Willis Polygon - Circle of Willis by A.Griguolo

Generality

The polygon of Willis is a circle of interconnected arteries, located at the level of the brain and deputed to oxygenate the cerebral hemispheres, the cerebellum and the brainstem.

Example of anastomosis, the Willis polygon results from the participation of the internal carotid arteries, the arteries of the vertebrobasilar system (ie the vertebral arteries and the basilar artery), of the anterior cerebral arteries, of the anterior communicating artery, of the posterior communicating arteries and posterior cerebral arteries.

Brief review of the meaning of Anastomosis

In anatomy, the term anastomosis generally indicates a connection between blood vessels; however, it can also refer to a connection between nerves, between connective fibers, between myocardial fibers or between two organs.

Remaining on the subject of anastomosis between blood vessels, this can be:

Arterial type, if it involves arteries,
Venous type, if involves veins,
Arteriovenous type, if it connects arteries with veins.

What is the Willis Polygon?

The polygon of Willis, or circle of Willis, is the arterial anastomosis constituting an important part of the circulatory system, obviously arterial, of the brain.

Origin of the name

Willis's polygon owes its name to Thomas Willis (1621-1675), that is the British doctor who identified it and described it for the first time.

Anatomy

The Willis polygon is a circle of interconnected arteries, located at the base of the skull, near the optic chiasm, the hypothalamus and the so-called pituitary stem .

Structure

The Willis polygon results from the interconnection of two important arterial systems:

The system of internal carotid arteries, which includes the right internal carotid artery, the left internal carotid artery and the branches of these, and
The vertebrobasilar system, which is formed by the right and left vertebral arteries, the basilar artery and the branches of the latter.

SYSTEM OF INTERNAL CAROTID ARTERIES

The system of internal carotid arteries represents the anterior portion of the Willis polygon as well as the part of the latter closest to the forehead and more distant from the nape.

Its constituent elements are:

The right internal carotid artery, which originates from the right common carotid artery, at the level of the IV cervical vertebra;

The left internal carotid artery, which originates from the left common carotid artery, always at the level of the IV cervical vertebra;
The right anterior cerebral artery and the left anterior cerebral artery . Branch of the right internal carotid artery, the first, and branch of the left internal carotid artery, the second, are the branches that, flowing into the anterior communicating artery, close the circle of Willis anteriorly;
The anterior communicating artery, which is an indirect branch of the internal carotid arteries and represents, as already mentioned, the connecting element between the right anterior cerebral artery and the left anterior cerebral artery;
The right posterior communicating artery and the left posterior communicating artery . Another branch of the right internal carotid artery, the first, and another branch of the left internal carotid artery, the second, are the branches that, through the posterior cerebral arteries, connect the system of carotid arteries internal to the vertebrobasilar system.

VERTEBRO BASILARE SYSTEM

The vertebro-basilar system constitutes the posterior portion of the Willis polygon as well as the part of the latter closer to the nape and more distant from the forehead.

Its components are:

The right vertebral artery, which originates from the right subclavian artery, goes up the neck, towards the encephalon, and, at the level of the Varolio Bridge, meets the left vertebral artery;

The left vertebral artery, which originates from the left subclavian artery, goes up the neck towards the encephalon, and, as already mentioned, at the level of the Varolio Bridge, joins the right vertebral artery;
The basilar artery, which is the fruit of the encounter between the right vertebral artery and the left vertebral artery;
The right posterior cerebral artery and the left posterior cerebral artery . Terminals of the basilar artery are, as previously announced, the elements connecting the internal carotid arteries system; to be precise, the right posterior cerebral artery joins the right posterior communicating artery, while the left posterior cerebral artery joins the left posterior communicating artery.

The polygon of Willis is ideally comparable to a heptagon, in which the 7 sides are: the anterior communicating artery, the two anterior cerebral arteries, the two posterior communicating arteries and the two posterior cerebral arteries.

