Circle of Willis
The circle of Willis is a ring of interconnecting arteries located at the base of the brain around the optic chiasm or chiasma (partial crossing of the optic nerve – CNII; this crossing is important for binocular vision), infundibulum of the pituitary stalk and the hypothalamus. It provides blood to the brain and neighbouring structures.
This arterial circle is more accurately referred to as “the polygon of Willis” by the French. Although it was noticed briefly and incompletely by ancient doctors, it is described completely by an English doctor called “Thomas Willis” in his book – Cerebri Anatome in 1664. Hence the circle was named after him.
Circulus arteriosus is formed by two interconnecting arterial sources – the internal carotid arteries and the vertebro-basilar system (which is formed by two vertebral arteries and the basilar artery). This anastomosis is in an “anteroposterior order”, with the anterior source from the internal carotid arteries and their branches, and the posterior source from the vertebral arteries and their branches (specifically, from the vertebral arteries, basilar arteries and their tributaries).
The circle of Willis is formed by two group of arteries - the internal carotid arteries and two vertebral arteries. These arteries provide the anterior and posterior circulation of the brain respectively.
Posterior Source (Vertebral Arteries)
This part (or half) of the circle provides the posterior circulation and basically supplies the cerebellum, brainstem and the posterior aspects of the cerebral hemispheres. Two vertebral arteries, originating from the subclavian artery, course upwards through the neck and ascend on the anterolateral aspect of the medulla to unite at the lower border of the pons (part of brainstem) forming a single artery called the basilar artery (and by extension, forming the “vertebrobasilar system”). The vertebral arteries also give-off the following branches which run somewhat downwards:
- the posterior inferior cerebellar arteries
- anterior spinal arteries
- posterior spinal arteries
The basilar artery runs upwards in the midline and ventral to the pons, from the level of the lower border of the pons, giving off the pontine branches (pontine arteries), superior cerebellar arteries, and anterior inferior cerebellar arteries. It finally terminates by bifurcating, at the level of the upper border of the pons (or lower border of midbrain, which is also a brainstem structure), into its terminal branches – the right and left posterior cerebral arteries.
Anterior Source (Internal Carotid Arteries)
The anterior part of the circle provides the anterior circulation of the brain and basically supplies the most part of the cerebral hemispheres and its deep structures like the caudate-putamen (striatum) as well as neighbouring structures of the cerebrum, like the orbit.
The internal carotid arteries originate from the common carotid arteries (left and right common carotid arteries), at the bifurcation of these arteries (common carotids) at the level of the 4th cervical vertebra. The internal carotid artery then courses upwards within the carotid sheath, and enters the cranial cavity through the carotid canal of the temporal bone.
Once in the cranial cavity, it runs anteriorly through the cavernous sinus, giving off a branch – the ophthalmic artery, then curls back to lie in the front half of the roof of the cavernous sinus. The artery then turns vertically upwards to the anterior perforated substance where it divides into the middle cerebral branch (middle cerebral artery) and the anterior cerebral branch (anterior cerebral artery).
At the anterior perforated substance, it also gives off other branches to complete its half of the circle. It gives off the striate arteries (these arteries are terminal branches of the middle cerebral artery, and they supply the striatum – which is the input unit of the basal ganglia), the anterior choroidal artery, anterior communicating artery (which is a very short artery connecting the right and left anterior cerebral arteries) and the posterior communicating arteries. The posterior communicating arteries anastomose with the posterior cerebral arteries from the vertebrobasilar system, and thus connecting the internal carotid arteries to the vertebrobasilar system to complete the circle of Willis.
Course & Distribution
Details of the course and distribution of the circle's tributaries are given below:
From the Internal Carotid Artery
- Ophthalmic artery - the internal carotid artery gives off this branch after emerging from the roof of the cavernous sinus. The ophthalmic artery supplies the structure of the orbit. For example, it supplies the eyelids through its palpebral branches. It also supplies the retina.
