The arterial blood supply to the brain derives from the vertebral and internal carotid arteries. The vertebral arteries are paired, each arising from their respective subclavian artery, and ascend deep in the neck. They travel through the transverse foramina of the cervical vertebrae at the levels of C6 through C1. At the foramen magnum of the occipital bone, they enter the posterior cranial fossa and the subarachnoid space. The vertebral arteries join at the ventral aspect of the medulla oblongata to form the basilar artery, which interconnects to the internal carotid arteries to form the posterior part of the circle of Willis. The internal carotid arteries dominate the circulation of the brain.
The circle of Willis or cerebral arterial cycle is a polygonal-shaped network of anastomosing arteries located at the cranial base and encircle the pituitary stalk. It is primarily formed by four paired and one single artery that supplies numerous brain branches. It also serves as a safety mechanism in cases of arterial blockage due to arteriosclerosis.
Numerous anatomical variations are documented in the literature for the circle of Willis and its branches. Many of these variations carry clinical significance in surgery and radiology, highlighting the need for awareness. The circle is typically formed by the internal carotid arteries, anterior cerebral arteries, anterior communicating artery, posterior communicating arteries, posterior cerebral arteries, and the basilar artery.
The internal carotid arteries arise from the common carotid arteries bilaterally. They pass through the carotid canals of the temporal bones located at the skull base. They supply the frontal, parietal, and temporal lobes of the brain, as well as the diencephalon. The anterior cerebral arteries are terminal branches of the internal carotid artery, and they anastomose together via the anterior communicating artery forming the anterior portion of the circle of Willis. The anterior cerebral arteries supply oxygenated blood to the medial parts of the frontal and parietal lobes of the cerebrum.
The middle cerebral arteries are also terminal branches of the internal carotid artery. They provide blood to the lateral parts of the frontal, temporal, and the parietal lobe of the cerebrum, part of the basal ganglia, and the internal capsule. The middle cerebral arteries are the most common sites involved in ischemic strokes.
The posterior cerebral arteries are the terminal branches of the basilar artery and supply the occipital lobes and posteromedial temporal lobes. They anastomose with the posterior communicating arteries on each side and, together with the basilar artery, create the posterior portion of the circle of Willis.
The basilar artery is shaped from the union of the two vertebral arteries. Altogether they formulate the vertebrobasilar arterial system. The basilar artery represents the most common site of intracranial aneurysms. It proceeds across the anterior surface of the pons. It supplies the cerebellum, the brainstem, and the posterior brain regions. Along its course, the basilar artery gives rise to the superior cerebellar arteries, anterior inferior cerebellar artery, the pontine arteries, and the labyrinthine arteries, which supply the inner ear. In the majority of the population, the labyrinthine arteries arise from the anterior inferior cerebellar arteries.
The superior cerebellar arteries supply the cerebellum and parts of the midbrain. The anterior inferior cerebellar arteries supply the pons and the cerebellum. The posterior inferior cerebellar arteries maintain an essential role in the blood supply of the cerebellum, and they are the most common area of cerebellar ischemic strokes.
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