerve

➡ MCA bifurcation (~20%)

➡ Basilar tip and others (~15%)

Associations to remember:

➡ Autosomal dominant polycystic kidney disease (ADPKD)

➡ Ehlers-Danlos syndrome (type IV)

➡ Coarctation of the aorta

➡ Smoking, hypertension, family history

Extracranial arterial supply

External carotid branches — mnemonic Some Anatomists Like Freaking Out Poor Medical Students:

➡ Superior thyroid

➡ Ascending pharyngeal

➡ Lingual

➡ Facial

➡ Occipital

➡ Posterior auricular

➡ Maxillary (terminal)

➡ Superficial temporal (terminal)

The occipital artery runs superficially in the posterior scalp — vulnerable to injury during stereotactic frame placement or posterior burr holes.

The middle meningeal artery is a branch of the maxillary artery (a terminal branch of the ECA). It enters the skull via the foramen spinosum and is the classic source of an extradural haematoma following pterion fracture.

Venous drainage of the brain

Cerebral veins drain into dural venous sinuses — endothelium-lined channels between the two layers of dura mater. They have no valves and no smooth muscle.

The principal pathway:

➡ Superior sagittal sinus → confluence of sinuses (at the internal occipital protuberance) → transverse sinus → sigmoid sinus → internal jugular vein (exits via the jugular foramen)

The cavernous sinus drains the orbit and central face. Because the facial vein communicates with the cavernous sinus via the ophthalmic veins, infections of the danger triangle of the face can spread retrogradely to cause cavernous sinus thrombosis.

Subclavian steal syndrome

Stenosis of the proximal subclavian artery (proximal to the vertebral origin) causes blood to flow retrograde down the ipsilateral vertebral artery to supply the arm during exertion — at the expense of the posterior circulation.

Clinical features:

➡ Arm claudication (cramping, fatigue) on exertion of that arm

➡ Posterior circulation symptoms: dizziness, syncope, diplopia, drop attacks

➡ BP difference > 15–20 mmHg between arms

➡ Bruit above the clavicle

First-line investigation: arterial duplex of the aortic arch and its branches. CT angiography and MRA come later for surgical planning.

Test yourself

[Image: MCQs banner]

Which of the following is the first branch of the internal carotid artery as it emerges from the cavernous sinus?

((Anterior cerebral::Terminal branch of the ICA, not the first branch out of the cavernous sinus.))
((Middle cerebral::The other terminal branch, at the ICA bifurcation.))
((Anterior choroidal::Arises after the posterior communicating, well distal to the ophthalmic.))
((Posterior communicating::Arises after the ophthalmic and superior hypophyseal.))
((Ophthalmic::☑️ First branch of the ICA after it exits the cavernous sinus.))

👩‍⚕️ ICA order mnemonic: Caroticotympanic, Vidian, Meningohypophyseal, Inferolateral, Ophthalmic, Superior hypophyseal, PCom, Anterior choroidal, ACA, MCA.

A man is undergoing stereotactic frame placement for radiosurgery on a cerebral AVM. During insertion of a posterior pin, arterial bleeding occurs. Which artery was most likely punctured?

((Ascending pharyngeal::Lies deep in the neck, not in the posterior scalp.))
((Middle meningeal::Intracranial, in the temporal region — not vulnerable to a scalp pin.))
((Occipital::☑️ Runs superficially in the posterior scalp; classic vessel injured by posterior pins.))
((Posterior cerebral::Intracranial artery of the Circle of Willis.))
((Posterior communicating::Intracranial — part of the Circle of Willis.))

During arch aortography, the vertebral artery would be seen to arise directly from which of the following?

((Arch of the aorta::Rare anatomical variant for the left vertebral; not the standard origin.))
((Brachiocephalic artery::Gives off right subclavian and right common carotid, not the vertebral directly.))
((Common carotid artery::Bifurcates into ICA and ECA only.))
((Internal carotid artery::Has no branches in the neck.))
((Subclavian artery::☑️ Vertebral is the first branch of the first part of the subclavian.))

👩‍⚕️ Subclavian branches: VIT C+D to become A — Vertebral, Internal thoracic, Thyrocervical, Costocervical, Dorsal scapular, then Axillary.

The vertebral artery is a branch from which part of the subclavian artery?

