Structure of the Circle of Willis
The bilateral internal carotid arteries (ICA) enter the cranial cavity via the carotid canals and become the anterior cerebral artery (ACA) and middle cerebral artery (MCA). An anterior communicating (ACOM) artery connects the 2 anterior cerebral arteries, 1 from each side of the body. This system represents the anterior circulation of the circle of Willis.
The vertebral arteries from each side entering the cranial cavity via the foramen magnum join to form the basilar artery, which branches out into the left and the right posterior cerebral arteries (PCA), constituting the posterior circulation. A communicating artery known as the posterior communicating (PCOM) artery connects the 2 posterior cerebral arteries.
The PCAs complete the circle of Willis by joining the internal carotid system anteriorly via the posterior communicating (PCOM) arteries.
Vessels Comprising the Circle of Willis
- Anterior circulation:
- Left and right internal carotid arteries (ICA)
- Horizontal segments of the left and right anterior cerebral arteries (ACA)
- Single anterior communicating (ACOM) artery
- Left and right posterior communicating (PCOM) arteries
- Horizontal segments of the left and the right posterior cerebral arteries (PCA)
- Single basilar artery
Anterior cerebral artery
The anterior cerebral artery is divided into various segments. The first segment starts from the bifurcation of the internal carotid artery (ICA) until the junction of the artery with the anterior communicating artery in the longitudinal fissure.
Branches of the first segment include:
- Medial lenticulostriate artery, which supplies the anterior hypothalamus, anterior commissure, fornix, striatum, optic chiasm, and optic nerves
- Perforator branches of the anterior communicating artery supplying the hypothalamus and optic chiasm
The second segment of the ACA extends from the anterior communicating (ACOM) artery to the bifurcation of the ACOM artery forming the pericallosal and callosomarginal arteries, near the genu of the corpus callosum.
Branches of the second segment include the perforators to the frontal lobe, as well as the recurrent artery of Heubner, which is a large lenticulostriate vessel supplying the caudate nucleus, internal capsule, and putamen. The orbitofrontal and frontopolar arteries are also the branches of the second segment.
The third segment, also known as the pericallosal artery, includes all the branches of the ACA that originate after the pericallosal and callosomarginal arteries distally. The 3rd segment ends in parietal arteries, which supply the corpus callosum and precuneus.
Middle cerebral artery
The middle cerebral artery is divided into 4 segments.
The first segment originates in the internal carotid artery until its bifurcation (or trifurcation).
The middle cerebral artery may bifurcate or trifurcate in different individuals. Branches of the 1st segment include:
- Lenticulostriate arteries that supply the anterior commissure, internal capsule, caudate nucleus, putamen, and globus pallidus
- Anterior temporal artery that supplies the anterior temporal lobe
This segment originates in the bifurcation of the middle cerebral artery into the circular sulcus of the insula. It extends until the margin of the insula within the Sylvian fissure.
The third segment originates in the circular sulcus of the insula and terminates at the surface of the Sylvian fissure. It is made up of cortical branches. It travels over the surface of the frontal and temporal opercula and reaches the external surface of the Sylvian fissure.
The 2nd and 3rd segments generate cortical branches supplying the cerebral cortex.
This segment of the middle cerebral artery gives off cortical branches, which supply the frontal, parietal, temporal and occipital lobes. These include:
- Orbitofrontal branch
- Prefrontal branch
- Pre-central branch
- Central branch
- Anterior and posterior parietal branch
- Angular branch
- Temporo-occipital branch
- Temporal branch
- Temporopolar branches
Posterior cerebral artery
This artery is also divided into 4 segments.
The first segment, which starts from the basilar artery bifurcation till the junction of the posterior communicating (PCOM) artery, gives off perforators to the midbrain, pons, and medulla. These are called the posterior thalamo-perforators.
Anterior thalamo-perforators are the vessels that originate in the PCOM artery. The direct perforators supply the thalamus, brainstem and the internal capsule. Circumflex arteries arising from the first segment supply the thalamus and midbrain. The meningeal branch supplies the inferior surface of the tentorium cerebelli.
The second segment starts from the posterior communicating PCOM artery and terminates at the posterior aspect of the midbrain. It gives off direct perforating branches to the thalamus, internal capsule and optic tract. The branches of the second segment include:
- Posteromedial choroidal artery that supplies the midbrain, pineal gland, thalamus, and medial geniculate body
- Postero-lateral choroidal artery supplying the choroid plexus, thalamus, geniculate bodies, fornix, cerebral peduncle, pineal body, corpus callosum, tegmentum, and temporal occipital cortex.
- A single hippocampal artery may also be present.
The third segment, which starts from the posterior aspect of the midbrain till the calcarine fissure, bifurcates terminally into the calcarine artery and the aforementioned parieto-occipital artery.
The fourth segment includes the distal branches of the artery to the anterior aspect of the calcarine fissure. It often includes one of the two main terminal branches of the PCA. The other terminal branch originates in the second or third segment.
Basilar artery is the major artery formed by the junction of the left and right vertebral arteries at the base of the brain. It moves anteriorly near the brainstem and gives off various branches to the base of the brain and cerebellar branches including the superior cerebellar artery (SCA) and the anterior inferior cerebellar artery (AICA).
The superior cerebellar artery originates at the basilar artery immediately before its bifurcation. It also is in close contact with the trigeminal nerve. The branches of this artery supply the tectum, the vermis and the medial aspect of the cerebellar hemisphere.
The anterior inferior cerebellar artery moves toward the cerebellopontine angle.
The posterior inferior cerebellar artery (PICA), the largest branch supplying the cerebellum, supplies the medulla, cerebellar tonsils, vermis, and inferolateral cerebellar hemisphere. Pontine branches supply the brainstem.