Circle of Willis

If there is any Neurosurgical Question you will get asked it will be about the blood supply for sure. Either in the way of labelling structures or even asked to draw it out even at registrar selection

The Circle of Willis is extremely detailed and not for the level of a medical student, so we will keep this simple. I have added extra-information- for the level of medical school, should you choose to ignore the italics than its more than acceptable

This might be an ‘old school’ type of picture but it contains a representation of what you should know at your level for medical school.



This is an arterial anastomosis occurring at the optic chiasm and pituitary stalk in the supra-sellar cistern. It consists of both the anterior and posterior circulation

Anterior Circulation

Internal Carotid Artery: AcA, AComm, MCA, and PComm

Posterior Circulation

Vertebro-Basilar Circulation: 2 Vertebrals’ Basilar Artery, Anterior Spinal, PICA, AICA, Superior Cerebellar Artery, Posterior Cerebral Artery

Anterior Cerebral Artery

The anterior cerebral artery divided into three parts A1,A2,A3.

A1: origin of the ICA to the ACOM (horizontal bit). This gives rise to three arteries: Anterior Communicating Artery (runs closely with the optic nerve), Recurrent Artery of Heubner and Medial Lenticulostriate Artery

A2: from the ACOM to the origin of the Callisomarginal Artery. This gives rise to the orbito-frontal and fronto-polar artery

A3: distal to the Callisomarginal Artery: This gives rise to the Peri-callosal artery and calliso-marginal artery




Clinically Applied Anatomy

The anterior cerebral artery arises just lateral to the optic chiasm and travels rostrally through the anterior hemispheric fissure. It supplies blood to the medial aspect of the cerebral hemispheres. It supplies blood to the medial aspect of the pre-central and post-central gyrus. Because the medial aspect of the gyrus represents the ‘leg’ any vascular injury can cause sensory and motor insult to the contralateral leg.

Importantly there is a close relationship with the anterior communicating artery and the optic nerve. A.Comm Aneurysms can be associated with the optic nerve palsy.

As the Anterior Cerebral Artery  travels in the anterior interhemispheric fissure any aneurysmal rupture will present as blood in the anterior hemispheric fissure on CT.

Intra-cranial aneurysms often occurr at the Bifurcation of the A.Comm and Anterior cerebral artery

Recurrent Artery of Heubner and Lenticulo-striate arteries supply the basal ganglia, thalamus and internal capsule. These perforating branches are subject to microaneurysmal formation (Charcot-Bouchard aneurysms) that are often ruptured in patients with hypertensive haemorrhages.

Middle Cerebral Artery

The middle cerebral artery like the ACA can also be divided into segments. If you remember the anatomy of the temporal lobe, there was an area of the brain lying deep to the sylvian fissure called the uncus dividing the frontal and temporal lobes. This has clinical relevance, because accessing this area via the Pteronial Craniotomy is used for MCA aneurymal clipping. To learn the segments requires some knowledge of the Insula


As you can see the Sylvian fissure has been retracted and the superior and inferior temporal gyri removed. The Limen of the Insula is of clinical relevance as this is where the M2 segment runs. Make note of the Operculum areas of the brain cortex lying superfical to the insula.

M1 Branch: From the origin to the bifurcation of the MCA within the Limen Insula. This segment gives rise to the lenticulo-striate arteries that perforate the basal ganglia.

M2 Branch: Limen Insula to make a hairpin bend to join M3. Note the majority of aneurysms occurr at the bifurcation of the MCA hence called the M2 segment. Therefore clinically access to the insula cortex is required via Pternial craniotomy which will expose the 2nd part of the MCA. The bifurcation often splits the MCA into superior and inferior division. Examples of the superior branches include the pre-central (pre-rolandic) and central (rolandic) that supply blood to the pre-central gyrus and central gyrus. Inferior branches would include the temporal arteries etc

M3 Branch: Called the Opercula branches: running within the Sylvian Fissure

M4 Branch: Cortical branches that run from the Sylvian Fissure and spread onto the cortex




Clinically Applied Anatomy

The majority of MCA Aneurysms occurr at the bifurcation the M2 segments.

The MCA supplies the lateral convexity of the brain (including parietal, temporal and part of the occipital lobe).Therefore any ischaemia to this area will affect blood flow to the lateral convexity of the homonculus affecting motor and sensory input to the arm.

Posterior Communicating Artery- Clinically Applied Anatomy



The key important aspect of this is the relationship of the 3rd nerve and the artery. You will also notice the relationship between the basilar artery and the superior cerebellar artery. All 3 aneurysms can cause a 3rd nerve palsy!

Posterior Cerebral Artery

Is divided into 4 segments P1-P4. I won’t dwell into too much detail with this. From a clinical point of view rememeber an ischaemic event will cause a contra-lateral hemianopia with macular sparing due to supply to the macula via branches of the Posterior Cerebral Artery. Supplies the occipital lobe and the midbrain

Superior Cerebellar Artery

Supplies to the Midbrain

Basilar Artery

Large Artery running along the Pons. Patients who have a large pontine haemorrhage, can suffer from a condition called Locked in Syndrome. These patients are only able to communicate with vertical eye movements. This is because of the large descending tracts, and the centres for controlling horizontal eye movements, the centres in the medulla for speech, being abolished. The midbrain is spared controlling vertical eye movements whose blood supply is via the superior cerebellar and posterior cerebral artery


Cerebellum and Medulla. Note the PICA supplies the lateral medulla. This has clinical relevance in the Wallenberg’s syndrome. This will be covered in more detail under Cranial Nerve 5.

Anterior Spinal Artery

Supplying the vasculature to the spine a branch directly of the vertebral artery.