((1st part of the subclavian artery::☑️ Medial to scalenus anterior; gives V, I, T branches.))
((2nd part of the subclavian artery::Behind scalenus anterior; gives the costocervical trunk.))
((3rd part of the subclavian artery::Lateral to scalenus anterior; gives the dorsal scapular.))
((Brachiocephalic artery::Bifurcates into right common carotid and right subclavian.))
((Arch of the aorta::Only a rare variant origin of the left vertebral.))

The vertebral artery enters the transverse foramina at which level?

((C3::Vertebral passes through but does not enter here.))
((C4::Wrong level of entry.))
((C5::Vertebral passes through C5 but enters one level below.))
((C6::☑️ Enters the transverse foramen at C6, ascends to C1, then enters the cranium via foramen magnum.))
((C7::C7 transverse foramen transmits accessory vertebral veins only.))

In subclavian steal syndrome, retrograde flow is seen in which artery?

((Subclavian artery::This is the site of the stenosis, not the vessel reversing flow.))
((Vertebral artery::☑️ Proximal subclavian stenosis reverses flow down the vertebral to perfuse the arm.))
((Internal carotid artery::Not directly involved in classical subclavian steal.))
((Brachiocephalic artery::Brachiocephalic steal is a separate entity.))
((Common carotid artery::Not typically involved.))

((CT brain::Assesses parenchyma, not flow direction in the vertebral.))
((Venous duplex of the neck::Tests for venous thrombosis, irrelevant here.))
((Arterial duplex of the aortic arch and its branches::☑️ Non-invasive first-line; demonstrates retrograde vertebral flow.))
((CT angiogram of the aortic arch::Useful for planning intervention but not the initial investigation.))
((MR angiography::Second-line; duplex comes first.))

👩‍⚕️ Look for an inter-arm BP difference >15–20 mmHg and a supraclavicular bruit.

A 65-year-old man with AF presents with paralysis of his right lower limb. Which arterial territory is most likely involved?

((Left anterior cerebral::☑️ ACA supplies the medial motor strip for the contralateral leg.))
((Left middle cerebral::MCA affects contralateral face and arm, not isolated leg.))
((Right anterior cerebral::Would cause left, not right, leg weakness.))
((Right middle cerebral::Would affect the left face and arm.))
((Right posterior cerebral::Supplies the visual cortex, not motor strip.))

👩‍⚕️ ACA = legs, MCA = face and arms, PCA = vision.

A man has sudden right upper-limb weakness and Broca's aphasia. Which artery is most likely occluded?

((Left anterior cerebral::ACA affects the contralateral leg, not arm or speech.))
((Left middle cerebral::☑️ Supplies lateral frontal cortex including Broca's area; causes contralateral arm/face weakness with expressive aphasia.))
((Right anterior cerebral::Would cause left leg weakness.))
((Right middle cerebral::Would cause left-sided weakness without aphasia in a right-handed patient.))
((Right posterior cerebral::Causes left homonymous hemianopia.))

A patient presents with a middle cerebral artery blockage. What is the most likely presenting symptom?

((Contralateral face weakness sparing the forehead::☑️ UMN pattern — frontalis has bilateral corticobulbar innervation.))
((Contralateral lower limb weakness::ACA territory.))
((Ipsilateral gaze preference::Can occur in large MCA strokes but not the defining feature.))
((Dysphagia and hoarseness::PICA territory (nucleus ambiguus).))
((Contralateral homonymous hemianopia::Hallmark of PCA occlusion.))

What differentiates a facial nerve LMN lesion from UMN in facial palsy?

((Inability to raise eyebrow::☑️ Forehead is spared in UMN lesions; paralysed in LMN lesions (Bell's palsy).))
((Inability to blow cheeks out::Impaired in both UMN and LMN lesions.))
((Loss of chin reflex::Jaw jerk tests the trigeminal/pyramidal tract, not CN VII.))
((Inability to close the eye::Impaired in LMN but not specific.))
((Inability to open mouth against resistance::Trigeminal function (CN V), not facial.))

A 60-year-old man has trouble forming long sentences (Broca's aphasia). Which artery is most likely occluded?

((Anterior cerebral::Supplies medial cortex for leg motor function.))
((Posterior inferior cerebellar::Supplies cerebellum and lateral medulla, not speech.))
((Posterior cerebral::Supplies visual cortex and medial temporal lobe.))
((Middle cerebral::☑️ Broca's area sits in the inferior frontal gyrus, supplied by the left MCA.))
((Basilar artery::Causes brainstem signs, not isolated aphasia.))

