Circle of Willis
Circle of Willis is a combined name given to the arteries which supply Brain.
Brain receives its arterial supply from two pairs of vessels, the vertebral arteries, and internal carotid arteries. These vessels are interconnected in the cranial cavity to form a cerebral arterial circle called a circle of Willis. The circle of Willis lies in the interpeduncular fossa at the base of the brain. It is named after a physician Thomas Willis.
Circle of Willis is formed by the anastomosis of arteries which provide a backup of arterial circulation when an artery is occluded. This interconnection of arteries prevents the supplied area of the brain from ischemia.
Location of the circle of Willis:
This structure is the circle of interconnected arteries and it is located at the base of the brain. Its location is near to the pituitary stalk, optic chiasm, and hypothalamus.
Anatomy overview of the circle of Willis
The circle of Willis lies in the interpeduncular fossa at the base of the brain. Circle of Willis is arterial ring just at the base of the brain (around eye level). It is formed by the anastomosis between the two internal carotid arteries and the two vertebral arteries. The anterior communicating, anterior cerebral, internal carotid, posterior communicating, posterior cerebral, and basilar arteries all contribute to the circle.
Circle of Willis is formed between the two major arteries.
- Two Internal Carotid Arteries
- Two vertebral arteries.
Other contributing arteries of the circle of Willis are
- The anterior communicating artery.
- Anterior cerebral arteries.
- Internal carotid arteries.
- Posterior communicating arteries.
- Posterior cerebral arteries
- Basilar arteries
Circle of Willis formation:
Common carotid arteries give rise to internal carotid arteries. The left and right internal carotid arteries give rise to left and right posterior communicating arteries it gives its terminal branches called anterior and middle cerebral arteries. The two vertebral arteries enter the cranial cavity through the foramen magnum and just inferior to the pons fuse to form the basilar artery. Basilar artery gives rise to posterior cerebral arteries. The anterior communicating artery connects the two anterior cerebral arteries. Note that middle cerebral arteries supplying the brain, are not considered part of this circle of Willis. The circle of Willis allows blood that enters by either internal carotid or vertebral arteries to be distributed to any part of both cerebral hemispheres. Cortical and central branches arise from the circle and supply the brain substance.
Function of the circle of Willis:
Circle of Willis is a very important anastomotic structure in the brain that provides blood supply to the fore-brain and hind-brain. This network of arteries provides blood to be distributed to both hemispheres from the internal carotid arteries or vertebral arteries. The circle of Willis provides safety to the brain. circle of Willis allows blood to pass blood if an artery is occluded on either side. This function allows collateral circulation to the affected side. Circle of Willis have an important role in autoregulation of the blood coming from the vessels of the circle of Willis.
Variations of the circle of Willis
There are considerable variations exist in the circle of Willis. In one variation there is a narrowing of proximal part of the posterior cerebral artery and the ipsilateral posterior communicating artery is large so the internal carotid artery supplies the posterior cerebrum. In another variation, there is large anterior communicating artery so that a single internal carotid artery supplies both anterior cerebral arteries.
Cerebral circulation arteries of the circle of Willis
Internal carotid arteries
The internal carotid artery begins at the bifurcation of the common carotid artery. At that place, there is localized dilatation, called the carotid sinus. It passes through the carotid canal at the base of the skull of the temporal bone. The artery then runs horizontally forward through the cavernous sinus and emerges on the medial side of the anterior clinoid process by perforating the dura mater. It now enters the subarachnoid space by piercing arachnoid mater.
Branches of internal carotid arteries
The ophthalmic artery:
This artery arises when the internal carotid artery emerges from the cavernous sinus. It enters the orbit through the optic canal below and lateral to the optic nerve. It supplies the eye and other orbital structures. The terminal branches of ophthalmic artery supply the frontal area of the scalp, the ethmoid and frontal sinuses, and the dorsum of the nose.
The posterior communicating artery:
This is a small artery that originates from the internal carotid artery close to its terminal bifurcation. The posterior communicating artery runs posteriorly above the oculomotor nerve and joins the posterior cerebral artery, thus forming part of the circle of Willis.
