Diagrams
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.

Penetrating branches of PCA participate in supplying the following key functional areas:
  • Diencephalon including thalamus, subthalamic nucleus, and hypothalamus
  • Midbrain including cerebral peduncle, third nerve and nucleus, red nucleus and its connections, superior cerebellar peduncle, reticular formation

Cortical branches of PCA participate in supplying the following key functional areas:
Posterior branches to the parietal and occipital lobe
  • Optic radiations and striate cortex (the primary visual cortex may be entirely supplied by PCA, or the tip of the occipital lobe where the fovea is mapped may be located in the border zone shared by PCA and MCA)
  • Splenium of the corpus callosum (these crossing fibers participate in the transfer of visual information to the language-dominant hemisphere)
Anterior branches to the medial temporal lobe
  • Hippocampal formation and the posterior fornix (these structures are critical for laying down new declarative memories

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.