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B. Vertebrobasilar System

1. Vertebral artery

It arises from the proximal subclavian artery and ascends through the transverse foramina of the first cervical vertebra. It then passes posteriorly around the articular process of the atlas to enter the skull through the foramen magnum. The two vertebral arteries join each other at the level of the pontomedullary junction to form the basilar artery. The vertebral artery gives rise to anterior and posterior spinal arteries, the posterior inferior cerebellar artery and branches to the medulla (Francke et al., 1981).

2. Basilar artery

It is formed by the two vertebral arteries joining each other in the midline. It ascends along the ventral aspect of the pons. It ends at the ponto-midbrain junction where it divides into two posterior cerebral arteries. It gives rise to anterior inferior cerebellar artery, superior cerebellar artery and numerous paramedian, short and long circumferential penetrators. The internal auditory (labyrinthine) artery arises from the basilar artery in about 20 % of the population whereas in the remainder it arises from the anterior inferior cerebellar artery.

3. Posterior cerebral artery (PCA):

The basilar artery ends by dividing into the two posterior cerebral arteries. They encircle the midbrain close to the occulomotor nerve at the level of tentorium cerebelli and supply the inferior part of the temporal lobe and the occipital lobe  (Marinkovic et al., 1987). Soon after their origin, they anastomose with the posterior communicating arteries to complete the circle of Willis. Many small perforating arteries arise from PCA to supply the midbrain, the thalamus, hypothalamus and geniculate bodies. In fifteen per cent of the population, the PCA is a direct continuation of the PoCA, its main blood supply then comes from the ICA rather than from the vertebrobasilar system.

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Article reviewed by:	Dr. Tamer Fouad, M.D.
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