The brain receives its blood supply through two basic arterial systems:

The anterior circulation depends on the carotid arteries and consists of the internal carotid arteries, anterior and middle cerebral arteries, and their branches. It supplies the orbit, and both cerebral hemispheres except for most of the occipital lobe and the thalami.

The posterior circulation consists of the vertebral, basilar, and posterior cerebral arteries and their branches. It supplies all of the brainstem, occipital lobes, most of the thalami, and medial-inferior temporal lobes.

This division of anterior and posterior arterial territories is a general one: arterial territories overlap. The overlaps and interconnections ("collaterals") allow one system to compensate for the other in disease states.

The common carotid arteries (CCA) arise from the aortic arch; the right one arises indirectly, through the brachiocephalic trunk (innominate artery). Each CCA bifurcates at the angle of the mandible, usually at the level of C3-4 (range C1-T2). The external carotid artery (ECA) supplies the face and scalp; the internal carotid artery (ICA), the brain and the eye.

The ICA ascends in a neurovascular bundle (with the IJV and vagus nerve) beneath the sternocleidomastoid muscle and enters the cranium through the carotid canal, where it is then called "petrous". Initially directed cephalad, it bends anteromedially (genu) to the apex of the petrous bone and enters the cavernous sinus. After a sinuous course inside the sinus (the siphon) it ascends just medial to the anterior clinoid process, pierces the dura and becomes the "supraclinoid" portion of the ICA.

The first branch of the ICA is the ophthalmic artery (OA), which supplies blood to the eye and orbit. It arises from the ICA as it emerges from the cavernous sinus ("infraclinoid" ICA).

The supraclinoid ICA divides into its major end-arteries: the anterior cerebral artery (ACA), middle cerebral artery (MCA) and anterior choroidal artery (AChoA). An anastomotic channel connecting the ICA with the posterior cerebral artery (PCA), arises posteriorly at this level and is called the posterior communicating artery (PCoA).

The ACA supplies the anterior two-thirds of the medial surface of the hemispheres. It anastomoses with the contralateral ACA by the anterior communicating artery (ACoA). Branches of the ACoA supply the septal and median paraolfactory nuclei, optic chiasm and nerves, corpus callosum genu, columns of the fornix, septum pellucidum, anterior cingulum, and the anterior hypothalamus. The segment between the point of origin from the ICA and the origin of the ACoA is called the A1 or horizontal segment; distal to the AcoA are the A2, A3 and A4 segments, in that order. The medial lenticulostriate arteries, arising from the A1 segment, supply the anterior perforated substance, parts of the optic chiasm and tract, anterior commissure, septum pellucidum, hypothalamus and paraolfactory structures. The recurrent artery of Heubner, one of the medial lenticulostriate arteries, most commonly arises (78% of the time) from the A2 segment and supplies the anterior limb of the internal capsule, parts of the globus pallidus and the head of the caudate nucleus. Beyond the A2 segment the ACA branches out into: orbitofrontal, frontopolar, callosomarginal and pericallosal arteries.

The middle cerebral artery (MCA) supplies the lateral aspect of the hemisphere (ie the convexity), including the frontal, parietal, and temporal lobes, as far back as the association area between parietal,temporal and occipital lobes. The M1 segment ends at the trifurcation into anterior, posterior and deep (lateral lenticulostriate arteries) divisions. The deep trunk branches out into some 20 lateral lenticulostriate arteries that supply the basal gnaglia, internal capsule and caudate nucleus. The superficial divisions (anterior and posterior) form slightly more superiorly, distal to the deep branch, in the sylvian fissure. These two divisions have multiple and variable branches that I will not discuss here. Approximately 25% of the population has a middle division of the MCA.

The AChoA supplies the anterior thalamus, posterior limb of the internal capsule and medial temporal lobe; its territory greatly overlaps with that of the posterior choroidal artery (PChoA), a branch of the PCA.

The PCAs are the end-branches of the basilar artery. The P1 segment is that betweent he origin and the PComA; the P2 segment arises beyond the PComA. Each PCA supplies the medial occipital lobe, medial-inferior temporal lobe, cerebral peduncles and the thalamus, and has meningeal branches to the posterior falx ( and anterior branchand a posterior branch called artery of Davidoff and Schechter). A branch of the P2 segment, the medial posterior choroidal artery (MPChA) supplies parts of the thalamus, pineal, and the third and lateral ventricular choroid plexi. The artery shares much of its territory with those of the AChoA and PChoA. There are other branches that you should read about.

The basilar artery (BA) arises from the "fusion" of the vertebral arteries, at the rostral medullary level. This artery has three types of branches:

(a) paramedian perforators

* unnamed branches supplying the susbstance of the brainstem

(b) lateral perforators (short circumferential) and

*wind around the brainstem, to supply most lateral structures

(c) long circumferential branches

*anterior inferior cerebellar (AICA), and superior cerebellar arteries (SCA) and PCA

Note: this classification of arterial branches can indeed be applied to all cerebral arteries but easier to visualize for the BA.

Each vertebral artery (VA)arises independently from a subclavian artery, penetrates the vertebral foramen of C6 and ascends throughout the more cephalad vertebral foramina up to the foramen magnum, where it enters the cranium. Before entering the cranium, it gives rise to segmental arteries that supply the vertebrae and supplement the spinal circulation, deep muscular rami, and the occipital artery. The major branches of the intracranial vertebral artery are: the anterior spinal artery, posterior meningeal artery, direct bulbar branches to the medulla, and the posterior inferior cerebellar artery (PICA). Occasionally, it may give rise the posterior spinal artery and small basal meningeal arteries.

The PICA arises intracranially in 80% of cases. It supplies the lateral medulla, cerebellar tonsil, choroid plexus of the fourth ventricle, inferior vermis (medial branches) and the lateral-posterior-inferior cerebellar hemisphere.

 On the high convexity of each hemisphere, there are portions supplied by more than one artery, the so-called "watershed" or "distal zones" territories. The end-branches of the MCA and ACA meet anteriorly whereas the end-arteries of the ACA, MCA and PCA meet posteriorly. Thus, disease in one artery may not produce an ischemic lesion if collateral flow though this system is effective. These are common sites of strokes in cases of severe hypotension or severe proximal stenoses.

Another collateral system exists between the ICA and the ECA, through the eye. For example, when severe ICA stenosis exists, the ECA may supply the entire ICA territory through the ophthalmic artery.

The ACAs are joined by an anterior communicating artery (ACoA); this allow side-to-side collateral flow in cases of severe stenosis or occlusion in one carotid system. A common normal variant, hypoplasia of the A1 segment, may limit the value of this system in some patients. Clinical tip: sometimes, hypoplasia of the A1 segment is associated with an aneurysm of the ACoA...an infortunate coincidence in patients who develop subarachnoid hemorrhage with vasospasm! (See Subarachnoid Hemorrhage)

The PComA also serves as a collateral channel between the anterior and posterior circulations. The flow may be in either direction, dictated by pressure gradients. Unfortunately, 18% of the population does not have a patent PComA and counts with one less compensating mechanism in cases of hemodynamic challenges.

The Circle of Willis is an arterial polygon formed by the AComA, A1 segments of the ACAs, ICAs, PComA, and proximal (P1) segments of the PCAs. It serves for collateralization of flow. There are many variants and the "classical" configuration, with all involved vessels patient and of normal caliber, is found in only 20% of the population.