POLYGON OF WILLIS: LOCATION, ANATOMY AND FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2025

The polygon of Willis, also called the ring of Willis or the cerebral arterial circle, is a heptagon-shaped arterial structure that is located at the base of the brain. This structure is made up of two groups of arteries: the internal carotid arteries and the vertebrobasilar system. The latter is composed of two vertebral arteries and the basilar artery.

This network is organized anteroposteriorly. That is, the carotid arteries and their branches supply the anterior area and the vertebral arteries and their branches are in the posterior part.

The Polygon of Willis surrounded in the image. John A Beal, PhD Dep't. of Cellular Biology & Anatomy, Louisiana State University Health Sciences Center Shreveport / CC BY (https://creativecommons.org/licenses/by/2.5)

This arterial polygon is the main responsible for cerebral irrigation. That is, it supplies blood to the brain and surrounding areas. It is often described as an anastomosis system. This means that it is made up of a network of connections between arteries.

Most individuals have a complete Willis polygon. However, good communication between their structures has been identified in just under half of the population.

This structure had previously been observed by other physicians. Although it was the English physician Thomas Willis (1621-1675) who described it most clearly in his book Cerebral Anatome, in 1664.

Location

The polygon of Willis is at the base of the brain. It surrounds the stalk of the pituitary gland, the optic chiasm, and the hypothalamus.

The brain has a more complex blood supply composed of four large arteries that form two large vascular systems: the vertebrobasilar system and the anterior system. The fusion of both gives rise to the Willis polygon.

Representation of the cerebral arterial circuit or polygon of Willis

Anatomy

The polygon of Willis is an anatomical structure shaped like a heptagon. It is made up of an anastomosis (connection) between the arterial structures of the anterior and posterior circulatory system. This polygon is made up of the following arteries:

Polygon of Willis anterior

It is made up of the internal carotid artery and supplies blood flow to the front of the brain. It irrigates most of the cerebral hemispheres. As well as some deep structures such as the caudate nucleus, the putamen or nearby structures such as the orbit.

The internal carotid arteries come from the left and right common carotid arteries. Specifically, they arise from the bifurcation of the common carotid arteries at the level of the fourth cervical vertebra.

The internal carotid arteries give rise to different branches:

- Ophthalmic artery: this irrigates the part of the orbit. Thus, it supplies blood to the eyelids and the retina, among others.

- Middle cerebral artery: it is the largest and most direct branch of the internal carotid artery, being the most vulnerable to embolisms. It supplies blood to the cortex of the insula and other adjacent areas.

- Anterior cerebral artery: it supplies motor areas of the brain such as Brodmann's areas 4 and 6, and sensory areas such as Brodmann's 1, 2 and 3. They also supply the orbitofrontal area of ​​the frontal lobe, as well as the nuclei of urination and defecation.

- Striated arteries: they supply blood to the internal capsule, the thalamus and the basal ganglia.

- Anterior choroid artery: provides blood flow to the choroid plexuses. Through its branches it also supplies the optic chiasm, the optic tracts, the internal capsule and the lateral geniculate nucleus.

- Anterior communicating artery: consists of a very short artery that connects the anterior, right and left cerebral arteries.

- Posterior communicating arteries: these join the internal carotid artery and the posterior cerebral artery.

Artery at the base of the brain. User Uwe Gille Derivative work: Skeptic0 translations by Dalton2 / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Posterior polygon of Willis

It is made up of the vertebral arteries. This half of the polygon provides blood circulation. Mainly to the cerebellum, brain stem, and back of the cerebral hemispheres.

Two vertebral arteries originating from the subclavian artery join at the lower border of the brain stem, forming a single artery: the basilar artery. All its components make up the vertebrobasilar system. The following branches depart from the basilar artery:

- Pontine artery: they involve small branches of the basilar artery. They supply blood to the ventral part of the pontine nucleus and to the lateral part of the pons.

- Superior cerebellar artery: regulate the blood circulation of the pons, the midbrain and the upper area of ​​the cerebellum.

- Anterior cerebellar artery: delivers blood to the lower surface of the cerebellar hemisphere.

- Posterior cerebral artery: it supplies the cerebral peduncles and the optic tract, as well as the inferomedial part of the occipital and temporal lobes. It also supplies blood to visual areas (Brodmann areas 17, 18 and 19).

On the other hand, the following branches arise from the vertebral artery:

- Inferior-posterior cerebellar artery: it is the major branch of the vertebral artery. It allows blood flow in the colloid plexus of the fourth ventricle. The adjacent area of ​​the medulla and the posterior area of ​​the cerebellar hemispheres.

- Anterior spinal artery: it is located in the middle fissure of the spinal cord and supplies the entire anterior spinal cord, as well as the posterior gray column.

- Posterior spinal artery: this supplies blood to the posterior columns of the spinal cord.

Function

This circle generates essential communication in the blood supply between the forebrain and the hindbrain. It also allows blood flow to equalize between the two sides of the brain (left and right hemispheres).

The primary function of the circle of Willis appears to be to provide an alternative route if there is occlusion of the blood supply in the usual route. For example, if blood flow in the left internal carotid artery is blocked, blood cannot reach the left front of the brain.

Top view of the Circle of Willis.

Thanks to the circle of Willis, blood can reach this area through the anterior communication artery from the right internal carotid artery.

This network of arteries has the function of allowing a correct distribution of cerebral circulation in the event of damage or decreased blood flow in one or more nearby vessels. This redistribution depends on the presence and size of the existing blood vessels.

Willis polygon involvement

Circle of Willis with the most common locations of ruptured aneurysms marked. Source: Gray's Anatomy of the Human Body, originally published in 1918. Public domain file

If the blood flow to any part of this structure is obstructed, the irrigating areas are left without oxygen and nutrients. This leads to brain lesions that can manifest themselves through various symptoms, depending on the affected area.

Some consequences of this are paralysis or weakness in the middle of the body, personality changes, aphasia, loss of sensation of the extremities, visual problems such as hemianopia, etc.

References

1. Circle of Willis. (sf). Retrieved on April 11, 2017, from KENHUB: kenhub.com.
2. Circle of Willis. (sf). Retrieved on April 11, 2017, from Wikipedia: en.wikipedia.org.
3. Gaillard, F. e. (sf). Circle of Willis. Retrieved on April 11, 2017, from Radiopaedia: radiopaedia.org.
4. Madrid Muñis, C. e. (sf). Study of the variants of the Polygon of Willis. Retrieved on April 11, 2017, from EPOS: posterng.netkey.at.
5. Tubbs Shane, R. (June 3, 2013). Circle of Willis Anatomy. Obtained from Medscape: emedicine.medscape.com.