👩‍⚕️ Broca's = expressive, non-fluent (slow, effortful, telegraphic); comprehension preserved.

A man has nystagmus, slurred speech, and cannot walk straight. What is the most likely diagnosis?

((PICA syndrome::☑️ Lateral medullary infarct — ipsilateral cerebellar signs plus dysphagia/hoarseness from nucleus ambiguus.))
((MCA occlusion::Face and arm weakness with possible aphasia, no cerebellar signs.))
((Lacunar syndrome::Pure motor or pure sensory — no cerebellar features.))
((ACA occlusion::Contralateral leg weakness.))
((PCA occlusion::Contralateral homonymous hemianopia.))

A patient presents with nystagmus, dysmetria, and ataxia. Which artery is involved?

((Anterior cerebral::Contralateral leg weakness.))
((Middle cerebral::Face/arm weakness with aphasia.))
((Posterior inferior cerebellar::☑️ PICA supplies cerebellum + lateral medulla; ipsilateral cerebellar signs.))
((Posterior cerebral::Contralateral homonymous hemianopia.))
((Ophthalmic artery::Monocular blindness.))

A 55-year-old presents with slurred speech, tremors, dysdiadochokinesia, and bilateral nystagmus. Which is the affected artery?

((Anterior cerebral::Affects the lower limbs, not cerebellum.))
((Middle cerebral::Affects face, arm and speech areas of lateral cortex.))
((PICA::☑️ DANISH signs localise to cerebellum; PICA infarct = Wallenberg syndrome.))
((Posterior cerebral::Causes visual field defects.))
((Basilar artery::Causes bilateral brainstem signs / locked-in, not isolated cerebellar signs.))

👩‍⚕️ Wallenberg (lateral medullary) syndrome — crossed signs: contralateral body pain/temp loss, ipsilateral face pain/temp loss, ipsilateral Horner's, ipsilateral cerebellar signs, dysphagia, hoarseness.

Which is the most common site for a ruptured berry aneurysm?

((Anterior communicating artery::☑️ ~35% — most common site; classic cause of subarachnoid haemorrhage.))
((Posterior communicating artery::~30% — second most common; classically causes painful CN III palsy with fixed dilated pupil.))
((MCA bifurcation::~20% — third most common.))
((Basilar tip::Less common; presents with brainstem signs if it ruptures.))
((Anterior cerebral artery distal segment::Uncommon site; aneurysms favour Circle of Willis branch points.))

👩‍⚕️ Associations: ADPKD, Ehlers-Danlos type IV, coarctation, smoking, hypertension.

Revision summary

➡ ICA branches (in order): Caroticotympanic, Vidian, Meningohypophyseal, Inferolateral, Ophthalmic (first after cavernous sinus), Superior hypophyseal, PCom, Anterior choroidal, terminal ACA + MCA. No branches in the neck.

➡ Subclavian branches: VIT C+D to become A. Vertebral arises from the 1st part, enters the transverse foramen at C6, enters the skull via the foramen magnum, joins the contralateral vertebral to form the basilar.

➡ Circle of Willis: ACAs + anterior communicating + ICAs + posterior communicating + PCAs + basilar. Complete in fewer than half the population.

➡ Stroke syndromes: ACA = contralateral leg. MCA = contralateral face + arm, aphasia if dominant, forehead spared. PCA = contralateral homonymous hemianopia with macular sparing. PICA = ipsilateral cerebellar signs + Wallenberg's crossed sensory loss + Horner's + dysphagia. Basilar = locked-in.

➡ UMN vs LMN facial palsy: UMN spares the forehead (bilateral corticobulbar input to frontalis); LMN (Bell's) paralyses the whole ipsilateral half-face.

➡ Berry aneurysms: AComm > PComm > MCA bifurcation. Associations — ADPKD, Ehlers-Danlos, coarctation, hypertension, smoking. PComm aneurysm = painful CN III palsy with blown pupil.

➡ Venous drainage: Superior sagittal sinus → transverse → sigmoid → internal jugular vein. Cavernous sinus drains the face — danger triangle.

➡ Subclavian steal: Proximal subclavian stenosis → retrograde flow down ipsilateral vertebral → arm claudication + posterior circulation symptoms on exertion. Inter-arm BP difference >15–20 mmHg. First-line investigation: arterial duplex.