Border Zones vs Core Territories

Concept of border zone versus core territory of the cerebral vessels 

We have seen that the cerebral cortex is supplied by the cortical branches of MCA, ACA, and PCA. These vessels reach the cortex, divide, and give off branches that penetrate the cortex perpendicularly. Long arteries and arterioles pass through the gray matter and penetrate the white matter to a depth of 3-4 cm, where they supply the core of the hemisphere's white matter. These vessels intercommunicate very little, and thus constitute many independent small systems. The deeply penetrating vessels that branch directly from major arterial stems (like the lenticulostriate branches of MCA) also do not interconnect. Therefore if one of these vesselsis occluded, the tissue it normally supplies will be infarcted since it has no other source of blood flow.

By contrast, the surface arteries supplying the cortical gray matter do anastomose. The anastomoses form a continuous network of tiny arteries covering the perimeters or border zones between core (central) territories of the 3 major cerebral arteries. Since these anastomoses are usually tiny, they carry little blood. Therefore, if one of the major arteries is occluded, the anastomoses cannot carry enough blood to rescue the core territory of that artery from infarction. However, they may be able to supply the border zone, so that the size of the infarction is reduced a little. Conversely, if there is a dramatic impairment of systemic blood pressure (secondary to cardiac arrest or massive bleeding), or decreased pressure specifically in the carotid system (due to an occlusive event), the regions of each vascular territory farthest from the core will become ischemic first. Thus there is risk of infarction specifically in the border zones.