This curious way of describing the Willis polygon excludes the internal carotid arteries and the vertebrobasilar system, but only to simplify the representation; if it included these elements, it would place:

The right internal carotid artery on the vertex where the right anterior cerebral artery and the right posterior communicating artery meet;
The left internal carotid artery on the vertex where the left anterior cerebral artery and the left posterior communicating artery meet;
The vertebrobasilar system, to be precise the basilar artery, on the vertex where the right and left posterior cerebral arteries meet.

Anatomical variants

The circle of Willis is a structure of the circulatory system of which there are numerous anatomical variants and that only in 25-35% of people has the characteristics reported in this article and corresponding to normal (or classical) anatomy.

According to several epidemiological studies, the anatomical variants of the Willis polygon would most often have as its object the posterior portion, ie the vertebrobasilar system.

EXAMPLES OF VARIANTS

Among the most common anatomical variants of the Willis polygon are:

The polygon of Willis with hypoplasia of one or both posterior communicating arteries (NB: in medicine, "hypoplasia" is synonymous with "reduced or incomplete development of an organ or anatomical structure");
The Willis polygon with hypoplasia or absence of a particular segment of anterior cerebral artery;
The polygon of Willis with the absence of the anterior communicating artery;
The Willis polygon with the posterior cerebral arteries that originate from the internal carotid arteries and with hypoplasia or absence of a particular tract of the posterior communicating arteries.

Function

Through the arteries that constitute it, the polygon of Willis contributes to supplying oxygenated blood, fundamental for cellular survival, both hemispheres of the brain, the cerebellum and the brainstem .

Wanting to deepen, using the system of internal carotid arteries, the polygon of Willis distributes oxygenated blood to the cerebral lobes of the mid-anterior section of each hemisphere ( frontal lobe and parietal lobe ) and to the structures underlying the aforesaid lobes; while, through the vertebro-basilar system, it provides to spray the encephalic trunk, the cerebellum, the mid-posterior portions of each hemisphere (including the temporal lobe and the occipital lobe ).

Did you know that ...

The spraying action of the Willis polygon is combined with the spraying action of the middle cerebral artery, an equal artery of the circulatory system of the brain that originates (therefore it is a branch of it) from the internal carotid artery.

Although it is in the immediate vicinity of the Willis polygon, the middle cerebral artery is not part of it, but it constitutes an arterial vessel in its own right.

The Willis Polygon is an intelligent and functional structure

Evolution gave birth to Willis's polygon, because it had to guarantee the brain with a blood supply system, in which, even in the presence of an occlusion, it could still effectively circulate the blood.

In fact, a system of arteries such as the Willis polygon, in which there is a total interconnection of the various components, makes it possible for the blood to take an alternative route, when the canonical one presents an obstacle.

The polygon of Willis is, therefore, a structure of the "intelligent" and extremely "functional" cerebral circulatory system.

The brain is a very important organ, therefore it must always receive the right blood supply, through the arterial vessels in charge.

diseases

The Willis polygon can be the protagonist of various pathological states; these pathological states include thrombosis, aneurysm and subclavian theft syndrome .

Thrombosis

In medicine, the term "thrombosis" indicates the pathological process of formation of an abnormal blood clot, the specific one of which is thrombus, on the wall of an artery or vein.

Thrombosis is a dangerous phenomenon, because it produces an obstacle to the circulation of the blood and because, if it detaches from the place of formation, it could circulate freely in the blood and completely obstruct a blood vessel of the same size.

Episodes of thrombosis affecting the Willis polygon can hinder blood circulation, especially when they simultaneously involve different points in the arterial system in question.

Aneurysm

An aneurysm is a permanent abnormal dilation of a short segment of artery, which can, due to the fragility of its wall, break and give rise to an often fatal internal hemorrhage.

Whether they relate to the Willis polygon or to other arterial vessels (eg: aorta), aneurysms are conditions that are difficult to manage therapy, as they are incurable or their treatment presents many more risks than benefits.

Subclavian Theft Syndrome

Subclavian theft syndrome is a set of symptoms and signs, which arises from a subclavian artery stenosis and reduces blood circulation within the Willis polygon to maintain adequate blood supply to the upper limb.