- Middle cerebral artery and striate artery - the middle cerebral artery is the largest and most direct branch of the internal carotid artery and therefore most subject to embolism (obstruction of an artery by an embolus like air-bubble, or clot of blood). It runs deep into the lateral sulcus (lateral fissure) to supply the cortex of the insula and overlying opercula. With its terminal branches called the striate arteries, it supplies the internal capsule, thalamus and the basal ganglia (basal nuclei). Note: that the striate branches derived their name from the “Striatum” which is the input unit of the basal ganglia. It is also important to note that some authors describe the striate branches as branches from the anterior and middle cerebral arteries.
- Anterior cerebral artery - this branch leaves the internal carotid at the anterior perforated substance and passes forwards above the optic nerve, running in the interhemispheric fissure or medial longitudinal fissure to supply the whole of the medial surface of the cerebrum above the corpus callosum as far back as the parieto-occipital sulcus and the orbital part or orbital surface of the frontal lobe. Its distribution extends over the superior border to meet the area supplied by the middle cerebral artery. Hence the anterior cerebral artery supplies areas of the cerebrum including the motor areas (area 4- and part of area 6 of Brodmann) and sensory (area 1-, 2- & 3 of Brodmann) areas for the opposite leg and perineum , as well as the micturition and defaecation centres.
- Anterior choroidal artery - it supplies the choroid plexus, passes below the optic tract to enter the inferior extremity of the choroid fissure. Through its branches, it also supplies the optic chiasm, optic tract, lateral geniculate body and the internal capsule.
- Posterior communicating artery - it connects the internal carotid artery to the posterior cerebral artery. That is, it joins the anterior half of the Willis circle (contributed primarily, by the internal carotid arteries) to the posterior half (from the vertebrobasilar system).
- Anterior communicating artery - this is a very short single artery. It connects the left and right anterior cerebral arteries to each other.
From the Basilar Artery
- Pontine artery - the pontine arteries are several small branches from the basilar artery. They run in groups to supply the medial portion of the ventral part of the pons as paramedian branches, supply the lateral portion of the ventral part of the pons as short circumferential branches, and the dorsal part of the pons as long circumferential branches.
- Superior cerebellar artery - the left and right superior cerebellar arteries are formed just before the basilar artery bifurcates into the posterior cerebral arteries. It curls around the upper margin of the pons. It supplies the pons, midbrain and superior surface of the cerebellum.
- Anterior inferior cerebellar artery - arises from the basilar artery at the lower part of the pons and passes back on the inferior surface of the cerebellar hemisphere, supplying this surface and the adjacent flocculus.
- Posterior cerebral artery - this artery curls back around the cerebral peduncle, supplying it (cerebral peduncle) and the optic tract. It then passes back above the tentorium to supply the inferomedial surface of the temporal and occipital lobes. The visual area (areas 17, 18 and 19 of Brodmann) for the opposite field of vision lies wholly within the supply of the posterior cerebral artery.
From the Vertebral Artery
- Posterior inferior cerebellar artery - this is the largest branch of the vertebral artery. It arises ventrally from the vertebral artery, near the lower end of olive, and spirals back around the medulla below the hypoglossal rootlets and then between the rootlets of the glossopharyngeal nerve and vagus nerve. It supplies the choroid plexus of the 4th ventricle, the adjacent part of the medulla and the posterior part of the cerebellar hemispheres.
- Anterior spinal artery - this is a single artery formed by the two vertebral arteries, and runs inferiorly in the anterior median fissure of the spinal cord. It is formed by union of the anterior spinal branch from each vertebral artery. It supplies the whole spinal cord anterior to the posterior grey column.
- Posterior spinal artery - this artery runs inferiorly on the posterior aspect of the spinal cord. It supplies the grey and white posterior columns of its own side (posterior side of the spinal cord).
The circle of Willis allows equalization of blood flow between the left and right cerebral hemispheres, and can allow anastomotic circulation if parts are occluded. That is, the circle serves as a back-up system or a bypass, allowing for an alternative route if there is an occlusion in the normal route of supply to an area. For example, if there is an obstruction of blood supply through the left internal carotid artery, and blood cannot reach the front of the left side of the brain through this artery, blood will be routed to this area, through the anterior communication artery, from the right internal carotid artery.