The vertebral arteries and internal carotid arteries are responsible for the arterial supply of the brain.
These vessels are interconnected in the cranial cavity to form this cerebral arterial circle called the polygon of Willis.
The polygon of Willis, also called the circle of Willis, is a heptagonal arterial structure located at the base of the brain that supplies blood, which plays an important role in the supply of oxygen and nutrients to the brain and surrounding structures.
This anastomosis of the arteries provides a backup of arterial circulation when an artery is occluded. This interconnection of arteries prevents ischemia in the supplied area of the brain.
The Polygon of Willis is named after Thomas Willis, an eminent English physician, who described the arterial ring present at the base of the brain 400 years ago.
Willis polygon location
This structure is the circle of several interconnected arteries and is located in the interpeduncular fossa, an arterial ring just at the base of the brain. Its location is close to the pituitary stalk, and forms structures such as the optic chiasm, the hypothalamus, and the pituitary.
Description of the anatomy of the polygon of Willis
The polygon of Willis is more specifically, a circulatory anastomosis, that is, an interconnection in a network of parts or elements, such as between arteries, veins or between an artery and a vein.
In this case, the differentiated arteries that surround the stalk of the pituitary gland and allow the distribution of blood to the brain and nearby structures.
The polygon of Willis is formed by the anastomosis between the two internal carotid arteries and the two vertebral arteries.
The anterior communicating artery, anterior cerebral arteries, internal carotid arteries, posterior communicating arteries, posterior cerebral arteries, and basilar arteries also contribute to the circle.
Circulations of the brain of the arteries of the circle of willis
Cerebral circulation begins at the bifurcation of the internal carotid artery, here there is a dilation called the carotid sinus.
Then it passes through the carotid canal which is located in the temporal bone at the base of the skull.
This artery runs horizontally forward through the cavernous sinus and pierces the dura mater and emerges on the medial side of the clinoid process.
Finally it pierces the arachnoid matter entering the subarachnoid space.
Formation of the polygon of Willis
The formation of the circle of Willis begins with the anastomosis between the internal carotid arteries and the vertebral arteries.
The vertebral arteries arise from the first part of each subclavian artery in the lower part of the neck and pass superiorly through the transverse foramina of the six upper cervical vertebrae.
The basilar artery is formed by the union in the brainstem of the two vertebral arteries. It ascends around the vertebrae in a groove on the anterior surface of the pons and there divides into the two posterior cerebral arteries.
The posterior cerebral arteries supply the lower and deep areas of the temporal and occipital lobes, and their function is to supply blood to the brainstem, occipital lobe, and other posterior regions of the brain.
The common carotid arteries give rise to the internal carotid arteries when they enter the skull. Later it branches into the anterior and middle cerebral arteries.
They are responsible for the supply of blood (oxygen and nutrients) to the anterior part of the brain: cortex and cortical structures.
The posterior communicating arteries allow communication between the internal carotid and the posterior cerebral artery, these also have their origin near the terminal bifurcation of the internal carotid artery.
The anterior communicating artery connects the right anterior cerebral artery and the left anterior cerebral artery, serving as a bridge between both hemispheres
The bifurcations of the internal carotid artery give rise to its terminal branches, which are called the anterior and middle cerebral arteries.
The middle cerebral artery is the branch most vulnerable to occlusions. The blood supply of this artery reaches the orbital, frontal, parietal and temporal regions.
The anterior cerebral artery is located along the optic nerve and enters the longitudinal fissure of the brain and eventually anastomoses with the posterior cerebral artery. This is the smallest terminal branch of the internal carotid artery.
The middle cerebral arteries that supply blood to the brain are not considered part of this circle of Willis. They are the largest branch of the internal carotid, supplying the entire motor area except the “leg area.”
The polygon of Willis surrounds the stalk of the pituitary gland and provides important communications between the forebrain blood supply and the hindbrain, that is, between the internal carotid and the vertebro-basilar systems after obliteration of the primitive embryonic connections .
Although a full circle of Willis is present in some individuals, it is rarely seen radiographically in its entirety.
Anatomical variations are very common and in less than half of the population a well-developed communication is identified between each of its parts.
The blood flow in the circle of Willis begins at the right and left internal carotid artery, enters the cranial cavity and divides between the two main branches: the anterior cerebral artery and the middle cerebral artery.
The anterior cerebral arteries join and blood can flow through the anterior communication artery.
The basilar artery gives off small branches to the pons, and this labyrinth artery supplies the inner ear.
Willis polygon function
The polygon of Willis is a very important anastomotic structure in the brain that provides blood supply to the forebrain and hindbrain through it flows the blood that supplies much of the brain.
This network of arteries provides blood to be distributed to both cerebral hemispheres from the internal carotid arteries or vertebral arteries, balancing the blood supply.
The polygon of Willis provides security to the brain, it acts as a safety valve for the brain, it allows the passage of blood to the brain even if there is an alteration or damage somewhere in the artery.
This function allows collateral circulation to the affected side. The Polygon of Willis has an important role in the self-regulation of the blood that comes from the vessels located in this polygon.
Thus if one of the vessels supplying blood to the brain becomes narrowed or completely blocked, the other vessels can preserve the cerebral perfusion pressure (that is, the net pressure gradient that causes cerebral blood flow to the brain) well enough to prevent the onset of a condition called ischemia (that is, restriction of blood supply to tissues).
Variations of the circle of Willis
It should be noted that, anatomically, the polygon of Willis is not the same in all individuals, it is found to have abnormalities in almost 50% of people.
There are considerable variations in the circle of Willis such as:
- In one variation there is a narrowing of the proximal part of the posterior cerebral artery and the ipsilateral posterior communicating artery is large, so the internal carotid artery supplies the hindbrain.
- In another variation, there is a large anterior communicating artery, such that a single internal carotid artery supplies both anterior cerebral arteries.
Clinical significance of the circle of Willis
The cerebral arterial circle plays a crucial role in blood circulation within the cortical area.
Arterial blood reaches the brain through the two internal carotid arteries and the two vertebral arteries, the internal carotid arteries are the main arterial blood supply.
The polygon of Willis provides arteries of distribution: the anterior, middle, and posterior cerebral arteries pass over the outer surface of the brain and anastomose with each other. They give rise to branches that penetrate the brain at right angles.
It is estimated that cerebrovascular disease remains responsible for approximately half of all neurological hospital admissions in adults.
Diseases that can cause blockages
Traumatic injuries, an aneurysm, or a cardiovascular accident in this region can cause an obstruction.
If an obstruction appears in the polygon, the irrigated areas are left without oxygen and the consequences can be the loss of mental and physical functions, sensitivity or motor capacity and even death.
Morphological abnormalities in vessels can be causal factors in the development of aneurysms.
Congenital aneurysms most often occur where two arteries meet in the circle of Willis formation.
At the site of the aneurysm, there is a deficiency in the tunica media layer of the vessel and this is complicated by the development of an atheroma, which weakens the arterial wall so much that local dilation occurs.
This local dilation can press on neighboring structures and produce signs or symptoms, or it can suddenly rupture in the subarachnoid space.
Subarachnoid hemorrhage is usually due to a leak or rupture of a congenital aneurysm present in the circle of Willis.
Less commonly, it can occur from an angioma or contusion and laceration of the brain and meninges.
Sometimes a blood clot is affected by the blood vessels and compromises the blood supply to the supply area and this leads to serious complications.