The choroidal artery:
Choroidal artery is a small branch which originates from the internal carotid artery close to its terminal bifurcation. The choroidal artery passes close to the optic tract and ends in the choroid plexus. It gives off numerous small branches to surrounding structures, including the crus cerebri, the lateral geniculate body, the optic tract, and the internal capsule.
The anterior cerebral artery:
This is the smaller terminal branch of the internal carotid artery. It runs along the optic nerve and enters the longitudinal fissure of the cerebrum. Here, it is joined to the anterior cerebral artery of the opposite side by the anterior communicating artery. Anterior cerebral artery then curves backward over the corpus callosum and, finally, anastomoses with the posterior cerebral artery. The cortical branches supply all the medial surface of the cerebral cortex. The anterior cerebral artery thus supplies the “leg area” of the precentral gyrus. Central branches helps to supply parts of the lentiform and caudate nuclei and the internal capsule.
The middle cerebral artery:
Though it is not part of the circle of Willis, it is the largest branch of the internal carotid, runs laterally in the lateral cerebral sulcus. Cortical branches supply the entire lateral surface of the hemisphere, except for the narrow strip supplied by the anterior cerebral artery, the occipital pole, and the inferolateral surface of the hemisphere, which are supplied by the posterior cerebral artery. Middle cerebral artery supplies all the motor area except the “leg area”.
Each vertebral artery arises from the first part of each subclavian artery in the lower part of the neck and passes superiorly through the transverse foramina of the upper six cervical vertebrae. It enters the skull through the foramen magnum and pierces the dura mater and arachnoid to enter the subarachnoid space. It then passes on the medulla oblongata. At the lower border of the pons, it joins the vessel of the opposite side to form the basilar artery.
Branches of the Cranial Portion
- The meningeal branches:
- The posterior spinal artery
- The anterior spinal artery
- The posterior inferior cerebellar artery
- The medullary arteries
The basilar artery is formed by the union of the two vertebral arteries.It ascends in a groove on the anterior surface of the pons. At the upper border of the pons, it divides into the two posterior cerebral arteries.
- The pontine arteries
- The labyrinthine artery
- The anterior inferior cerebellar artery
- The superior cerebellar artery
- The posterior cerebral artery
Clinical significance of circle of Willis
Circle of Willis perfuse the brain. The arterial blood reaches the brain through the two internal carotid arteries and the two vertebral arteries, the internal carotid arteries are the major supply of arterial blood. Circle of Willis provides distributing arteries—the anterior, middle, and posterior cerebral arteries pass over the outer surface of the brain and anastomose with one another. They give rise to branches that penetrate the brain at right angles. It is estimated that cerebrovascular disease is still responsible for about half of all adult neurologic hospital admissions.
Congenital aneurysm :
Cerebral aneurysm in the circle of Willis
Congenital aneurysms occur most commonly at the site where two arteries join in the formation of the circle of Willis. At aneurysm site, there is the deficiency in the tunica media layer of the vessel and this is complicated by the development of atheroma, which so weakens the arterial wall that a local dilatation occurs. This local dilatation may press on neighboring structures and produce signs or symptoms or may suddenly rupture into the subarachnoid space. Bleed in subarachnoid space results in severe pain. Death may occur quickly, or the patient may survive the first bleeding only to die a few days or weeks later. Clipping or ligating the neck of an aneurysm gives the best chance of recovery.
Subarachnoid hemorrhage usually results from leakage or rupture of a congenital aneurysm present on the circle of Willis. Less commonly, it may occur from an angioma or contusion and laceration of the brain and meninges. The symptoms include a severe headache, stiffness of the neck, and loss of consciousness. The diagnosis is established by computed tomography (CT). The dense areas of the blood in the subarachnoid space can be identified.
Sometimes a blood clot gets impacted by blood vessels and compromises blood supply to the supplying area and this leads to serious complications.
Circle of Willis