Posterior Cerebral Artery

At the level of the midbrain the basilar artery bifurcates to form the two posterior cerebral arteries (PCA). If an embolus travels in a vertebral branch it may stop where the vertebral arteries join to form the basilar artery. More often, however, it keeps going, traversing the basilar (which has a large diameter) and is arrested at the upper bifurcation of the basilar into the posterior cerebral arteries or in a PCA branch.

As each PCA passes around the cerebral peduncles, it forms a series of branches to the midbrain, and gives rise to a series of long, slender penetrating arteries that supply much of the hypothalamus and thalamus. The circle of Willis partially surrounds the hypothalamus, and additional small perforators from its other vessels also help to supply the hypothalamus. After the basal ganglia and internal capsule, the thalamus is the next most frequent site of hypertensive intracerebral hemorrhage and lacunar infarcts. Thalamic hemorrhages may be confined to the thalamus, or the bleeding may involve the neighboring internal capsule, subthalamic nucleus and midbrain, and even rupture into the third ventricle.

The cortical branches of PCA supply the posterior medial parietal lobe and the splenium of the corpus callosum, inferior and medial part of the temporal lobe including the hippocampal formation, and the medial and inferior surfaces of the occipital lobe. 

Recall that atheromatous plaques form at branchings or curves in vessels. In the vertebral-basilar circulation they are most frequent in the cervical part of the vertebral arteries and at their junction to form the basilar artery. They are also common in the PCA as it swings around the midbrain on its way to medial parts of the parietal, occipital, and temporal lobes.